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Primary Textures and Mineral Associations 

·in the Ultramafic Zone of the 

Stillwater Complex 

Montana 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 358 . 

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Primary Textures and Mineral Associations 

in the Ultramafic Zone of the 

Stillwater Complex 

Montana 

By EVERETT D. JACKSON 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 358 

The petrographic features and origin of the layered 

chromitites, bronzitites, and harzburgites 

in the lower part of the 

Stillwater complex 

UNITED STATES G.OVERNMENT PRINTING OF.FICE,PWASHINGTON : 1961 i '. : ~. ~ •. . : ..... f ... .. -~ ~ ·~ .. .. . . ; ' . f • 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

The U.S. Geological Survey Library has cataloged this publication as follows: 

Jackson, Everett Dale, 1925-

Primary textures and mineral associations in the Ultramafic zone of the Stillwater complex, Montana. Washington, U.S. Govt. Print. Off., 1961. 

v, 106 p. illus., map, diagrs., tables. 30 em. (U.S. Geological Survey. Professional paper 358) 

The petrographic features and origin of the layer.ed chromitites, 

bronzitites, and harzburgites in the lower part of the Stillwater com·-

plex. 

Bibliography : p. 100-101. 

1. Petrology-Montana. 2. Mineralogy-Montana. 3. Rocks, Igneous. I. Title: Stillwater complex, Montana. (Series) 

Fo~ sale by the Superintendent of Documents, U.~. Government Printing Office 

Washington 25, D.C. - Price $1 (paper cover) 
CONTENTS 

Abstract--------------~----------------------------

Introduction __________ ·,- ___________________________ _ 

Fieldwork, acknowl~dgments, and scope of report_ __ 

General geology of the complex.------------------ Composition of the rocks ___________________________ _ 

~1ineralogY-------~----------------------------

Rock types in the Ultramafic zone _______________ _ 

Lithologic similaritiEis and contrasts ______________ _ 

Textures of the rocks ___ .:. ___________________________ _ 

Primary precipitate~ ___________________________ _ 

Shape of the settled crystals _________________ _ 

Euhedra __ ~---------~------------------ Subhedra and anhedra __________________ _ 

Discussion:. _______ ~ ___________________ _ 

Distribution of: the settled 'crystals ___________ ~ 

Basal zon~----------~------------------

Ultramafic zone, Peridotite member ______ _ 

Ultramafic zone, Bron~itite member ______ _ Discussion ____________________________ _ 

Internal character of the settled crystals ______ _ Inclusions~:-- _______________________ .. __ 

Oriented intergrowths __________________ _ 

Zoning ___ ~ _________________ · __________ _ 

Fractures-~----------------------------

Undulatory extinction.,. _________________ _ 

Grain size aild grain-size distribution ___ · ______ _ Grain size_; ______________ :... _____________ _ 

Sorting ___ ~---------~------------------ Size-density relations ___________________ _ 

Discussion·~ ___________________________ _ 

Orientation of the settled crystals ____________ _ 

Bronzite __ ·~ _________________________ ~ __ 

Olivine _____________ :... _________________ _ 

Discussion.:. _________ ~ _________________ _ 

Relations of the pri~nary preci~itate to· the interprecipitate materiaL~ ___________________________ _ 

Reaction replacement ____ ,_ _________________ _ 

Olivine--~ronzite ____ . __________ ~· ______ _ 

Olivine--6hrorriian augite ____________ ~ __ 

Bronzite__;..chromian ~ugite _____________ _ 

Volume relations _____ · __________________ _ 

Discussion _______________________ .,. ____ _ 

Secondary enlargement ________________ ~ ____ _ 

Poikilitic harzburgite and dunite ________ _ Bronzitite _____________________________ _ 

Chromitit~--- _____________ "" _ ""- ________ _ 

Olivine chromitite arid poikilitic chromite 

harzburgite _______ · __________________ _ 

Olivine bronzitite and granular harzburgite_ 

Volumetric relations ____________________ _ 

Stratigraphic and areal distribution ______ _ Discussion ____ :. ____ ~ ____ :.: __ ~ _______ .. __ _ 

Page 

1 Textures of the rocks-Continued 

2 Relations of the primary precipitate-Continued 

2 Initial porosity ____________________________ _ 

2 Bronzitite _____________________________ _ 

4 Poikilitic harzburgite _____ ~--------------

4 Chromitite ____________________________ _ 

5 Granular harzburgite_...: _________________ _ 

8 Olivine chromitite and poikilitic chromite 

11 harzburgite _______ -·~ _ ~: _____________ _ 

13 Discussion ____________________________ _ 

13 Interprecipitate materiaL ----------------------- 13 Shape of the interstitial materiaL ____________ _ 

13 Shape of the interstices _________________ _ 

~3 Shape of the poikilitic crystals ___________ _ 

13 Discussion ____________ ~ __________ · _____ _ 

14 Distribution of the interprecipitate materiaL __ _ 

14 Poikilitic harzburgite: ___ ~ ____ : --·-------- 14 Granular harzburgite ___________________ _ 

14 Bronzitite _____________________________ -

15 Chromitite __ · ____ --.:-- __________________ _ 

15 Olivine chromite and poikilitic chromite 

17 harzburgite ____________ · _____ - _-------

18 Stratigraphic distributio~- ______________ _ 

18 Discussion __ .., _________________________ -

18 Internal character of the interprecipitate ma- 20 teriaL ___ ~ ________ ~ _____________________ _ 

21 Inclusions _____________________________ _ 

28 Oriented intergrowths __________________ - 30 Zoning _______________________________ _ 

35 Size and size distribution of poikilitic crystals __ _ 37 Grain size ___ :_.:- _______________________ --

39 Size distribution_~ _______________ ------- 42 Discussion ____________ ~ _______________ _ 

45 Orientation of poikilitic crystals ________ c-. ____ _ 

De~elopment of dominant textures in the -Ultramafic 

47 zone _______________________________________ _ 

4 7 Automorphic-poikilitic texture,.. _______________ -

47 Hypautomorphic texture ____________________ _ 

4 7 Xenomorphic texture_~ ___ ~ _________ ~ _______ _ 

50 Development of the Ultramafic zone __ ~ ______________ -

50 Conditions of crystallization and deposition ________ -

52 Predepositional conditions ___________________ -

52 The magma ____________ -.- _____________ -

54 Primary crystallization products _________ -

54 Depositional conditions _______ ~ ___ .- _________ -

54 Agents of transportation and deposition __ -

Origin of the layering_ ~ __ · _________ - -- -- -

58 Postdepositional conditions ________________ --

58 Character of the crystal mush __ · ________ --

58 Interprecipitate magma _________________ _ 

60 Crystallization of. the interprecipitate ma- 61 terial ___________________ · __ - - - - - - - - - - -

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IV CONTENTS 

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Development of the Ultramafic zone-Continued Development of the Ultramafic zone-Continued 

Mineral associations in the Ultramafic zone _______ _ 86 Bottom crystallization __________________________ _ 

Primary precipitate ________________________ _ 86 Intermittent crystallization _____________________ _ 

Interprecipitate materiaL ___________________ _ 88 Intermittent magma injection _______________ _ 

Classification of rock products _______________ _ 90 Variable-depth convection __________________ _ 

Significance of the depositional cycles _______ ----- _ 

Summary of salient features ______________ -_--

91 Similarity to chemical sediments _________________ _ 

91 References cited _______________ · ____________________ _ 

Chemical aspects _________________________ - _ 92 Index ____________________________________________ _ 

History of the cycle ________________________ _ 92 

ILLUSTRATIONS 

FIGURE 1. Geologic index map of the Stillwater complex ______________________________________________________ _ 

2, 3. Photographs of bronzitite and granular harzburgite hand specimens. 2, Bronzitite; 3, Granular harzburgite __ 4. Photograph of poikilitic harzburgite hand specimen _________________________________________________ _ 

5, 6. Field photographs of primary structures in the Ultramafic zone. 5, Secondary dunite cutting layered granular 

harzburgite. 6, Planar structure in bronzitite ____________________________________________________ _ 

7, 8. Photomicrographs of pegmatitic segregations in layered ultramafic rocks. 7, Volume-for-volume replacement of olivine by gabbro pegmatite; 8, volu~e increase during replacement of olivine by olivine gabbro 

pegmatite ________________________________________________________________________ -.-- _________ _ 

9, 10. Field photographs of primary structures in the Ultramafic zone. 9, Layering in chromitite and poikilitic 

harzburgite; 10, Layering in chromitite and olivine chromitite ______________________________________ _ 

11. Stratigraphic variation of settled minerals in a typical cyclic unit of the Peridotite member ______________ _ 

12-15. Photomicrographs showing internal characteristics of settled minerals. 12, Inclusions in chromite crystals; 

13, Fine-grained chromite inclusions in olivine; 14, Clinopyroxene lamellae in euhedral bronzite; 15, Clinopyroxene lamellae in partially replaced bron:zite ___________________________________________________ _ 

16-18. Photomicrographs showing undulatory banding in olivine grains. 16, Early deformation texture in poikilitic 

harzburgite; 17, Weak undulatory banding of olivine in poikilitic harzburgite; 18, Compound undulatory 

banding of olivine in olivine chromitite __________________________________________________________ _ 

19. Stratigraphic distribution of mean diameters of settled bronzite and olivine crystals in the Peridotite member_ 

20-21. Cusp texture illustrated. 20, Photomacrograph of cusp texture at chromitite-poikilitic harzburgite contact; 

21, Field photograph of cusp texture ____________________________________________________________ _ 

22-23. Field photographs of layered structures in the Ultramafic zone. 22, Size-graded bedding in olivine chromitite; 

23, Local incon"formities in chromitite-dunite section ______________________________________________ _ 

24. Stratigraphic distribution of mean diameters of settled bronzite crystals in bronzitite ____________________ _ 

25. ·Stratigraphic distribiltio~ of mean diameters of settled chromite crystals in chromitite ___________________ _ 

26. Grain-size distribution curves of chromitite and sandstone ___________________________________________ _ 

27. Comparison of size-frequency distributions in layered rocks containing only one settled mineraL __________ _ 

28. Comparison of size-frequency distributions of coexisting olivine and bronzite crystals in layered granular harzburgites __________________________________________________________________________________ _ 

29: Comparison of size-frequency distribution of coexisting olivi.ne and chromite crystals in structureless olivine 

chromitites and poikilitic chromite harzburgites _________________ -: ______ _,_ __________________________ _ 

30. Photomacrograph of layered olivine chromitite-dunite stratigraphic sequence ___________________________ _ 

31. Comparison of size-frequency distributions of olivine and coexisting olivine and chromite in current-bedded 

dunites and olivine chromitites ____________________________________________ -.- ___________________ _ 

32. Ratios of sorting coefficients of coexisting olivine and chromite in olivine chromitite and poikilitic chromite 

harzburgite __________________________________________________________________________________ _ 

33. Ratios of mean diameters of coexisting olivine and chromite in olivine chromitite and poikilitic chromite 

harzburgite ___________________________________________________________________________________ _ 

34-37. Photomicrographs of settled mineral distribution in the Ultramafic zone. 34, Coexisting settled olivine and 

bronzite in granular harzburgite; 35, Sharp change in grain size within a chromitite layer; 36, Clustering of 

olivine crystals in olivine bronzitite; 37, Chain structure in chromitite _______________________________ _ 

38. Fabric diagrams of 80 anchiequidimensional bronzite grains in bronzitite _________________ -. _____________ -

39. ·Fabric diagrams of 80 prismatic bronzite grains in olivine bronzitite ___________________________________ _ 

40. Fabric diagrams of 80 broad prismatic bronzite grains in bronzitite ____________________________________ _ 

41. Fabric diagrams of 80 broad prismatic bronzite grains in olivine bronzitite _______________________ -.,. ___ --

42. Azimuthal distribution of bronzite Z=c axes in specimens showing Z girdles ____________________________ _ 

43. Fabric diagrams of 80 anchiequidimensional olivine grains in poikilitic chromite harzburgite ______________ _ 

44. Fabric diagrams of 80 anchiequidimensional olivine grains in poikilitic harzburgite _________________ ------

45. Azimuthal distribution of olivine Y = c axes in specimens showing Y girdles ________________________ -----

46. Distribution of undulatory olivine grains in poikilitic harzburgite ______________________________ --------

47. Fabric diagrams of average positions of 50 flattened olivine grains in poikilitic chromite harzburgite _______ _ 

48. Azimuthal distribution of olivine Z =a axes in the layering plane __________________________________ -----

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CONTENTS v 

Page 

49. Distribution of undulatory olivine_________________________________________________________________ 45 

50-52. Photomicrographs of olivine---7bronzite and olivine---7augite reaction textures. 50, Reaction replacement of 

settled olivine in poikilitic harzburgite; 51, Reaction replacement of settled olivine by bronzite; 52, Reaction replacement of settled olivine and bronzite in granular harzburgite___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 48 

53. Photomacrograph of reaction replacement texture in poikilitic chromite harzburgite_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 49 

54-55. Photomacrographs of bronzite---7augite reaction textures. 54, Reaction replacement of settled bronzite in 

bronzitite; 55, Reaction replacement of settled bronzite by augite____________________________________ 51 

56. Relations of volumes of settled minerals lying between pyroxene oikocrysts to volumes of settled minerals re- placed within pyroxene oikocrysts_____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 53 

57-59. Photomacrographs of olivine-rich rocks showing progressively advanced degrees of secondary enlargement. 

57, Poikilitic harzburgite virtually unaffected by secondary enlargement; 58, Poikilitic harzburgite affected 

by intermediate secondary enlargement; 59, Dunite showing complete secondary enlargement___________ 55 

60-62. Photomacrographs of bronzitites showing progressively advanced degrees of secondary enlargement. 60, 

Bronzitite virtually unaffected by secondary enlargement; 61, Bronzitite affected by intermediate secondary enlargement; 62, Bronzitite showing complete secondary enlargement_____________________________ 56 

03-64. Examples of heterogeneous secondary enlargement in chromitites. · 63, Field photograph of massive and disseminated clots in chromitite layer; 64, Photomicrograph of massive and disf?eminated clots in chromitite_ _ 57 

65-68. Photomicrographs showing details of secondary enlargement and relations between interprecipitate minerals 

in the Ultramafic zone. 65, Secondary enlargement of settled olivine in olivine chromitite; 66, Secondary 

enlargement of settled bronzite in olivine-rich granular harzburgite; 67, Details of contact between interstitial plagioclase and interstitial bronzite in poikilitic harzburgite; 68, Details of contact between interstitial plagioclase and interstitial augite in bronzitite_---------------------------------------------- 59 

69. Frequency distribution of volume of interstitial material, by rock type_________________________________ 60 

70. Relations between thickness of correlative subunits and amount of secondary enlargement_________________ 61 

71. Sectional shapes of interstitial minerals in poikilitic harzburgite. A, Interstitial plagioclase surrounding 

olivine crystals; B, Interstitial bronzite surrounding olivine crystals__________________________________ 65 

72-75. Photomicrographs of textures of interprecipitate minerals in the Ultramafic zone. 72, Plating of chromite 

by bronzite; 73, Plating of chromite by plagioclase; 7 4, Augite replacing bronzite in poikilitic harzburgite; 

75, Biotite in poikilitic harzburgite___ _ __ _ _ __ _ _ __ _ _ _ _ ______ _ _ _ _ _ _ _ _ _ _ ___ __ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ 68 

76-79. Photomicrographs of final interprecipitate crystallization products in rocks of the Ultramafic. zone. 76, 

Granular harzburgite with pockets of plagioclase characterized by extreme zoning; 77, Interstitial quartz 

in bronzitite; 78, Grossularite-pyrope associated with quartz in bronzitite; 79, Grossularite-pyrope associated with plagioclase in bronzitite______________________________________________________________ 70 

80. Diagrammatic. illustrations of order of crystallization from the interprecipitate magma___________________ 74 

81-84. Photomicrographs of textures of interprecipitate minerals in the Ultramafic zone. 81, Interstitial augite 

replacing interstitial bronzite in chromitite; 82, Clinopyroxene lamellae in secondary enlarged bronzite crystal; 83, Clinopyroxene lamellae in interprecipitate bronzite; 84, Orthopyroxene lamellae in interprecipitate 

augite--------------------~----------------------------------------------------~---------------- 75 

85. Stratigraphic distribution of diameters of bronzite oikocrysts in poikilitic harzburgite layers_______________ 78 

86. Relation between size of settled olivine and size of poikilitic bronzite in poikilitic harzburgite _ _ _ _ _ _ _ _ _ _ _ _ _ 78 

87-88. Examples of ophitic texture in the chilled gabbro and layering in the Ultramafic zone. 87, Photomicrograph 

of chilled gabbro at base of the Stillwater complex; 88, Photograph of contact between poikilitic harzburgite and granular harz burgi te _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 84 

89. Ratios of settled minerals in rocks of the Ultramafic zone_____________________________________________ 87 

90. Ratios of principal interstitial minerals in rocks of the Ultramafic zone__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 90 

91. Hypothetical crystallization behavior in a thick convecting sill_________________________________________ 95 

92. Diagrammatic illustrations of proposed mechanism for crystallization in the lower part of the Stillwater 

complex-------------------------------------------------------------------------------------- 97 

TABLES 

Page 

TABLE 1. Average modes of principal rock types ___________________________ ·--------------------------------------- 5 

2. Arithmetic mean diameters of chromite crystals in a 7-inch-thick massive chromitite layer------------~-------- 26 

3. Relative settling velocities of coexisting olivine and chromite ____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 34 

4. Constituents within and between pyroxene oikocrysts in poikilitic harzburgite and bron~itite, in volume percent____ 50 

5. Average amount of enlargement. of settled crystals in the principal rock types, in volume percent_______________ 58 

6. Intracycle stratigraphic distribution of amount of secondary enlargement in the Mountain View section of the 

Peridotite member, in volume percent________________________________________________________________ 60 

7; Average amount of interstitial and secondary enlargement material in correlative subunits of the Ultramafic zone__ 61 

8. Settled and interstitial minerals in Ultramafic zone rocks having relatively small amounts of secondary enlargem~nt_ 69 

9. Diameter of poikilitic crystals in rocks of the Ultramafic zone______________________________________________ 77 

10. Relative stratigraphic thicknesses of harzburgite and bronzitite in three complete sections of the Ultramafic zone__ 86 
PRIMARY TEXTURES AND MINERAL ASSOCIATIONS IN THE ULTRAMAFIC ZONE OF THE 

STILLWATER COMPLEX, MONTANA 

By EVERETT D. JACKSON 

ABSTRACT 

The Stillwater complex can be divided into several major 

stratiform zones. The Ultramafic zone, with which this paper 

is concerned, lies near the lower margin of the complex along 

its entire exposure and has an average stratigraphic thickness 

of about 3,500 feet. The Ultramafic zone can be further subdivided into two members: the Peridotite member, which forms 

the stratigraphically lower two-thirds of the zone, and which 

is composed of conformably interlayered harzburgite, chromitite, bronzitite, and dunite; and the Bronzitite melll'ber, which 

forms the upper one-third of the zone and which is composed 

of a single thick layer of bronzitite. 

~rhe constituent grains of the layered rocks of the Ultramafic zone can be divided into two groups on the basis of texture: those that are commonly euhedral and occur in the rocks 

llS uniform-sized individual crystals; and those that are molded 

on these individual crystals. Minerals of the first group represent the ,primary precipitate, which crystallized in the magma 

column, settled, and accumulated on the floor of the magma 

chamber. Minerals of the second group represent the interprecipitate material, which crystallized in place in the pore 

spaces surrounding the primary precipitate. 

The three primary precipitate minerals of the Ultramafic 

zone--olivine, bronzite, and chromite--make up about twothirds by volume of the rocks. These settled crystals are euhedral and closely packed, and they would be self-supporting if 

the interprecipitate material were removed. The grain size 

ranges of any one settled mineral in any given layer are remarkably small, and size distributions are virtually lognormal. 

In layers containi'ng two settled minerals, each mineral tends 

to have lognormal size distributions, but tll.e two minerals are 

not necessarily in hydraulic equivalence. Planar structures are 

formed in the rocks wherever settled minerals are elongate or 

flattened; however, lineation is weak or absent. Changes in 

grain size, relative proportions, and presence or absence of the 

three settled minerals define layering planes. Of these factors, 

pres~nce or absence of the settled phases is most important, and 

the Peridotite member is made up of repetitive layers caused 

by the cyclic appearance and disappearance of olivine, chromite, and bronzite in regular sequence. 

Enhedral shapes of the settled crystals and simple relations 

between the primary precipitate and interprecipitate material 

have been obscured -in many rocks by reaction replacement and 

secondary enlargement. Three reaction pairs are recognized: 

olivine-bronzite, olivine-augite, and bronzite-augite. Secondary 

enlargement has occurred where settled crystals continue to 

grow after deposition, and the extent to which this process 

operated appears to be inversely related to the rate of crystal 

accumulation. Making allowance for these processes, the initial porosity of the crystal mush appears to have ranged between 20 and 50 percent and to have averaged about 85 

percent. 

The interprecipitate material makes up about one-third of 

the volume of the rocks. It can be divided into three types: 

( 1) secondarily enlarged, optically continuous rims on settled 

crystals; (2) material that has partially replaced settled 

crystals by postdepositional reaction replacement; and (8) 

material that simply fills interstitial cuspate cavities between 

euhedral or secondarily ·enlarged settled crystals. New phases, 

which occur as interstitial but not as settled constituents, include plagioclase, chromian augite, and (in some rocks) minor 

amounts of biotite, quartz, and grossularite-pyrope. The presence or absence, proportions, and order of crystallization of 

these minerals are largely dependent on the settled phases 

present in the rocks and on the amount of secondary enlargement. Interstitial minerals have strong tendencies to be poikilitic, and the average size of .oikocrysts appears to be related 

to the grain size of settled minerals in the rocks. Oikocrysts 

are randomly distributed and oriented ; they bear no relation 

to the layering plane. 

The textures of the rocks of the Ultramafic zone show every 

gradation between automorphic-poikilitic and xenomorphic, 

and this textural variation is largely dependent on the amount 

of secondary enlargement that has occurred. Rocks with little 

enlargement have interposition fabrics with euhedral settled 

crystals and contain relatively large amounts of predominantly 

poikilitic interstitial material; rocks with much enlargement 

have mosaic fabrics and contain little or no interstitial 

material. 

The three settled minerals have a strong tendency to occur 

singly rather than in combination with one another. Settled 

olivine and bronzite occur together in about 12 percent of the 

rocks of the Ultramafic zone; they are mutually exclusive. in 

the remaining 88 percent. Although quantitatively less important, chromite commonly occurs as the only settled mineral 

in chromitites. Settled chromite and bronzite are generally 

antipathetic, but chromite and olivine occur together in all 

proportions. In contrast to the wide proportional variation 

of the settled minerals in the various layers, the bulk composition of the interstitial minerals is relatively constant. In 

a general way, the composition of the interstitial material 

approaches that of the chilled gabbro at the base of the 

complex. 

The layered rocks of the Ultramafic zone are believed to 

have formed during crystallization of a single saturated basalt 

magma by accumulation of early crystal precipitates that fell 

layer on layer, to the floor of the magma chamber a~d afte~ deposition were enlarged or cemented by the magma from 

which they had crystallized. The textures and structures of 

the rocks suggest that the magma near the floor of the intru1 
) 

2 STILLWATER COMPLEX, MONTANA 

sion was essentially stagnant throughout the accumulation of 

the Ultramafic zone. Other relations indicate that crystallization of the primary precipitate took place near the bottom of 

the magma chamber, and that the cyclical compositional layers 

directly reflect changing crystallization products of the magma 

with time. It is proposed that the textures, mineral associations, and cyclical rock distributions in the .Ultramafic zone 

can best be explained by a mechanism involving continuous 

but variable-depth convection, which caused periodic refreshment of the stagnant magma undergoing crystallization in the 

lower part of the intrusion. Each set of cyclic compositional 

layers is therefore believed to be the product of a period of 

stability in the lower magma, preceded and succeeded by 

overturn. 

INTRODUCTION 

FIELDWORK, ACKNOWLEDGMENTS, AND SCOPE OF 

REPORT 

This paper presents some results of work <;lone between 1951 and 1955 as a part of a comprehensive 

investigation of the stratigraphy, geochemistry, and 

petrography of the Ultramafic zone of the Stillwater 

complex by the U.S. Geological Survey. Detailed 

mapping of the chromite deposits of the complex was 

done between 1939 and 1943, under the supervision 

of J. W. Peoples. Reports of this work, and geologic 

maps of most of the complex, have been published: 

Peoples and Howland (1940); Wimmler (1948); 

Howland, Garrels, and Jones (1949); Peoples, Howland, Jones, and Flint (1954); Jackson, Howland, 

Peoples, and Jones (1954); Howland (1955); Jones, 

Peoples, and Howland (1960). The present investigation has called for additional mapping and study of 

many problems which had to be bypassed because of 

the pressure of wartime urgency. I spent about 15 

months in the field during the summers of 1951 

through 1955 ; P. R. Vail assisted in the field work in 

1952 and 1953; and R. L. Christiansen assisted in 1954. 

Two members of the Geological Survey have been 

especially helpful to the conduct of this investigation. 

Arthur L. Howland introduced me to the Stillwater 

complex and worked closely with me during the first 

three summers of field~ork. The critical advice and 

ideas of Arthur H. Lachenbruch on the subject of heat 

relations have contributed substantially to the conclusions on the origin of tl).e Stillwater complex outlined 

in this report. 

GENERAL GEOLOGY OF THE COMPLEX 

The Stillwater complex is a differentiated "gravitystratified" igneous sheet, which strikes northwest 

across the northern margin of the Beartooth Mountains, in Stillwater, Sweetgrass, and Park Counti~s, 

Mont. (fig. 1). The exposed strike length of the complex is about 30 .miles, but it is terminated at both 

ends by faults. The maximum exposed stratigraphic 

thickness is 18,000 feet; Hess ( 1940, p. 377) estimates 

that the original thickness was 25-45 percent greater. 

In general, the rocks of the complex are unaltered 

and "\Yell exposed. Locally, the more mafic rocks have 

been serpentinized, but preservation of primary textures generally permits mapping of the original rock 

types. Five deeply glaciated canyons cut the complex 

nearly at right angles to its strike, thus exposing it 

continuously through a vertical distance of 5,000 feet. 

The general structural and age relations between 

the complex and adjoining formations have been described in a paper by Jones, Peoples, and Howland 

( 1960). In Precambrian time the parent magma of 

the complex was intruded as a horizontal sill into 

pelitic sedimentary rocks of unknown age, which were 

metamorphosed to cordieri te-hypersthere-biotite-quartz 

hornfels (Howland, 1954, p. 1264-1265). After it 

crystallized, the complex was locally intruded by granite, tilted about 25 degrees in the eastern part, beveled 

by erosion, and buried by sediments ranging from 

Middle Cambrian through M:esozoic in age. All these 

rocks were later deformed during the Laramide orogeny, and the complex now stands nearly vertical. 

~1uch faulting accompanied the rotation of the complex to its present position. 

Internally, the complex consists of a series of conformably layered subsilicic rocks that range in composition from dunite to norite and anorthosite. Peoples (1936, p. 358) has divided these rocks into four 

major stratiform units: the Basal chilled zone, Ultrabasic zone, Banded zone, .and Upper zone. According 

to Jones, Peoples, and Howland (1960), Hess has 

abandoned the terms Banded zone and Upper zone and 

has proposed a five-unit stratigraphic section for the 

rocks above the Ultramafic zone. 

Peoples' terminology is the basis for the informal 

stratigraphic nomenclature used in this report. The 

terms Basal zone, Ultramafic zone, and Banded zone 

correspond to Peoples' Basal chilled zone, Ultrabasic 

zone and Banded zone, respectively. In addition, the 

Ultramafic zone has been divided in this report into 

two members: the stratigraphically lower Peridotite 

member and the stratigraphically higher Bronzitite 

member. 

In the usage of this paper, the Basal zone includes 

those rocks that underlie the stratigraphically lowest 

harzburgite layer and is composed of pyroxene gabbros, norites, and feldspathic bronzitites. This zone is 

irregularly developed along the south margin of the 

complex. Locally it is absent, but at its maximum 

measured exposure it is 700 feet thick. Where well 

developed, the lower part of the Basal zone is composed of fine-grained ophitic gabbro containing many 
45°30' 

25' 

('! 45°20' 

110°15' 

EXPLANATION ----

m 

Sedimentary rocks 

Granite 

C'd 0 1- 1,..'7 

N , ban,, ~J,., >L"1 1."'"::>L 

"Oil> 

]~{ E]'c<,, 

; §: Norite, gabbro, and anorthosite ~~~~ . 

"1-t 

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l:l 

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5 

Bronzitite member 

Peridotite member 

Bronzitite and hai:zburgite 

Metamorphic 

-

rocks 

Basement complex 

10' 

u u 

}

2 0 2 Ozo 

We{ (f) 

.J w ~ ~ 

z 

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ID 

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<{ 

u 

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5' 

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Contact 

Dashed where approximately located 

110°00' 55' 

APPROXIMATE 

~ MEAN 

DECLINATION. 1959 

50' 109°45' 

MONTANA 

0 Livingston 

I 

I 

._,_('~ 

IDAHO I 

Billings 0 

~aparea 

WYOMING 

Ocody 

INDEX MAP 

5 0 5 10 Miles 

Fault 

Dashed where approximately located 

1-

Thrust fault 

T, upper plate 

FIGURE I.-Geologic index map of the Stillwater complex. 

Simplified from Jones, Peoples, and Howland, 1960 

~ ~ 

0 

t:l 

q 

n 

~ 0 z. 

(J.j 
4 STILLWATER COMPLEX, MONTANA 

inclusions; of hornfels. Upward the rock is coarser 

grained, has fewer .inclusions, and contains less clinopyroxene ··and plagioclase. The upper rocks of the zone 

are, for th,e most part, fine- to medium~grained layered 

bronzitite~ with automorphic-granular textures; but in 

several arkas a few lenticular layers of norite occur 

within th~ bronzitite. 

The IDtramafic ·.zone, with which this paper is 

largely c~ncerned, qontains dunites, chromitites, harzburgites, a:nd bronz~tites, and it overlies comformably 

the Basal.;zone, whe!l'e present, or lies directly on base- · 

ment. The Ultram~fic zone averages about 3,500 feet 

in thickness, includ~ng the rocks between the base of 

the stratigraphically lowest harzburgite in the complex 

and the bfl,se of the. stratigraphically lowest norite in 

the Banded zone; both the upper and lower cont3tcts 

are sharp and can ~e traced the entire length of the 

complex. The lower two-thirds of the Ultramafic 

zone, here called the Peridotite member, is composed 

of alternating, conformable layers of dunite, chromitite, harzburgite, arid bronzitite; the upper one-third 

of the zone, here called the Bronzitite member, is composed of a single th:ick unit of bronzitite. Most of 

these rocks have m~dium- to coarse-grained automorphic- to hypautomorphic-granular or poikilitic textures, but ~orne hav~ a xenomorphic-granular texture. 

The layered rock~ lying stratigraphically above the 

Ultramafic zone have a maximum exposed thickness of 

about 14,QOO feet and are composed of alternating 

layers of ~orite, gabbro, and anorthosite. N orite and 

gabbro are most abundant in the lower part of the section-; anorthosite and olivine gabbro are more common 

in the upper part. i The rocks are medi urn to coarse 

grained, ~nd most have hypautomorphic-granular or 

poikilitic textures. :· 

The most striking feature of the complex is the regular and 'lpersistent. layered character· of its rocks. 

Hess (194p, p. 377) has shown that the compositions 

of the miJieral phases in the complex change in a systematic fashion upward from the base. Superimposed 

on this gr~dual mineral compositional change are the 

rem·arkably continuous zones formed by abrupt 

changes in~ mineralogic associations. Within the zones 

are many alternating compositional layers defined by 

the prese11~e and absence. of individual minerals. Superimposed on the compositional layers are concordant 

layers defined by change in proportions, grain size, or 

· habit of n}inerals. The layering is remarkably similar 

to sedimen;tary bedding and is recognized and rna pped 

by the sa:rp.e criteria, such as change in mineralogic 

composition, proportion of minerals, texture, grain 

size, and orientation of constituent grains. Although 

certain similarities between .these layered rocks and 

detrital sediments exist, and although textural comparisons between the two are made in the text of this 

report, the writer proposes that the origin of the Stillwater rocks more nearly resembles that of chemical 

sediments. · 

The complex is believed to be a product of the fractional crystallization of,. a basaltic magma, formed by 

the settling of crystals, layer on layer, to the nearly 

horizontal floor of the magma chamber. The constituent grains of the layered rocks can be divided into 

two categories: well-sorted individual crystals, which 

make up about 65 percent of the layered rocks; and 

anhedral, commonly poikilitic grains, which are molded 

on the well-formed individual crystals, and which 

make up the remaining 35 percent of the rocks. The 

i~dividual crystals define the layering plane by upward changes in proportions or size; the interstitial 

material has no direct relation to the layering plane. 

Hess ( 1938a, p. 264-268) deduced from these and other 

textural considerations that the well-formed individual 

grains represent primary precipitates from the main 

magma, successively accumulated on the floor of the 

magma chamber, and that the interstitial grains represent material crystallized from the magma surrounding these settled crystals. I believe this to be the only 

tenable theory to explain the textures and structures 

of the layered rocks of the complex. 

The purpose of this paper is threefold: to describe 

the distribution and interrelations of the primary minerals of the Ultramafic zone of the complex; to point 

out the dominantly sedimentary nature of the texture 

of the rocks within the zone; and to discuss some of 

the genetic implications of these distributions and textures. Subsequent publications will describe the primary structures, stratigraphy, and geochemistry of the 

ultramafic rocks of the complex. 

COMPOSITION OF THE ROCKS 

MINERALOGY 

The rocks of the Stillwater complex are composed 

essentially of only five primary solid solution minerals: olivine, orthopyroxene, clinopyroxene, plagioclase, 

and chromite. In addition, very minor amounts of 

primary biotite, sulfide minerals, and quartz occur in 

some ro.cks. The compositions of the essential minerals 

and their overall isomorphous variations within the 

complex have been described in a series of papers by 

Hess ( 1939, p. 431; 1940, p. 377). 

All five of the essential solid solution minerals occur 

in the Ultramafic zone of the complex. Detailed compositional studies within the zone have not yet been 

completed, but the general range of composition is: 

bronzite, En75-9o; olivine, Foso-95; plagioclase, Aris5-s5; 
COMPOSITION OF THE ROCKS 5 

and chromite, in Thayer's (1946, p. 205) terminology, 

Cr52A1so (:l\1g5t)-CraoA12a (Mgso.5). Two clinopyroxenes from ti1e Ultramafic zone were determined by 

fiess (1949, p. 646 and 647) to be chromian augites 

with the compositions Cas7Mg5aFe7 and Ca4oMg52Fes. 

ROCK TYPES IN THE ULTRAMAFIC ZONE 

In the field, the layered rocks of the Ultramafic zone 

have been divided into four major compositional 

types: bronzitite, harzburgite, dunite, and chromitite 

(Peoples and Howland, 1940, p. 378-380). Although 

these rock names, with appropriate mineralogic and 

textural modifiers, have been found convenient for 

field classification, many of the rocks do not conform 

strictly to the definitions given in standard texts. The 

local usage of the terms is described below. In addition to the four types of layered rocks, small irregular 

intrusive bodies of dunite and of noritic, gabbroic, and 

troctolitic pegmatite occur within the zone. 

Bronzitite is used as a general term to designate the 

equigranular, medium- to coarse-grained rocks whose 

principal constituent mineral is bronzite (figs. 2, 60-

62) ; these rocks, by definition, contain no olivine. An 

average mode for bronzitite is given in column 1 of 

table 1. Bronzite constitutes 50-99 percent of bronziTABLE I.-Average modes of principal rock types 

Rock types 

(1) (2) (3) (4) (5) (6) (7) 

PoikiOli- Gran- Poiki- Iitle OliDron- vine ular litic chro- vine Chrozitite bronzi- harz- harz- mite chromi- mitite 

tite burgite burgite harz- tite 

burgite 

-------------- Number or modes 

averaged_____________ 38 ~ 22 16 10 22 

Brom.lto_______________ 83.2 83.1 8 17.6 26.7 16.7, 4. 2 

Olivine •••••••••••••••. -------- 4. 6 3 70.4 40.7 19.4 3. 9 

Chromlto______________ . 2 1. 0 . 2. 0 27.0 58.4 80.3 

Plagioclase............. 12.3 8.5 9.1 7.0 1.9 2.5 6.2 

Chromlan augite....... 4. 1 2. 8 3. 4 2. 8 3. 5 2. 8 5, 1 Biotite................. . 1 ····---- . 1 . 2 . 2 . 2 . 3 

Quartz................. . 1 ••••••.. -------- -------- _______________________ _ 

'l'otal............ 100.0 100.0 100.0 100.0 100.0 100.0 100. o 

tite, and occurs as individual equidimensional or broad 

prismatic grains or crystals, mostly between 1 and 4 

mm in diameter. Chromite is absent from most bronzitite but occurs in some in amounts up to 3 percent as 

1.1- to 0.4-mm octahedra (or clusters of octahedra), 

which are located between, rather than within, the 

bronzite crystals. All bronzitite contains some plagioclase and 'bright green chromian augite, invariably· 

interstitial to, and molded on, bronzite crystals. Plagiochise makes up from less than 1 percent to 35 percent of the rocks; chromian augite, from less than 1 

percent to 16 percent. Both these minerals are commonly poikilitic, enclosing large- numbers of bronzit~ 

grains. Interstitial quartz, generally accompanied by 

minute amounts of grossularite-pyrope garnet, occurs 

in bronzitite in the Basal zone and in the top of the 

Bronzitite member of the Ultramafic zone. In the 

bronzitite of the Basal zone, quartz occurs in amounts 

up to 5 percent; in the Bronzitite member of the 

Ultramafic zone, it is present in amounts less than 0.1 

percent. Biotite occurs in some bronzitite in. amounts 

up to 2 percent. 

Rocks which are texturally and mineralogically 

similar to bronzitite but which contain olivine in measurable amounts up to about 10 percent are termed olivine bronzitite, and the average mode for a rather 

. limited number of specimens is listed in column 2 of 

table 1. Olivine occurs as single crystals 1-2 ~m long, 

or as clusters of crystals almost always slightly smaller 

than the contiguous bronzite crystals. Olivine bronzitite grades into granular harzburgite on inc,rease of 

olivine to amounts greater than 10 percent. 

Harzburgite is used as a general term to ~esignate · 

the medium- to coarse-grained rocks ~omposed principally of olivine and bronzite, both in amounts. exceeding 10 percent by volume. Harzburgite has been divided into two types on the basis of· texture: granular 

harzburgite and' poikilitic harzburgite (figs. 3, 4) ; the 

average modes of each are given in .columns 3 and 4, 

table 1. Granuiar harzburgite is texturally identical 

with bronzitite and differs from it compositionally 

only by the substitution of olivine for bronzite. The 

volumetric sum of these two minerals makes up 65-95. 

percent of the rock. Harzburgite with an olivine con,.. 

tent of less than 10 percent grades into olivine:: bronzitite. Chromite is more commonly present in granular 

harzburgite than in bronzitite, but it does not occur in 

amounts greater than 4 percent in individual specimens. Plagioclase and chromian augite are ubiquitous 

interstitial constituents of granular harzburgite, in · 

amounts up to 20 and 10 percent, respectively. Quartz 

or grossularite-pyrope has not been observed~ ·aJthough 

biotite occurs in minor amounts in many specimens of 

granular harzburgite. Poikilitic harzburgite is compositionally similar to granular harzburgite but differs 

texturally. In poikilitic harzburgite, brop.zite,: occurs 

only in irregularly spherical oikocrysts, genera.lly %, -

4 inches in diameter, which enclose large numbers of 

olivine grains. Olivine crystals, generally ranging 

from 1 to 4 mm in diameter, are. scattered evenly 

throughout the rock, making up 50-90 percent of its 

volume.· Chromite is considerably more abundant in 

poikilitic harzburgite than in bro~zitite ~r :granular 

harzburgite, and almost all t~e chrq!fiitite layers in the 

com11lex lie within poikilitic harzbutgite layers. Chromite occurs in av proportioii.'s- with olivine, as small 
6 STILLWATER COMPLEX, MONTANA 

FIGURE 2.-Bronzitite hancl specimen. Dark euheclral grains are bronzite; darker areas are augite oikocrysts; white 

interstitial material is plagioclase. 

FIGURE 3.-Granular harzburgite hancl specimen. Black euhedral grains are bronzite; gray euheclral grains are olivine: 

white interstitial material is plagioclase. 

PHOTOGRAPHS OF BRONZITITE AND GRANULAR HARZBURGITE HAND SPECIMENS 
COMPOSITION OF THE ROCKS 

FIGU&E 4.-Photograph of poikilitic harzburgite hand specimen. Gray euhedral grains evenly distributed through rock are olivine; 1- to 2-inch 

gray and black ovals are bronzite oikocrysts; white interstitial material is plagioclase. 

7 
8 STILLWATER COMPLEX, MONTANA 

octahedra evenly scattered between the olivine grains. 

Poikilitic harzburgite co~1taining more than 5 percent 

chromite is termed poikilitic chromite harzburgite. 

Rocks in which chromite has displaced olivine as the 

principal constituent are termed olivine chromitite. 

Poikilitic harzburgite contains between 5 and 40 percent of poikilitic bronzite. Plagioclase is commonly 

interstitial to olivine between bronzite oikocrysts, making up as ·much as 20 percent of some rocks. Chromian augite is a less abundant constituent of poikilitic 

harzburgite than of the other rocks, but it does occur 

in a few rocks in amounts up to 12 percent. Minor 

amounts of biotite are present in all poikilitic harzburgite, but neither quartz nor grossularite-pyrope has 

been observed. 

Dunite is used as a .general term to designate rocks 

containing more than 95 percent olivine. Dunite is 

not abundant in the Stillwater complex, but it does 

occur locally, both as parts of primary layers and as 

secondary bodies cutting irregularly across the layering (fig. 5). The .. secondary dunites have been de-

. scribed by Hess ( 1938b, p. 33~339) and will be treated 

only incidentally in this paper. The layered dunites 

commonly contain accessory chromite and interstitial 

grains or small oikocrysts of bronzite. Dunite, on increase of interstitial bronzite, grades along the strike 

of the layering into poiki~itic harzburgite. In some 

dunites, bronzite is absent and the rock consists of an 

interlocking mosaic of olivine grains (fig. 59). Some 

dunite contains a few percent of interstitial plagioclase.· 

Olivine ch.romitite.is a term used to designate rocks 

whose principal constituent is chromite, but which 

contain lesser amounts of olivine as individual crystals 

scattered through the rock (fig: 9, central part). An 

average mode is given in column 6, table l. Chromite 

occurs as individual octahedral crystals, generally 1-4 

mm in diameter, which surround or are located between the olivine crystals. The olivine crystals are 

generally about 10 times the diameter of the chromite 

crystals, and on·increase in olivine abundance the rocks 

grade into poikilitic chromite harzburgite (fig. 53). 

Bronzite is generally present as an interstitial mineral 

in amounts up to 35 percent, and interstitial plagioclase and chromian augite may be present in amounts 

·up to 11 and 7 percent, respectively. Biotite is nearly 

always present in small amounts, but quartz has not 

been recognized. 

Chromitite is a term designating those rocks whose 

principal constituent is chromite, but which contain 

less than 5 percent individual olivine crystals (fig. 10; 

upper part of fig. 53). An approximate average composition is given in column 7 table 1. Chromite makes 

up from about 55 to 90 percent of such rocks and, as 

in other rock types, generally occurs as 1- to 4-mm 

octahedra. l\iost chromitite contains no individual 

crystals of olivine, but olivine may ·occur as an interstitial constituent; as such, it appears in amounts up · 

to 20 percent in individual specimens. Bronzite and 

plagioclase are common but not ubiquitous interstitial 

constituents, in some rocks occurring in amounts up to 

15 and 17 percent, respectively. Chromian augite is 

slightly more abundant in chromitite than in other 

rocks, in amounts up to 27 percent. Biotite is nearly 

invariably present up to a maximum of 2 percent, but 

generally in amounts less than 0.5 percent. Quartz 

does not occur. 

N orite, gabbro, and troctolite pegmatites occur 

within the Ultramafic zone, but not as layered rocks. 

Although not abundant, pegmatites occur as irregular 

bodies, with intrusive contacts, which locally cut the 

layering plane but generally are elongate parallel to 

it. Most commo.nly these bodies are irregularly developed along the base of massive chromitite layers; in 

general, the thicker the chromitite, the larger and 

coarser grained the pegmatite. Only those pegmatites 

associated with layered chromitite ·contain chromite, 

and these chromite crystals have identical sizes and 

size distributions with those of the layered chromitite 

invaded. Chromite crystals in pegmatite are therefore 

present as included and redistributed xenocrysts rather 

than as primary crystallization products. In the larger 

bodies, pyroxene and plagioclase cryst~ls attain lengths 

in excess of a foot, but olivine crystals greater than 

3 inches in diameter have not been obseJ"ved. Good 

examples of both volume-for-volume replacement (fig. 

7) and expansion during crystallization (fig. 8) have 

been observed. 

LITHOLOGIC SIMILARITIES AND CONTRASTS 

The various lithologic types of the layered rocks 

of the Ultramafic zone have a common compositional 

feature: all consist principally of individual crystals 

of bronzite, olivine, or chromite, or.a mixture of these 

three constituents, set in a gabbroic mesostasis of plagioclase, bronzite, and augite. The volumetric sum of 

the bronzite, olivine, and chromite crystals ranges from 

50 to 99 percent, regardless of the proportions in which 

they occur. Mixtures of olivine and chromite occur 

in all proportions; mixtures of olivine and bronzite 

occur through the range from 99-20 percent bronzite 

to 1-80 percent olivine, but chromite and bronzite are 

generally antipathetic. Although such mixtures do 

occur, there is an extreme tendency for these minerals 

to occur singly. Rocks composed only of bronzite and 

mesostasis constitute about 65 percent of the volume of 
COMPOSITION OF THE ROCKS 

FrouuE 5.-Secondary dunite cutting layered granular harzbnrglte. du =dunite; gh =granular harzburg!te. 

FIGUH>~ 6.-Planar structure in bronzitite. Note that foli ation of bronzite is not perfect but that many grains stand at 

high angles to the layering plane and others appear to be propped up by underlying grains. Highly refl ec ting area a t 

left center is augite oikocryst. 

FIELD PHOTOGRAPHS OF PRIMARY STRUCTURES IN THE ULTRAMAFIC ZONE 

9 
10 STILLWATER COMPLEX, MONTANA 

FIGURE 7.-Volume-for-volume replacement of olivine by gabbro pegmatite. Chromite grains are unaffected, and 

the delicate layering is not disturbed. aug= augite; pc=plagioclase. Crossed nicols. 

FiGURE 8.-Volume increase during replacement of olivine by olivine gr.bbro pegmatite. Chromite grains are 

unaffected, but have been redistributed, destroying the layering within the area occupied by the pegmatite. 

Layers above the segregation have been bowed upwards. br=bronzite; ol=olivine; pc=plagioclase. 

Crossed nicols. 

PHOTOMICROGRAPHS OF PEGMATITIC SEGREGATIONS IN LAYERED ULTRAMAFIC ROCKS 
TEXTURES OF THE ROCKS 11 

the Ultramafic zone; rocks composed only of olivine 

and mesostasis constitute another 25 percent; rocks 

containing mixtures of olivine and bronzite make up 

only about 10 percent of the zone. Although rocks in 

which chromite is a major constituent make up only 

a small part of the volume of the Ultramafic zone, 

many of these rocks consist only of chromite and mesostasis. 

The volumetric sum of the interstitial grains ranges · 

from 1 to 50 percent, regardless of the volumetric proportions present. In contrast to the wide proportional 

variation of the peridotitic crystals in the various 

rocks, the composition of the mesostasis is relatively 

constant; plagioclase seldom if ever constitutes more 

than 35 percent of any rock in the zone, clinopyroxene 

seldom more than 25 percent, and interstitial bronzite 

seldom more than 50 percent. Plagioclase and clinopyroxene occur interstitial to all the peridotitic major 

constituents, but bronzite occurs interstitially only in 

rocks which contain few or no individual bronzite 

crystals. Olivine is not common as an interstitial constituent and when it does occur is confined to chromitites containing no individual olivine crystals. 

TEXTURES OF THE ROCKS 

The minerals of the Ultramafic rocks can be divided 

into two general groups on the basis of shape and 

mutual grain relations: those \vhich are commonly 

euhedral and always occur as individual grains or 

crystals, and those which are invariably anhedral and 

molded around these individual crystals. Olivine and 

chromite belong to the first group, plagioclase, clinopyroxene, quartz, and biotite to the second. Bronzite 

is unique, being an important constituent of both 

groups in the Ultramafic zone, but bronzite of the two 

habits does not generally occur together. Minerals of 

the first group are considered to be the primary precipitate, which crystallized and settled from the molten 

magma and successively accumulated at its floor; minerals of the ·second group crystallized at a later stage, 

in the pore space surrounding the crystal accumulate. 

The distinction between the two groups in analogous 

to the distinction made in clastic sedimentary rocks 

between detrital grains and cement: thus, the former 

came into existence outside of, and prior to, the formation of the rock of which it now forms a part; the 

latter was formed in the places it now occupies in the 

rock. 

In distinguishing between these two genetic groups 

in the Stillwater complex, Hess (1939, p. 430-432) 

used the term "settled crystals" to describe those grains 

that fell to form a crystal mush, and the term "interstitial mineral" to describe the material crystallized 

from the interstitial liquid between the settled crystals. 

Wager and Deer (1939, p. 127-132), who encountered 

similar textural relations in the layered rocks of the 

Skaergaard intrusion, used the terms "primary precipitate" and "interprecipitate material." They defined (p. 127) the primary precipitate as "· .. discrete 

crystals or small glomeroporphyritic groups which 

separated from the overlying magma ... " and the 

interprecipitate material as " ... crystallized from tlre 

magma surrounding the primary precipitate." 

In this report the terms "settled crystals" and "primary precipitate" will be used synonymously to describe those crystals that grew in and were separated 

by settling from the overlying magma. The term 

"interprecipitate material'~ will be used to include all 

minerals that grew in place in the rocks. I prefer to 

reserve "interstitial" for a descriptive term without 

genetic connotation. 

The primary precipitate of olivine, bronzite, and( or) 

chromite makes up about two-thirds of all the layered 

rocks of the Ultramafic zone. The remaining onethird is composed of ( 1) interprecipitate enlargements 

of settled crystals, and (2) interstitial plagioclase, 

bronzite, and clinopyroxene "cement." The settled 

crystals were self supporting prior to development of 

the interprecipitate material. Settled crystals that are 

not equidimensional tend to lie with their long axes 

in a plane parallel to the fl9or of the intrusion (fig. 6) ; 

however, lineation in the foliation plane is weak or 

absent. Changes in grain size or proportions of the 

three settled minerals upward in the section produced 

layering in the rocks (fig. 9), and such layers are remarkably continuous along the strike. 

The interprecipitate material occurs as enlarged 

rims on settled crystals, in interstitial grains, and in 

large, sponge-shaped oikocrysts. The oikocrysts bear 

no relation to the layering plane and commonly cut 

across the delicate planar structures exhibited by the 

settled crystals (fig. 10). 

The distinction between individual euhedral settled 

crystals and space-filling interprecipitate material is 

evident in many rocks by change in mineralogy. Two 

processes have, however, operated after crystal settling 

to obscure such clear-cut relations: secondary enlargement and reaction replacement. Where crystals continued to grow after deposition, interference boundaries developed, and the original euhedral shapes of 

the settled crystals were obscured by addition of optically continuous material. Where settled crystals were 

partially replaced by reaction with the interprecipitate 

magma, the originally euhedral crystals were rounded 

and embayed. 
12 STILLWATER COMPLEX, MONTANA 

FIGURE 9.-Layering in chromitite and poikilitic harzburgite. The layering is defined by changes in grain size 

and mineralogy, and the unit of layeri ng is one grain t hick. Lower light gray layer is poikilitic harzburgite; cen tral darker layer is chromitite and olivine chromitite; upper light gray layer is poikilitic chromite harzburgite. Note rhythmic layering with "dense" chromitite at base and ''less dense" olivine chromitite at top, repeated twice in photograph. 

FIGURE 10.-Layering in chromitite and olivine chromitite. The layering is produced by the primary precipitate 

minerals olivine and chromite. The highly r efl ecting areas are bronzite oikocrysts cutting across layers. 

Lower black layer is chromitite; upper layers are olivine chromitite containing varied proportions of olivine 

and chromite. 

FIELD PHOTOGRAPHS OF PRIMARY STRUCTURES IN THE UL'l'RAliiAFIC ZONE 
TEXTURES OF THE ROCKS 13 

The layered rocks have some textures that can be 

described in ordinary igneous terminology, and others 

more accurately described in terms usually reserved 

for sedimentary rocks. In the following discussion, 

both igneous and sedimentary terminology will be used 

to describe appropriate textural features; but these 

terms will be used, in so far as possible, in their descriptive, rather than genetic, sense. 

PRIMARY PRECIPITATE 

SHAPE OF THE SETTLED CRYSTALS 

In many rocks of the Ultramafic zone, the individual 

grains of chromite, olivine, and bronzite are _completely 

bounded by crystal faces, but every gradation between 

euhedral and anhedral grains exists. Euhedral grains 

are associated with rocks that include relatively large 

amounts of interstitial material. With reduction in 

the amount of interstitial material due to enlargement 

of the settled constituents, these crystals develop generally polygonal interference boundaries. The degree 

of automorphism varies from layer to layer, but it is 

relatively constant in any given layer and can be predicted by knowledge of the amount of interstitial material within that layer. Independently of this effect, 

settled olivine and bronzite have rounded and embayed 

forms wherever they are in contact with interstitial 

pyroxene. 

EUHEDRA 

The euhedral crystals show only a limited number of 

preferred forms. The dominant form of chromite is 

the octahedron, although dodecahedral modifications 

·occur in some rocks, particulary mi the larger crystals. Strongly distorted crystals are uncommon, although in rocks where chromite crystals are clumped 

there is restricted development along join surfaces. 

In rocks where chromite is a minor constituent, the 

octahedral outlines as seen in thin section are generally 

sharp and angular; but in most chromitite, the edges 

of chromite crystals are rounded (fig. 72). The radius 

of rounding is generally less than one-tenth the long 

diameter o£ the octahedra. Olivine crystals are do-· 

matic and nearly equant, commonly slightly elongated 

parallel to the c axis, and slightly flattened on the 

( 010) crystal face. Brachydomes are well developed 

and basal terminations uncommon. Many olivine crystals have slightly rounded terminations. Bronzite 

crystals are domatic, generally equant to broad prismatic, less commonly tabular. Crystals are simple, 

generally formed by eight nearly equally developed 

faces in the prism zone terminated by flat domes. 

Typical prismatic forms are elongate parallel to the 

c axis, and the short axis is b (using the orthopyroxene 

orientation preferred by Hess and Phillips [1940, p. 

271-272]). Although most bronzites are stubby and 

nearly equidimensional, crystals with dimensions of 

1: 5: 10 are not uncommon. Bronzite crystals in any 

one layer generally have the same habit (fig. 60). 

Terminations of bronzite crystals as seen in thin section are generally sharper than those of chromite and 

olivine. 

SUBHEDRA AND ANHEDRA 

The subhedral and anhedral grains may be divided 

into two types: those that share mutual interference 

boundaries with each other, and those that are anhedral against the interstitial material. Chromite, olivine, and bronzite may all have interference boundaries in rocks containing·1ess than about 30 percent 

interstitial material. In rocks containing less than 10 

percent gabbroic mesostasis, the peridotitic grains have 

polygonal outlines and equigranular mosaic textures 

(figs. 59, 62). Only olivine and bronzite have anhedral boundaries with interstitial material: olivine has 

irregular, embayed contacts where surrounded by interstitial bronzite; and bronzite is embayed where it 

is in contact with clinopyroxene. 

DISCUSSION 

Textural evidence, which wfll subsequently be discussed, indicates that all of the settled crystals were 

originally euhedral, and that those not currently euhedral were altered in shape after deposition, either by 

continued growth or by reaction with the interprecipitate magma. 

The euhedral development and tendency toward 

equidimensional shapes are typical of crystals which 

have grown freely suspended in a saturated solution, 

and it is difficult to believe that rocks composed o£ 70 

percent of perfectly formed olivine or chromite or 

bronzite could possibly have crystallized in place. The 

rounded corners of some olivine and chromite crystals 

were probably caused by partial resorption prior to 

deposition. According to Buckley ( 1951, p. 45, 257), 

such effects are not uncommon in artifidally prepared 

crystals and can be caused by slight changes in temperature, pressure, viscosity, and other factors. 

DISTRIBUTION OF THE ·sETTLED CRYSTALS 

The distribution of the three settled minerals differs 

with stratigraphic height above the floor of the complex, and the repetitive occurrence and absence of these 

constituents give rise to the well-developed compositional layering. Along planes parallel to the layers, 

the kind and proportions of the settled minerals remain essentially unchanged. Description of distribution of the primary precipitate is therefore largely 

concerned with stratigraphy, a subject which will be 
14 STILLWATE-R COMPLEX, MONTANA 

outlined here, but which will be considered in more 

detail in a subsequent publication. 

Distribution of settled minerals within the various 

rock types is reasonably simple. Poikilitic harzburgites and dunites essentially contain only settled olivine, 

although a small percentage of settled chromite is present in almost all of them. In many chromitites, 

chromite is the only settled constituent, although a 

small percentage of settled olivine is present in some. 

In most bronzitites, bronzite is the only settled constituent. Mixtures of settled olivine and bronzite are 

present in olivine bronzitites and granular harzburgites, as are mixtures of settled olivine and chromite in 

olivine chromitites. A characteristic feature of the 

settled mineral distribution throughout the entire section is that settled phases do not persist vertically 

across the layering· but appear, disappear, and reappear. 

BASAL ZONE 

Along much of the base of the complex, where the 

chilled gabbro is separated from the Ultramafic zone 

by fine-grained bronzitite, settled bronzite is the first 

primary precipitate mineral of the Basal zone and of 

the complex. In several areas bronzite is joined by 

settled plagioclase in a few thin, lenticular hiyers within 

the bronzitite. In those areas where Basal zone bronzitite is not developed, the first precipitate is olivine of 

the Ultramafic zone. 

ULTRAMAFIC ZONE, PERIDOTITE MEMBER 

In the lower part of the Ultramafic zone,· the settled 

phases not only appear but also disappear in a regular 

order. Detailed field mapping has shown that the rock 

sequ-ence-poikilitic harzburgite, chromitite, olivine 

chromitite, poikilitic harzburgite, granular harzburgite, bronzitite-is repeated, with minor vari.ations, 

some 15 times within the Peridotite member. Several 

examples of these rock sequences, or cyclic units t, are 

illustrated in figure 19. A typical example of the variation within cyclic units, in terms of stratigraphic 

appearance and disappearance of settled minerals, is 

shown in figure _11. Olivine and chromite appear 

abruptly at the base of the unit, with olivine comprising about 99 percent of the settled material and chromite about 1 percent. No settled bronzite is present. 

About midway in the poikilitic harzburgite section, 

olivine abruptly disappears, succeeded by settled chromite alone. In this particular cyclic unit the accumu1 The term "cyclic unit," used here to describe repeated sequences of 

sharply defined layers, should be distinguished from the term "rhythmic 

unit," used by Br()wn ( 1956, p. 8) t() describe layers characterized by 

gradually changing pr()p()rti()ns ()f p.rimary precipitate minerals. 

lated chromite forms a layer 7 inches thick. At the 

top of the chromitite layer chromite is joined by olivine, and the two minerals appear together, with 

olivine gradually increasing in amount and chromite 

decreasing, to the top of the olivine chromitite layer. 

In many cyclic units, however, the decrease and disappearance of chromite and concomitant increase of 

olivine is not gradational but abrupt, with repeated 

alternations upward of chromite-rich and olivine-rich 

rock. At the base of the granular harzburgite layer, 

olivine and chromite are joined abruptly by relatively 

large quantities of settled bronzite (fig. 88), and 

throughout the remainder of the granular harzburgite 

layer, settled bronzite increases in amount at the expense of olivine. Chromite generally disappears completely within the granular harzburgite layer. At the 

top of the granular harzburgite layer, olivine abruptly 

ceases to be a settled constituent of the rock, and only 

settled bronzite crystals remain. In other cyclic units, 

however, the contact between granular harzburgite and 

bronzitite is smoothly gradational; in still others, the 

disappearance of olivine is oscillatory. At the top of 

the bronzitite layer, bronzite disappears abruptly, and 

the section is overlain by the poikilitic harzburgite 

layer of the succeeding cyclic unit. 

Ignoring accessory chromite in poikilitic harzburgites, the order of appearance of settled minerals within 

each cycle is olivine, chromite, bronzite; the order of 

disappearance is chromite, olivine, bronzite. The appearance of each new phase is abrupt, but the disappearance may be abrupt or gradational. 

ULTRAMAFIC ZONE, BRONZITITE MEMBER 

Essentially, the only settled constituent of the thick 

Bronzitite member of the Ultramafic zone is bronzite. 

The section begins at the upper contact of a granular 

harzburgite layer, as do the thinner bronzitite layers 

of the Peridotite member, and differs from them only 

in thickness. About 100 stratigraphic feet below the 

top of the. Bronzitite member, bronzite is joined by a 

few percent settled chromite, and this gradually diminishes in amount, disappearing before the bronzite 

is abruptly joined by large quantities of settled plagioclase at the base of the layered norites. 

DISCUSSION 

The direction of top and bottom in the complex can 

be demonstrated by slump structures, crystal sorting, 

and other gravity-controlled phenomena, so that the 

stratigraphic order of appearance and disappearance 

of the settled minerals can be said to be their order 

of accumulation. Further, relative ages of accumula-
TEXTURES OF THE ROCKS 15 

PARTIAL 

COLUMNAR 

SECTION 

MOUNTAIN VIEW 

AREA 

THICKNESS, 

IN FEET 

PERCENT OF SETTLED 

MINERALS 

OLIVINE ~ 

0 10 20 30 40 50 60 70 80 90 100 

~ BRONZITE AND CHROMITE 

100 90 80 70 60 50 40 30 20 10 0 

Bronzite 

Olivine 

Chrom· 

Olivine 

200 

FIGURE H.-Stratigraphic variation O·f settled minerals in a typical 

cyclic unit of the Peridotite member. 

tion of the settled phases, based on the law of superposition, are from older at the base to younger at the 

top. In this sense, the oldest crystal accumulate along 

most of the base of the complex is bronzite, succeeded 

within a very short stratigraphic distance by olivine 

and chromite, and, much higher in the section, by settled plagioclase ahd· augite. Within the cyclic units 

of the Peridotite member, the older settled minerals 

are olivine and chromite, followed by younger bronzite. 

In order to demonstrate that the order of accumulation is identical with the order of crystallization of 

the Stillwater magma, it is necessary to show that the 

magma crystallized in a closed system, that crystal 

products did not accumulate except at the floor, and 

that long-term suspension or floating of crystallization 

products did not occur. These conditions are discussed 

elsewhere in this report. 

INTERNAL CHARACTER OF THE SETTLED CRYSTALS 

INCLUSIONS 

Inclusions in settled olivine, bronzite, and chromite 

crystals are present but relatively uncommon. In all 

three minerals the distdbution of inclusions is related 

to the layering plane, in that thin layers containing 

abundant inclusions are separated by much thicker 

sections in which inclusions are rare. 

Inclusions in chromite can be divided into tw.o types, 

both relatively uncommon: (1) those which are made 

up of the same material that surrounds the chromite 

grain, and which are commonly in optical continuity; 

and (2) those which are made up of individual crystals that have no relation to the surrounding material. 

Inclusions of the first type occur in large erratic chro-

·mite grains, very sparsely scattered among normalsized chromite crystals, and seerri to be made up o£ a 

shell of smaller chromite grains. The large, compound 

grains tend to have euhedral shapes (fig. 12) but are 

commonly embayed. These crystals are believed to 

have formed in the magma as clusters of chromite 

grains that continued to grow on their outer surfaces 

while suspended. After deposition, their open irregular cavities were filled by interprecipitate minerals. 

Inclusions of the second type have smooth margins, 

are centrally located within the chromite grain, and 

are composed o£ olivine, usually accompanied by small 

amounts of pleochroic brown mica. The inclusions 

bear no relation to the material surrounding the 

grains; in some rocks, the inclusions are unaltered 

where interstitial material is partly serpentinized. 

The~e inclusions do not occur in rocks where chromite 

is a minor constituent, and they are rare even in most 

chromitites. In a few chromitite layers, however, from 

10 to 20 percent of the chromite grains contain these 

olivine inclusions. The distribution of tl~is type of 

inclusion suggests that small olivine crystals acted as 

nucleation centers for chromite growth. The olivine 

crystals apparently did not continue to grow after 

chromite crystallization began, for such inclusions are 

confined to massive chromitite layers. 

Olivine crystals, like those of chromite, rarely contain inclusions. Chromite is the only included mineral 

that was observed, and it occurs in three different distributions: ( 1) as single euhedra in the central part 

of olivine crystals; (2) as abundant small crystals 

around the periphery of olivine crystals ; and ( 3) as 

abundant small crystals evenly distributed throughout 

the olivine crystals. Although chromite occurs mostly 

between olivine grains rather than within them, sparse 

inclusions of the first type occur in all rock types containing the two. minerals. Where chromite crystals 
16 STILLWATER COMPLEX, MONTANA 

FIGURE 12.-Inc1usions in chromite crystals. chr=chromite; pc= 

plagioclase. Crossed nicols. 

FIGURE 14.-Clinopyroxene lamellae in euhedral bronzite. Lamellae 

pinch out at margin. Crossed nicols. 

FIGURE 13.-Fine-grained chromite inclusions (black) in olivine (white 

ovals). Olivine chromitite. Plane polarized light. 

FIGURE 15.-Clinopyroxene lamellae in partially r eplaced bronzite. Lamellae pinch out adjacent to replacing augite. Crossed nicols. 

PHOTOMICROGRAPHS SHOWING INTERNAL CHARACTERISTICS OF SETTLED MINERALS 
TEXTURES OF THE ROCKS 17 

occur both within and between olivine crystals, the 

included chromite crystals are generally smaller. Olivine crystals with inclusions of the second and third 

type occur only in a few olivine chromitite layers, but 

where they occur, each one in the layer has the same 

internal ~)1aracter. In some of these layers, the olivine 

crystals have a clear core and marginal chromite inclusions; in others the olivine is crowded with chromite 

inclusions throughout, and these become slightly larger 

in size toward the margin (fig. 13). In both cases the 

size break between the included chromite crystals and 

those packed between the olivine crystals is sharp, the 

included crystals usually being about one-fourth as 

large. Th~ included crystals tend also to have perfect 

terminations,. whereas those between oliyine crystals 

are slightly rounded. Inclusions· of all three types are 

believed to represent envelopment of chromite by olivine during the growth and differential settling of the 

crystals through the magma column. The central location of many of the chromite crystals of the first type 

may indicate that some olivine. formed around a nucleus ·of chromite. Inclusions of the second and third 

type occur only where chromite is an important constituent of the rock and probably indicate simultaneous crystallization of the two minerals. In the second type, olivine crystallization preceded that of chromite, and in the third, chromite apparently preceded 

olivine. 

Rare inclusions of olivine, chromite, and plagioclase 

have been qbserved in bronzite crystals. Settled bronzite crystals with olivine inclusion.s are confined to rocks 

in which olivine also occurs as discrete crystals. Included olivine grains are rounded and embayed, suggesting partia~ . replacement by bronzite;. discrete olivine crystals in the same rocks are larger and euhedral. In keeping with the general antipat~1y of ·chromite and settled bronzite, chrom:ite inclusions are extremely uncommon, being confined to rocks in which 

chromite also occurs between settled bronz.ite crystals 

In the few examples observed, crystals .of· chromite 

within and between those of bronzite appear to be 

about tl~~ same size. In one sp_ecimen cut from the 

Bronzitite member of the Ultramafic zone at its contact with the s~ratigraphical_ly lowest norite layer,. 

plagioclase .inclusions in bronzite were observed, but 

other specimens from .the same horizon do not contain 

them. In this specimen, the bron~ite crystals average 

about 2 ·mm in diameter, and about one-half .. of them 

contain one or more prisms of plagioclase, 0;2 mm in 

length, near tl~eir margins. No ·individual settlecl. 

plagioclase occurs in the rock, but that in the norite 

layer· above is the same size as the bronzite. It would 

appear that in this specimen plagioclase and bronzite 

had crystallized 'in part simultaneously. 

Certain inferences as to the paragenesis of settled 

minerals coexisting in the rocks can be drawn from the 

nature of their inclusions. It has been demonstrated 

that, in some cases, olivine and chromite crystallized 

simultaneously. The absence of inclusions in olivine 

and chromite in most rocks containing mixtures of 

these minera1s can be explained in two ways: (1) olivine and chromite in all such rocks crystallized simultaneously, with widely spaced crystallization centers 

and with no tendency for one mineral to act as nuclei 

for the formation of the other; or, (2) olivine and 

chromite in most of these rocks crystallized at slightly 

different compositional horizons in the magma. _ 

A somewhat different problem exists in rocks containing mixtures· of settled olivine and bronzite. The 

well-known reaction relation between olivine and 

bronzite has been established in the Stillwater magma 

by the presence of embayed olivine grains in the centers of bronzite crystals. In most granu~ar harzburgite 

and olivine bronzitite, .however, oliv'ine individuaJs are 

not rimmed by bronzite~ but the two minerals occur 

side by side as euhedral crystals (fig. 34). These relations may have occurred in two ways: (1) the coexisting olivine and bronzite in the rocks represent the assemblage present at ·the reaction boundary between 

olivine and bronzite in a homogeneous magma; or, (2) 

the olivine and bronzite in the rocks crystallized alone 

at slightly different compositional horizons in the 

magma. In the first case, either reaction rims around 

olivine or rounded olivine grains would normally be 

expected, especially. on those individual crysta~s located in. the upper parts of the layers. In the ~econd 

case, assuming that bronzite must fall through magma 

in equilibrium with olivine,_ some evidence of melting 

of brohzite crystals might be seen, unless the settling 

velocity was rapid compared to the d.istance of fall. 

ORIENTED INTERGROWTHS 

Reguhir intergrowths in unaltered ch'romite have not 

been observed.· Many crystals of olivine, 4owever, 

contain minute, opaque blades of metallic rilinerals 

uniquely oriented on ( 100) of the host. The blades 

are discontinuous, generally average about O.o5· mm in 

length, and presumably represent exsolution of an iron 

or titanium oxide. 

·Hypersthene in the chilled gabbro at the base: o£ the 

Stillwater complex contains clinopyroxene of two 

types : ( 1) abundant coarse, discontin qous lamellae· or 

blebs,- 0.1 mm thick, oriented on irrational planes in 

the host ; and ( 2) · narrow; more regular lamellae ori.:. 
18 STILLWATER COMPLEX, MONTANA 

en ted parallel to ( 100) of the hypersthene. Poldervaart and Hess (1951, p. 472-489) and Brown (1957, 

p. 529-534) consider lamellae of the first type to be 

indicative of primary crystallization of pigeonite, 

which, on cooling, exsolved augite-for the most part 

oriented on ( 001) of the pigeonite-and, on further 

cooling, inverted to hypersthene. The presence of relict pigeonite lamellae is considered to indicate that 

these pyroxenes crystallized above the orthopyroxeneclii{6pyroxene inversion temperature (Hess, 1941, p. 

58~584). . 

Bronzite crystals in the layered rocks of the Basal 

zone and throughout the entire Ultramafic zone, on the 

other hand, contain only regular, well-developed clinopyroxene lamellae oriented parallel to (100) of the 

bronzite. These lamellae are commonly 0.001-0.002 

mm thick and are generally spaced 0.005-0.01 mm 

apart. No relict pigeonite lamellae have been observed. Bronzite with simple (100) lamellae has been 

described by Hess and Phillips (1938, p. 450-456); 

Poldervaart and Hess interpret this as indicating primary crystallization of orthopyroxene and exsolution 

of clinopyroxene on slow cooling. Pyroxene crystallization temperatures throughout the upper part of the 

Basal zone and the entire Ultramafic zone were, accordiJ?.g to Hess (1941, p. 582-584), below the orthopyroxene-clinopyroxene inversion temperature for 

their range in composition. 

Some indication of the relative sequence of clinopyroxene exsolution in settled bronzite can ·be obtained 

by examination of the margins of the crystals. In a 

zone aqout 0.05 mm thick around the peripheries of 

euhedral crystals, the ( 100) lamellae pinch out (fig. 

14). The tips:· of the narrowing lamellae extend to the 

terminating crystal face. Presumably, this is a primary border effect caused by the local reduction of 

unit volume from which Ca+2 can migrate during the 

exsolution. Similar pinching effects also occur around 

chromite inclusions in bronzite, and along a very few 

apparently pre~exsolution fractures. Lamellae in relict, partially resorbed bronzite surrounded by interstitial augite also pinch out at the bronzite-augite contact (fig. 15). This suggests that exsolution in settled 

bronzite occurred after crystallization of much of the 

interprecipitate magma. 

In summary, the character of the la.mellae indicates 

that all of the settled bronzite o~ the Basal and Ultramafic zones crystallized below its inversion temperature as primary orthopyroxene, cooled very slowly, 

and exsolved clinopyroxene lamellae at some time after 

at least the major part of the interstitial material had 

crystallized. 

ZONING 

Zoning has not been observed in euhedral settled 

crystals within the Ultramafic zone. In a few nearly 

monomineralic layers, however, anhedral olivine and 

bronzite grains have narrow, gradational selvages of 

slightly more iron-rich material, which are believed to 

be related to continued growth of these crystals after 

deposition. Zoned chromite has not been observed, although in a few highly altered rocks, rims of secondary magnetite around chromite grains can be seen in 

polished section. · FRACT.URES 

Almost all the minerals are intimately fractured on 

a microscopic scale, and many of these fractures are 

loci for whatever alteration the minerals exhibit. The 

great majority of fractures cut settled and interstitial 

material alike and are related to postconsolidation 

deformation. In a few rocks, however, tongues of 

interprecipitate material extend into fractures that cut 

settled crystals; equally uncommon, exsolution lamellae in bronzite grains pinch out on either side of 

prominent cracks. These relations suggest that, in a 

few places, settled crystals were fractured prior to the 

complete solidification of the interprecipitate magma, 

and prior to the exsolution of clinopyroxene from the 

bronzite. 

UNDULATORY EXTINCTION 

In several parts of the complex, near the base of the 

Ultramafic zone, the rocks are characterized by plastic 

deformation of layers, drag folds, and broken layers 

healed by dunite. Although the same compositional 

rock types present in the main part of the Ultramafic 

zone can be recognized in these areas, the rocks have 

xenomorphic textures, and the constituent grains show 

brecciated margins, patchy extinctions, b~n.t pyroxene 

exsolution lamellae, rupture on cleavage planes, and 

other evidence of postdepositional deformation (fig. 

16). It is in these areas that the crosscutting dunite 

segregations described by Hess ( 1938b, p. 334-340) 

occur, and these are also characterized by xenomorphic 

textures and deformed minerals. Although typical 

interstitial minerals are generally absent from these 

rocks, plagioclase and interstitial bronzite, where present, are highly strained, showing patchy extinctions 

and rupture surfaces on cleavage planes. Undulatory 

extinction is most highly developed in the olivine 

grains, all of 'vhich show repeated undulatory banding 

on planes nea.r (100), or continuous extinction changes, 

or patchy strain shadows. Differences in extinction 

position in the same olivine exceed 30 degrees in many 

arains but translation lamellae have not been observ- b ' ed. Chromite in some of these rocks is slightly anisotrophic. 
TEXTURES OF THE ROCKS 

FIGURE 16.-Early deformation texture in poikllitlc harzburglte. Grains are strongly undulatory with patchy extinctions and brecciated margins. ol=olivine; br=bronzite; pc=plagioclase. Crossed nicols. 

FIGURE 17.- Weak undulatory banding of olivine in poikilitic 

harzbnrgite. About one-third of the oliYine in the specimen 

shows some banding. ol==olivine; pc=plagioclase. Crossed 

nicols. 

FIGURE 18.-Compound undulatory banding of olivine in oli,•ine 

chromitite. .\11 olivine grains are undulatory and in some 

grains banding is repeated on ruptures. ol=olivine; chr= 

chromite. Crossed nicols. 

PHOTOMICROGRAPHS SHOWING UNDULATORY BANDING IN OLIVINE GRAINS 

541905 0--61--2 

19 
20 STILLWATER COMPLEX, MONTANA 

In contrast, the layered rocks that form the main 

bulk of the Ultramafic zone-stratigraphically above 

the deformed areas where present-are characterized 

by abundant interstitial plagioclase and pyroxene, undeformed layering, and absence of crosscutting ultramafic bodies. The plagioclase and pyroxene of these 

rocks show no evidence of intragranular strain, but 

olivine crystals in many of the layers show comparatively weak undulatory banding. The bands, which 

are 0.1-1.0 mm thick, are sharply defined along nonrational planes subparallel to (100). A complete range 

exists from rocks which contain no undulatory olivine 

to those in which all the olivine grains have extinction 

bands; however, the latter are rare. The number of 

bands per grain ranges from 2 or 3 in rocks in which 

only a small proportion of the olivine grains are undulatory, to 10 or 15 in rocks where all are undulatory. 

In olivine with 2 or 3 extinction bands, the crystallographic orientation of the subindividuals commonly 

differs by 1 to 3 degrees (fig. 17). In olivine with 

multiple extinction bands, the extinction positions in 

the subindividuals commonly change consistently in 

the same direction from one side of the grain to the 

other. In a few rocks where bands are highly developed, this consistent change may be interrupted by 

ruptures and repeated several times (fig. 18), so that 

a total extinction difference of 20 degrees may be produced. However, these ruptures-as well.as patchy or 

continuous extinction changes and brecciation-are extremely uncommon in the main body of the lJltramafic 

zone. Within these rocks, undulatory extinction is best 

developed and most common in layers containing mixtures of olivine a.nd chromite, and in adjacent poikilitic 

harzburgite. · It is less common and more poorly developed in granular harzburgite and olivine bronzitite. 

· Turner (1942, p. 280-300) considers undulatory extinction in olivine to be an expression of combined 

flexure gliding and incipient rupture of the olivine 

space lattice, and he believes that the presence of undulatory olivine is evidence of intragranular deformation after the complete solidification of the rock which 

contains them. This is abundantly substantiated by 

field evidence in the locally deformed areas near the 

base of the Ultramafic zone, and it seems likely that 

such deformation took place above serpentine stability 

temperatures (Bowen and Tuttle, 1949, p. 455). · In 

many of the rocks from the main part of the zone, 

however, undulatory olivine is the only evidence for 

internal deformation. The rocks are not sheared; they 

retain their depositional fabrics, and coexisting chromite and bronzite retain their euhedral shapes. Furthermore, undulatory and internally homogeneous olivine can be found in all proportions in .the rocks, apparently without favored orientations for either type 

(fig. 46). It would thus appear that olivine is very 

sensitive to deformation. 

A great deal more work would be necessary to account for the localization of undulatory minerals both 

areally and within samples, or to relate the preferred 

orientations to deformation mechanisms, or to the period of deformation. From field evidence, such as 

incompetent behavior of layers and healing of fractures, it seems reasonable to assign the major deformation of the local areas near the base of the Ultramafic 

zone to a period before complete solidification of the 

mush, and perhaps before the major part of the rocks 

of the zone were deposited. Very early deformation 

would also account for the general paucity of interstitial minerals in these rocks, for the trapped magma 

would have been squeezed out during deformation by 

filter press action. In the main part of the IDtramafic 

zone, it may be significant that undulatory olivine is 

more commonly developed in olivine chromitite than 

in other rock types. It seems possible that the relatively fine grain size of the chromitite makes it less 

competent than other rocks in the section. The single 

specimen investigated from the main part of the zone, 

which contained ·strongly undulatory olivine, proved 

to be a tectonite (fig. 47). Possibly this structure 

is related to the known Precambrian tilting, or to 

Laramide folding of the complex to its present position. If so, the effects are local only, for not only do 

many layers contain undeformed olivine, but interstitial quartz in the Basal zone and Bronzitite member 

shows no strain shadows or undulatory extinction. 

GRAIN SIZE AND GRAIN-SIZE DISTRIBUTION 

The superficial similarities of grain-size variation 

and distribution between settled crystals of the Ultramafic zone and clastic grains of sediments are striking. 

Layers formed by changes in grain size are parallel 

to the compositional layering, and in some layers 

change in grain size accompanies compositional change. 

Grain sizes are remarkably consistent along the layering plane. The distribution of grain sizes of settled 

olivine, bronzite, or chromite within almost all compositionally homogeneous layers shows a positive skew-

'riess that is largely eliminated by logarithmic transformation, and sorting characteristics most nearly resemble those of beach sands. ~ore detailed study, 

however, reveals many features of ~ize distribution distinctly different from tho~e of clasti_c sediments. Many 

rocks, for instance, contain mixtures of settled olivine 

and either bronzite or chromite. Each of the settled 

minerals in such rocks has approximately lognormal 

size ·distributions with small sorting coefficients, but 
TEXTURES OF THE ROCKS 21 

these settled minerals are most commonly not in hydraulic equivalence. Further, the sedimentary structures usually associated with coarse sands are not abundant . Scour and local unconformity occur but are 

extremely rare. uriequivocal crossbedding has never 

been observed in the ·ultramafic zone. Sn1all-scale sizegraded bedding is i;are and, in the few places where 

recognized, is just as commonly coarse-zone up as 

coarse-zone down. . The origin of the rocks of the 

Ultramafic zone is 'Qelieved to have been more similar 

to that of chemical than that of detrital sediments. In 

the following section, however, comparisons are largely 

with detrital rocks,, for size and sorting in chemical 

sediments have not been investigated to the same 

extent. · 

In order to describe adequately the grain-size variation in the closely p~cked settled crystals of the Ultramafic zone, quantitative size measurements were made 

using standard sed1mentary techniques. Because of 

the coherent nature of the rocks, all size measurements 

were m.a.de in the following manner: for the finer 

grained rocks, areas to be measured were blocked out 

on 2- by 2-inch thin· sections, ~nd the diameter of each 

grain within the area was measured. by means of the 

micrometer ocular 9f a mict:oscope; for the coarser 

grained rocks, measprements :were made with a millimeter scale on etched rock slabs. For each specimen, 

the apparent diameter of each grain as seen in section 

was measured and s~parately tabulated. In specimens 

that contained non~quidimentional grains, the maximum and minimum :_diameters of the grains were read 

and averaged. 'Vh~re more than one of the three settled minerals was present in, a specimen, grain sizes 

of each mineral were measured separately. In rocks 

for which mean diaineters only were desired, between 

75 and 100 grains of each mineral species were measured. In rocks for ~hich size~distribution curves were 

to be constructed, between 150 and 400 grains of . each 

mineral species were counted: and individually tabulated. In each case; grains were counted until cumulative arithmetic n1eans of ·the measurements were 

nearly stabilized. In additioi1, the maximum 95 percent confidence limit of· the inean was determined on 

one ''7

ell-sorted and one more poorly sorted sample and 

was found to be 8 and 12 p~rcent respectively. The 

tabulated measuren~ents wer~ assigned to :Y2 Udden 

size grades, number percentages for each interval were 

calculated and plotted as hi$.tograJ?S and cumulative 

curves, and quartile measures were read graphically. 

It should be recognized that grains measured in section 

show not only their true dia1neters, but diameters less 

than the true ones. Corrections for sectioning have 

not been applied to these me~surements, both because 

the results were to be used only for comparative purposes within a single· study and because of doubt as 

to the applicability of sectioning corrections to grains 

not spherical or ellipsoidal in shape (Greenman, 1951a, 

p. 271-272). The general effect of an arithmetic mean 

correction such as that proposed by Krumbein ( 1935, 

p. 482-496) would be to increase the reported arithmetic mean diameters about 25 percent. If the cumulative size distribution curves were corrected by the 

method proposed by Greenman (1951b, p. 447-462), 

the quartile sorting coefficients would be lowered about 

6 percent. 

GRAIN SIZE 

In the layered rocks of the Ultramafic zone, olivine 

crystals range in diameter from 0.5 mm to more than 

40 mm, but the great majority <?f grains are 1-4 mm. 

Bronzite crystals range from 0.5 mm to 15 mm, but, 

like olivine, most commonly are 1-4 mm. Chromite 

crystals are always s·maller than associated bronzite 

and olivine; in most rocks they are 0.1-0.4 mm in diameter, but they occur with observed extreme diameters 

of 0.02 mm and 5 mm. 

The vertical distribution of grain sizes within the 

Ultramafic zone is complex, and study of the variation 

has been confined to stratigraphic sections in the eastern 15 n1iles of exposure. 'Vithin the Basal zone of 

the complex in this area, there is a gradual, but somewhat irregular, increase in grain size from the chilled 

lower margin up to the base of the Ultramafic zone. 

The average grain size of the chilled marginal gabbro 

itself is about 0.3 mm, and the bronzitite layers above 

it contain euhedral bronzite crystals ranging in average diameter from about 0.7 mm to 1.0 mm. Upward, 

with the appearance of the first settled olivine (the 

base of the Ultramafic zone) a sharp change in si~e 

was observed, and the lowest ultramafic layer has an 

average grain size of about 2 mm. 

Within the Ultramafic zone, there are three types of 

sharp vertical size change: (1) A decrease in size invariably occurs from silicate to chromitite layers, because grains of chromite are always smaller than those 

of bronzite or olivine. (2) A size cha.nge occurs at the 

base of eac~1 poikilitic harzburgite layer, the overlying 

poikilitic harzburgite being coarser. grained than the 

layer stratigraphically below it. (3) Sharp size 

changes occur within some layers of bronzitite, harzburgite, and chromitite without a change in mineral 

constitution of the rock. Each of these types of size 

.variation is illustrated in figure 19. The chromite crysstals in chromitite in the lower part of cyclic unit 2 

have an average grain size about one-twelfth that· of 

the olivine on either side, and the chromite crystals in 

the lower part of cyclic unit 15 have about the· same 
. i 

22 

Cyclic Stratigraphic height 

unit above base of ultramafic zone, in feet 

Columnar 

section 

STILLWATER COMPLEX, MONTANA 

Specimen 

No. 

Volume ratio 

of settled 

bronzite 

to settled 

olivine 

1.7:1 

1.9:1 

1:1 

7.7:1 

3.7:1 

2.4:1 

0.4:1 

0.3:1 

5.7:1 

2.7:1 

2.2:1 

5.2:1 

Arithmetic mean diameter, 

in millimeters 

Settled 

chromite 

0.23 

0.20 

Settled 

bronzite 

2.0 

2.4 

3.8 

3.7 

2.1 

0.8 

1.4 

1.3 

2.7 

2.3 

2.4 

2.0 

1.9 

1.4 

Settled 

olivine 

2.2 

2.6 

2.7 

2.9 

2.5 

1.7 

1.5 

1.5 

2.3 

2.2 

2.4 

2.4 

1.9 

1.3 

Plot of diameters of bronzite 

and olivine crystals within 

cyclic units, in millimeters 

~Bronzite 

Olivine ________ --..o 

\ 

~ 

\ 

~ 

I or · I/ 1vme 

I 

I 

6 

I 

I 

I 

-c. 

EXPLANATION 

Bronzitite 

Poikilitic harzburgite 

Granular harzburgite 

Chromitite 

100 

200 

300 

--400 

500 

FIGURE 19.-Stratigraphic distribution of mean diameters of settled bronzite and olivine crystals in the Peridotite member in the Mountain 

View area . 
TEXTURES OF THE ROCKS 23 

size ratio. In detail, the boundary surfaces between 

silicate layers and layers of the finer chromitites are 

cuspate, analogous to conglomerate-sand contacts in 

sedimenbtry rocks (figs. 20, 21) .

2 Five examples of 

sharp increases in size between the poikilitic harzburgites and the rocks underlying them can be seen in figure 19, and these size breaks coincide with the compositional layering contacts. In the same figure, an 

example of a sharp grain size change within a compositional layer is shown in the bronzitite of cyclic unit 

2. Contacts at which such changes within compositional ]ayers occur are parallel to the boundaries of 

the layer. ~1ost such intralayer size changes observed 

are, like the example in cyclic unit 2, from coarse to 

fine upward in the section, and the grain size of the 

stratigraphica1ly higher, finer grained upper portion 

typically :increases gradationally away from the contact. Sharp grain size changes also occur in some 

chromiti tes (fig. 35), but in these, no grading above 

the contact has been observed. · 

In addition to the sharp variations in grain size, 

there are three types of gradual vertical grain-size 

changes within the Ultramafic zone: (1) small-scale 

size-graded bedding,, both right side up and upside 

down; (2) gradual increases an4 decreases in grain· 

size at intervaJs of 100 to 500 stratigraphic feet, which 

are related to the cycles; and (3) a gradual overall 

increase h1 grain size to near the center of the Ultramafic zone, followed by a decrease to the top. 

Small-scale size-graded beds are r~re features of the 

Ultramafic zone; only a few unequivocal graded beds 

have been seen in the field (fig. 22), and laboratory 

work has failed to turn up additional examples. The 

graded beds observed rm~ge from 1 to 3 inches in thickness, and in about one-half of the occurrences grain 

size diminished downward-that is, the graded beds 

were upside down. In the layered norites .and gabbros 

above the Ultramafic zone, where small-scale graded 

beds are slightly more abundant, the ratio of upside 

down to right side up is about the same. Densitygraded beds, or "rhythmic layering," which is a prominent feature of the entire complex, should not be confused with size-graded beds, even though the densitygraded beds have been compared with size grading in 

sedimentary rocks by some authors ('Vager and Deer, 

1939, p. 271; Cooper, 1936, p. 32; Hess, 1938a, p. 265). 

The second type of gradual grain-size variation occurs within the cyclic units of the Peridotite member, 

and the size behavior of settled olivine and bronzite in 

three of these cycles is shown in figure 19. Olivine 

crystals within the three cycles gradually increase in 

2 I nm not ncquninted with previous descriptions of this relation, nnd 

the term '·cusp texture" is here coined to describe it. 

size stratigraphically upward to a point near the base 

of the granular harzburgite layers, then decrease. The 

abrupt appearance of bronzite as a settled crystal at 

the bases of the granular harzburgite layers has no 

· pronounced effect on the gradual increase or decrease 

in size of the olivine. As the bronzite exceeds olivine 

in volume, the sizes of olivine grains decrease sharply. 

The earliest bronzite crystals in each cyclic unit, which 

are those near the bases of the granular harzburgite 

layers, are also relatively fine· grained. Like the olivine, these gradually increase in size to a point in the 

lower part of the bronzitite layers, then decrease in 

size toward the upper parts of the sections. In the 

lower part of cyclic unit 9, however, this pattern is 

complicated by an initial diminution in grain size of 

the bronzite. The gradual variation in bronzite grain 

size shows no discontinuity as olivine ceases to be a 

constitutent of the rock. The maximum and minimum average size attained by olivine and bronzite in 

a given cycle are very nearly equal but occur at different positions in the cycle. Further, the maximum and 

minimum sizes attained by olivine and bronzite within 

a cycle are not related to the thickness of the cyclic 

unit. A major violation of this general pattern of size 

variation occurs at the base of a few cyclic units, including the one containing the main chromitite layer. 

In these, the basal poikilitic harzburgite is composed 

of extremely coarse olivine crystals up to the base 

of the chromitite, but above the chromitite the normal 

size-variation pattern of. olivine obtains. 

The grain-size variation of chromite in· single massive chromitite layers is probably much like that of 

olivine and bronzite in harzburgite and bronzitite. 

The size variation of the chromite is so small, however, 

·and the number of measurements necessary to sample 

adequately a particular horizon is so large, that such 

a vertical grain -size variation cannot be said to be 

general. About 2,000 measurements were made on one 

7-inch-thick chromitite layer, and the results are summarized in table 2. Several other similar massive 

layers were more cursorily examined, and generally 

were slightly coarser grained near the center than at 

either margin. In some simple chromitite-olivine 

chromitite layers, where the amount of chromite in 

the chromitite decreases upwards, there is a gradual 

diminution of chromite grain size toward the top; but 

this variation again cannot be said to be general. It 

has been noted, however, that although the compositional break between chromitite and olivine chromitite 

in such rocks is generally extremely sharp, there is no 

size difference between the chromite grains at the top 

of the chromitite and the bottom of the olivine chromitite. The chromitite-olivine chromitite contacts are 
24 STILLWATER COMPLEX, MONTANA 

FIGURID 20.-Photomacrograf'h of cusp texture at chromitite-poikilitic harzburgite contact. The chromite crystals have 

sifted down between the larger olivine grains. ol=olivine; br=bronzite; pc=plagioclase; aug= augite. Cl"ossed 

nicols. 

FIGURE 21.-Field photograph of cusp t exture. The poikilitic harzburgite above the chromitite (dark layer) is considerably finer grained than the poikilitic harzburgite below it. Light gray areas are lichen. 

CUSP TEXTURE ILLUSTRATED 
TEXTURES OF THE ROCKS 

FIGURE 22.-Size-graded bedding in olivine chromitite. Lower layer is chromitite. Olivine grains decrease in size stratigraphically upward: chromitite gmins are the same size throughout. 

FwunE 23.-Local unconformiti es in chromitite-dunite section. Lowest dunite is cut out to ri ght of photograph; centra l 

dunite Is cut out to left. du=dunite; ol chr=olivine chromitite. 

FIELD PHOTOGRAPHS OF LAYERED STRUCTURES IN THE ULTRAMAFIC ZONE 

25 
26 STILLWATER COMPLEX, MONTANA 

MOUNTAIN VIEW AREA WEST FORK AREA 

x .... 

Q.) 2 x" 2 Q.) ~ Q.) 

c: 0.. E - c: c. Eui 0 0 .... E ro.!!2 0 0 .... E ro- :;:; .!:O ·- ro :;:; J::o ·- ro ro z "0...., -o- tlOu en ro z oou en Q; .... u c: 'Qj- C:>, u 'Qj- c: >, Q.) .2 .!:0 ro .._en .2 c: J::O ro .._ en .c .c Q.) Q.) u .... Q.) Q.) u .... 

E c: E u Q.) Ea.>2 c: E u Q.) Ea.>2 E Q.) ·-en Q.) ·-en Q.) Q.) E '(3 .!: ro .~ Q.) '(3 J::ro .~ Q.) 

~ Q.) c. .a ~ ~.~ E Q.) c. .a ~ ~.~ ~ '(3 c. roa.>...., a.>o= '(3 c. ~~~ Q.) a.>o= (j') 

·~~~ ~.a .E Q.) (j') c. c. :.t::iO~ ~.a .E (j') ro .c c: --c: (j') ro .c c: ·'E 0 ·= .!= ro._ ·;:: o._ .!= ro._ (j') ex: (j') ex: 

Millimeters Millimeters 

ljO li5 2i0 2i5 \0 li5 2i0 2i5 

v ( 7 L 

., r ~ ' " ~- en r L- Q.)- L 

0 52WF-47 1.1 

0 51MV-18 6420 1.5 0 52WF-44 2905 1.3 

0 51MV-13 5540 1.6 0 52WF-40 2175 1.8 

':Oe~ 1625 

«'e«' / 

0 52WF-31 1475 

1.8 ~~O// ~i'"'•(/ -1125 0 51MV- 7 4550 

~~,·r-- . ·'/ 0 52WF-20 1025 1.6 

/ ,~~ / 

4140 ·~'e/ 0 52WF-12 725 1.7 

0~ / 

x:--<./ 0 52WF- 8 475 1.3 

0 55MV-82 3810 2.3 ·~<:--/ ~~ 0 130 

52WF- 2 125 1.2 

/ 0 

3105 FEET 

0 55MV-32 3020 2.2 0 
TEXTURES OF THE ROCKS 27 

thus analogous to the contacts between poikilitic harzburgite and granular harzburgite, in that the grain 

size of chromite or olivine is unaffected by the abrupt 

appearance of another phase, which is larger in the 

first type of contact and smaller in the second. In 

complex chromitite layers, where the section is composed of many alternating layers of massive and disseminated chromitite, no simple size variation has been 

found. 

TABJ .. E 2.-Arithmetic mean diameters of chromite 

crystals in a 7-inch-thick massive chromitite layer 

Specimen Height above Arithmetic mean No. base or chromi- diameter or chrotltc (inches) mite crystals(mm) 

7UU ___ ------- 6. 5 0. 143 7UL __________ 5. 25 . 161 7MU _________ 4 . 151 7ML _________ 3 . 149 7LU __________ 1. 25 . 146 7LL __________ 0 . 137 

The third type of gradual size variation in the 

Ultramafic zone is an overall change in average grain 

diameter superimposed on all the minor variations 

thus far discussed. This variation is not readily apparent unless some consistent horizon within any of 

the alternating compositional layers is compared 

throughout the Ultramafic zone. Figure 24 illustrates 

the stratigraphic variation in grain size in two sections 

about 5 miles apart. Sizes of bronzite crystals in 

bronzitite layers were chosen for comparison in each 

case so that the section could be extended through the 

Bronzitite member of the Ultramafic zone. Within 

the Peridotite member, specimens were chosen from 

near the centers of bronzitite layers, but in the Bronzitite member, specimens were chosen at nearly equal 

stratigraphic intervals. Grain size of the settled 

bronzite gradually increases nearly to the base of the 

Bronzitite member, then decreases to the top of the 

Ultramafic zone. One significant sharp size change 

within the Bronzitite member, not shown in figure 24, 

occurs about 100 feet below the top of the member 

wherever this section is exposed. Grain size of bronzite individuals at this horizon increases from 1.1-1.4 

mm to 1.8-1.9 mm, and bronzite continues at this 

size into the lowest norite layer. Grain sizes of olivine 

in the Ultramafic zone have the same overall variation 

as bronzite, if olivine crystals from the same relative 

stratigraphic position within the cycles are compared. 

Olivine crystals attain their maximum size near the 

base of the Bronzitite member and a;bove this horizon 

are, of course, no longer constituents of the rock. 

The overall size distribution of chromite is slightly 

different. In figure 25 the average size of the settled 

chromite crystals at the footwall of the various chromitite layers in the Mountain View area is compared 

with the stratigraphic position of the chromitite. The 

maximum size is reached about a thousand feet below 

the base of the Bronzitite member and the grain size 

of the chromite gradually decreases from this point 

upwards. Although the maximum grain size is at the 

base of the thickest chromitite, the variation as a whole 

has no relation to thickness of layers. It should be 

emphasized that these samples were taken from a consistent stratigraphic horizon in each chromitite layer 

(the footwall). The overall variation would be concealed with random sampling because the internal 

vertical size variation within the main chromitite layer 

is as great as the overall size variation shown in figure 

25. 

No systematic study of lateral size variation has been 

made. On a small scale, grain sizes are remarkably 

OJ 

c: 

0 

N 

u 

;;: 

ro 

E 

~ ::> 

Q) 

..c 

E 

OJ 

:E 

Basal zone 

c: 

0 

:.;:::; 

ro 

u 

.2 

c: 

OJ 

0 

z 

c: 

OJ 

E 

·c:; 

OJ 

Q. 

(/) 

55MV·64 

55MV·62 

507H·1 

501 G-1 

52MV·7 

55MV·5 

54MV·26 

x 

OJ 

a. .... E ..co 

oou 

·w-

..co 

uOJ 

·-Ill 

..cro 

Q..D 

~~~ 

~.8~ .!::; (0,_ 

(/) 

3690 

3530 

3415 

3105 

2470 

770 

655 

205 

130 

0 

.27 

.29 

.34 

.35 

.16 

.15 

.20 

.12 

Millimeters 

0 0.1 0.2 0.3 

FEET 

0 

2000 

0 

0 

0 

FIGURE 25.-Stratigraphic distribution of mean diameters of settled 

chromite crystals in chromitite in the Mountain View area. 
28 STILLWATER COMPLEX, MONTANA 

constant along the layering plane. In the Benbow 

area, for. exampl~, an uncommonly coarse-grained 

chromitite layer, 2' inches thick, near the base of the 

Peridotite member was follo·wed for 11/2 miles along 

the strike. At its. westernmost exposure the average 

grain siz~: of its chromite crystals is 1.1. mm; 7,000 feet. 

to the east their mean diameter is 1.2 mm. On a gross 

scale, Wager and D:eer (1939, p. 263) were able to show 

that the average size of crystals in the layered rocks 

of the Skaergaard intrusion increased in size from the 

margins of the intrusion toward the center. No such 

gross lateral variation has been observed in the field in 

the Stillwater complex. The data given in figure 24 

seem to indicate that, at any given stratigraphic position, the ~rocks in the thinner West Fork section are 

slightly finer grained than those of the thicker Mountain View section. ··The grain sizes of a third section, 

intermedi~te betwe·en these· two in thickness but east 

of the Mountain V1e,v section in position, were determined, and the ave~age bronzite diameters were found 

to be very close "to those in the Mountain View section. 

A great Jj:lany more measurements would be necessary 

to determine if this apparent lateral variation is related to stratigrap4ic thickness, to lateral position, or 

to neither. 

SORTI}ofG 

Size-fr~quency distributions of the settled crystals 

within cdmpositionally homogeneous layers are symmetrically disposed when plotted on a logarithmic 

scale, an4 most di~tributions appear as straight-line 

curves wl}en plotteq. on logarithmic probability paper. 

Such distributions ·are said to be lognormal and are 

typical of_"size distributions in many sedimentary rocks 

(I{rumbe1n, 1938, p. 84-90). For comparison, the size 

distributions of sand grains in a well-sorted sandstone 

~ 30 u· 

z 

wl5~ 20 wu 

O::o:: LLw 

5~ 10 CD_ 

:2: 

::::> 

z 

~ 30 (.) 

z 

wl5~ 20 wu 

O::o:: LLw 

5~ 10 CD_ 

:2: 

::::> 

z 

0.08 0.16 . 0.24 0.32 0.40 0.48 0.56 0.64 

DIAMETERS, IN MILLIMETERS 

Size distribution of chromite crystals in 52WF-28b chromitite 

·o.o8 o.16 o.24 o.32 o.4o 0.48 o.56 o.64 

DIAMETERS, IN MILLIMETERS 

Sand grains in St. Peter sandstone (after Krumbein, 1935) 

Arithmetic size groupings of grains in layered chromitite and in sandstone 

0.9 

0.8 

0.7 

0.6 

0.5 

~ 0.4 w 

1-

~ 0.3 

j 

~ 0.2 

en 

5 

1-

~ <t 0.1 

Ci 0.09 

0.08 

0.07 

0.06 

0.05 

0.04 

(the St. Peter) and chromite grains in a chromitite 

layer are plotted graphically in·· figure 26. The distribution curves for the St. Peter sandstone were constructed from measurements made in thin section by 

l{rumbein (1935, p. 486). Neither set of measurements is corrected for sectioning effects; both are 

grouped into the same arithmetic intervals; and both 

are based on more than 300 measurements. The similarity between the distribution curves is obvious. The 

chromitite is slightly coarser grained, but both rocks 

have the same degree of sorting,3 as shown by the 

equal slopes of their cumulative curves. 

Sorting in most of the layered rocks that contain 

only one settled mineral is remarkably good, and sorting indices are nearly constant, regardless of grain 

size. For illustration, distribution curves and parameters of the three varieties of such rocks is given in 

figure 27. In each example,. size-frequency distributions are plotted on a logarithmic base using one-half 

Udden scale class limits, and both histograms and cumulative curves are shown. Medians and sorting coefficients were obtained from quartile measures, but arithmetic means were calculated directly from the individual measurements. Quartile sorting coefficients range 

from 1.27 to 1.50 in most of the layered ultramafic 

rocks, and normally there are 3 or 31f2 U dden classes 

that contain more than 1 percent of the total individuals. These sorting characteristics are remarkably like 

those of the marine beach sands described by Martens 

(1939, p. 207-218), although the average grain sizes of 

beach sands are considerably finer. No difference in 

average sorting has been found between poikilitic harza The term sorting is used here in a statistical sense, without implication as to the agency responsible for the spread (Pettijohn, 1957, 

p. 37). 

CUMULATIVE NUMBER FREQUENCY, IN PERCENT 

'-(:)_ 

.._ 

2 

~ ~ Chro. b- f171tecr_y 

~ ~< an0 ~ · 1

n S<w. era;ns . . F.<'8o 

------ In St ""' Chro .. 

_sanoston . " ~ . I" :::t---- ~terkru ~be;n 

.._ 

• 1935; ~ 

.._., v r---

10 20 40 60 80 90 

Cumulative size distributjon curves of grains in layered 

chromitite and in sandstone plotted on log probability paper 

........ 

98 

FIGURE 26.-Grain-size distribution curves of chromitite and sandstone. 
TEXTURES OF THE ROCKS 29 

100 100 

> (.) 80 80 z 

UJ._ 

=>z 

O'UJ 60 60 UJ(.) 

O::o:: 

Lt...UJ 

0::0. 

~z40 

::i!-

40 

::l 

z 20 

0 0 

8 I I 

2 2 

DIAMETERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS 

Size distributions of olivine crystals in specimens of poikilitic harzburgite 

100 

t)- 80 

z 

~~--

8'~60 

E~ UJ 

0::0.40 

~~ ::l 

z 20 

8 I 

2 

DIAMETERS, IN MILLIMETERS 

I 

2 

DIAMETERS, IN MILLIMETERS 

Size distributions of bronzite crystals in specimens of bronzitite 

·. 4 i6 

DIAMETERS, IN MILLIMETERS 

I 

i6 

DIAMETERS, IN MILLIMETERS 

Size distributions of chromite crystals in specimens ofchromitite 

Specimen Arithmetic mean Median diameters, Sorting 

diameters, in millimeters coefficient No. In millimeters 'V""fh7~ 

55MV-26 2.57 2.53 1.31 

55MV·27 2.68 2.53 1.37 

55MV·32 2.18 2.07 1.48 

55MV·33 1.74 1.54 1.40 

52MV·38 .25 .23 1.36 

52WF·28 .24 .22 1.32 

FIOUIUD 27.-Compnrisou of size-frequency distributions in layered 

rocks containing only one settled mineral. 

burgite, bronzitite, and chromitite, nor has a consistent 

variation in sorting with change in grain size, stratigraphic position, or areal location been discovered. 

In olivine chromitite, poikilitic chromite harzburgite, olivine bronzitite, and some granular harzburgite 

-the four principal rock types that consist of two 

varieties of settled crystals-sorting is poor or bimodal 

if, as is the common practice in sizing sediments, no 

distinction is made between n1ineral types. If, however, the two settled constituents are 1neasured and 

tabulated separately, each is found to be well sorted, 

but generally different in average size. The size-frequency distribution curves in figure 28 illustrate the 

general range and type of sorting variations in four 

selected specimens of rocks containiil.g both settled olivine and settled bronzite. As previously noted, the 

average diameter of the two constituent minerals can 

be roughly correlated with their relative abundance 

and stratigaphic position. No such correlations of degree of sorting can be made, nor is the sorting coefficient related to absolute or r:elative grain size. As illustrated in figure 28, the slopes of the distribution curves 

of the two minerals in the same rock tend to be similar, 

and in some rocks the sorting coefficients are identical. 

In· rocks where slight differences in spread exist, either 

olivine or bronzite may have the steeper curve. The 

size-frequen.cy relations between coexisting olivine and 

chromite in structureless olivine chromitite and poikilitic chromite harzburgite are similar to those of olivine and bronzite, with two exceptions: ( 1) the chromite is invariably Inuch smaller than the olivine, never 

exceeding it in average diameter; and, ( 2) the distribution curves of coexisting olivine and chromite are 

generally more similar in both shape and slope than 

the distribution curves of coexisting olivine and bronzite. Figure 29 illustrates the size-distribution variations in four selected specimens containing both settled 

olivine and chromite. The distributions as a whole are 

strikingly bimodal, although each constituei.1t has a 

narrow spread. The similarity in distribution-curve 

slopes of olivine and chromite in the same rock is very 

close compared to the range in slope between rocks. 

One specimen of coarse-grained poikilitic harzburgite 

containing accessory chromite has been included to illustrate the independence of sorting index with respect 

to average grain size and relative abundance of the 

chromite and olivine. 

In a·few places in the Ultramafic zone, current structures have been observed. These structures are: most 

easily recognized in the chromitite layers because of 

the contrast in size ai~d. color of the constituents. Figure 23 illustrates scour and local unconformity in the 

upper part of the main chromitite layer. Typically, 

the grain size of the settled crystals is finer than average, and ·the rock consists of alternating laminae of 

dunite' and olivine chromitite, which is in contrast to 

the relatively thick bedding of sections that show no 

current structures. The size distributions in a sequence 

of these layers were investigated and found to be con., 

siderably different from those of structureless olivine 
30 STILLWATER COMPLEX, MONTANA 

100 100 

55MV-29 

u 

>-" 80 80 80 

z 

IJ.Jf-

=>z 

ow 60 60 60 60 WU 

0:::0::: 

u..w 

c:::C. 

wz 40 40 40 40 

ID-

~ 

::::> 

z 20 20 20 

0 0 0 0 

8 4 2 1 8 4 2 8 4 2 1 8 4 2 1 2 2 2 

DIAMETERS, IN MILLIMETERS DIAM~TERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS 

Specimen Arithmetic mean Median diameters, Quartile sorting Volume ratio of 

No. Mineral diameters. in millimeters coefficient bronzite to 

in millimeters olivine in rock 

Olivine 1.54 1.36 1.52 

55MV-7 Bronzite 2.05 1.97 1.44 2.7 

55MV-28 Olivine 2.58 2.58 1.30 0.3 Bronzite 2.47 2.45 1.26 

55MV-29 Olivine 1.91 1.82 1.28 2.4 Bronzite 1.99 1.96 1.37 

55MV-30 Olivine 2.01 1.93 1.29 3.7 Bronzite 2.53 2.46 1.29 

FIGURE 28.-Comparison o~ size-frequency distributions of coexisting olivine and bronzite crys·tals in layered granular harzburgites. 

chromitite and granular harzburgite. The layered sequence investigated is illustrated in figure 30 and sizedistribution data are given in figure 31. 

The most obvious feature of the sequence is that the 

fine-grained olivine is associated in layers with the 

chromite, and that these layers are separated by 

coarse-grained dunite. The· distribution curves of coexisting olivine and chromite (layers B, H, J, and L), 

in distinction to the distribution curves of these minerals in structureless olivine chromitite, are unlike, the 

chromite being considerably more poorly sorted than 

olivine. The olivine of these olivine chromitite layers 

has nearly symmetrical distribution, but the chromite 

is strongly skewed to the coarser size. The size distribution of olivine grains in the coarse dunite (layers 

C, K, and M) is similar to that of the chromite in the 

olivine chromitite layers. Despite the large variations 

in arithmetic mean diameters of olivine grains in the 

various layers, all their. modes are the same, as are 

those of the chromite. If it is assumed that the modes 

of olivine and chromite in these layers represent the 

modes of lognormal distributions of available chromite 

and olivine before deposition, then current action has 

removed the fine fraction of olivine from the coarse 

dunites and the fine fraction of chromite from the 

olivine chromitites.. The coarse dunite layers would 

have been deposited from the strongest currents, which. 

suspended the finer olivine and all the chromite. The 

olivine chromitites would have been deposited by the 

weakest currents, which allowed deposition of all the 

olivine but which still suspended or carried off the 

finer chromite fractions. Regardless of interpretations 

as to mechanisms of origin, the most significant feature 

of the distribution is that where olivine chromitite or 

poikilitic chromite harzburgite appears internally 

structureless in the field, sorting indices of olivine and 

chromite in the rock are nearly equal, but, in the few 

localities where current structures can be observed in 

the field, chromite is much more poorly sorted than 

the associated olivine. This relation is shown graphically in figure 32. 

SIZE-DENSITY RELATIONS 

The common association of small, dense chromite 

crystals with larger, lighter olivine crystals suggests 

that these minerals may have had nearly equal settling 

velocities. Rittenhouse (1943, p. 1725-1780), who 

studied the size distributions of light and heavy minerals in fluvial sands, found the equivalent hydraulic 

size of two minerals to be closely related to their densities, the coarser light minerals being associated with 

the coarser heavies, and the finer light minerals with 

the finer heavies. As illustrated in figure 33, this relation does not obtain in either the internally structureless or the current-bedded olivine chromitites. On 

theoretical grounds, none of the coexisting olivine-
TEXrURES OF THE ROCKS 31 

100-

~ UJ 80 

~ UJ 

c.. 

z 

;: 60- (.) . z 

UJ 

::l 

0' 

~ 40 u.. 

0: 

UJ 

CD 

::!: 

~ 20 

1-

z 

100-

~ 80 

UJ 

c.. 

!: 

t 60 

z 

UJ 

::l 

0' 

UJ e: 40-

0: 

UJ 

CD 

::!: 

::l 

z 20 

53MV-16 

Poikilitic chromite 

harzburgite 

52NL-19 

Olivine chromitite 

4 

1 ! ! ! 

2 4 8 

DIAMETERS, IN MILLIMETERS 

1 ! ! 2 4 

DIAMETERS, IN MILLIMETERS 

I 

i6 

I 

16 

I 

32 

I 

32 

100 

so 

60 

40 

20 

100 

80 

60 

40 

20 

51MV-4 

Poikilitic chromite 

harzburgite 

4 1 ! ! 2 4 

DIAMETERS, IN MILLIMETERS 

53BE·15 

Poikilitic harzburgite )l',..A 

with accessory chromite/ 

, 

I 

I 

'/Olivine 

I 

I 

I 

J 

~ 

16 8 4 2 1 

DIAMETERS, IN MILLIMETERS 

I 

i6 

.. 

I 

32 

Specimen Arithmetic mean Median diameters, Quartile sorting Volume ratio of 

Mineral diameters, chromite to No. in millimeters in millimeters coefficient olivine in rock 

53MV·16 Olivine 1.42 1.37 1.43 0.60 Chromite .31 .29 1.43 

51MV·4 Olivine 1.83 1.79 1.29 0.11 Chromite .23 .22 1.25 

52NL-19 Olivine 2.32 2.27 1.31 1.00 Chromite .24 .23 1.30 

53BE-15 Olivine 9.40 9.20 1.38 0.02 Chromite 1.48 1.41 1.39 

FIGURE 29.-Compa.rison of size-fre.quency distribution of coexisting ollvine and chromite crystals in structureless ollvine chromitltes a.nd 

poikilitic chromite harzburgites. 

chromite associations would appear to be hydraulically 

eqt1iva1ent. According to Christiansen (1935, p. 480), 

Stokes' law is valid where the Reynolds number 

( 2rvp ) is Jess than 0.2. The Reynolds number of 

'I] • • • d ( the largest ohv1ne grains measure 9.4 mm average 

diameter) exceeds 0.2 only at viscosities of less than 

20 poises, and grains of more normal diameter exceed 

0.2 only at viscosities of less than 2 poises, so that 

Stokes' law should adequately express the settling rates 

of all the particles considered here. In table 3 the 

r~lative settling rates of coexisting olivine and chromite are tabulated, assuming a magma density of 2.7. 

It is apparent from the wide variation in values that 

the differences in size between olivine and chromite is 

not compensated by differences in density, and that the 

particles are not hydraulically equivalent. It should 

be noted, however, that, on the average, the currentqedded olivine chromitites approach hydraulic equivalence more closely than structureless rocks containing 

the two minerals. It is also obvious from inspection of 

figure 19 that coexisting olivine and bronzite in granular harzburgite and olivine bronzitite are not hydraulically equivalent. Olivine and bronzite have nearly 

the same density in the composition range existing in 

the IDtramafic zone and, theoretically, should have the 
32 

• I 

•-:: 

STILLWATER COMPLEX, MONTANA 

,': •·· c. .. I . La 

M Dunite 

. - > •• -----+----+----------j 

'A- , .. • • 

0 .2 1nch 

L 

K 

J 

H 

G 

F 

E 

D 

c 

B 

A 

Chromite 

dunite 

Dunite 

Olivine 

chromitite 

Dunite 

Olivine 

chromitite 

Dunite 

Dunite 

Dunite 

Olivine 

chromitite 

Dunite 

Olivine 

chromitite 

Dunite 

Top 

< t 

Trace of 

layering 

FIGURE 30.-Photomacrograph of layered oliYine chromitite--dunite stratigraphic sequence A-M. Black grains are chromite, 

gray grains are olivine, light gray areas are plagioclase. Plane polarized light. 
TEXTURES OF THE ROCKS 

100 ... - 100 / 

Layer K / Layer L I I 

' ' 80 80 I 

>" I I u I :z I L.Jt- I ::l:Z 60 ~ 

(olivine 60 O'W L.JU 

cro: 

"'-W 

cro... 

w:z 

a:l- 40 40 

~ 

::l 

:z 

20 20 

DIAMETERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS 

100 ,.,. 100 ;... 

I ., Layer I Layer J I I I 

80 I 80 I I I ~ z /Aiivine L-olivine WI- I ::lZ 60 I 60 f ow wU I o:cr I .... L.J I 0:0... 

w:z I 40 40 a:l- ' ~ 

::l I :z 

20 20 

I 

2 

DIAMETERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS 

100 100 _... _. Layer C / Layer H / 

80- I 80 I ' >" f I u ~livine J....--Oiivine :z L.Jt- I ::l:Z 60 - I 60 O'i.J wU I o:cr I I .... L.J 0:0... I L.J:Z 40- 40 a:l-

~ 

::l 

:z 

20- 20 

I I 

16 32 

DIAMETERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS 

100 

- 100 ,. fl"' / Layer A / Layer B 

I I 80 ! 80 I >" 

u ~Olivine ~livine :z 

I.Jt-

::l:Z I I 60 60 O'W L.JU ~ I 0:0: I , "'-W 

cro... I I L.J;z 40 40 a:l-

~ 

::l 

:z 

20 20 

0 

I I 

ii 16 

DIAMETERS, IN MILLIMETERS DIAMETERS, IN MILLIMETERS 

"' 2l 

CD 

:§ 

.E 

c: 

~ 

~ :.c 

I8 

5 

4 

3 

4 

5 

4 

4 

2 

5 

2 

3 

100 

Layer M 

80 

60 

40 I 

I 

I 

20 

0 

I 

r-

; 

1 

I 

I 

I 

I 

rOiivine 

I 

2 

DIAMETERS, IN MILLIMETERS 

Rock 

type 

Mineral 

... 

2l 

CD 

E 

., 

:0 

c: 

., "' E~ ¥ .§ <~>= 

~ .E .... c: ~ ·-

·= 

CD 

CD 

E ~ 

-~ $ "CCI> 

c: E 10:.: 

'g'E 

:2: 

N Chromitite Chromite 0.21 

M Dunite Olivine 1.53 1.28 

Chromite Olivine 0.75 0.75 L dunite Chromite .14 .12 

K Dunite Olivine 1.05 0.85 

Olivine Olivine 0.75 0.75 

chromitite Chromite .18 .13 

Dunite Olivine 0.94 0.87 

Olivine Olivine 0.72 0.70 H chromitite Chromite .22 .18 

G Dunite Olivine 1.44 

Dunite Olivine 1.09 

Dunite Olivine 0:92 

Olivine Olivine 0.84 D chromitite Chromite .26 

c Dunite Olivine 1.06 0.90 

Olivme Olivine 0.73 0.71 B chromitite Chromite .22 .17 

A Dunite Olivine 0.74 0.75 

SAMPLE 55DR·1 

·= 

0.84 

0.84 

.11 

0.84 

0.84 

.11 

0.84 

0.84 

.11 

0.84 

0.84 

.11 

0.84 

~ ·u 

::: 

CD 

0 

(J 

00 

c: 

:e 

0 

C/) 

1.38 

1.16 

1.38 

1.23 

1.25 

1.45 

1.38. 

1.26 

1.63 

1.34 

1.34 

1.53 

1.30 

33 

5.3 

4.2 

3.3 

3.2 

3.3 

Fzounm 31.-Compu.rlson of size-frequency distribution of olivine and coexisting olivine and chromite In current-bedded dunites and olivine 

chromitites. 
34 STILLWATER COMPLEX, MONTANA 

1.7 

LLJ 

t: 1.6 ~ 

0 

0:: 

:::t 

c..> 

L.i.. 

0 

z "' 

i= 

0:: 

0 

U) 

L.i.. 

0 

1-

z 

LLJ 

u 

Li: 

L.i.. 

1.&.1 1.2 0 

<.,) 

0 550R-1/ 

· /0 55DR-1B / 

/ ~: :4MV 72 / 

/ (! p53MV-16 

0 53MV-18 EJ ,.....,lj}1>3 BE-15 

55DR-ll . y52MV-36 

A 52NL-13 

52NL·l9 

52MV-38 

[.l (!] 52BE-26 

~ [!] -~ / -lMV-4 52WF-28 

COEFFICIENT OF SORTING OF OLIVINE 

EXPLANATION 

0 

Current-bedded rocks 

[!] 

Internally structu reless rocks 

FIGURE 32.-Ratios of sorting coefficients of coexisting olivine ancl chromite in olivine chromitite. and poikilitic chromite harzburgite. 

same diameter in equivalent association, but coexisting 

olivine-bronzite diameter ratios range from 1.3:1 to 

0.5 :1. At the one extreme the Stokes settling rate for 

olivine would be 1.8 times that of bronzite, and at the 

othei· extreme bronzite ·would theoretically settle 3.5 

times faster than olivine. 

TABLE 3.-Relative settling velocities of coexisting olivine and 

chromite 

Specimen No. 

Arithmetic mean Settling diameter (millimeters) velocity 

olivine: 

settling Chromite Olivine velocity 

chromitc 1 

------------------------------1--------1-------------

52MV -38. __ ----- __ ·-------------------------- 52BE-26 .. ------------ _ ------------- _ ------ __ _ 52NL-19 ___ ----------------------- ____ --------

13 ..... -------------------------.-------- 52,VF-28. ____ ------------------- ___ ------- __ _ 51MV -4. _ --------- ___ --------------- ________ _ 54MV -72 2 ____ •• ____ ----------------------- __ _ 53B E-15. _____ • ______________________________ _ 

53MV -18. -··-- ______ ----- __ ------------------- 55DR-1L 2 

53MV -16. --~~~================================ 55DR-1J 2 __ ------------------------ ___ ------- 1B 2 __________________________________ _ 

1H2 

52MV -1~- ~ ~==================~~============== 

0. 22 

. 21 

.24 

. 32 

. 21 

.23 

.20 

1.48 

.27 

.14 

. 31 

.18 

. 22 

.22 

.26 

. 26 

3. 03 2.38 

2. 32 2. 87 1. 68 

1. 83 

1. 55 9. 40 1. 62 

. 75 1. 42 

. 75 

. 73 

.72 

. 84 

. 68 

67 

45 

33 

28 

22 

22 

21 

11 

13 

10 

7. 4 6.1 

3. 9 3.8 

3. 7 2.4 

I Compu_ted from Stokes' law assuming the density of the magma to be 2.7; olivine 

3.3; chrom1te 4.1. 2 Current-bedded olivine chromitite. 

- LLI 

!::: 

~ 

0 

a:: 

:I: 

(.) 

0.40 

~ 0.30 

ffi~ 

ti~ ~LI.J 

<~ 

Ci:::l 

;;:::~ 

~z 

~- 0.20 (.) 

i= 

LLI 

~ 

:I: 

!::: 

a:: 

<( 

E) 0 55DR-1D 

52MV-36 

55DR-1B 

<l:)55DR·l H 

055DR-1J 

0 55DR·lL 

1.0 

[) 53MV-16 

[) 53MV-18 

E) 52NL-19 

G 51MV-4 

0 54MV·72 

EJ 52BE-26 [!] 52WF-28 

2.0 

E) 52NL·13 

[) 52MV-38 

3.0 

ARITHMETIC MEAN DIAMETER OF OLIVINE, 

IN MILLIMETERS · 

EXPLANATION 

0 

Current-bedded rocks 

Internally structureless rocks 

FIGURE 33.-Ratios of mean diameters of coexisting olivine and chromite in olivine chromitite and poikilitic chromite harzburgite. 

From textural evidence in the Ultrmnafic zone, it 

seems doubtful that the effective size, and therefore 

the settling rate, is modified to any extent by groups 

or clusters of crystals sinking together. Effects of 

clumping are most easily evaluated in rocks containinO' • b 

m1xtures of two settled phases, and preaccmnulation 

clustering apparently has occurred in a few olivine 

bronzitite layers, where groups of 4 or 5 olivine crystals welded at their contacts are irregularly distributed throughout the rock (fig. 36). In the great Jnajority of olivine bronzitite and granular harzburgite 

specimens, however, individual crystals of bronzite and 

olivine are evenly distributed throughout the rocks 

(fig. 34). A few compound chromite grains fonned 

by clusters of smn1ler crystals have been recognized, 

but these are easily separated frmn their individual 

associates and, where observed, have been 1neasured in 

aggregate. Synneusis texture in chromite as described 

by Vogt ( 1921, p. 321) has not been observed in the 

Stillwater chromitite, although a superficially si1nilar 

texture has been· developed in olivine chromitite by 

close packing of large olivine and small chromite crystals (fig. 53). Chain texture, identical with that described by Sampson (1932, p. 126-127) frmn the Bushveld complex. occurs in some Stillwater chrmnitite. 
TEXTURES OF THE ROCKS 35 

The long, branching, generally curved "chains" seen in 

thin section consist of as many as 30 contiguous grains, 

and the elongation of these bears no relation to the 

layering plane (.fig. 37). Bastin (1950, p. 7) suggests 

chain texture n1:ay be a variety of synneusis texture, 

but I doubt that they represent preaccumulation aggregn.tions. The curved, branching groups of crystals 

seen in two dimensions a.re proba.bly not chains, but 

com.p]ex three-climensional structures. In the Ultramafic zone, chain texture occurs only where the interstitial material between chromite grains makes up 

:tbout 25-30 percent of the rock; where interstitial mater.ial makes up more of the rock, most chromite grains 

do not touch; n.nd where interstitial material is less 

abundn.nt, all the grains mutua.Jly interfere. It seems 

most probable, therefore, that the chain effect is caused 

by :fortuitous sectioning of tightly packed chromite 

octn.hedra. 

DISCUSSION 

The size-distribution data have important bearing 

on the problem of mechanism of crystal accumulation, 

part:icularly in respect to the effect of convection current as agents of transportation and deposition. Gravity sinking of crystals to the floor of a magma chamber 

seems to be generally accepted as an essential condition for the formation of layered intrusions of this 

type, and gravity settling in basaltic liquids has been 

abundantly demonstrated (l\1acDonald, 1944, p. 177-

189; Fuller, 1939, p. 303-313) ·and experimentally verified by Bowen ( 1915, p. 175-191). Wager and Deer 

( 1939, p. 262-289), in one of the best documented discussions of crystal ~tccumulation, pi'opose that, in addition to settling by gravity, the minerals of the Skaerg:tard intr1ision were transported and deposited by 

convection currents sweeping across the floor of the 

magma chamber. The existence of currents was deduced from igneous htmination, lineations, and trough 

banding in certain parts of the intrusion (1939, p. 

262-289), and, as evidence of their effectiveness, the 

authors found an enrichment of heavy melanocratic 

minerals near the margin of the intrusion ( 1939, p. 40, 

263), and a gradual increase in the average size of 

pla.gioclase crystals away from the margin toward the 

center of the mass (1939, p. 71, 263). Pulsatory variation in the velocity of conve~tion currents was considered responsible for variation in the proportion of 

light and heavy minerals deposited in any place, giving rise to rhythmic layering. In the Stillwater complex, the very local scour and . unconformity effects 

indicnte that currents were present at certain times 

and in certn.in parts of the magma chamber during 

the period of crystal accumulation. 

It does not seem to me, however, that currents in the 

magma were important in either the transportation 

or deposition of the primary precipitate minerals in 

the Ultramafic zone for the following reasons : ( 1) no 

trough banding has been observed; (2) lineation ·of 

elongate minerals is weak or absent; ( 3) although a 

wide range o:f grain size of settled crystals is recognized in some rocks, size-graded bedding is almost nonexistent; ( 4) in nearly all rocks containing two coexisting settled phases, the two minerals are not in 

hydraulic equivalence; and ( 5) in the rocks in which 

current bedding can be observed, the size-distribution 

relations are markedly different from those of the 

structureless rocks. It should be noted that rhythmic 

layering or "gravity stratification" in the Ultramafic 

zone is developed in rocks containing two settled constituents of different density. In many olivine chromitites, for instance, "gravity stratification" is properly 

developed with denser chromites concentrated at the 

base, and these gradually give way to less dense olivines at the top (fig. 9). Because size affects settling 

velocity as the square and density only to a power of 

one, -however, every one of the "gravity-stratified" 

layers so constructed is hydraulically upside down. 

I do not wish to imply that no convection currents 

existed in the Stillwater magma, and am aware of the 

problems of extremely slow settling rates in thick 

stagnant magmas, but I do wish to point out here that 

current transportation and deposition of the primary 

precipitates in the lower part of the Stillwater ·complex is incompatible with the textures of the rocks. 

The lognormal distribution of grain sizes in many 

sedimentary rocks has suggested origin by some natural process concerned with grading by fluid movemmtt, and I(rumbein ( 1938, p. 89-90) has suggested 

that fluctuation in wind velocity is one process capable 

of producing such distributions. The effectiveness of 

such a mechanical sorting process is largely dependent 

on the viscosity o:f the fluid and on _the density difference between the particles and the fluid, and Kuenen 

( 1951, p. 25-26) has experimentally determined that 

the degree of sorting is greatly reduced in fluids of 

high viscosity and density. In a highly viscous 

magma, therefore, it seems doubtful that mechanical 

sorting of precipitates with a large size range could 

account :for the narrow size spread in the rocks. 

In a magmatic system, where the suspending fluid 

is also producing the particles by crystallization, it 

seems possible that logarithmic size distributions might 

be produced by crystal growth as well as by fluid 

movement. Loveland and Trivelli (1927, p. 193-217) 

found logarithmic size-frequency distributions of silver bromide crystals in photographic emulsions. They 
36 STILLWATER COMPLEX, MONTANA 

FIGURE 34.-Coexistlng settled olivine and bronzite in granular harzburgite. The two minerals occur side by side 

as euhedral crystals, evenly distributed throughout the 

rock. ol=olivine; br=bronzite; pc=plagioclase. Crossed 

nicols. 

FIGURE 36.-Clustering of olivine crystals in olivine bronzitite. Contacts between most olivine crystals in cluster 

are welded. ol=olivine; br=bronzite; pc=plagioclase; 

aug=augite. Crossed nicols. 

FIGURE 35.-Sharp change in grain size within a chromitite 

layer. Black grains are -chromite; white mesostasis is 

plagioclase. Crossed nicols. 

FIGURE 37.-Chain structure in chromitite. Many of the 

chromite crystals are connected into two-dimensional irregular "chains." Mesostasis is plagioclase, bronzite, and 

augite. Crossed nicols. 

PHOTOMICROGRAPHS OF SETTLED MINERAL DISTRIBUTION IN THE ULTRAMAFIC ZONE 
TEXTURES OF THE ROCKS 37 

reasoned that the distribution is a result of crystal 

growth proportional to the attained size of the crystals 

nt ~tny given instant, and predicted that the logarithmic form would be found to be uniquely involved in 

the case of precipitates. 

I therefore propose that the close approach to lognormal distributions of the three primary precipitate 

minerals in the Ultramafic zone is a phenomenon produced by crystallization and supply, and that the sorting or spread is a direct measure of the range of size 

of the crystallization products of the magma immediately above the continually rising floor of the magma 

chamber. If two minerals were crystallizing simultaneously or at slightly different horizons in the melt, 

their relative average diameters would probably depend largely on their concentrations, solubilities, and 

relative distances of fall to the floor. Simple crystal 

sinking of two minerals which had inherently different 

sizes would explain the apparently anomalous observed 

relations of narrow, symmetrical size distributions of 

different minerals in the same rock, where these minerals are not in hydraulic equivalence and have no consistent size relation in different rocks. Any currents 

strong enough to cause suspension of hydraulically 

lighter particles would tend to skew or destroy the 

lognormality of the distribution and, at the same time, 

move the minerals closer to hydraulic equivalence. 

This is in accord with the field and laboratory observations. 

. A corollary of this argument is that crystalliz~tion, 

and therefore supply, must be continuous. Should 

crystallization suddenly cease while only one mineral 

was being supplied, the finer crystals of that constituent would fall at a slower rate than the coarser crystnJs, and a normal graded bed would occur. Should 

crystallization cease while two· minerals were present 

in the supply then the hydraulically heavier mineral 

would be preferentially deposited at the base and gradually give way to the lighter constituent at the top. 

Should crystallization begin again after all the individuals of the ·previous cycle had settled, then the first 

of the new crystals to be deposited on the floor would 

be smaller than those higher up because they would 

have fallen a shorter distance through the magma, and 

a reverse graded bed would result. This has been illusti·ated by Bowen ( 1915, p. 175-191) and Bowen's figure 

1a shows such reverse graded bedding of olivine crystals in an artificial 1nelt. 

The factors influencing the size of crystals collected 

on the floor of the magma chamber are many and complex; these include: ( 1) the rate of heat loss from the 

intrusion, (2) the viscosity, temperature, and composition of the magma, ( 3) the solubility of minerals crystallizing from it, and ( 4) the rate and distance of fall 

of crystals to the floor. Despite the abundance of variables, some speculations as to the origin of the various types of size changes described can be made. The 

relatively fine grain size of the chromite may be related 

to the relatively low concentration of chromium in the 

magma, and perhaps· to the solubility of chromite in 

the magma. The sharp size breaks at the bases of the 

poikilitic harzburgite layers are believed to represent 

hiatuses in crystallization, and the relatively fine grain 

size of olivine crystals in the lower part of these units 

is believed due in part to slight supercooling probably 

necessary to inaugurate crystallization ( Overbeek, 

1952, p. 63). The gradual increase in grain size of 

these olivine crystals upward in the section is· probably 

caused by the relatively longer growth times of crystals that began crystallizing farther from the floor, as 

described by Bowen (1915, p. 175-191). The large-: 

scale size breaks within compositional layers, such as 

the one illustrated in cyclic unit 2, figure 19, where 

gradual increases in size occur upward in the overlying 

unit, would seem to be caused by the same processes. 

The gradually diminishing grain sizes of olivine in 

the granular harzburgite layers is not a result of resorption, for the olivine crystals retain their euhedral 

character even where present in very small ~mounts. 

It is possible that olivine crystallization is restricted 

to increasingly narrower zones above the floor, and 

that this is reflected in the gradually smaller proportion and grain size of the olivine crystals. A similar 

exp~anation might account for the decreasing size of 

bronzite crystals in the bronzitite layers. It seems 

likely that the overall increase in grain size upward 

in the Peridotite member is a function of the decrease 

in cooling rate from the walls of the magma ·chamber 

as the thermal gradients are gradually flattened, so 

that each cycle is repeated with a slightly coarser grain 

size throughout. 

ORIENTATION OF THE SETTLED CRYSTALS 

Planar orientation of platy or elongate minerals in 

the layering plane is sporadically developed throughout the complex, as has been described by Jones, Peoples, and Howland (1960); but lineation, or alinement 

of long axes within the layering plane, is poorly developed .. 

Dimensional orientation of settled crystals is not 

commonly observed in the Ultramafic zone in the field, 

because of the tendency for all three precipitate minerals to occur in nearly equidimensional habits. In 

some layers iri. the lower two-thirds of the zone, however, bronzite crystals assume a broad prismatic habit, 

and the broadest surfaces of the crystals tend to lie in 
38 STILLWATER COMPLEX, MONTANA 

the layering plane. Average attitudes obtained on the 

broad surfaces of such crystals are parallel to the compositional layering. Such orientations are never perfect, and in places crystals can be observed at high 

angles to the layering, giving the appearance of being 

propped up on underlying individuals (fig. 6). The 

perfection of the dimensional orientation seems to be 

related to the degree of elongation or flattening of the 

bronzite in the rock. This is best illustrated in the 

uncommon rocks that contain bronzite crystals of both 

equidimensional and tabular habits (fig. 54). The 

equidimensional crystals show no preferred orientation, but the layering plane is marked by the broad 

surfaces of the tabular crystals. 

Because the settled crystals are commonly euhedral 

and because the habit and the ratio of crystal dimensions of any mineral type in a given layer is relatively 

constant, it is possible to investigate the degree of perfection of dimensional orientation by optical methods. 

Hanrbleton 4 studied the optical orientations of principal minerals in 6 specimens from the Ultramafic 

zone. In one specimen each of an anorthosite pod and 

a poikilitic harbzurgite layer the fabric diagram of 

plagioclase and olivine showed little significant preferred orientation. In the four bronzitite specimens 

investigated, one reported as being composed of equidimensional bronzite crystals showed no obvious preferred orientation. The other three bronzitite specimens are reported as having "lath"-shaped crystals 

with the flat (100) 5 crystal face parallel to the layering plane. In each of these three specimens, therefore, there is a strong concentration of X perpendicular to the layering, and Y and Z tend to lie in the 

layering plane. In one of Hambleton's specimens both 

Y and Z form girdles; in a second, the Y and Z diagrams are almost identical, showing two maxima about 

90° apart; and in the third, Y and Z show mutually 

perpendicular maxima in the layering plane. Hambleton concludes that the coincidence of the flat face 

of the bronzite crystals with the layering plane is best 

explained by movement of rigid particles in a fluid 

magma, and that the observed lineation suggests flow 

of the magma during deposition of the crystals. 

Because such marked lineation had never been observed in the field, and because planar structure is an 

intermittent phenomenon in the Ultramafic zone, the 

present investigation was directed toward three objectives: ( 1) to determine if crystal orientations were 

purely dimensional; ( 2) to determine the degree of 

"'Hambleton, W. w., 1947, A petrofabric study of layering in the 

Stillwater complex, Monta,na, un~published· Master of Science thesis, 

Northwestern University, 61 p. 5 (0110) of the orientation used in this report. 

perfection and, if possible, the relative abundance of 

linea ted fabrics; and ( 3) to determine whether the 

layering plane. could be found by petrofabric methods 

where it is not visible in the field. Optical orientations 

of bronzite in four specimens and olivine in three specimens were determined. In one of these specimens 

the optical orientation did not conform to the dimensional orientation and the rock is believed to be a tectonite. In the other six specimens the following relations were noted: ( 1) the perfection of the optical 

orientations are directly dependent on the habits of 

the crystals making up the rocks; (2) five of the specimens displayed concentrations of long axes in the 

layering plane; of these, one had a weak unimodal 

lineation of long axes, one had a weak bimodal lineation, and the remaining three had long axes evenly 

distributed in the girdle; and ( 3) three of the four 

megascopically structureless specimens studied showed 

significant planar structures. 

In rocks where olivine or bronzite occur in elongate 

habits, the long axes form a girdle in the layering 

plane; where bronzite occurs in flattened habits, the 

pole to the broadest face is perpendicular to the layering plane. These patterns, characterized by crystals 

lying in positions of least potential energy with reference to th~ layering surface, are interpreted as apposition fabrics. 

The seven specimens for which optical orientations 

were determined were collected in each case from near 

a compositional contact, so that the layering plane 

could be accurately located on the hand specimens, 

even within apparently massive layers. Three mutually perpendicular thin sections, including one in the 

layering plane, were cut from each specimen. Measurements of crystal dimensions of 30 to 50 individuals, 

oriented with a, b, or c perpendicular, were made for 

each specimen, using the three mutually perpendicular 

sections, and these were converted to ratios and averaged. The X, Y, Z axes of 80 crystals for each specimen was determined with a 4-axis Universal stage 

on thin sections cut in the layering plane, and axis 

positions were plotted on a Schmidt net. Petrofabric 

diagrams were constructed for each principal optic direction. In those specimens showing a concentration 

of long axes in the layering plane, points were replotted on polar coordinate paper and azimuthal distribution histograms constructed after the method described by l(rumbein (1939, p. 681-685). Eighty 

measurements for each optic direction seem to provide 

sufficient information for recognition of the essential 

features of the orientation patterns encountered, but 

individual submaxima are probably not significant. 
TEXTURES OF THE ROCKS 39 

BR ONZITE 

As previously noted, bronzite crystals occur in a 

wider variety of habits than do olivines but generally 

occur in a particular habit within a single layer. 

Specimen 54EDJ-71 was selected as typical of the 

bronzitites which contain stubby, anchiequidimensional 

bronzite crystals, and in which no macroscopic preferred crystal orientation can be detected. The average ratio of crystal dimensions of bronzite in the rocks 

was determined to be a: b: c equals 0.92 : 0.77: 1, using 

the orthopyroxene orientation preferred by Hess and 

Phillips (1940, p. 271) and equating c to unity. Diagrams of the distributions of the principal optic 

directions in 80 bronzite grains are shown in figure 38. 

Y axes appear to be randomly distributed, but a very 

weak preferred orientation of X and Z can be detected. 

About one-half of the bronzite crystals lie with Z near 

the layering plane, and of these about 40 percent are 

oriented with the short axis X = b nearly perpendicular to the plane. The orientation is, nevertheless, too 

weak to permit unequivocal location of the layering 

plane. 

Specimen 54EDJ-80, an olivine bronzitite, was selected to study the orientation of bronzite crystals 

with short prismatic habits. A preferred orientation 

was not detected in this specimen in the field, but a 

planar structure was observed on the hand specimen 

subsequent to slabbing. The average ratio of crystal 

dimensions a: b : c was determined to be about 

0.67: 0.62: 1. The fabric diagrams (fig. 39) show an 

excellent Z girdle in the layering plane, and little or no 

preferred distribution of X and Y, which correspond 

to the almost equally short a and b crystal axes. A 

histogram of azimuthal distribution of Z axes was prepared from polar coordinate plots as a check on preferred orientation with the girdle. Figure 42 shows 

a broad lineation over about 60° of the first and third 

quadrants, which was not apparent at the outcrop nor 

on etched slabs cut in the layering plane. 

Two specimens of bronzitite composed largely of 

broad prismatic bronzite crystals were investigated. 

One of these, specimen 54EDJ-100, was collected from 

a normal compositional layer with an average grain 

size of 2.2 mm. The second, 54EDJ-74, was finer 

grained, averaging 1.3 mm, and was collected 2 inches 

below a thin chromitite layer showing scour structures 

and believed to be current bedded. Both specimens 

showed a strong planar structure parallel to the compositional layering in the field. The average ratio of 

crystal diameters in 54EDJ-100 was 0.79: 0.51: 1; in 

54EDJ-74, 0.67: 0.39 : 1. The optical orientations of 

these specimens are shown in figures 40 and 41. Both 

Y=o 

EXPLANATION 

Contours 1 

in percent 

FIGURE 38.-Fabr!c diagrams of 80 anch!equ!dlmenslonal bronzite grains In bronzitite specimen 54EDJ-71. Sections parallel 

to layering. 
40 STILLWATER CO:MPLEX, MONTANA 

Z =c 

EXPLANATION 

Contours, 

in percent 

FIG URE 39.- Fabric diagmms of 80 prismatic bronzite grains in 

olivine bronzitite 54EDJ-80. Sectlon:s parallel to layering. 

Z =c 

EXPLANATION 

Contou rs, 

in percent 

FIGURE 40.-Fabric cliagrams of 80 broad prisma tic bronzi te grai ns in 

bronzitite 54ED.T-100. Sec tions parallel to layering. 
TEXTURES OF THE ROCKS 41 

Y= o 

EXPLANATION 

Contours, 

in percent 

FIGURE 41.-Fabric diagrams of 80 broad prismatic bronzlte 

grains In olivine bronzitite 54EDJ-74. 'Sections parallel to 

layering. 

show a 711z percent concentration of X axes perpendicular to the layering, well developed Z girdles perpendicular to the X maximum, and little or no preferred orientation of the intermediate Y axes. In both 

specimens there is a tendency for girdle development 

of X, and in 54EDJ-74 two X submaxima occur about 

equidistant from the pole to the layering plane. The 

significance of this is not apparent. Both the X maximum and the Z girdle are better developed in specimen 

54EDJ-74, which has the more extreme difference in 

average b: c dimensions. Azimuthal distribution plots 

of the bronzite Z axes in these two specimens (fig. 42) 

show little or no preferred direction of alinement. 

The orientation of the investigated bronzite appears to conform to the following patterns: (1) where 

crystals are nearly equidimensional they have little or 

no preferred orientation; ( 2) where crystals have a 

broad flat surface, that surface tends to lie in the layering plane; and ( 3) where crystals are elongate, the long 

20 

15 

10 

5 

Iz 

w 

u 

0:: 

w 

0... 20 

~ 

CJi 

w 

X 15 <( 

u. "' 

0 10 >-

u 

z 

w 

::J 5 0 

w 

0:: 

u. 

0:: 

w 

CD 

::;E 

::J 20 z 

15 

10 

0 

54EDJ-80 

54EDJ-100 

54EDJ-74 

20 40 60 80 100 120 140 160 180 

AZIMUTH WITH REFERENCE TO ARROW 

ON PETROFABRIC DIAGRAM 

FIGURE 42.-Azlmuthal distribution of bronzlte Z:=c axes In specimens 

showing Z girdles. 
42 STILLWATER COMPLEX, MONTANA 

axis tends to lie in the layering plane regardless of 

whether or not the crystal is flattened. A rude lineation was discovered in one specimen where it had not 

been suspected, but lineation was not detected in the 

other two specimens showing Z girdles. 

OLIVINE 

Of the three specimens studied, olivine crystals 

showed uniform extinction in one, were weakly undulatory in the second, and showed extreme undulatory banding in the third. 

Specimen 52WF -28 was selected as being typical of 

internally structureless poikilitic chromite harzburgite, 

within which no preferred dimensional orientation of 

olivine crystals could be detected either at the outcrop 

or on etched slabs. The olivine crystals, whose originally euhedral boundaries are marked by surrounding 

chromite crystals, have narrow selvages of bronzite. 

None of the olivine is undulatory. The average ratio 

of original crystal dimensions a: b : c was determined 

to be 0.76: 0.63: 1. The fabric diagrams illustrated in 

figure 43 show a fairly well developed Y girdle in the 

layering plane and almost no preferred orientation of 

X and Z, although there is a slightly greater concentration of the shorter X axis perpendicular to the section. An azimuthttl plot of Y axes (fig. 45) shows 

little or no preferred orientation within the layering 

plane. 

Specimen 54EDJ-79 was selected for study as a 

typical poikilitic harzburgite. The rock is composed 

of about 80 percent olivine in stubby anchiequidimensional crystals, and appears structureless in hand specimen. No dimensional orientation of crystals could 

be detected at the outcrop or on etched slabs. The 

average ratio of olivine intercepts was 0.78: 0.64: 1, 

nearly identical with the average intercepts of specimen 52vVF -28. The fabric diagrams, figure 44, show 

a discontinuous Y girdle in the layering plane and 

little or no preferred orientation of X and Z. The Y 

fabric diagram and the azimuthal distribution of Y 

axes, plotted in figure 45, show two Y concentrations 

about 65° apart. In this specimen 37 of the 80 measured olivine crystals showed undulatory extinction. 

Most undulatory grains are composed of 2 or 3 extinction bands and the difference in orientation between 

the subindividuals ranges from 1° to 3°. Interstitial 

plagioclase and bronzite show no evidence of deformation, and, because of the similarity to specimen 52WF28, it is concluded that the original fabric of the rock 

has not been significantly altered. Undulatory grains 

and grains with internally uniform extinction were 

plotted separately and their distribution is shown in 

Y=c 

EXPLANATION 

Contour s, 

in percent 

FIGURE 43.-Fabric diagrams of 1>0 anchiequidimensional olivine 

grains in poikilltic chromite har~<burgite 52WF-28. Sections 

parallel to layering. 
TEXTURES OF THE ROCKS 43 

I 

I 

I 

I 

l 

I 

\ 

/, 

I r //1 

) 

' 

,, ' 

\ 

I 

I 

) 

/ 

r: 

Y=c 

t I ' 

Z=o 

() / 

ol J 

I l 

I 

J jq 

J/ 

c.. I 

I \ 

j 

I 

I 

EXPLANATION 

Contours, 

in percent 

FIGURE 44.-Fabric diagrams of 80 anchiequidimensional olivine 

grains in poikilitic harzburgite 54EDJ-79. Sections parallel to 

layering. 

541905 0-61--3 

1--

z 

w 

u 

a:: 

w 

a_ 

z 

cii 

w 

X 

<( 

).. 

"-0 

>-

u 

z 

w 

::::> 

a 

w 

a:: 

"-

a:: 

w 

CD 

:2: 

::::> 

z 

20 

15 

10 

0 

20 

15 

10 

0 

0 20 40 60 80 100 120 140 160 180 

AZIMUTH WITH REFERENCE TO ARROW 

ON PETROFABRIC DIAGRAM 

]'IGURE 45.-Azimuthal distribution of olivine Y=c axes in specimens 

showing Y girdles. 

figure 46. No significant preferred position of either 

type can be established. 

Specimen 52~fV -41, a poikilitic chromite harzburgite, contained the most strongly undulatory olivine of 

any rock examined from the main part of the Ultramafic zone. Nearly every olivine grain consists of 5 

to 15 subindividual bands. Some of the grains are 

brecciated at the margins. Chromite crystals between 

the olivine are euhedra] and undeformed, but slightly 

anisotropic in polished section. The olivine grains are 

round and nearly equidimensional in the layering 

plane, but flattened parallel to it, giving the rock a 

foliated appearance. The fabric diagrams illustrated 

in figure 47 represent the average axis positions for 50 

grains. The diagrams show a well-developed Y girdle 

at right angles to the layering, and a strong Z maximum in the layering plane nearly perpendicular to the 

girdle. Although the Z maximum is split in the fabric 

diagram, the azimuthal histogram illustrated in figure 

48 shows only a single maximum, and a greater number of measurements would probably fill in the fabric 

diagram. X shows no obvious preferred orientation. 

In most of the 50 grains, the orientation of the subindividuals changes consistently from one side of the 

grain to the other. The most widely divergent subindividuals of each grain were measured and plotted 

separately, and in some grains, prominent intermediate 

bands were also determined. The positions of the 
44 STILLWATER COMPLEX, MONTANA 

.· 

•• 0 

0 • 

\ 

\ 

\ 

• 0 

0 -t 

.. · 

.. • 6 • • • 

·.· 

Y=c 

EXPLANATION 

-- Internally homo gene~us olivine crystals 

t Olivin e crystals with un dulatory extinction bands 

Z'O 

FIGURE 46.-Distrlbutlon of undulatory olivine grains in polkilitlc 

harzburglte 54EDJ-79. 

Y=c 

Z=a 

EXPLANATION 

Contours , 

in percent 

FIGURE 47.-Fabrlc diagrams of average positions of 50 flattened olivine grains In polkilitic chromlte harzburgite 52MV41. Sections parallel to layering. 
TEXTURES OF THE ROCKS 

25 

C/l 

w 

X 

<! 20 " ..._ 

0 

>-fuz 15 zw wU 

=>"' 

ow wo._ 10 o::z 

u..-

0:: 

w 

(l) 5 :::;, 

::::> 

z 

0 

0 20 40 60 80 100 120 140 160 180 

AZIMUTH WITH REFERENCE TO ARROW 

ON PETROFABRIC DIAGRAM 

FIGURE 48.-Azimuthal distribution of olivine Z==a axes in the layering plane. 

measured undulatory bands for each of the 50 grains 

are shown in figure 49. In most grains Y shows the 

least divergence, and Z is generally more divergent 

than X. In all grains where more than two extinction 

bands were measured, the Z axis changes in a consistent direction from one side of the grain to the 

other, but both X andY are observed to rotate in some 

grains (that is, 6, 22, 31, 39, 49 ... ) . Some regularity 

in the direction of divergence of X and Y can be observed, but the trends of divergence do not appear to 

be so pronounced as those of the Z axes. If, as in the 

undeformed olivine fabrics investigated, the Y axes 

originally lay preferentially in the layering plane, then 

these axes have revolved up to 90°, at the same time 

turning to bring Z axes to a maximum at right angles 

to the Y girdle. 

The orientation patterns of the two undeformed 

olivine-rich rocks are dimensional, and the slightly 

elongate Y axes are preferentially oriented in the layering plane. Olivine crystals are not sufficiently flattened on b to produce X maxima perpendicular to the 

layering. As previously noted, the shape of olivine 

crystals seems to change very little throughout the 

Ultramafic zone, and presumably these patterns are 

typical of all olivine-rich rocks throughout the zone. 

The relative abundance of Stillwater complex rocks 

whose primary fabrics have been altered by deformation is unknown, but such effects are thought to be 

minor because of the scarcity of rocks with strongly 

developed undulatory olivines. Although many rocks 

contain a few olivine grains with poorly developed 

undulatory bands, deformation has apparently not 

been severe enough to alter their fabrics. 

DISCUSSION 

The fabrics of settled olivine and bronzite in the 

undeformed rocks of the Ultramafic zone are dimensional, and the degree of perfection of preferred grain 

As 

/35 'Y/-.1. ~ 

;. ~ 3~ 

y ~ 

;?{. "\ 4 23 

~ ;Y y J6 ["' < ,_., 

3\ ~~ 

~ 2'0'- . \ ' [' 

z ~ y 

<. 34 • 33 

.. ....--:::? ....- 8 30 

'? \ 42 17 

25 

\ 

_;:. 

I /' 

\ ~ 

..--(, 

11 < 

\ 

Y, 

]4~/J 

~ 38 J 27 

\'' 

Y=c 

~ ""32 

-:-r: 2 

49 

..--. 

... \' 

~ 

> J6 

. \9 

~ 

~ 

--........_ 

Z =a 

\ 

" 

'\4 

2l 

-

6 

2~ 

-.1... 

ly 

7 / 

10 \ :;-.. 

19 

"\ 

~ 

4.· 

4 :; 

~·M ~~42 

y_%i 

} 

/ 

FIGURE 49.- Distribution of undulatory olivine in 

specimen 52MV-41. Dots connected by straight 

lines represent observed positions of indicatrix 

axes in subindividuals of the same crystal. 

45 
46 STILLWATER COMPLEX, MONTANA 

orientation is dependent on the degree of anisotropism 

of grain shape. Fabrics of this type, which imply 

orientation prior to the solidification of the magma 

containing the crystals, might be accomplished by at 

least four mechanisms: laminar mush flow; magma 

currents; simple gravity settling; or postdepositional 

compaction. 

Phillips (1938, p. 130-135) and Turner (1942, p. 

280-300) have described certain peridotites and chromite-rich dunites in which the optical orientation of 

olivine is governed by its external shape and have proposed that the fabric of these rocks originated by laminar flow in a largely crystalline "magma" in the process of intrusion. Huang and Merritt ( 1952, p. 865-

868) find a dimensional and consequent optical orientation in certain troctolites that they interpret as being 

flow banded. There are several strong objections to 

origin of layering in the Stillwater complex by laminar mush flow: ( 1) the extreme lateral continuity of 

layers only a fe,v inches thick; (2) the prevalence of 

gradational layering and regular cyclical size changes 

throughout the layered sequence; and (3) the abundance of delicate textural features, such as layers one 

crystal thick and cusp textures. Furthermore, many 

of the fabrics assigned by these authors to origin by 

flow of a largely crystalline mass show a strong linear 

alinement of long axes in the layering plane, a feature 

very poorly developed in Stillwater rocks. 

Magma flow has been proposed by several authors 

as a mechanism for obtaining dimensional fabrics in 

layered igneous rocks. Current-bedded sedimentary 

rocks have strongly lineated fabrics (ICrumbein, 1942, 

p. 1355-1402; Nanz, 1955, p. 130), and preferred elongations are also produced where sand grains are deposited by flowing air and water in laboratory experiments (Dapples and Rominger, 1945, p. 246-261). It 

would seem probable, therefore, that magma currents, 

if capable of producing a dimensional orientation of 

grains, should also produce a strong lineation of long 

axes. In the Skaergaard intrusion, where evidence for 

magma currents has been abundantly demonstrated, 

Wager and Deer (1939, p. 271-275) found lineation 

of plagioclase crystals wherever such crystals had developed rectangular shapes. In the Merensky Reef, 

Schmidt ( 1952, p. 233-279) found long axes of orthopyroxenes and plagioclases alined parallel to the sides 

of circular depressions whose origin he ascribes to 

swirling magma currents, whereas the orthopyroxenes 

in the undisturbed Reef show no directional alinement 

in the layering plane. In the Ultramafic zone of the 

Stillwater complex, current structures have been observed locally but no systematic study of orientations 

within these areas has been made. By analogy, however, the two rudely lineated fabrics encountered in 

the present study, as well as those described by Hambleton (see footnote 4, p. 38) , seem best assigned to 

current action. 

It seems reasonable to expect that platy or elongate 

crystals sinking through quiet magma would tend to 

accumulate with their largest surfaces parallel to the 

floor of the magn1a chmnber. The perfection of such 

orientation would depend on the size and habit of the 

crystals, their density difference from the magma, and 

the viscosity of the 1nagma. The efficacy of this process has, however, been questioned by several authors. 

Turner ( 1942, p. 295) concluded, on the basis of his 

examination of a specimen from the differentiated sill 

of Lugar, Ayrshire, that settling of olivine crystals 

under the influence of gravity as they separate from a 

basic magma under static conditions fails to produce 

a preferred orientation of the olivine space lattice. 

Huang and l\1erritt ( 1952, p. 866) similarly failed to 

find preferred orientation in olivine from a troctolite 

in the 'Vichita l\1ountains, Oklahoma, which they believed frmn independent evidence to have been formed 

by gravity settling. Both authors, however, reported 

equant olivine in these specimens, and it would seem 

from the present investigation that randon1 fabrics in 

gravitationally settled rocks may be caused simply by 

lack of dimensional anisotropisn1 of the minerals in-

\rolved. vVager and Deer (1939, p. 268) doubted that 

the platy parallelism of tabular plagioclase in the 

Skaergaard intrusion resulted from direct sinking of 

crystals, because of the feebleness of orienting forces 

acting in a viscous magma of nearly the same density 

as the crystals. Van den Berg (1946, p. 199), however, had no hesitation in assigning dimensional fabrics of plagioclase and bronzite in Bushveld gabbros 

to crystal settling, and, in the Ultramafic zone of the 

Stillwater complex, the olivine and bronzite grains 

must have had considerably greater densities than the 

magma through which they fell. The process of orientation by simple settling and toppling leads to nonlineated fabrics, 'vhich thus would explain the position 

of grains in those rocks, which contain elongate crystals randomly disposed in the layering plane. 

The role of postdepositional compression in orienting grains is difficult to evaluate. Borg and Maxwell 

(1956, p. 71-81) have produced dimensional fabrics in 

loose quartz sands by compressional deformation under conditions simulating burial at depths of 16,000 

to 35,000 feet, but the relative amount of orientation 

caused by initial packing of the sand into cylinders 

and the amount caused by the subsequent deformation 

were not determined. If, as seems likely, settled crystals were very loosely packed at the surface of the floor 
TEXTURES OF THE ROCKS 47 

of the magma chamber, considerable compaction and 

consequent rotation of crystals could be accomplished 

by jarring or from compressive weight of burial, and 

such rotation should at least intensify any dimensional 

grain orientation already present. 

Several inferences as to conditions of crystal deposition in the Ultramafic zone may be made from the observed fabrics. Rocks containing elongate settled crystals that tend to be evenly disposed in the layering 

plane were probably deposited in the absence of 

magma currents. The disposition of the crystals is 

probably a result of simple gravitational forces that 

caused grains to assume their most stable position on 

the floor of the magma chamber. This process was 

assisted to an unknown extent by postdepositional 

compaction. Rocks containing elongate settled minerals that tend to be concentrated in one or two directions in the layering plane were probably influenced 

by weak magma currents during deposition. Rocks 

with little or no preferred orientation of settled crystnJs are composed of nearly equidimensional grains, 

and these rocks give little information as to conditions 

during deposition. 

RELATIONS OF TH'E PRIMARY PRECIPITATE TO THE 

INTERPRECIPITATE MATERIAL 

The original relative volumes of settled crystals and 

of the interprecipitate 1nagma between them can be 

measured directly in a few rocks, but in n1ost the boundaries of the primary precipitate have been obscured 

either by reaction with the i1~terprecipitate magma or 

by continued growth after deposition. 

REACTION REPLACEMENT 

Textural relations in the Ultramafic zone suggest 

that under the particular conditions of crystallization 

obtaining, reaction relations existed between olivinebronzite, olivine-augite, and bronzite-augite. Resorption textures have only been observed between settled 

and interprecipitate minerals. It should be noted that 

settled bronzite coexists with settled olivine in the 

Ultramafic zone without resorption textures, and that 

settled orthopyroxenes similarly coexist with settled 

clinopyroxenes in parts of the Banded zone. 

Textural evidence for the three reaction .Pairs mentioned above may be summarized as follows: (1) 

'Vherever settled olivine is surrounded by interstitial 

or poikilitic bronzite or augite, and wherever settled 

bronzite is surrounded by interstitial or poikilitic augite, the normally euhedral settled crystals are rounded 

and embayed. (2) vVhere augite or bronzite occurs as 

poikilitic host crystals, the number of centers of included settled crystals within and between the oikocrysts is the same, but volume of settled crystals within 

the oikocrysts is much reduced. ( 3) Single settled 

crystals at the edges of pyroxene oikocrysts are embayed where in contact with pyroxene and euhedral 

at the emergent ends. The reaction relation between 

olivine and bronzite is well known, and the textural 

features between these minerals are explained as reaction replacement of settled olivine in contact with 

the interprecipitate magma to forn1 bronzite. The 

similar textural relations where augite is the interstitial mineral suggest that olivine-augite and bronziteaugite are likewise reaction pairs, at least within the 

Ultramafic zone. 

Reaction textures involving chromite have not been 

observed. · OLIVINE ~ BRONZITE 

Replacement of olivine by interstitial bronzite is 

most highly developed in poikilitic harzburgites, which 

are composed of essentially equigranular olivine in a 

mesostasis of poikilitic bronzite and interstitial phtgioclase. Olivine grains within the bronzite oikocrysts 

are, without observed exception, rounded, embayed, 

and in some cases separated into islands (fig. 50). Between the poikilitic bronzite crystals, on the other 

hand, where the interstitial material is plagioclase, 

olivine crystals are commonly euhedral and invariably 

larger in size than those included in bronzite in the 

same hand specimen (fig. 50). At the margins of 

bronzite oikocrysts, single olivine crystals can be ob~ 

served which are euhedral at the ends that protrude 

into plagioclase, but which are embayed. at the ends 

that are in contact with bronzite (fig. 51). In rocks 

where chromite is an essential constituent, the original 

shapes of the olivine crystals before resorption are 

outlined by rims of formerly contiguous chromite (fig. 

53). Textural evidence that the replacement of olivine 

by bronzite was postdepositional is provided by thin 

chromite layers within the poikilitic harzburgites, 

which pass continuously through the rock without regard for the composition or orientation of the interstitial material (fig. 10). In most rocks the centers of 

the roughly spherical bronzite oikocrysts contain 

smaller olivine remnants than do the edges. 

OLIVINE~ CHROMIAN AUGITE 

Chromian augite oikocrysts, about one-half inch in 

diameter, are rare rather than essential constituents of 

poikilitic harzburgite and, where present, are accompanied by interstitial bronzite. They are more commonly associated' with olivine in granular harzburgite, 

where bronzit.e oikocrysts are not developed, and, in 

these rocks, augite oikocrysts contain embayed bronzite 
48 STILLWATER COMPLEX, MONTANA 

FIGURE 50.-Reaction replacement of settled olivine in poikllit!c harzburg!te. Olivine crystals surrounded by interstitial 

plagioclase (in lower part and at right of photograph) are euhedral; olivine grains within bronzite oikocrysts (upper 

left) are rounded, embayed, and islanded. ol=olivine; br=bronzite; pc=plagioclase; aug=augite. Crossed nicols. 

FIGURE 51.-Reaction replacement of settled ollvine by 

bronzite. Single olivine crystal at lower left of photograph shows crystal faces against plagioclase, but is 

partially replaced where in contact with bronzite. ol= 

olivine; br=brouzite; pc=plagioclase. Crossed nicols. 

FIGURE 52.-React!on replacement of settled olivine and 

bronzite in granular harzburgite. Settled minerals 

surrounded by interstitial plagioclase are euhedral; 

both olivine and bronzite within augite oikocryst are 

rounded and em bayed. ol=olivine; br=bronzlte; 

pc=plagioclase; aug= augite. Crossed nicols. 

PHOTOMACROGRAPHS OF OLIVINE ~ BROi\-ZITE AND OLIVINE ~ AUGITE REACTION TEXTURES 
TEXTURES OF THE ROCKS 49 

FIGURE 53.-Photomacrograph of reaction replacement texture in poikilitic chromite harzburgite. The original size and shape of the olivine 

crystals is outlined by the rims of former!~· contiguous chromite crystals. In Rome areaR outlined by chromite, olivine has been completely 

reacted out. ol =olivine. Crossed ntcols. 
50 STILLWATER COMPLEX, MONTANA 

as well as olivine. "·Textural relations between olivine 

and augite are identical with those between olivine and 

bronzite: olivine crystals within augite oikocrysts are 

rounded, embayed, and smaller than those surrounded 

by interstitial plagioclase (fig. 52). Examples of 

chromitite layers a few grains thick passing through 

augite hosts without interruption have been observed. 

BRONZITE--,) CHROMIAN AUGITE 

All bronzitite contains o/8 - to o/8 -inch clinopyroxene 

oikocrysts scattered evenly throughout the rock. Textural relations within and between the oikocrysts are 

identical with those described between olivine and 

bronzite and between olivine and augite (fig. 54). 

Single settled crystals of bronzite at the margins of 

poikilitic augite crystals show euhedral forms against 

plagioclase, and embayed contacts against clinopyroxene (fig. 55). Similar reaction textures between 

hypersthene arid clinopyroxene in the main zone of 

the Bushveld complex have been described by Lombaard (1934, p. 26-27), Van den Berg (1946, p. 163), 

and Wells (1952, p. 919-92_9). 

VOLUME RELATIONS 

Relative proportions of settled olivine and bronzite 

within and bet,veen the bronzite and augite oikocrysts 

have been determined by rileans of separate modal 

counts on large etched slabs by the method described 

by Jackson and Ross ( 1956, p. 648-651). A total of 

53 specimens 'of p_oikilitic·: harzb~rgite and bronzitite 

were investigated, and results are tabulated in table 4. 

TABLE 4.-Volume percentages of constituents within and between pyroxene oikocrysts in poikilitic harzburgite and bronzitite 

Poikilitic harzburgite 

Constituents Constituents Constituents Amount of 

between py; within bronz- within chro· olivine re- roxene oik- ite oiko~rysts mian augite placed within 

Specimen ocrysts. oikocrysts oikocrysts No. ------ -- Plagi- B·ronz- Chro- Bronz- ChroOlivine oclase Olivine ite Olivine mian ite mian 

augite augite 

--------------------- 52WF-4 _______ 94 6 68 32 -------- -------- 28 14 ______ -------- 80 20 ·so 50 -------- -------- 37 18 -------- ______ 82 18 67 33 -------- -----3i" 18 ------iii 27------ 85 15 55 45 69 35 29 ______ 9(1 10 57 43 71 29 37 21 52BE-8 ________ 82 18 48 52 66 34 41 20 15 ______ - 87 13 71 29 -------- 18 -------- 52AL-4 ________ 75 25 42 58 55 45 44 27 

52NL-13 ___ --- 7!1 22 34 66 57 43 56 27 52MV-8 _______ 70 30 36 64 50 50 49 29 20 ______ 91 9 55 45 70 30 40 23 55MV-l _______ 78 22 53 47 -------- -----42" 3 _______ 32 -------- 86 14 54 46 58 37 33 25 ______ 87 13 58 42 ·------- -----33" 33 --·----- 26 ______ 89 11 55 45 67 38 25 

27------ 81 19 50 50 63 37 38 22 63 ______ 82 18 57 43 -------- -------- 30 69 ______ -------- 83 17 52 48 -----7i" -----29" 37 -------- 72 ______ 85 l.'i ---·---- -----58" 1 

MEDJ-73.---- 72 28 30 70 -------- -------- --------6 

Specimen No. 

Bronzitite 

Constituents 

between chromian 

augite oikocrysts 

Constituents Amount 

within chromian of 

augite oikocrysts bronzite 

replaced 

1---~---1-------1 within 

Bronzite Plagioclase 

chromian 

Bronzite Chromlan augite 

augite oikocrysts 

-·---------1--------------- 51MV -7 _- ___ -- ________________ 87 13 50 50 43 

9 __ --------------------- 91 "9 53 47 42 11 .. ------------ _·_ ------- 87 13 48 52 45 

13. ---------------------- 90 10 52 48 42 15. ~ --------------------- 88 12 51 49 42 55MV -9 .. _____________________ 81 19 48 52 41 10.---------------------- 76 24 37 63 51 12 __ --------------------- 79 21 31 69 61 

32.---------------------- 88 12 55 45 38 

33 __ --------------------- 91 9 63 37 31 

68 __ --------------------- 87 13 54 46 38 77----------------------- 91 9 61 39 33 

78.---------------------- 89 11 57 43 36 52WF -2 _________ . _____________ 90 10 60 40 33 

8_---------------------- 99 1 64 36 35 

12_---------------------- 99 1 73 27 26 

20_ ---------------------- 93 7 62 38 33 

35_---------------------- 93 7 66 34 29 

40_ -·--------------------- 89 11 48 52 46 

42_---------------------- 90 10 49 51 46 

44_ ---------------------- 89 11 45 55 49 

47----------------------- 85 15 42 58 51 

48_---------------------- 80 20 34 66 57 52B E-29 ____________ -----.- ____ 85 15 48 52 44 3L ________ - _____________ 85 15 51 49 40 

34 _____ ------------------ 88 12 46 54 47 

35 ___ ------------ ~---- --- 83 17 46 54 45 '55B E-2 ____ -- __________________ 66 34 12 88 82 

4_----------------------- 59 41 9 91 85 

5 ___ --------------------- 73 27 25 75 66 7------------------------ 85 '15 38 62 55 

53IM -12 .. ___ -- _ -- _- _- ___ ----- _ 87 13 50 50 43 

EDJ-52-7 _ -------------------- 85 15 42 58 51 

Modal counts outside of oikocrysts, where the interstitial material is plagioclase, are reproducible within 

about 2 percent, but counts within the poikilitic crystals are no better than about 5 percent. In most rocks 

the oikocrysts contain higher proportions of included 

bronzite or olivine near the margins, and adequate 

samples within nonhomogeneous spheres larger than 

about one-half inch in diameter cannot be made, even 

on large slabs. Some of the rocks investigated contain 

from 1-3 percent chromite, which was assigned during 

counting to the mineral within which it occurred, so 

that only two constituents need be dealt with. 

In the 20 specimens of poikilitic harzburgite listed 

in table 4, the average volume of olivine between oikocrysts is 83 percent, the average volume of olivine 

within bronzite oikocrysts is 52 percent, and the average volume of olivine within augite oikocrysts, where 

these are present in the rocks, is 63 percent. In the 

33 specimens of bronzitite investigated, the average 

volmne of bronzite between the augite oikocrysts is 85 

percent, and the average volume of bronzite within the 

oikocrysts is 48 percent. Assuming that the settled 

bro-nzite and olivine crystals in the individual rocks 

were the same size and shape when deposited in the 

layers, and that the volume percentage of olivine 

where surrounded by plagioclase represents the volume 

percentage of olivine in the crystal mush before crystallization of the trapped interprecipitate magma, then 
TEXTURES OF THE ROCKS 

FIGURE 54.-Reaction r eplacement of settled l:H·onzlte in bronzitite. Bronzite crystals snrrounded by plagioclase 

(upper right half of photograph) are euhedral; bronzite grains within augite oikocryst (lower left ) are 

rounded, embayed and islanded. br=bronzite; aug= augite; pc=plagioclase. Note subparallel alinement 

of two elongate bronzite crystals. Crossed nicols. 

FrGU RB 55.-Reaction r eplacement of settled bronzite by a ugite. Single bronzite crystals around periphery of 

a ugite oikocryst show crystal faces against plagioclase, but they are partially replaced where in contact 

with augite. br =bronzite; aug= augite ; pc=plagioclase. Crossed nicols. 

PHOTOMACROGRAPHS OF BRONZITE ~ AUGITE REACTION TEXTUREJS 

51 
52 STILLWATER COMPLEX, MONTANA 

the averages indicate that more than 35 percent of 

olivine in contact with bronzite has been replaced, that 

about 25 percent of olivine in contact with augite has 

been replaced, and that more than 40 percent of bronzite in contact with augite has been replaced. The 

ability of augite t~ replace more bronzite than olivine 

is also suggested qualitatively in granular harzburgites, where settled bronzite in augite oikocrysts is noticeably smaller and more embayed than contiguous 

olivine in the same oikocrysts. 

The amounts of settled olivine and bronzite resorbed 

within the area of replacing pyroxenes have been computed for each specimen and are tabulated in table 4. 

These values hav~ been plotted against the volume 

percentage of settied minerals lying between the pyroxene oikocrysts for the individual specimens in figure 56. A rough but significant linear trend can be 

observed. in each of the three sets of data. The slopes 

of these trends for olivine-bronzite and bronzite-augite 

relations~ are the same, and although the data for olivine-augite relations are not definitive, a similar slope 

seems p~ssible. Again assuming that the volume of 

settled rp.inerals lying between the oikocrysts represents the":volume of olivine and bronzite present prior 

to the crystallization of either interstitial pyroxene or 

plagioclase, then these curves indicate that the higher 

the poro~ity of the crystal mush, the more olivine and 

bronzite ·:were resorbed within the area of the oikocrysts, regardless of the relative percentages of the 

two interstitial minerals in the rock. 

DISCUSSION 

In vie'Y of the well-known reaction relation between 

olivine a~d orthopyroxene, the poikilitic bronzite enclosing and partly replacing olivine is best explained 

as a reaction product between settled olivine crystals 

and the ihterprecipitate magma with which they were 

in contac_t. The trapped magma at the same time precipitated,lbronzite between the olivine crystals, and the 

reaction $topped when the proportionally small volume 

of magm,a had exhausted its mafic constituents. Plagioclase 6rystallizing from the interprecipitate magma 

simply filled interstices between olivine grains without 

reaction. The extension of this mechanism to explain 

identical textures between olivine-augite and bronziteaugite is hampered by incomplete data on the crystallization :behavior . of clinopyroxenes. In synthetic 

melts, cliiwpyroxenes form solid solutions from clinoenstatite to diopside, and Osborn and. Tait (1952, p. 

429) stat'e that clinopyroxenes over most of the composition range in the system diopside-forsterite-anorthite-silica have reaction boundaries with olivine. Poldervaart and Hess (1951, p. 472-489), however, consider that only limited solid solution exists in the pyroxene field of natural magmas, and that lime-rich 

pyroxenes are separated from olivines and lime-poor 

pyroxenes by cotectic boundaries. They conclude that 

although olivine may have a reaction relation with orthopyroxene, neither olivine nor orthopyroxene has a 

reaction relation with augite. vVithout questioning 

the general validity of this conclusion, it would appear 

that if cotectic relations between augite and olivinebronzite were invariably operative, then augite crystallizing from the interprecipitate magma in the Ultramafic zone would simply fill interstices between settled 

olivine and bronzite as plagioclase is observed to do. 

The consistent development of replacement textures 

would seem to indicate that, under the particular conditions obtaining, augite was formed in part by reaction of previously settled bronzite and olivine with 

the interprecipitate magma. 

Relative volume measurements indicate that only 

about one-third of the interprecipitate pyroxene in the 

Ultramafic zone fills spaces between settled crystals, 

and that the remainder crystallized at the expense of 

the settled crystals by reaction replacement. The 

measurements also suggest that the ability of the 

trapped magma to react with and replace the outer 

peripheries of settled crystals is restricted by low 

porosity in the crystal mush and enhance·d by high 

porosity. 

SECONDARY ENLARGEMENT 

Individual crystals of chromite, olivine, and bronzite, 

although as a general rule euhedral, are commonly 

observed with partially developed mutual interference 

boundaries and, in a few rocks, are completely anhedral against each other. Mutual interference between 

these crystals is inversely related to the amount of interstitial minerals present in the rocks, so that crystals 

range from perfectly euhedral, in rocks with 30 percent or more matrix, to completely anhedral, in rocks 

with less than 5 percent matrix. 

In certain areas of the complex characterized by 

plastically deformed layers, undulatory olivine, and 

strained bronzite, these relations are believed to be 

caused by deformational filter pressing prior to the 

final crystallization of the interstitial material. In 

other areas, however, where no evidence of early deformation exists, mutual interference seems best explained as being caused by continued growth of the 

settled crystals in certain layers after deposition, as 

proposed by Hess (1939, p. 431). The term secondary 

enlargement is used here to describe phenomena believed caused by this second mechanism. 

Several independent criteria indicating secondary 

enlargement are recognized: ( 1) All gradations exist 
UJ 100 z UJU.I 

XI-

~a >-a:: c..:::l Ill ZN UJO:: 

U.lc( 

all-

~~ 90 

UJ::::i 

~;;2 2:o ...JQ.. 

oz 

1-- 80 Zen UJtuen 0::>- UJO:: 

C..(.) 

...JO 

<(~ 

go 

:E 70 

UJ 100 

z UJU.I 

XI-

~a >-a:: c..:::l Ill ZN U.IO:: 

U.lc( 90 ~:I: 

UJ~ 

allUJ::::i 

z-

-~ 2:o ...JQ_ 

oz 

1-- 80 Zen UJtuen 0::>- UJO:: 

C..(.) 

...JO 

<(~ 

go 

:E 70 

UJ 

1-

a 

:;:, 

c( 

z 

c( 90 

i 

0 a::UJ 

:x:t: 

UlZN UJZ UJO 

~g; 80 

~~ 

UJen 1-tN~ Za:: 

Ou 

a::o 

Ill~ 

t-- zO 

UJ 

(.) 

a:: UJ 

c.. 

...J 

c( 

Cl 

0 

:E 

10 

TEXTURES OF THE ROCKS 

~ 052WF-4 

052BE-15 

052WF-18 

20 

~ 52WF-290 052MV-20 

055MV-2~ 55MV-26 

055MV·3 

52WF-27 · 

55MV-690 055MV-63 · 52BE-8 

~055MV-27' 

055MV·l 

52WF·l4 .......... 

0 52NL-13 

052AL·4~ 

30 40 50 60 

MODAL PERCENT OLIVINE REPLACED WITHIN BRONZITE OIKOCRYSTS 

~ 52WF·290 052MV-20 

~ 055MV-26 

70 

MODAL PERCENT OLIVINE REPLACED WITHIN CHROMIAN AUGITE OIKOCRYSTS 

~F-42 0 52WF-8 

52WF-350 ~2WF-20 lMV-4 

55MV-330 055M "r.'J51MV-13 52WF-20 (Y 852WF-42 51MV·l~~WF-40 · 0 52WF-44 

55MV-520 Mv:i.~-52l3E-34 

55MV·680glf~~i~ · 51MV-1152WF-4? 

52BE-310 0526 · ~-;::_ 055BE-7.. 

52BE-350 DJ52-7' ' 

055MV-9 '- •·• • 

055MV-78 '-05?.WF:48 

~055MV-12 

055MV-10 

1.: 

80 90 

~-

055BE~55BE·2 

055BE-4 

~ 

40 50 60 70 80 90 

MODAL PERCENT BRONZITE REPLACED WITHIN CHROMIAN AUGITE OIKOCRYSTS 

Fxouam 56.-Relations of volumes of settled minerals lying between pyroxene oikocrysts to volumes of settled minerals -':'epla~~d 

within pyroxene oikocrysts. 

53 
54 STILLWATER COMPLEX, MONTANA 

between rocks with perfectly euhedral settled crystals 

and those with mosaic textures. ( 2) The degree of 

euhedrism of individual crystals of olivine, bronzite, 

or chromi~e is related to the layering plane, so that a 

bronzitite with euhedral bronzite may be succeeded in 

the section by a dunite with mosaic texture, which may 

itself be succeeded by a granular harzburgite with partially interfering olivine and bronzite, each of these 

units retaining its character along the strike. (3) Individual crystals of olivine and bronzite in a few rocks 

have euhedral cores surrounded by slightly more ironrich,. mutually interfering rims, which are suggestive 

of authigenic overgrowths in sedimentary rocks. The 

general absence of peripheral zoning in rocks where 

secondary enlargement is believed to have occurred is 

attributed to the growth of the settled crystals in contact with magma from which crystals of the same composition were being precipitated. 

The simplest and most regular development of secondary enlargement occurs in rocks composed of only 

one settled mineral: poikilitic harzburgite, dunite, 

bronzitite, and ·chromitite. In rocks composed of two 

settled minerals (olivine chromitite, poikilitic chromite 

harzburgite, olivine bronzitite, and granular harzburgite), secondary enlargemei1t is dependent on the relative proportions of ~settled minerals present, and relations are more complicated. 

POIKILITIC HARZBURGITE AND DUNITE 

The transition from rocks containing automorphic 

to those containing xenomorphic olivine is accompanied by concomitant decrease in matrix minerals. For 

example, the specimen shown in figure 57 is composed 

of 70 percent perfectly euhedral olivine crystals and 

30 percent interstitial plagioclase. In the rock illustrated in figure 58 the olivine-plagioclase ratio has increased to 82: 18, and the olivine shows some mutually interfering boundaries. The plagioclase, however, still perfectly fills interstices between the partially interfering olivine, and shows no evidence of 

being replaced. The specimen illustrated in figure 59 

contains no interstitial plagioclase, and the olivine is 

completely anhedral. The absence of .replacement textures between olivine and plagioclase is considered to 

indicate that the mutual interference between olivine 

grains occurred prior to the crystallization of the interstitial material, and that interprecipitate magma 

was. displaced in the process of overgrowth. 

In a few .rocks the outermost margins of secondarily 

enlarged. olivine are slightly more iron rich than the 

core·s, but, in genera1, no zoning can be detected. 

BRONZITITE 

Progressive increase of interference boundaries with 

decreasing interstitial material content occurs with 

bronzite exactly as it does with olivine (figures 60"762). 

Zoning of peripheral parts of secondarily enlarged 

bronzite is, as with olivine, rare, but it is somewhat 

more easily recognized because of bronzite's lower birefringence. In rocks where such zoning is observed 

(fig. 82), the central core of the bronzite commonly 

has a euhedral shape. Exsolution lamellae generally 

extend continuously from the cores into the peripheries 

of the grains, indicating that enlargement occurred 

prior to exsolution. 

CHROMITITE 

In some chromitite layers chromite behaves like olivine and bronzite in that mutually interfering grains 

are distributed evenly throughout the layer. In others, 

however, the distribution of mutually interfering chromite grains is heterogeneous and the layers are composed of two textural types : ( 1) aggregations of 

nearly pure, massive chromite grains with polygonal 

shapes; and (2) normal euhedral or subhedral chromite crystals, which comprise about 70 percent by volume of the rock, and which are separated by interstitial silicates. Chromite crystals in the massive clots 

are about one-third larger than those in the more disseminated areas. Transitions between the massive and 

more disseminated chromitite areas are sharp, and the 

irregular attitudes of their contacts have no relation 

to the layering plane (figs. 63, 64). Wagner ( 1923, 

p. 228, 232) describes a similar texture in the chromitites of the Bushveld complex, which he calls "pseudoporphyritic poikilitic", and concludes that the more 

disseminated clots fell to the floor as aggregates, 

whereas the chromite crystals of the more massive 

areas fell as individuals. Several aspects of this texture in the Stillwater complex do not seem to conform 

with "\V agner's hypothesis : ( 1) Through-going layers 

of olivine crystals and thin chromitite layers parallel 

to the general ·structure cut across both massive and 

disseminated clots. ( 2) In many occurrences the more 

disseminated clots are very irregular in shape and long 

dimensions are not in the layering plane. ( 3) The 

dense packing and polygonal mutual boundaries of the 

chromite grains in the massive areas would seem to 

preclude an origin by simple accumulation of individuals. It seems most likely that the "pseudo-porphyTitic poikilitic" texture of Stillwater chromitites is 

caused by irregularly distributed secondary enlargement sub~equent to deposition. The chromitite layers 

probably originally consisted of closely packed chro-
TEXTURES OF THE ROCKS 

FIOURE 57.-Poikilitic ha r zburgite virtually unaffected by secondary enlargement. Euhedral olidne crystals are separated 

uy interstitial plagioclase. Texture is automorphic poikilitic. Cro,sed ni cols. 

FIGURE 5S.~ oikil iti c harzburgite affected by intermediate secondary enlargement. Subhedral olivine crystals show some 

mutual interference, but arc partially sepa rated by interstitial plagioclase. 'l'exturc is hypautomorphic granular. 

Crossed nicols. 

FIGURE 59.-Dunite showing complete secondary enlargement. Anhedral olivine grains show well-developed mutual 

interference. 'J.'he rock contains no plagioclase. Texture is xenomorphic granular. Crossed nicols. 

PIIOTOi\IACROGRAPHS OF OLIVINE-RICH ROCKS SHOWING PROGRESSIVELY ADVANCED DEnRmm~ OF SECONDARY ENLARGEMENT 

55 
56 STILLWATER COMPLEX, MONTANA 

FIGURE 60.-Bronzit!te virtually unaffected by secondary enlargement. Euhedral bronzite crystals are separated by Interstitial 

plagioclase. Texture Is automorphic poikilitlc. Crossed nicols. 

FIGURE 61.-Bronz!tite affected by Intermediate secondary enlargement. Subhedral bronzite crystals show some mutual interference but a re partially separated by Interstitial plagioclase. Texture is hypautomorphi c granular. Crossed nlcols. 

FIGURE 62.-Bronzitite showing complete secondary enlargement. Anhedral bronzite grains show well-developed mutual 

interefence. The rock contains no plagioclase. Texture is xenomorphic granular. Crossed nicolR. 

PHOTOMACROGRAPHS OF BRONZITITES SHOWING PROGRESSIVELY ADVANCED DEGREES OF 

SECONDARY ENLARGEMENT 
TEXTURES OF THE ROCKS 

. -

Pp-n;ilitic harzburgite . ,...., ' ~: .. -.. ·' 

FIGURE 63.- F!eld photograph of massive and disseminated clots in chrom!tite layer. 

FIGURE 64.-Photomicrograph of massive and disseminated clots In chrom!t!te. Light material interstitial to chrom!te 

grains in disseminated clots is pyroxene and plagioclase. Plane polarized light. 

EXAMPLES OF HETEROGENEOUS SECONDARY ENLARGEMENT IN CHROMITITES 

57 
58 STILLWATER COMPLEX, MONTANA 

mite euhedra containing about 40 percent interstitial 

magma. In part these chromite crystals were secondarily enlarged, and elsewhere the undisplaced interstitial magma crystallized to silicate minerals. Zoning 

at the outer margins of enlarged chromite inaividuals 

has not been observed. · 

OLIVINE CHROMITITE AND POIKILITIC CHROMITE 

HARZBURGITE 

In many specimens of olivine chromitite and poikilitic chromite harzburgite, chromite crystals are comnlonly euhedral, whereas olivine commonly shows evidence of secondary enlargement. In many of the rocks 

the peripheral parts of olivine grains are crowded with 

inclusions of chromite identical in size with the chromite located between olivine grains (fig. 65). The 

clear inner core of such olivine is thought to represent 

the original settled crystal, and the periphery is believed to represent secondary enlargement of the olivine after deposition. Enlargement of the chromite 

in many. of these rocks;· however, apparently did not 

occur. 

In other olivine chromitite and poikilitic chromite 

harzburgite specimens, particularly in those with high 

chromite-olivine ratios, these relations a.re reversed. 

Chromite grains commonly show interference boundaries with each other, and olivine crystals are nearly 

euhedral or are· partially replaced by interprecipitate 

bronzite (fig. 53). In these rocks chrorriite 'vas apparently precipitated from the interstitial liquid or at the 

mush surface, whereas olivine crystals were not enlarged. 

In solne·olivine chromitite both olivine and chromite 

show evidence of having been enlarged subsequent to 

deposition. It is not clear from textural evidence 

whether enlargement of olivine and chromite occurred 

simultaneously or sequentially. 

OLIVINE BRONZITITE AND GRANULAR HARZBURGITE 

The character of enlargement in rocks containing 

mixtures of settled olivine and bronzite, like that in 

olivine-chromite rocks, seems dependent on the relative 

proportion~ of settled minerals in the layer. In olivine-rich granular harzburgites, olivine crystals conlinonly show considerable mutual interference. Bronzite crystals in these rocks are also enlarged, but the 

outer peripheries have replacement textures against 

the olivine (fig. 66). In olivine bronzitite, olivine crystals are generally euhedral, whereas the bronzite may 

show considerable mutual interference (fig. 36). In 

both types, olivine enlargement, where present, appears 

to have been completed before bronzite enlargement 

began. 

VOLUMETRIC RELATIONS 

The original pore space in th_e _crystal mush that 

formed at the floor of the intrusion durinO' the accu- . b 

mulatwn of the Ultramafic zone is now cmnposed of 

material that was added to settled crystals after deposition, plus interstitial precipitates of other 1ninerals. 

If the original pore-space volume in the crystal mush 

is known, a 1neasure of the amount of secondary enlargement in any rock can be obtained by subtracting 

the volume of interstitial material from the initial 

porosity. 

Variations in the initial porosity of the crystal mush 

are discussed in the following section, and estimates of 

the average initial porosities for the various rock types 

are listed in table 5. No single average value for initial porosity of olivine chromi1tites can be given because 

of their bimodal size characteristics and variable proportions of olivine and chromite. 

Th1easurements of the amount of interstitial material 

in specimens of the various rock types, made by modal 

counts, are presented graphically in figure 69. The 

amount of interstitial material in harzburgite and 

bronzitite was determined on those parts of large 

etched slabs where plagioclase was in contact with the 

settled 1ninerals; areas within pyroxene oikocrysts, 

where much of the material formed by reaction replacement, \vere avoided. Interstitial chromite in 

these rocks was tabulated as settled material, even 

though it occupies parts of the interstices between 

the larger settled olivine and bronzite crystals. Measurements in chromitite, where no reaction textures 

have been observed, were made on the entire area of 

2- by 2-inch thin sections. The frequency histoO"rams 

in figure 69 suggest a normal distribution of the 

amount of interstitial material, but the number of 

measurements is, in all cases, insufficient to establish 

this definitely. Arithmetic mean percentages of the 

average amounts of interstitial material are listed in 

table 5. 

TABLE 5.-Aver~ge _amount of enlargement of settled crystals in the 

pnnc·tpal rock types, in volume percent 

Rock type 

Poikilitic barzburgite _____________ ------------ Bronzitite .. ______ . _____ . ____________________ _ 

Chromitite .. _____ ---------------------------- Granular harz"Qurgite and olivine bronzitite __ _ Olivine chromitite and poikilitic chromite harzburgite .. _______________________ . ______ . 

Estimated 

average 

initial· 

porosity 

35 

35 

40 

30 

25-40 

Average 

amount of 

interstitial 

material 

15 

14 

20 

10 

14 

Average 

amount of 

secondary 

enlargement 

20 

21 

20 

20 

11-26 

The average amount of secondary enlargement for 

the rock types is calculated by subtraction in table 5. 

Differences in estimated initial porosity tend to bal-
TEXTURES OF THE ROCKS 

FIGURE 65.-Secondary enlargement of settled olivine In olivine 

chromltite. Olivine has grown outward after deposition to 

Include neighboring settled chromlte grains. Some enlarge· 

men ta of chromite have also occurred. ol=ollvlne; chr= 

chromlte; pc=plagioclase. Crossed nlcols. 

FIGURE 67.-Detalls of contact between Interstitial plagioclase 

and Interstitial bronzlte In polkllltic harzburgite. Where the 

two minerals are In contact, small-scale mutual embayment 

occurs. m=·blotlte; other symbols as In figure 66. Crossed 

nlcols. 

FIGURE 66.-Secondary. enlargement of settled bronzlte In olivine-rich granular harzburglte. Bronzlte has grown out to 

fill Interstices and partially to replace olivine. ol=ollvine; 

br=bronzlte; pc=plagloclase. Crossed nlcols. 

FIGURE 68.-Detalls of contact between Interstitial plagioclase 

and Interstitial augite In bronzitite. Locally the augite has 

developed segments of prism and dome faces against plaglo· 

clase, but a long most of the contact there Is mutual embayment. aug.= augite ; other symbols as in figure 66. Crossed 

nlcols. 

PHOTOMICROGRAPHS SHOWING DETAILS OF SECOJ\TDARY ENLARGEMENT AND RELATIONS BETWEEN 

INTERPRECIPITATE MINERALS IN THE ULTRAMAFIC ZON'E 

59 
60 STILLWATER COMPLEX, MONTANA 

I- 50 z 

LLJ 

() 

5 40 a.. 

z 

~ 30 z 

LLJ 

:::> 

8' 20 0::: 

Lt... 

0::: 

~ 10 

::2: 

:::> 

z 0 

I- 50 z 

LLJ 

() 

5 40 a.. 

z 

~ 30 () 

z 

LLJ 

:::> 

<Y 20 LLJ 

0::: 

Lt... 

5 10 CD 

::2: 

:::> 

z . 0 

I- 50 z 

LLJ 

() 

0::: 

~ 40 

~ 30 z 

LLJ 

:::> 8' .20 0::: 

Lt..." 

0::: 

~ 10 

::2: 

:::> 

z 0 

Poikilitic 

harzburgite, 

21 specimens 

VOLUME PERCENT INTERSTITIAL 

MATERIAL 

Bronzitite, 

38 specimens 

Granular harzburg- ite and olivine bronzitite, 

24 specimens 

50 

VOLUME PERCENT INTERSTITIAL 

MATERIAL 

Olivine chromitite and poikilitic chromite harzburgite, 25 specimens 

10 20 30 40 50 10 20 30 40 50 

VOLUME PERCENT INTERSTITIAL VOLUME PERCENT INTERSTITIAL 

MATERIAL MATERIAL 

Chromitite, 31 specimens 

VOLUME PERCENT INTERSTITIAL 

MATERIAL 

FIGURE 69.-Frequency distribution of volume of interstitial material, 

by rock type. 

ance the differences in average amounts of interstitial 

constituents. Within the limits of measurements and 

assumptions, therefore, there seems to be no significant . 

difference in the amount of secondary enlargement between the five principal rock types of the Ultramafic 

zone. 

. STRATIGRAPHIC AND AREAL DISTRIBUTION 

Determinations of the amount of secondary enlargement material within the cyclic units of the Peridotite 

member. show no smooth pattern of variation. Of the 

three cyclic units shown in table 6, units 9 and 15 show 

ref\SOnably consistent values co~sidering the limits of 

accu~acy of the method used to determine the amount 

of se~ondary enlargement, and cyclic unit 2 shows an 

irregular variation. Furthermore, no simple upward 

TABLE 6.-Intracycle stratigraphic distribution of amount of 

secondary enlargement in the Mountain View . section of the 

Peridotite member, in volume percent 

Cyrlic Secondary unit Specimen Interstitial enlarge- No. No. material rncntmate·· rial I 

-- Bronzitite ___ --------------------- 55MV-77 9 26 

Granular harzburgite __ ----------- 55.1\1V-76 10 20 

-75 12 18 

·15 -74 10 20 

Poikilitic harzburgite_ ------------ 55MV-73 13 22 -72 15 20 

-69 12 23 

Bronzitite._---------------------- 55MV-33 9 26 

-32 12 23 

-31 13 22 Granular harzburgite __ -------- ___ 55MV-30 8 22 9 -29 8 22 -28 8 22 

Poikilitic harzburgite_ -------- ____ 55MV-27 17 18 

-26 9 26 

-25 9 26 

Bronzitite: fine-grained. ______ ... ____________ 55MV-12 21 14 -10 24 11 coarse-grained ___________________ 52MV-11 17 18 

55MV-9 19 16 2 Granular harzburgite ______________ 55MV-8 15 15 -7 19 11 ii2MV-9 14 16 

Poikilitie harzburgite_ ------------ 52MV-8 29 f) 

55MV-3 12 23 -2 14 21 -1 20 15 

I Assuming an initial porosity of 35 percent in bronzitite and poikilitic harzhurgite, and 30 percent in granular harzburgite. 

variation between cycles could be detected in either the 

West Fork or Mountain View sections. If simple intercycle variation does exist, an excessively large number of measurements would be required to define it 

because of the large deviation in amount of interstitial 

material within single layers. 

Although a pattern of stratigraphic variation in the 

amount of secondary enlargement cannot be established, gross changes in the amount o:f interstitial material along the strike were observed in the field and 

have been confirmed to some extent by measurements. 

In the field, it was observed that in areas where the 

Peridotite member is stratigraphically thick, the rocks 

contained more interstitial material than rocks in thinner sections. Within the Bronzitite member of the 

Ultramafic zone, which is considerably more consistent 

in thickness than the Peridotite member, no significant 

changes in interstitial material content along the strike 

were detected in the field. Measurements were made of 

the amount of interstitial material in a number of specimens from correlative subunits in the Mountain View 

and West Fork sections, and the amount of secondary 

enlargement was calculated. These values are listed 

in table 7. In the lower part of the Peridotite member, the West Fork section is about one-third as thick 

as the Mountain View section and contains 1.5 times 

as much secondary-enlargement material; above the 

main chromitite layer, the West Fork section is about 

one-half as thick and contains about 1.1 times as much 
TEXTURES OF THE ROCKS 61 

TABLE 7.-Average amount of interstitial and secondary enlargement material in correlative subunits of the Ultramafic zone 

Subunit Stratigraphic 

thickness 

(feet) 

Bronzitite member _______________________________ 

Peridotite member: 

2, 120 

Main chromitite to top of Peridotite member ____ 500 

Base of Peridotite member to main chromitite ____ 1, 000 

enlargement material; in the Bronzitite member, the 

vVest Fork section is about two-thirds as thick as the 

Mountain View section and contains only very slightly 

more enlargement material. These relations are illustrated graphically in figure 70. 

The average amount of enlargement material in the 

bronzitite of the Basal zone, which is sporadically 

developed between the chilled gabbro and the base of 

the Peridotite member, is considerably less than the 

average anywhere within the Ultramafic zone. The 

average of four detern1inations from the Benbow area, 

where the basal bron-zitite reaches its maximum development, is 7 percent enlargement material, and the 

range is 20 percent to none. Lateral variation within 

the basal bronzitite was not investigated. 

Secondary enlargement of settled crystals in the 

Ultramafic zone may be summarized as follows : ( 1) 

No significant difference in amount of enlargement 

between rock types can be established. ( 2) The amount 

of enlargement may change abruptly and on a small 

0 3.0 

i= 

<:: 

0:: 

Vl 

Vl 

LLI 

z 

::£ 

~ 

J: 

1-

::£ 

0:: 

0 

1.1... 

1- 2.0 Vl 

LLI 

~ 

0 

1-

~ LLI 

> 

z 

·~ 

1-

z 

::::> 

0 

0 

~ \ 0 

\ 

\ 0 

~ 1.0 -+---..------.-----,,----, 

0.6 0.7 0.8 0.9 

MOUNTAIN VIEW TO WEST FORK 

ENLARGEMENT RATIO 

1.0 

FIGUH!ll 70.-Relations between thickness of correlative subunits and amount of secondary enlargement. 

West Fork section Mountain View section 

Secondary Secondary 

Intersti- enlarge- Strati- Intersti- enlarge- tial ment Number of graphic tial ment Number of 

material material specimens thickness material material spechnens (volume (volume averaged (feet) (volume (volume averaged percent) percent) percent) percent) 

12 23 6 3,060 13 22 6 

12 21 12 1, 035 14 19 19 

9 24 15 2, 975 16 17 28 

scale with stratigraphic position. (3) No systematic 

trends in this change within layers, within cyclic units, 

or between cyclic units could be established. ( 4) The 

average amount of secondary enlargement seems to be 

related to relative thickness of stratigraphic section, 

so that in two correlative sections believed to have been 

formed in the same interval of time, the thinner stratigraphic section shows more enlargement tha~ the 

thicker. ( 5) There is considerably less enlargement in 

the bronzitite of the Basal zone than 111 any investigated part of the Ultramafic zone. 

DISCUSSION 

Wager and Deer (1939, p. 127-128) postulate that 

primary precipitate minerals in the Skaergaard intrusion had been enlarged subsequent to their deposition. 

They state: 

That part of the interprecipitate material which crystallized 

as the same minerals as the primary crystals, is found as outer 

zones of increasingly lower temperature solid solutions, and 

such outer zones often occupy interstices. Neglecting the outer 

fringe, the primary precipitate may be found to have an idiomorphic relation to a certain mineral, but taking into consideration the outer fringe, the crystals would be put down as 

allotriomorphic to the same mineral. 

Enlargement of settled minerals caused only by continued precipitation of. the same minerals from the 

trapped interstitial magma should be limited in amount 

and characterized by strong peripheral zoning. Such 

a mechanism cannot account for completely monomineralic dunites, bronzitites, and chromitites in the Ultramafic zone of the Stillwater complex without calling upon olivine, bronzite, and chromite magmas, for 

which there is no other evidence of existence. To account for these relations, Hess ( 1939, p. 430-431) has 

proposed a more complicated mechanism involving 

diffusion. On the basis of slump structures in the gab-· 

broic rocks, Hess considers the unconsolidated mush on 

the floor of the complex to have been about 10 feet 

thick throughout the period of accumulation. Diffusion 

between the main body of magma and the magma 

trapped within this crystal mush allowed crystalliza-
62 STILLWATER COMPLEX, MONTANA 

tion of solid phases in the interstices which were identical in' composition with the settled crystals. Hess 

h3ts concluded that the amount of diffusion was controlled by the rate of accumulation of crystals: where 

the rate of accumulation was slow, diffusion was operative; where rapid, the original magma was effectively 

trapped. 

The relative efficacy of diffusion in causing secondary enlargement in the IDtramafic zone seems open to 

some question. The quantity of a material that diffuses in the steady state is proportional to its concentration at the front (Garrels, Dreyer, and Howland, 

1949, p. 1826-1827) ; assuming that diffusion is capable 

of supplying olivine or bronzite to dunite or bronzitite 

layers in the mush, the supply of chromite to associated chromitites should be much less. The average 

amount of secondary enlargement in chromitite, however, does not seem to be significantly less than in silicate layers. In some sequences, 1- to 2-inch chromitite 

layers contain less interstitial material than the olivine-rich rocks immediately above and below, and the 

mechanism for accomplishment of this selective overgrowth after burial is difficult to visualize. Furthermore, detailed mapping of the Ultramafic zone has resulted in discovery of small-scale slump structures involving units as thin as six inches. It would therefore 

seem possible that some secondary enlargement, at 

least, took place at or near the surface of the mush, 

in contact with the magma from which the crystals 

were precipitating. 

Regardless of whether optically continuous overgrowth occurred at the surface of the mush or at 

depths .of several feet, Hess' concept of a direct relation between amount of overgrowth and rate of crystal 

accumulation seems equally valid. Field estimates 

throughout the Ultramafic zone as well as laboratory 

measurements indicate that in areas where the stratigraphic section is relatively thin, the rocks contain less 

interstitial material than rocks in areas where the 

stratigraphic section is relatively thick. Because these 

"shelf" and "basin" sections are believed to have accumulated in the same interval of time, the amount of 

overgrowth would seem to be related to rate of accumulation. 

Assuming that the measurements are representative 

and that a qualitative relation between secondary enlargement and accumulation rate is established, the 

following observations can be made: ( 1) Accumulation 

of bronzite to form the bronzitite of the upper part of 

the Basal zone was sufficiently more rapid than crystal 

accumulation with the Ultramafic zone, proper, to appreciably inhibit secondary crystal enlargement. (2) 

In some parts of the Ultramafic zone the rate of crystal 

accumulation changed erratically on a small scale. ( 3) 

The average rate of crystal accumulation in the Mountain View section was greatest near its base and gradually decreased throughout the section, whereas the 

average rate in the 'Vest Fork section was initially 

very slow but increased near the main chromitite and 

maintained a steady rate at least through the Bronzitite member (table 7). 

INITIAL POROSITY 

The original porosity of settled crystals in the accumulating mush can be estimated in some rocks by 

means of modal counts of individual crystals and interstitial material, but distinction between what were 

originally settled crystals and interprecipitate material 

is commonly difficult because of secondary enlargement 

and reaction replacement of the settled material. The 

apparent porosity is, of course, decreased where settled 

material has been enlarged, and increased where the 

settled material has been resorbed. Modal counts of 

areas where relations are n1ost clearly definable indicate that original porosities in the crystal mush ranged 

between about 20 and 50 percent by volume, and that 

the average for the zone was probably about 35 percent. Packing relations are similar to those in comparable sedimentary rocks : the finer· grained chromitites apparently had slightly higher initial porosities 

than the coarser harzburgites and bronzitites, and bimodal associations of chromite and olivine in general 

had lower initial porosities than assemblages containing minerals of uniform size. 

Estimates of 1naximum, minimum, and average porosities are considered below. For a given rock type, 

the maxiinum original porosity is based on the maximum observed amount of interstitial material in any 

rock of that type; the minimum estimate is based on 

the amount of interstitial material in rocks showing 

very slight n1utual interference effects between settled 

crystals; and the estimated average porosity is a guess 

limited by these two extremes. 

BRONZITITE 

In bronzitite composed of anchiequidimensional 

bronzite, the greatest measured amount of interstitial 

material was 41 percent; the amount of interstitial 

material in bronzite showing the beginnings of mutual 

interference boundaries between bronzite crystals is 

about 30 percent. The range of original porosity was 

probably, therefore, about 10 percent. Differences in 
TEX~URES OF THE ROCKS 63 

original porosity in bronzitite of this type are more 

probably a result of differences in packing than of 

differences in grain size or sorting, because the observed ranges in the latter properties are relatively 

small. The average porosity of bronzitite with nearly 

equidimensional grains is estimated to have been 35 

percent. 

The much less abundant bronzitite composed of tabular or broad prismatic bronzite is observed to have 

considerably less interstitial material than bronzitite 

with nearly equidimensional grains. This is presumably an effect of grain shape on original porosity similar to the decrease in porosity of sands with diskshaped particles observed by Fraser (1935, p. 937). 

No numerical estimate of the average original porosity 

of bronzitite with flattened bronzite crystals can be 

given because the ratio of intercepts of these minerals 

varies widely among individual specimens, and, presumably, so would the initial porosity. The largest 

n,mount of interstitial material measured in bronzitite 

of this type was 15 percent, but the specimen showed 

some n1utual interference between the bronzite grains. 

The initial porosity of this bronzitite is estimated to 

have been about 20 percent. 

POIKILITIC HARZBURGITE 

The maximum amount of interstitial material measured in poikilitic harzburgite was 30 percent, and this 

specimen shows slight effects of mutual interference 

between the olivine crystals. Presumably, both the 

maximum porosity and the average porosity were 

greater than 30 percent. The grain shape, sorting, and 

size range of olivine in poikilitic harzburgite do not 

differ appreciably from those of anchiequidimensional 

bronzite in bronzitite. Furthermore, the average 

amount of interstitial material for all measured poikilitic harzburgite is slightly greater than the average 

for bronzitite, a difference that may be related to the 

inclusion of several specimens of bronzitite with broad 

prismatic crystals into the average. It seems reasonable, therefore, that the porosity of crystal mush composed of olivine crystals and liquid was very nearly 

equal to the porosity of anchiequidimensional bronzite 

crystals and liquid, and the original porosity of the 

poikilitic harzburgite is assumed to have been about 35 

percent. Neither grain size, sorting, nor shape variations within the poikilitic harzburgite seem large 

enough to have caused much variation in original porosity. IIowever, the presence of accessory chromite 

in poikilitic harzburgite lowered the original porosity 

slightly, because of its location between olivine grains. 

CHROMITITE 

The maximum amount of interstitial material in any 

massive chromitite investigated was 50 percent; beginnings of mutual interference boundaries are observed 

in · chromitite with about 30 percent interstitial material. The amount of interstitial material in three of 

the specimens measured exceeded the 41 percent recorded for the maximum amount in the bronzitite. 

The range of porosity in chromitite, therefore, exceeded that of bronzitite, and the average porosity was 

probably higher. An average porosity of 40 percent 

would seem to be a reasonable estimate. That finegrained aggregations of chromite crystals should have 

a higher porosity than aggregations of olivine or bronzite crystals 10 times as large is in accord with the observations of Ellis and Lee ( 1919, p. 121), Trask, 

( 1931, p. 273), and others, that, contrary to the theoretical behavior of spheres, the porosity of sands decreases as grain size inc~eases. Variations in grain 

size, shape, and sorting within the chromitite, as in 

the poikilitic harzburgite, is probably too small to 

effect ,porosity changes of any great magnitude. between layers. If an original porosity range of 15 to 

20 percent did exist, as indicated, the major part of the 

variation, is most likely caused by differences in packing and bridging. 

GRANULAR HARZBURGITE 

The maximum observed amount of interstitial material in a granular harzburgite is 19 percent, in a specimen that shows considerable mutual interference. The 

average amount of interstitial material in all specimens of granular harzburgite measured is 3 percent 

lower than the average for bronzitite (fig.· 69). In 

granular harzburgite composed of bronzite and olivine 

crystals of equal size, the original porosity should have 

been in the same range as the original porosity in 

bronzitite and poikilitic harzburgite. In most granular harzburgite, however, the settled olivine and bronzite crystals have different sizes (fig. 19), and, although these minerals considered individually have 

good sorting, the degree of sorting of the rock as a 

whole is redticed by the inclusion of two sizes of material (fig. 28). The effect of this poorer sorting 

should be lowered original porosity (Fraser, 1935, p. 

922-930) ; the amount of lowering is dependent on the 

relative proportions and the relative sizes of settled 

olivine and bronzite in the rock. The decrease in average original porosity caused by these bimodal size· distributions was probably relatively small, because at 
64 STILLWATER COMPLEX, MONTANA 

the observed size ratios between olivine and bronzite 

most of the smaller grains are incapable of filling 

voids between the larger ones. The average porosity 

of granular harzburgite, therefore, should have been 

somewhat less than that of bronzitite and poikilitic 

harzburgite, possibly about 30 percent. 

OLIVINE CKROMITITE AND POIKILITIC CHROMITE HARZBURGITE 

The maximum observed amount of interstitial material in a rock composed of a mixture of settled olivine and chromite is 26 percent. This specimen, which 

has an olivine chromite ratio of 69: 31, shows no mutual interference, and the observed amount of interstitial material is believed to agree closely with the 

initial porosity. The average amount of interstitial 

material in the 20. specimens measured was 14 percent, 

about the same as that for bronzitite. 

It seems reasonable that mushes composed of mixtures of olivine and chromite had considerably lower 

porosities tha~ mushes composed of either mineral 

·alone. The sorting of olivine chro!!litite is strictly 

bimodal (fig. 29) and the average size ratios of chromite to olivine range from 0.07 to 0.38, with an average 

of 0.19 (table 3). In all these rocks, the chromite is 

small enough to exist in voids between the olivi~e crystals; the size ratios are less than what Fraser ( 1935, 

p. 919) calls the "critical ratio of occupation." In 

most of the 16 olivine chromitite specimens for which 

size measurements have been made, the chromite-olivine diameter ratio is also within. what Fraser ( 1935, 

p. 919) calls the "critical diameter of entrance" and 

finds t~ be 0.414 for loosest packing, and 0.154 for 

tightest ·packing (in the case of spheres). Assuming 

that octahedral chromite grains and nearly equidimensional olivine did behave essentially as spheres, the 

observed diameter ratios indicate that chromite grains 

in most olivine chromitite are small enough to have 

passed between tightly packed olivine crystals and are 

therefore capable, at least, of having been sifted down 

in the mush. At any rate, the original porosity of 

olivine chromite was much reduced by small· chromite 

grains filling spaces between olivine grains at the expense of interstitial riiagma. The extent of this reduction would be greatest where olivine constitutes about 

65 percent of the rock, and the porosity of the mush 

with these proportions would seem to have been about 

25 percent. With decrease in chromite content, the 

porosity would approach that of poikilitic harzburgite. 

With increase of chromite the porosity would approach that of chromitite. The range of porosity was 

probably· between 25 and 40 percent. 

DISCUSSION 

The original porosity variation of the mush, which 

is from 20 to 50 percent in the Ultramafic zone, 'is 

most probably caused by two independent factors: 

( 1) the physical properties of the crystal aggregate 

making up the mush, and (2) the amount of compaction of the mush prior to cementation. Crystal shape 

differences are apparently responsible for the porosity 

difference between equidimensional and tabular collections of bronzite; average size differences for porosity differences between silicate layers and massive 

chromitite layers; and differences in sorting for porosity differences between olivine-chromite mixtures and 

monomineralic accumulations. Indicated porosity differences of 10 to 20 percent within bronzitite and 

chromitite with nearly equal size and shape, however, 

cannot be accounted for by changes in physical properties of the grains, and these differences would seem 

to be caused by variations in amount of compaction 

prior to cementation. . 

Bronzitite and poikilitic harzburgite make up about 

88 percent of the Ultramafic zone, and the remaining 

12 percent is composed largely of granular harzburgite. The estimated average porosity of the Ultramafic zone as a whole, therefore, is only slightly less 

than 35 percent. In consideration of the high viscosity 

and density of the magma, and the generally uniform 

sizes and nearly equidimensional shapes of the settled 

crystals, even an average porosity of 35 percent seems 

somewhat low compared with measured porosities of 

experimentally settled and freshly deposited sands, 

which, in the 0.2 to 1.0 mm size grades, generally range 

between 40 and 46 percent (Ellis and Lee, 1919, p. 

121; Trask, 1931, p. 273; Hamilton and M:enard, 1956, 

p. 756). Fraser (1935, p. 936), however, has shown 

that the porosity of quietly settled wet natural sands 

can be reduced by 5.5 to 8 percent by continued 

jarring. 

The porosity of the complex as a whole is probably 

considerably less than 35 percent because of the prevalence of broad prismatic and tabular minerals in the 

thick section of norites, gabbros, and anorthosites stratigraphically above the Ultramafic zone. In contrast, 

the Ultramafic zone is characterized by a preponderance of nearly equidimensional settled crystals. 

Within the limits imposed by a small number of 

significant measurements, the settled crystals in the 

Ultramafic. zone obey the general physical laws of 

particle packing with regard to grain size, sorting, and 

shape. Numerical estimates of the original porosity 

agree very well with porosity measurements of wellpacked sedimentary rocks in the same size range. The 
TEXTURES OF THE ROCKS 65 

generally good packing of the settled crystals, like the 

packing in sandstones, was probably effected near the 

surface· of deposition, perhaps by periodic jarring of 

the floor by minor tectonic disttirbances during deposition. Failure of the mush to pack is recorded in a few 

layers by n,bnormally large an1ounts of interstitial materinJ in the rocks. 

INTERPRECIPITATE MATERIAL 

The interprecipitate material is defined as including 

all miner~tls that crystallized in place in the rocks. 

Under this definition, it n1ay be divided into three 

types : ( 1) the material added on to settled crystals 

by secondary enlargement after deposition; (2) the 

material which simply fills space between the original 

or secondarily enlarged settled crystals; and ( 3) the 

material which has partly replaced settled crystals by 

postdepositional reaction replacement. The first two 

types of material, taken together, occupy what originally was pore space in the crystal mush. The third 

type, however, now occupies space which originally 

was taken up by precipitated crystals, and, although 

it is not descriptively "interprecipitate", it is so classif-ied because it formed in place. 

The relative proportions of these three types of material differ from layer to layer. Development of secondary enlargement and interstitial space-filling minerals are reciprocal: At one extreme are the completely 

monomineralic rocks whose entire original pore space 

has been filled by secondary enlargement of the settled 

crystals; at the other are certain layers of bronzitite 

and chromitite that contain 40 to 50 percent of interstitial space-filling material. Development of reaction 

replacement in rocks containing olivine or bronzite is 

greatest in rocks with minimal secondary enlargement. 

The exact distinction between the precipitate and 

interprecipitate constituents of every rock cannot be 

made by simple inspection because of the absence, in 

most rocks, of internal boundaries in secondarily enlarged crystals and in replacing minerals. The distinction between individual grains and interstitialmateria.l, however, is evident by change of minearology, 

and the following descriptive material will deal in 

large part with interstitial grains. 

SHAPE OF THE INTERSTITIAL MATERIAL 

The interstices between settled crystals are continuously interc~nnecting. Some interstitial · material 

is confined to the space defined by 3 or 4 adjacent settled 

crystals, which may be termed a unit interstice, and 

dif:fers frmn the material in the adjoining interstice 

either in composition or optical orientation. In 

other instances, the interstitial material is in common 

optical orientation over many hundreds of these interstices, and the texture is said to be poikilitic. A distinction must be made, therefore, between the shapes 

of the unit interstices, which may or may not define 

the boundaries of an optically continuous interstitial 

mineral, and the exterior shape of poikilitic crystals, 

where these occur. 

SHAPE OF THE INTERSTICES 

The interstices are invariably concave polygons, entirely bounded by the crystal faces of the settled minerals. In cross section the interstices are cuspoid, terminating in 4 to 8 sharp points, or continuing along 

narrow throats between settled crystals. Crystals that 

have continued to grow after deposition develop interference boundaries against their neighbors but retain 

their crystal faces where growing into pore spaces, so 

that in rocks with moderate. amounts of overgrowth 

material the interstices are the same in shape but decreased in volume. This effect is illustrated in figure 

71A, a tracing of olivine-plagioclase relations in a 

poikilitic harzburgite. Modal counts indicate that the 

olivine crystals of this rock have been enlarged about 

15 percent after deposition, and, although olivine crystals are in contact in many areas, the shapes of the 

interstices occupied by plagioclase retain their sharp 

boundaries. Where ·the interstitial material has not 

only filled spaces between settled crystals, but partially replaced them, the material is increased in volume and the interstices lose their characteristic shapes. 

In rocks where such replacement has been extensive, 

A 

B 

FIGURE 71.-Sectional shapes of interstitial minerals in poikilitic harzburgite. A, Interstitial plagioclase (black) surrounding olivine crystals (white) between bronzite oikocrysts; B, Interstitial bronzite 

(black) surrounding olivine crystals (white) within bronzite oikocrysts. Traced from etched slab. 
66 STILLWATER COMPLEX, MONTANA 

the interstitial material becomes the dominant element 

in the texture and is, in aggregate, sponge shaped. 

The sectional relations for settled olivine partly replaced by interstitial bronzite is shown in figure 71B. 

Local restriction of any of the interstitial minerals 

to one interstice has been observed, but the tendency 

to form connecting, poikilitic crystals is pronounced 

and varies with mineralogy and mineral abundance. 

Quartz has been observed to surround 3 or 4 bronzite 

grains,· but generally occurs in widely separated single 

grains. Plagioclase crystals generally occupy one to 

five connecting interstices but uncommonly develop 

into large poikilitic crystals. Bronzite, chrome augite, 

and olivine all have strong tendencies toward development of large oikocrysts and are rarely seen in single 

grains confined to individual interstices. Biotite, 

where relatively abundant, also prefers the poikilitic 

habit but commonly is restricted to single interstices 

because of its scarceness. 

SHAPE OF THE POIKILITIC CRYSTALS 

Bronzite oikocrysts in poikilitic harzburgite are perhaps the most striking textural feature of the Ultramafic zone. Less obviously, they are also well developed in chromitite. In both rock types, the poikilitic 

bronzite grains are, with rare exceptions, grossly 

spherical (fig. 4). In detail, the spherical boundaries 

are irregular; olivine crystals project across the contacts, and where the bronzite is in contact with interstitial plagioclase the boundary is generally irregular 

with small-scale mutual embayments (fig. 67). In 

some highly feldspathic poikilitic harzburgite, however, the bronzite develops smooth crystal boundaries 

along short segments of its contact with plagioclase. 

Several examples of elliptical rather than spherical 

development of bronzite oikocrysts occur where thin 

poikilitic chromite harzburgite layers exist between 

two chromitite layers. Some of the bronzite oikocrysts 

have. apparently been restricted in vertical development. 

Chromian augite oikocrysts tend to have prismatic 

rather than spherical shapes, so that cross sections are 

imperfect rectangles or squares. In detail the outer 

margins of augite oikocrysts, like those of bronzite, 

are irregular, with settled crystals extending across 

the contacts. In many rocks the boundaries of the 

poikilitic crystals against surrounding plagioclase are 

completely irregular, but euhedral crystal terminations 

of augite against plagioclase are developed to a greater 

extent than in the case of bronzite. In rocks containing minimal amounts of_ secondarily enlarged material, 

particularly in bronzitite, augite oikocrysts terminate 

against plagioclase with what appear to be sharp 

prism and dome faces (fig. 68). When examined in 

detail, however, the augite-plagioclase contacts ·are, 

for the most part, mutually embayed and curved and 

only short segments of the augite oikocrysts are actually euhedral. The augite oikocrysts, therefore, have 

a strong tendency to maintain a prismatic form but at 

their outer margins develop interference boundaries 

with plagioclase. In feldspathic bronzitite, where replacement of bronzite by augite has been extreme, these 

prismatic forms give the rocks the appearance of porphyritic texture. Where augite is in contact with interstitial bronzite, the boundaries of the bronzite are 

commonly embayed. 

Interstitial olivine occurs only in certain chromitite 

specimens; in these, it is commonly poikilitic. In the 

restricted number of specimens where they have been 

observed, olivine oikocrysts have prismatic, nearly 

equidimensional shapes. Boundaries with continuous 

olivine are irregular and interfering, and contacts with 

pyroxene are irregularly embayed. Contacts between 

interstitial olivine and plagioclase are not common in 

chromitite because of the tendency for all minerals to 

fill completely the smaller interstices, because much 

chromitite with relatively abundant interstitial olivine 

contains no plagioclase, and because olivine is commonly rimmed by pyroxene. In the few places where 

relations are observa!ble, intersttitial olivine is completely euhedral against plagioclase. 

Biotite occurs in very minor amounts in all the rock 

types, but poikilitic crystals have been observed only 

in a few poikilitic harzburgites and chro~itites. Oikocrysts tend to be strongly tabular parallel to the micaeeous cleavage. and are generally less than 0.1 mm 

thick. In sections parallel to the cleavage the biotite 

is roughly circular in shape but irregular in detail, 

and apparently it surrounds several settled crystals. 

Perpendicular to the cleavage, however, settled crystals 

extend through both sides of the oikocryst. Pinacoid 

faces, or segments of them, are developed against all 

the other interstitial materials, but prismatic or pyramidal terminations have not been observed. 

Poikilitic plagioclase, where it is seen, is roughly 

spherical in bronzitite and granular harzburgite. In 

poikilitic harzburgite and chromitite, however, the 

plagioclase oikocrysts are irregular in shape and tend 

to conform to hour-glass-shaped or large cuspate areas 

between pyroxene oikocrysts. The outer contacts of 

the poikilitic plagioclase are scalloped by extension of 

settled crystals across the contacts. Where it is in 

contact with other interstitial minerals, the plagioclase 

either has interference boundaries or is controlled by 

crystal boundaries of interstitial pyroxene; plagioclas.e 
TEXTURES OF THE ROCKS 67 

has not been observed to be terminated by its own 

crystal faces. 

Quartz, in its uncommon occurrences as a poikilitic 

mineral, has roughly spherical outlines and irregular 

boundaries against all the other interstitial constituents. 

Poikilitic olivine, bronzite, and augite crystals in 

chromitite, and, to a lesser extent, bronzite and augite 

crystals in harzburgite and bronzitite, are locally modified by narrow apophyses that envelop settled crystals 

far beyond the roughly spherical limits of the oikocryst. Narrow rims of these three minerals surround 

settled crystals at considerable distances from the centers of the oikocrysts, coating the walls of the interstices with optically continuous material (fig. 72). 

The centers of the interstices are filled with plagioclase. The reverse of this textural development has 

been observed in a few chromitite specimens, where 

plagioclase plates the faces of chromite crystals outwards frmn the margins of plagioclase oikocrysts, and 

augite or bronzite fills the central parts of the outlying interstices (fig. 73). Where these peripheral 

platings of settled crystals are well developed, the 

coating material decreases in abundance away from 

the center of the oikocrysts, so that the rims become 

increasingly narrow, although the whole remains in 

optical continuity. 

DISCUSSION 

There is a striking similarity between the sectional 

shapes of unit interstices defined by the closely packed 

settled crystals and the section shapes of unit voids 

between spheres illustrated by Graton and Fraser 

(1935, figs. 14, 17-20). If arcuate boundaries are substituted for angular ones, many of the shapes correspond exactly. 

The tendency of the interstice walls to maintain 

their angularity, even where the settled crystals have 

grown considerably after deposition, suggests that secondary enlargement was virtually completed by the 

time the interstitial minerals crystallized. Where 

bronzite forms a part of the interstitial material, for 

instance, its boundaries with plagioclase are for the 

most part irregular and interfering, whereas secondarily enlarged bronzite has euhedral terminations against 

plagioclase. :However, the homogeneity of interstitial 

material formed in part by reaction replacement suggests that reaction occurred simultaneously with space 

filling. Within the greatly enlarged interstices no 

trace of the original pore-space boundaries can be 

detected. 

The characteristic equidimensional or prismatic 

forms of bronzite, augite, and olivine oikocrysts dominate the texture of the interstitial material, suggesting 

541905 0-61-4 

that these minerals grew outward from central nuclei, 

displacing the interprecipitate magma without local 

entrapment. The generally irregular contacts of augite and bronzite with plagioclase at the peripheries 

of the oikocrysts suggest mutual interference in the 

final stages of crystallization. Local segments of crystal faces of these minerals against plagioclase probably 

record the local parts of the outer peripheries that 

completed crystallization prior to plagioclase. Although the relative power of automorphism of olivine 

and plagioclase cannot be evaluated, I prefer to believe that the interstitial olivine in chromitite completed crystallization prior to precipitation of plagioclase. 

Envelopment and plating of chromite crystals at the 

margins of some oikocrysts, although almost identical 

with reaction textures between chromite and gabbro 

des.cribed by Thayer ( 1946, p. 203-205), would seem 

to have a different origin for the. following reasons: 

( 1) No consistent zonal arrangement is recognized. 

(2) The enveloping rims are optically continuous with, 

and connected to, large oikocrysts, which fill the entire 

space· between chromite crystals. ( 3) The width and 

therefore volume of the rims decreases away from the 

oikocrysts. ( 4) The euhedral shape of the chromite 

is not altered. It seems JTIOSt likely that the enveloping 

rims are skeletal outgrowths from the poikilitic crystals with which they are connected, and that the 

magma was unable to supply enough of the constituent 

to fill the interstices completely. 

DISTRIBUTION OF THE INTERPRECIPITATE MATERIAL 

In most rocks of the Ultramafic zone, the interstiti~l 

material is evenly distributed along the plane of the 

layering and ranges in volume from less than 1 percent, in the uncommon rocks where secondary enlargement is nearly complete, to 50 percent, in abnormally 

porous chromitite with no secondary enlargement. Exceptions to the rule of even distribution of interstitial 

material occur in the clotted chromitite, where secondary enlargement is irregularly developed, and in some 

piokilitic harzburgite, which contains relatively large 

quantities of bronzite formed by reaction. 

The mineralogy and distribution of interstitial material varies with the amount of oriented overgrowth 

on settled crystals, and because of this they also vary 

areally and stratigraphically. In rocks in which the 

settled crystals have been considerably enlarged aftei· 

deposition, interstitial minerals do not ha.ve characteristic poikilitic textures. Oikocrysts become progressively smaller as enlargement increases, un~il, in nearly 

monomineralic rocks, the interstitial constituents are 

confined to single widely spaced interstices and are not 
68 STILLWATER C0~1:PLEX, MONTANA 

IG RE 72.-Plating of chromite by bronzite. Narrow shells of 

bronzite surround settled chromi te crystals nea r margin of 

bronzite oikocryst. br = bronzite ; pc=plagioclase. C1·ossed 

nicols. 

FIGURE 74.-Augite replaci ng bronzite in poikilitic harzburgite. 

Remnants of unHormly oriented bronzite oikocryst occur within 

augite oikocryst near bounda ri es of olivine crystals. ol = olivine; a ug=augite ; br=bronzite; pc=plagioclase. Crossed 

nicols. 

F IGURE 73.-Piating of chromite by pl agioclase. Narrow shells 

of plagioclase surround settled chromite ct·ystnls near margin 

of plagioclase oikocryst. br = L>ronzite ; pc = plagioclase. 

Crossed nicols. 

FIGURE 75.- Bioti te In poikilitic harzburgite. Biotite has pa rtly 

r eplaced included portion of chromite crystal. Note that plagioclase is strongly zoned near biotite. m =biotite; chr = 

chromite; ol =oli\ in~; pc=plagioclase. CTossed nicols. 

PHOTOMICROGRAPHS OF TEXTURES OF INTERPRECIPITATE MINERALS IN THE ULTRAi\IAlnC ZOKE 
TEXTURES OF THE ROCKS 69 

in contact or in optical continuity. Presumably this 

restriction of poikilitic development is caused by cementn,tion and consequent closing of the throats of 

interstices, thus decreasing the permeability of the 

mush. 

The character of the interstitial material also varies 

independently with change in mineralogy of the settled 

crystals and with stratigraphic position. The variation of the character of interstitial material with 

chn,nge in mineralogy of settled crystals seems the 

most fundamental, and for this reason the descriptions 

will be considered separately for the various rock 

types. The mineralogy of these rocks is summarized 

in table 8. · 

TABLE B.-Settled and interstitial minerals in Ultramafic zone 

rocks having relatively small amounts of secondary enlargement 

[Parentheses in~icate minerals not always present] 

Interstitial constituents 

Settled constiMajor rock types tuents Generally abund- ant (in order of Generally <I peraverage abund- cent of rock a nee) 

--

Poikllitio hnrzburgite ••. Olivine Bronzite Biotite 

Ohromite Plagioclase 

Granular harzburgite Olivine Augite 

Plagioclase (Biotite) 

and olivine bronzitite. Bronzite Augite (Ohromite) (Bronzitc) 

Bronzitite.----- ___ ----- Bronzite Plagioclase (Quartz) 

(Chromite) Augite (Biotite) 

(GrossulariteOhrorriite 

pyrope) 

Ohromitlte. ------------ ~Plagioclase) Biotite 

(Olivine) Augite) 

(Bronzitc) 

Olivine chromitltc and Ohromite (Olivine) Bronzite (Biotite) ~oikilltlc chromite Olivine (Augite) arzburgite. (Plagioclase) 

POIKILITIC HARZBURGITE 

The settled constituents of poikilitic harzburgite are 

olivine and accessory chromite; the recognized interstitial constituents are bronzite, plagioclase, augite, and 

very minor amounts ~of biotite~ Quartz has not been 

observed. Interprec:ipitate olivine occurs where settled 

crystals have been se9ondarily enlarged. 

Bronzite invariabl:r occurs in poikilitic crystals partially replacing included olivine in amounts up to 58 

percent. In some bt·onzite oikocrysts the amount of 

replacement seems constant throughout, but in others 

olivine :is more highly resorbed in the central part of 

the bronzite oikocrysts. Augite is present only in extremely minor amounts in many specimens of poikilitic 

harzburgite, and it is nearly always subordinate in 

amount to bronzite. Its most general form of occurrence is as minute grains in the cusps or throats of 

interstices occupied largely by. strongly zoned plagioclase. In rocks where augite is slightly more abundant, it also occurs "in poikilitic crystals very much 

smaller than, and generally located between, bronzite 

oikocrysts in the same rock. Poikilitic aug-ite partially 

replaces olivine, but to a lesser extent than bronzite 

does. In a few layers augite is nearly as abundant as 

bronzite and the rocks are properly poikilitic lherzolites. In these rocks augite occurs in oikocrysts the 

same size as those of bronzite, and spaced equally with 

them throughout the rock. l\1icroscopic examination 

shows that the central part of ·each augite oikocryst 

of this type contains irregular isolated inclusions of 

uniformly oriented bronzite (fig. 74). Apparently, 

therefore, the augite in these rocks is in part formed at 

the expense of bronzite, which had previously crystallized from the interprecipitate magma. Bro:q.zite and 

augite oikocrysts in individual specimens of poikilitic 

harzburgite tend to ha:ve equal size and roughly equidistant spacing along the strike of the layers. 

Plagioclase fills the interstices between pyroxene 

oikocrysts; for the most part, it occurs as small grains 

occupying a single interst5ce or as small poikilitic 

grains including, in section, 3 .or 4 olivine crystals. 

The poikilitic grains are gener~lly larger near the centers of the areas of plagioclase in the specimen-that 

is, at some distance from the pyroxene oikocrysts. 

Scattered throughout the area of interstitial plagioclase, but generally concentrated near the margins of 

pyroxene oikocrysts, are small pockets of plagioclase 

which occupy single interstices or parts of interstices, 

and which are characterized by extreme zoning (fig. 

76). In most poikilitic harzburgite interstitial pyroxene is about 3 times more abundant than plagioclase, 

but examples in which plagioclase is dominant are 

known. The problem of estimating the proportions is 

greatly complicated by the extremely large grain size 

of the bronzite oikocrysts; in some rocks a representative mode would involve an area of about 75 square 

feet. 

Biotite generally occurs within the pockets of 

strongly zoned plagioclase, either along the wall of an 

interstice in contact with olivine or adjacent to chromite grains, if these are present (fig. 75). It commonly occurs in single tablets 0.5 to 1.0 mm in diameter and 0.05 to 0.1 mm thick, so that the entire crystal 

commonly is confined within a single interstice. Development of larger poikilitic crystals which cross several interstices are not common, occurring only in poikilitic harzburgite where mica is abnormally abundant. 

Rarely, however, mica plates cut transversely across 

boundaries between plagioclase and olivine, where they 

have apparently formed by replacement of both minerals. A small percentage of chromite crystals in contact with mica are serrate at the margins (fig. 75), and 

small volumes of chromite are replaced. 
70 STILLWATER COMPLEX, MONTANA 

FIGURE 76.-Granular harzburglte with pockets of plagioclase 

characterized by extreme zoning. The two noses of augite 

that extend Into the Interstice at left center of photograph 

are also zoned. ol=ollvlne; br=bronzlte; aug=auglte; pc= 

plagioclase. Crossed nlcols. 

FIGURE 78.-Grossularlte-pyrope associated with quartz In 

bronzitite. Two crystals within Interstitial quartz are Illustrated. g=grossulartte-pyrope; br=bronzlte; q=quartz. 

Crossed nlcols. 

FIGURE 77.-Interstltlal quartz In bronzitite. Note that nearly 

all quartz-filled In terstlces con taln some biotite near their 

margins. m=blotite; q=quartz; br= bronzlte ; pc=plagioclase. 

Crossed nlcols. 

FIG URE 79.-Grossularlte-pyrope associated with plagioclase In 

bronzitite. Garnet is partly replacing bronzite. Note suggestions of dodecahedral shapes. br=bronzite; g=grossularite-pyrope; pc=plagloclase. Crossed nlcols. 

PHOTOMICROGRAPHS OF FINAL INTERPRECIPITATE CRYSTALLIZATION PRODUCTS IN ROCKS OF THE 

ULTRAMAFIC ZONE 
TEXTURES OF THE ROCKS 71 

GRAN_ULAR HARZBURGITE 

The settled constituents of granular harzburgite are 

olivine, bronzite, and accessory chromite; the interstitia,} constituents are plagioclase, augite, bronzite, and 

accessory biotite. Qun.rtz has not been observed. Varied amounts of interprecipitate olivine and bronzite 

occur where these minerals have been secondarily enlarged. 

In the upper pn.rts of gra,nular harzburgite layers, 

where settled bronzite is equal to or exceeds olivine in 

vo.lume, the bronzite tends to be euhedral toward plagioclase, even where it is secondarily enlarged, 

(Jigs. 34, 36). In the lower parts of granular harzburgite layers, however, where olivine is more abundant than bronzite, the bronzite is irregular, generally 

terminating in cuspoid shapes controlled by surrounding crystnJs (fig. 66). Such bronzite grains ha.ve interference boundaries where they are in contact with 

plagioclase, and have grown out to replace partly the 

neighboring olivines. These relations are interpreted 

to indicate that in olivine-rich granular harzburgite, 

settled bronzite continued to grow after deposition 

until joined by plagioclase, filling interstices between 

and partly replaci1ig settled olivine, whereas in the 

bronzite-rich granular harzburgite, bronzite enlargement was essentially completed before plagioclase crystallized, and no interstitial bronzite occurs in the rock. 

Chromian augite typicn.lly occurs ii1 small poikilitic 

crystals or in grains filling ·one or two interstices between settled crystals, with oikocrysts more commonly 

developed in the bronzite-rich granular harzburgite. 

~fargins of included settled silicates are partially replaced, bronzite to a greater extent than olivine (fig. 

52). In rocks where oikocrysts are deve~oped, the augite grains tend to have nearly equal size and spacing 

nJong the strike of the layering. Augite also occurs 

independently, in minute grains, in the cusps or throats 

of many interstices occupied by plagioclase, accompanied, in some rocks, by biotite. The plagioclase adjacent to these small augite crystals commonly is 

strongly zoned, and the areas are interpreted as pockets 

of relatively late crystallization. 

Plagioclase distribution is much 'like that in poikilitic harzburgite. It occurs in single grains and small 

poikilitic crysta~s, which are generally larger away 

from augite oikocry!:ltS; pockets of fine-grained, highly 

zoned plagioclase are also scattered throughout the· 

rocks. In granular harzburgite, plagioclas~ is considerably more abundant than interstitial pyroxene. 

Biotite is much less abundant in granular harzburgite than in poikilitic harzburgite, and it cannot be 

detected at all in 2- by 2-inch thin sections of many 

specimens. vVhere observed, it does not form poikilitic 

crystals but occurs as tiny tablets, generally within 

areas of strongly zoned plagioclase. 

BRONZITITE 

The settled constituents of bronzitite are bronzite 

and, in a few rocks, a little chromite; the observed 

interstitial constituents are plagioclase, augite, biotite, 

quartz, and extremely 1ninor amounts of grossularitepyr_ope. Of the interstitial constituents only_ plagioclase and augite are ubiquitous. Biotite is· spottily 

developed throughout the Ultramafic zone, but quartz 

and grossularite-pyrope, which generally occur together, occur only in the Basal_ zone and in the upper 

part of the Bronzitite ~ember. 

Augite very commonly occurs in poikilitic crystals 

replacing included bronzite in amounts up to 85 percent. As with bronzite oikocrysts in poi~ilitic harzburgite, augite commonly replaces larger amounts of 

bronzite near the center and lesser amounts near the 

edge of the oikocrysts. Poikilitic crystals of augite 

tend to have nearly equal size and spacing along the 

strike of the layers. Augite also occurs as thin selvages 

on bronzite in the throats and cusps of some interstices 

occupied by plagioclase. 

Plagioclase distribution in bronzitite is identical 

with that in the harzburgite. It is invariably more 

abundant than interstitial pyroxene. 

Biotite i? even less abundant -in bro~zitite than in 

granular harzburgite; where present, it is generally 

associated with strongly zoned plagioclase, as in the 

other rock types. Generally it is confined to the interstices between settled bronzites, but in some cases it 

has replaced the preexisting n1inerals. 

Quartz occurs in clear, internally homogeneous single grains or in small poikilitic crystals scattered 

throughout bronzite layers (fig. 77), and it is generally 

closely associated with biotite. Like biotite, quartz is 

slightly n1ore abundant near the margins of pyroxene 

oikocrysts, but it 1naintains a fairly even distribution 

through the rock even where it is an extremely minor 

constituent. 'Vhere quartz occurs in amounts less than 

about 0.5 percent, it shares the corners of interstices 

with strongly zoned plagioclase; where more abundant, it fills entire interstices. Quartz seems to be 

entirely space filling, and it has not been observed with 

replacement relation·s. . 

Very minor amounts of grossularite-pyrope garnet 

have been observed in six specimens of quartz-bearing 

bronzitite. In thin section the mineral is colorless, 

locally very weakly birefringent with sectoral extinction, and in places has suggestions of dodecahedral 

boundaries. Other physical properties of the garnet 

were difficult to detertnine because of the extreme scar-
72 STILLWATER. COMPLEX, MONTANA 

city of the mineral in the rocks in which it occurs. 

About one-fourth of a gram of material containing 

roughly 10 percent garnet and altered garnet was obtained from a 2-pound sample of bronzitite, by. hand 

panning in methlyene iodide. Garnet grains in this 

concentrate were pale pink to pale amber in color and 

ranged in refr~tctive index from 1.73 to 1.755, with 

single grains commonly variable over much of this 

range. Unit cell size, calculated from X-ray diffraction patterns, proved to be 11.7 4±0.04 angstrom units. 

Average values of n and a0 plotted on Sriramadas' 

( 1957, p. 294-298) determinative diagrams reveal two 

possible compositions: grossularite7 -pyrop~2o-almanditeto. and grossulariteso-pyropego-andraditeto· Winchell's (1958, p. 595-600) diagrams confirm these two 

alternatives and add a third: grossularites5-pyrope2ospessartite15. Although the physical properties do not 

provide a definitive composition, it seems clear that in 

first approximation the garnet is predominantly grossularite, with subordinate pyrope. The grossularitepyrope is closely associated with quartz, either occurring in 0.2- to 0.6-mm subhedral crystals within the 

quartz (fig. 78) or in plagioclase near quartz concentrations. In some i1~stances the garnet is concentrated 

at the contacts between plagioclase and settled bronzite, 

with its boundaries partly controlled by the outlines of 

the bronzite and partly replacing it (fig. 79). The textures, mineral associations, and stratigraphic distribution of this mineral are strong evidence for its occurrence as a primary constituent in the Ultramafic zone. 

CHROMITITE 

The interstitial constituents of chromitite include 

bronzite, augite, olivine, plagioclase, and small amounts 

of mica. Quartz and garnet have not been observed. 

Olivine occurs as an interstitial mineral only in 

about one-half of the chromitite examined. Where it 

does occur it is commonly in small poikilitic crystals. 

Chromitite containing interstitial olivine commonly 

also contains both pyroxenes, and the amount of olivine may exceed or be less than total pyroxene. In 

some specimens, interstitial olivine is separated from 

plagioclase by a shell of bronzite that commonly re-

.places the olivine near its periphery; but in a few, 

interstitial olivine is directly in contact with the feldspar. 

Bronzite and augite are apparently absent in some 

chromitite, but not simultaneously. Where present, 

bronzite occurs in evenly spaced poikilitic crystals 

about one-half inch in diameter, filling space between 

chromite crystals without evidence of replacement. 

·Augite typically occurs in irregular oikocrysts either 

rimming or partially enclosing poikilitic bronzite, with 

replacement textures where the two 1ninerals are in 

contact (fig. 81). As in the silicate rocks, augite is 

also found in tiny grains in corners of interstices. 

The two pyroxenes generally occur together and, in 

most rocks, in nearly equal amounts, although rocks 

with either in excess are known. 

Plagioclase in chromitite layers, although not appreciably different in grain size from that in silicate layers, is almost invariably poikilitic because of the relatively fine grain size of included chromite crystals. 

As in silicate layers, plagioclase occurs between the 

sharply defined pyroxene oikocrysts, but in chromitite, 

the size of the plagioclase oikocrysts commonly is 

nearly the same as poik~litic crystals of bronzite and 

augite. Most commonly plagioclase is about one-half 

as abundant as total interstitial ferromagnesian minerals, but layers are known where, at least locally, 

plagioclase is well in excess of total bronzite, augite, 

and olivine. In some chromitite, plagioclase is apparently absent. 

Biotite is relatively abundant in chromitite. Traces 

can be found in nearly all specimens and, in a few 

rocks, concentrations up to nearly two percent have 

been observed. As in the silicate layers, biotite 9ccurs 

as tiny grains between settled chromite crystals, and, 

much less commonly, as small poikilitic plates. Generally, biotite is associated with ·strongly zoned plagioclase, and locally replaces edges of contiguous chromite. 

OLIVINE CHROMITITE AND POIKILITIC CHROMITE 

HARZBURGITE 

The interstitial constituents of rocks containing mixtures of settled olivine and chromite are bronzite, augite, plagioclase, and biotite. Of these, only bronzite 

is invariably present. 

The proportions and distribution of interstitial n1inerals in these rocks are In ore similar to those of poikilitic harzburgite than chrmnitite. Olivine does not 

occur as an interstitial mineral but does surround some 

chromite as a consequence of secondary enlargment 

of settled olivine. Bronzite, which commonly occurs 

in 1- to 2-inch oikocrysts, partially replacing olivine 

and surrounding large numbers of chromite crystals, 

is considerably 1nore abundant than augite or plagioclase. Where present, augite occurs as in chromitite, 

but it is much less abundant and is absent in a few 

speCimens. 

Plagioclase occurs in very small poikilitic crystals, 

and is confined to the chromite-rich part of the rock 

between olivine crystals. Plagioclase is less abundant 

in olivine chromitite and poikilitic chromite harzburg-
TEXTURES OF THE. ROCKS 73 

ite than in any other rock types in the Ultramafic 

zone; it was not found in about one-third of the specimens examined. 

Biotite occuTs with strongly zoned plagioclase in 

chromite-rich parts of the rocks, but it is less abundant 

than in either chromitite or poikilitic harzburgite. 

STRATIGRAPHIC DISTRIBUTION 

As is apparent from the foregoing descriptions, the 

mineralogy, texture, and proportions of the interstitial 

constituents are strongly dependent on the character 

of the settled minerals present. Because the settled 

minerals tend to be regularly disposed in cycles, the 

interstitial minerals have, to the same extent, a cyclic 

stratigraphic variation. Thus, poikilitic bronzite is an 

important constituent of the poikilitic harzburgite in 

the lower parts of the cyclic units; it decreases in abundance in the lower part of the granular harzburgite 

layer in the central part of the unit; it is completely 

absent from the upper half where settled bronzite becomes an abundant mineral. Augite has a reciprocal 

distribution, being generally of minor importance in 

poikilitic harzburgite, increasing in size and abundance in the upper part of granular harzburgite, and 

reaching max~mum development in the bronzitite at 

the stratigraphic tops of the cycles. Plagioclase is 

' ubiquitous, but the ratio of plagioclase to interstitial 

pyroxene increases markedly upward in the cyclic unit. 

Although concentrations of biotite in excess of three 

percent in any rock have not been observed, it is nevertheless more abundant in the lower parts of cycles 

than in the upper. 

Several anomalous stratigraphic variations in distribution of interstitial minerals between cycles have 

been observed. In the western half of the complex, 

augite is abnormally abundant in the harzburgite immediately above the cyclic unit 2 chromitite layer in 

the lower middle part of the Peridotite n1ember. In 

this poikilitic harzburgite (properly a feldspathic 

lherzolite), augite and bronzite occur in nearly equal 

amounts; in the overlying granular harzburgite, augite 

also occurs in greater than normal abundance. A second anomaly occurs in the lower half of the Peridotite 

member in the vVest Fork: section, where plagioclase 

is unusually scarce, and the plagioclase-interstitial pyroxene ratio is exceptionally low. The chromitite in 

this area contains abundant interstitial olivine and 

little or no plagioclase. A third anomalous stratigraphic variation concerns the distribution of quartz 

and grossularite-pyrope, which have been observed 

only in the upper two-thirds of the Bronzitite 1nember 

and in the bronzitite of the Basal zone. Neither mineral has been found in rocks containing settled olivine 

or chromite, nor have they been observed in bronzitite 

within the Peridotite member. Where present in the 

Bronzitite n1ember neither 1nineral exceeds one-tenth 

percent by volume; in the Basal zone grossularitepyrope similarly does not occur in n1easurable amounts, 

but quartz is present in amounts up to 5 percent. 

Quartz ~tpparently occurs alone in some of these 

bronzitites, but grossularite-pyrope is found only in 

bronzitite that contains some quartz. 

DISCUSSION 

Several deductions concerning the order of crys-tallization in the interprecipitate magma can be made 

on the basis of mineral distribution. As previously 

pointed out, the idiomorphism of crystal faces on secondarily enlarged settled crystals contiguous to interstitial minerals indicates that enlargement was virtually complete before crystallization of the interstitial 

phases. This is not true, however, of bronzite enlargement in olivine-rich granular harzburgite, where the 

outer margins of enlarged bronzite crystals partially 

replace contiguous olivines, and interfere with plagioclase. Interstitial· bronzite in these rocks does not 

form new centers of crystallization as it does in the 

poikilitic harzburgite immediately below, but adds on 

to settled crystals and continues to crystallize with 

plagioclase. 

It is apparent from textures in the chromitite that 

the reaction replacement of settled olivine by interstitial bronzite continues to occur between these two 

minerals even where both are interstitial constituents, 

and thus crystallization of interstitial olivine must 

cease before bronzite can appear. Similarly, in the 

poikilitic harzburgite, where both augite arid bronzite 

appear as interstitial cmistituents, there is abundant 

evidence that augite occurs in part as ·a. replacement 

of bronzite. A reaction, rather than cotectic, relation 

between these minerals therefore continues in the interprecipitate magma, and interstitial bronzite ceased 

crystallizing prior to deposition of augite. 

'Vhen plagioclase began to precipitate in the crystallization sequence o.f the interstitial magma is not entirely clear. In chromitite containing interstitial olivine, the olivine apparently finished crystallizing prior 

to the appearance of feldspar, but both pyroxenes in 

these rocks show 1nutually interfering boundaries with 

plagioclase. In poikilitic harzburgite, the bronzite 

oikocrysts control the textures by their even-spaced 

poikilitic development, and it seems likely that bronzite began to crystallize before plagioclase. The same 

argument suggests that augite in bronzitite began to 

crystallize before feldspar. In rocks that contain both 

augite and bronzite as interstitial constituents, the 
74 STILLWATER COMPLEX, MONTANA 

bronzite has interference boundaries with plagioclase, 

indicating that feldspar must have begun to crystallize 

before augite. 

The pockets of strongly zoned plagioclase and isolated augite gains in all the rock types ·suggest that 

augite and plagioclase were the final crystallization 

products of the interstitial magma throughout the section, and their association with biotite, quartz, and 

grossularite-pyrope indicates that they were joined, in 

some rocks, by these new mineral phases. 

On the basis of the foregoing observations on shape, 

distribution, and mutual relations, diagrammatic sequences of crystallization for the principal rock types 

are presented in figure 80. 

INTERNAL CHARACTER OF THE INTERPRECIPITATE 

MATERIAL 

INCLUSIONS 

Settled crystals excepted, primary inclusions in interstitial minerals are rare. One example of inclusions 

of previously formed interstitial material has been described: uniformly oriented bronzite occurs in augite 

in a few augite:-rich poikilitic harzburgites. Minor 

inclusions of accessory minerals in the interstitial material are even less common. Grossularite-pyrope inclusions in quartz and plagioclase have been described. 

Apatite· crystals in plagioclase, although relatively 

abundant in the chilled border rocks, are not common 

within the -Ultramafic zone. 

ORIENTED INTERGROWTHS 

Clinopyroxene exsolution lamellae in interprecipitate bronzite, like those of settled· bronzite, are ob~ 

served only on ( 100) crystal planes of the host. In 

settled bronzite these lamellae are ubiquitous; but in 

parts of the interprecipitate bronzite, lamellae apparently ·are completely lacking~ Even where they are 

well developed, lamellae in interprecipitate bronzite 

tend to be slightly n·arrower and more widely spread 

than those of settled crystals. · 

The lamellae of bronzite that occurs as euhedral settled crystals. in granular harzburgite and bronzitite 

commonly extend all the way to the outer margin of 

the grain and pinch out sharply. In rocks where 

settled bronzite crystals have been secondarily enlarged, however, the lamellae begin to die out at considerable distances from the margin, leaving an outer 

shell of nonlamellar bronzite (fig. 82). In most grains 

this outer shell of clear bronzite is zoned, but in a few 

grains the bronzite appears to be in perfect optical 

continuity throughout the grain. 

The lamellae of bronzite that occurs as an interstitial 

mineral in poikilitic harzburgite and some granular 

Poikilitic harzburgite: 

Olivine 

Bronzite 

Augite 

Plagioclase 

Biotite 

Secondary 

enlargement 

Interstitial crystallization 

Olivine-rich granular harzburgite: 

Olivine 

Bronzite 

Augite 

Plagioclase 

(Biotite) 

Secondary 

enlargement 

Interstitial crystallization 

Reaction 

Bronzite-rich granular harzburgite: 

Secondary 

enlargement 

Olivine 

Bronzite 

Augite 

Plagioclase 

(Biotite) 

Bronzitite: 

.... I .... 

- -.1 

Reaction 

Reaction 

Secondary 

enlargement 

Bronzite 

Augite 

Plagioclase 

(Biotite) 

(Quartz) 

(Grossularitepyrope) 

Chromitite: 

Reaction 

Secondary 

enlargement 

Chromite 

(Olivine) 

(Bronzite) 

(Augite) 

Reaction 

Interstitial crystallization 

Interstitial crystallization 

Interstitial crystallization 

Reaction 

-

(Plagioclase) 

Biotite 

-----------1 

FIGURE 80.-Diagrammatic illustrations of order of crystallization 

from the interprecipitate magma. Parentheses indicate minerals not 

invariably present. 
TEXTURES OF THE ROCKS 75 

Fwua~ 81.-Interstltial augite replacing interstitial bronzlte In chromitite. Black grains are chromite. Tongues of augite extending Into 

bronzlte are zoned. br=bronzlte; aug= augite; pc=plagioclase. 

Crossed nlcols. 

l!'IGURE 83.-Cl!nopyroxene lamellae in interpreclpitate bronzite. Lamellae continue to margins of some included olivine grains but die 

out at considerable distances from others. Absence of lamellae corresponds with maximum zoning. ol=olivine; br=bronzite. Crossed 

nicols. 

FIGURE 82.-Cllnopyroxene lamellae in secondary enlarged bronzite 

crystal. Lamellae die out at considerable distances from the margin. 

Bronzite Is zoned and in photograph is darker (more Iron rich) at 

margin. br = bronzite. Crossed nicols. 

FIGURE 84.-0rthopyroxene lamellae In interprecipltate augite. Lamellae die out in areas of extreme zoning. br = bronzite; pc =plagioclase ; aug= augit~. Crossed nicols. 

PHOTOMICROGRAPHS OF TEXTURES OF INTERPRECIPITATE MINERALS IN THE ULTRAMAFIC ZONE 
76 STILLWATER COMPLEX, MONTANA 

harzburgit,e are most com1nonly continuous throughout 

the grain ~nd pinch out sharply at the grain margins 

and arourid inclusions of olivine. In many rocks, 

however, lamellae are absent from zoned parts of the 

interstitiai bronzite, and these clear areas are commonly conyentrated near the margins of the poikilitic 

crystals and in narrow rims around included settled 

crystals (fig. 83). In a very few layers, interstitia.! 

bronzite has poorly developed, irregularly distributed 

lamellae tl\roughout. The absence of lamellae in certain parts of interprecipitate bronzite must be related 

either to migration of the exsolved material during 

exsolution; or to a deficiency of diopside molecule in 

the origina1 crystallization product, inasmuch as the 

plutonic environment precludes retention of diopside 

molecule owing to rci,.pid cooling. Lime deficiencies in 

parts of the interp~ecipitate bronzite could occur in 

two ways'; ( 1) by crystallization of the nonlamellar 

bronzite i1l. a lime-free environment, as proposed by 

Walker ( i940, p. 1Q90) to explain similar bronzites 

in the Palisades diabase; or ( 2) by crystallization at 

low mag111a temperatures where diopsidic material is 

less soluble in orthopyroxene, as proposed by Hess and 

Phillips (1940, p. 282-283). Hess and Phillips' explanation is favored because plagioclase is almost invariably ·P.resent in the final crystallization product. 

It is also possible that exsolution began near the centers of br.onzite grains and that exsolved material 

moved from the margins toward the centers during 

this process, as proposed by Brown ( 1957, p. 521, 528) 

to explain ~~imilar relations in inverted pigeonites from 

the Skaergaard intrusion, but this hypothesis does not 

explain th.e general 'association of absence of lamellae 

with zoni~g. 

Narrow \orthopyroxene lamellae are abundantly developed on'. the (100) plane of interstitial augite, and 

these pinch out sharply at grain boundaries and around 

inclusions ,'in the same manner as do clinopyroxene 

lamellae in bronzite. Exsolution plates on ( 001) of 

augite, ·indicating primary exsolution of pigeonite, 

have not b~en observed. Augite oikocrysts tend to be 

peripherally zoned, and the outer margins of these 

zoned crystals are generally free of lamellae. On oikocrysl:s oriented perpendicular to Y of augite, where 

zoning carl be observed by change in interference colors, the lamellae become progressively less abundant 

and more ;poorly developed in areas of increasing birefringence (fig. 84). 

The absbnce of lamellae from the outer parts of 

many augite oikocrysts is probably due to a deficiency 

of orthopyroxene molecule in the original crystallization prod~ct. Again, relatively low temperatures during the final stages of consolidation of the rocks would 

decrease the solubility of the orthopyroxene molecule 

in augite and explain the distribution of lamellae-free 

areas. 

ZONING 

Compositional zoning in settled crystals is not at all 

common; in contrast, the interprecipitate mi:t?-erals 

characteristically show well-developed compositional 

variation. 

The .secondarily enlarged outer peripheries of some 

settled crystals show n1inor continuous compositional 

changes, and these seem to be related to the amount of 

overgrowth that has occurred. In some bronzitite and 

dunite, characterized by xenomorphic texture, the margins of bronzite and olivine become gradually more 

iron rich outward, especially where the amount of enlargement has been relatively great (fig. 82). Where 

the an1ount of enlargement has been relatively small, 

constituent grains of olivine and bronzite are homogeneous in composition throughout. Zoning in secondarily enlarged outer margins of chromites has not been 

observed, in either transmitted or reflected light. 

Smoothly gradational, nonoscillatory zoning is well 

developed in interstitial plagioclase, augite, and bronzite. In areas where plagioclase grains are confined to 

single interstices, the most calcic part of the grain generally lies ~djacent to a settled crystal forming part 

of the boundary, and the feldspar becomes more sodic 

toward the opposite side of the interstice. In some 

specimens, however, the more sodic part lies in the 

center of the interstice (fig. 76). The pattern of compositional change in larger, poikilitic plagioclases is 

more complex. In these, the n1ost calcic portion commonly lies adjacent to one of the included settled crystals, or in narrow areas between two adjacent settled 

crystals, located near the center of the oikocryst. Plagioclase of intermediate composition surrounds the 

calcic portion and fills the central parts of nearby 

interstices. The most sodic plagioclase surrounds the 

intermediate portion and thus lies adjacent to n1ost of 

the settled crystals included within the plagioclase 

oikocryst. 

The zoning pattern of poikilitic augite is similar to 

that of poikilitic plagioclase. Small but continuous 

increases in birefringence, 2V, and extinction angle 

occur roughly concentrically outward from the center 

of the a·ugite oikocrysts, accompanied by similar increase$ around the margins of many of the included 

settled crystals. The magnesium-poor zones at the 

edges of the oikocrysts and aratind settled crystals are 

commonly characterized by absence of orthopyroxene 

lamellae. Zoning of augite oikocrysts around settled 

crystals is about equally developed in chron1itite, where 

nearly all of the augite simply fills pore space in 
TEXTURES OF THE ROCKS 77 

bronzitite and harzburgite, where augite occurs partly 

at the expense of bronzite or olivine. In the latter 

case, zoning follows the embayed boundaries of the 

partially replaced chadacrysts. 

Bronzite oikocrysts appear to be considerably more 

uniform in composition than those of augite, but weak 

enrichment of iron near the extreme margins and surrounding settled inclusions can be detected in many 

individuals. 

If the advance of crystallization of poikilitic crystals 

of plagioclase and pyroxene was outward from the 

center, as indicated by their distribution and mutual 

boundaries, then the general concentric compositional 

distribution indicates a soda and iron enrichment outward during growth. The secondary pattern of zoning around many of the settled inclusions 1nay indicate 

that, once inaugurated, crystallization advanced more 

rapidly through the central portions of the interstices, 

so that crystallization lagged immediately adjacent to 

inclusions. 

The absence of zoning in settled crystals has been 

interpreted as indicating growth of these crystals 

while suspended in a large volume of 1nagma; the 

prevalence of zoned interstitial minerals is believed to 

be a result of growth from small volumes of trapped 

or partially trapped magma surrounding the settled 

crystals. The consequence of this entrapment is differentiation on a small scale, and it is interesting to 

note that the general chemical direction of differentiation of the complex as a whole is presaged in the interstices of its stratigraphically lowest layers. 

SIZE AND SIZE DISTRIBUTION OF POIKILITIC 

CRYSTALS 

The grain-size range within the interstitial material 

is extreme compared with that of settled crystals in 

the saine rocks. In many poikilitic harzburgites that 

contain settled olivines of ~ size range of about four 

Udden size grades, the sizes of single interstitial crystals range from bronzite oikocrysts 8 inches long to 

plagioclase crystals 0.004 inch in length. Despite this 

extreme size range, qualitative variations in average 

grain dimensio:ns can be observed, particularly when 

observations are confined to poikilitic crystals of one 

mineral in a particular rock type. 

The size of poikilitic crystals is relatively constant 

along the layering plane, and several consistent variations occur across the layering. The size distribution 

of poikilitic crystals is influenced by stratigraphic 

position, and by the grain size and amount of secondary enlargement in settled crystals. 

~{aximum and minimum dimensions of oikocrysts 

were measured, whenever possible,. in the field, and 

"average" dimensions and variability along the strike 

of layers were estimated at the same time. More refined descriptions of size and sorting were impractical 

because of the large crystal dimensions involved. 

GRAIN SIZE 

The occurrence and diameter ranges o~ poikilitic 

crystals of bronzite, augite, and plagioclas~ are summarized in table 9. The minimum observed diameters 

TABLE 9.-Diameter of poikilitic crystals in rocks of t~e Ultramafic zone 

Poikilitic mineral Principal occurrence 

Bronzite __________ . ___ Poikilitic harzburgite ___ ------- } 

Cbromitite ____ ---------------- Augite .. ---------- ____ Granular harzburgite ___________ } 

Bronzitite ___ -------·---------- Chromitite ___________ --------- Plagioclase ... _. _. ___ . All rock types in Ultramafic zone. 

Maximum 

observpd 

diameter 

range 

(inches) 

l}-(6··15. 

:rfs-10: 

Hs-8 

Normal 

diameter 

range 

(inches) 

%-4 

%-% 

%-% 

' for all three minerals are arbitrary; each of the minerals occupies single interstices in some rocks, and 

three-sixteenths inch is about the minimum diameter 

at which a crystal can be said to be poikilitic. 

SIZE DISTRIBUTION 

The most obvious feature of the size distribution of 

poikilitic crystals _is that the size variation· of oikocrysts along layers is less than one-half that across 

layers. Individual layers can therefore be characterized by "average" values for the sizes of their ,poikilitic 

crystals. 

Study of stratigraphic variation in poikilitic crystals has been confined to bronzite in poikilitic harzburgite, because such bronzite oikocrysts are easily 

observed in the field, and because they have a relatively large size range. Within individual poikilitic 

harzburgite layers, no consistent variation in oikocryst 

size has been observed. In some layers, bronzite' oikocrysts increase in size from bottom to top; in others, 

size decreases fron1 bottom to top; in· most :layers~ no 

perceptible change occurs. · 

A rough overall variatiOI1 in size of bronzite oikocrysts in poikilitic harzburgite throughout the Peridotite member seems to exist, as illustr_ated in figure 

85. In a general way, poikilitic harzburgite l,ayers in 

the lower part of the section contain smaller .bronzi~e 

oikocrysts than do such layers in the upper part. It 

has been noted that the settled olivine crystals 'in these 

po!kilitic harzburgite layers increase in mea~ size in 

much the same way. 

In areas where sharp breaks in size. of settled olivine 

crystals occur within layers, the size of bronzite oiko-
78 STILLWATER COMPLEX, MONTANA 

MOUNTAIN VIEW AREA 

2.!. 2 

3.!. 4 

3 

2450 1.! 4 

2000 3 

4 

3 

4 

600 )'I 

200 3 

130 4 

0 

·\''-e \\{0~ ~l'\i'0~'le{ ~ \'3 

CZI l; 

12trt o/ . :¢o/ .$"" 

~~ 

0~ 

~ 

WEST FORK AREA 

0 - .._Q) 

Q).~ 

-N 

a>c Eo ro"-

ii~~ Q)".;:;..c 

~~-~ a>Oc 

~ 0.--

FEET 

0 

-1000 

. 2000 

... 

Q) 

.c 

E 

Q) 

~ 

:FIGUHID 85.-Stratigraphic distribution of diameters of bronzite oikocrysts in poikilitic harzburgite layers in the Mountain View and 

West Fork areas. 

crysts responds in the same· direction, so that larger 

olivine crystals are associated with larger oikocrysts, 

and vice versa (fig. 21). Bronzite oikocrysts with 

diameters from 5 to 15 inches are largely confined to 

the poikilitic harzburgite layers immediately below the 

two thickest chromitite layers, and these harzburgite 

htyers are also characterized by the· extreme grain size 

of their settled olivine. In order to check the interdependence of size of settled olivine and interstitial 

bronzite in the same rocks, the average sizes •of both 

constituents in 22 specimens of poikilitic harzburgite 

were measured and are simultaneously plotted in figure 

86~-, Errors in determining the "average" size of oikocrysts are probably large; nevertheless, the plot suggests a broad direct relation. 

Oikocrysts are smaller in chromitite than in layers 

composed of settled olivine or bronzite, and it is --not 

common for crystals of any of the three principal 

poikilitic minerals to exceed one-half inch in diameter. 

In chromitite,- bronzite and augite tend, to· occur in 

nearly equal-sized crystals. 

In a general way, the size of poikilitic crystals decreases in rocks whose settled crystals have been secondarily enlarged. Poikilitic harzburgite and bronzitite that ·contain relatively small amounts of interstitial material are characterized by small poikilitic 

crystals of bronzite and augite. In dunite, where the 

interstitial material has been reduced to less than 10 

percent of the rock, bronzite oikoctysts are commonly 

less than one-half inch in diameter. 

Another type of variation in size of poikilitic crystals is apparently concerned with the amount of the 

interstitial mineral present in the rock. In rocks 

where biotite is relatively abundant, it occurs in poikilitic crystals; but where biotite is scarce, it occurs in 

scattered grains within single interstices. Similarly, 

augite oikocrysts tend to be larger in bronzitite, where 

augite is relatively abundant, then in harzburgite, 

where augite is less abundant. 

DISCUSSION 

The sizes of oikocrysts and settled crystals in poikilitic harzburgite appear to be interdependent. Two 

mechanisms for explaining this correlation are suggested: ( 1) changes in the magmatic environment 

caused increases or decreases in the size of both settled 

olivine and poikilitic bronzite; and (2) the size variations in accumulated settled olivine caused size variations in bronzite by changes in the permeability of the 

crystal 1nush. The relative influence of these two 

mechanisms is not clear. The consistently small oikocrysts in the relatively fine-grained and hence relatively impermeable chromitite suggest that the physi-

(/) 

I.J.J 

J: 3 (.) 

z 

~ 

vi 

1-

(/) 

>-

0: 

(.) 

~ 2 

0 

UJ 

IN 

z 

0 

0: 

OJ 

LL. 

0 

0: 1 I.J.J 

1-

I.J.J 

:::!: 

<t 

Ci 

Lu 

(.!) 

<t 

55MV-69 0 0 55MV-73 

0""'·" / 

0 52MV-43 

52MV-800 52MV~20 52MV-26 .../55MV-26 

51MV·4<:::0 • ~055MV-27 

52MV-28 52MV-31 5 MV·25 

55MV-180~ 055MV-23 

55MV-220 ~ 55MV-1 

52Mv-89/' 55MV-13 

/ 0 55MV-72 

0 55MV-34 

ffi > 0--+---------~----~--~----~--~~--~--~ 

1- 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 

ARITHMETIC MEAN DIAMETER OF SETTLED OLIVINE CRYSTALS, 

IN MILLIMETERS 

5.0 

FIGURE 86.-Relation between size of settled olivine and size of -poildlitic bronzite in 22 specimens of poikilitic harzburgite. 
TEXTURES OF THE ROCKS 79 

cal ·properties of the mush had some influence on 

poikilitic development. 

Two other factors apparently restrict the growth of 

poikilitic crystals: ( 1) secondary enlargement of settled crystals presumably limits size of poikilitic crystals by reducing the pore volume and permeability; 

and (2) the abundance of 1naterial available· to form 

the poikilitic 1nineral limits its final attained size. 

ORIENTATION OF POIKILIT'IC CRYSTALS 

No statistical study of orientation of poikilitic crystals had been made because of their extreme dimensions. On the basis of qualitative observations, however, no strong or consistent optical or dimensional 

orientation is believed to exist. 

No relation between the optical orientation of oikocrysts and the optical or dimensional orientation of 

settled crystals has ·been observed, even where there is 

a reaction relation between the two. Similarly, no 

relation between the optical orientation of oikocrysts 

a.nd the ]a.yering plane have been· observed. Cleavage 

reflections on poikilitic crystals within compositional 

layers are apparently ranaom. 

Except for the ellipsoidal dimensions of bronzite 

oikocrysts in a few chromitite specimens described on 

p. 66, dimensional orientation in bronzite oikocrysts 

has not been observed. 

DEVELOPMENT OF DOMINANT TEXTURES IN THE 

ULTRAMAFIC ZONE 

The coarse granular textures and abundant examples of crystal settling in the Ultramafic ·zone may be 

nttributed primarily to the extremely slow cooling of 

the thick intrusion, inasmuch as no abnormalities in 

composition or physical properties of the saturated 

basaltic parent magma are apparent. In· the lower 

part of the Basal zone, where the effects of rapid cooling can be observed, the rocks are fine grained and the 

texture is generally ophitic (fig. 87), pyroxene and 

plagioclase being present in about equal amounts. According to 'Valker (1957, p. 9), ophitic textures in 

rocks similar in composition to the chilled gabbro of 

the Stillwater complex indicate that both pyroxene 

and plagioclase began crystallization at a very early 

stage n.nd continued to crystallize in ophitic relationship throughout the cooling history of the rock. 

'Vithin the Basal zone above the chilled rocks, a rapid 

transition from ophitic to settled textures occurs, and 

the rocks consist of packed euhedral pyroxenes in a 

predominantly feldspar mesostasis. It is proposed 

that, in the Stillwater complex, this texture indicates 

primary precipitation, settling, and burial of bronzite 

prior to interprecipitate crystallization of plagioclase. 

The Ultramafic zone is characterized throughout by 

settled texture·s in which the primary precipitate and 

the interprecipitate 1naterial can generally be distinguished. However, modifications of simple settledinterstitial relations have occurred throughout the 

zone, and the dominant textures range from automorphic-poikilitic to xenon1orphic. Rocks with minor secondary enlargement have interposition fabrics, in 

which the settled crystals are euhedra or partially .resorbed euhedra and the interstitial material is largely 

poikilitic. Rocks with much secondary enlargement 

have mosaic fabrics and contain little or no interstitial 

. material. All gradations between these two textural 

extremes occur. 

The wide range of textural variation is caused by 

the behavior of the interprecipitate magma. In those 

parts of the crystal mush where this magma has crystallized constituents different from the settled crystals, 

textures are dominantly automorphic poikilitic; where 

the magma crystallized material of the same composition as the settled constituents, textures are xenomorphic. In many rocks, where the interprecipitate 

magma has partly crystallized as enlargements on settled crystals and partly crystallized as different phases, 

the textures are best described as hypautomorphicpoikilitic or intergranular. 

A UTOMORPHIC-POIKILIT'IC' TEXTURE 

Rocks composed predominantly of euhedral settled 

crystals are accompanied by poikilitic mesostasis textures. Such textures are common in each of the rock 

types of the Ultramafic zone except dunite (figs. 57, 

60, 72), but are perhaps best exemplified by chromitite 

because of the absence of resorption effects between 

precipitate and interstitial material. Iri. rocks where 

resorption textures are marked, particularly in poikilitic harzburgite, the automorphic character of the 

settled crystals is partially destroyed, but only those 

crystals that lie within the boundaries of the poikilitic 

reaction product are affected, and the other settled 

crystals in the rock retain their euhedral character. 

Automorphic-poikilitic textures are most abundant in 

the stratigraphically thickest sections of the Ultramafic zone. 

The euhedral nature of the settled crystals is attributed to their uninhibited crystallization in a large 

volume. of free magma, and the preservation of their 

euhedral forms is a result of the failure of the interprecipitate magma to crystallize large amounts of material of the same composition as the settled crystals, 

or to react with the crystals to form other minerals. 

Secondary enlargement was not effective in completely 

filling the interstices between settled crystals where 
80 STILLWATER COMPLEX, MONTANA 

burial was too rapid for diffusion to be operative between the main magma and the interprecipitate 

magma. The trapped interstitial liquid was therefore 

incapable of supplying much material the same composition as the settled crystals, and proceeded to differentiate in place, giving rise to new minerals. 

The absence of secondary enlargement in these rocks 

produced buried crystal mushes characterized by both 

high porosity and permeability. As a result, poikilitic 

crystallization of the mesostasis minerals was favored, 

and the rocks .are characterized by large volumes of 

coarse-grained interstitial 1naterial. Crystallization 

and differentiation in the trapped magma proceeded 

independently of, but in the same direction as, that 

of the free magma, and the interstices were filled with 

minerals not encountered as primary precipitates until 

much later in the crystallization history. 

HYPAUTOMORPHIC TEXTURE 

Rocks composed predominantly of subhedral settled 

crystals are, like those with euhedral settled crystals, 

generally accompanied by poikilitic mesostasis textures, but these are more poorly developed, and individual oikocrysts are commonly smaller in size. Most 

of the rocks of the Ultramafic zone are characterized 

by such textures, and all gradations between automorphic and xenomorphic textures are recognized (figs. 

58, 61, 13). 

The stibhedral nature of the settled crystals in these 

rocks is primarily due to partial mutual interference 

between grains ; this interference is believed to be 

caused by continued growth of settled crystals subsequent to deposition. Such loss in automorphism by 

secondary enlargement is confined to those parts of 

settled crystals which are in contact with neighboring 

crystals, and. those parts in contact with interstitial 

minerals retain their crystal faces. From this and 

other evidence it is concluded that secondary enlargement was completed prior to precipitation of new minerals from the interprecipitate magma. The rate of 

burial in rocks with textures of this type was appare.ntly slow enough to allow some enlargement of settled crystals, either at the surface of the mush, or, as 

suggested by Hess ( 1939, p. 431) by diffusion between 

the main magma and the interprecipitate magma. The 

volume of the interprecipitate magma was reduced by 

continued crystallization of the settled minerals. The 

remaining interprecipitate magma was trapped by continued burial, so that it crystallized and differentiated 

in much the same way as in rocks with no secondary 

enlargement. 

Porosity and permeability in the buried crystal mush 

were reduced proportionally to the amount of secondary enlargement, and both volume and grain size of 

interstitial minerals are smaller than in rocks with 

little or no enlargement. Where the amount of enlargement is small, poikilitic textures are well developed; where relatively large, the tendency for interstitial minerals to occupy adjoining interstices is reduced and textures of the rocks are perhaps better 

described as hypautomorphic-intersertal. 

XENOMORPHIC TEXTURE 

Rocks predominantly composed qf anhedral settled 

crystals generally contain little or no mesostasis nlinerals. Such rocks, whether dunite, harzburgite, bronzitite or chromitite, have typical peridotite even-grained 

mosaic textures (figs. 59, 62, 64). Rocks with xenomorphic textures can be correlated with rocks of the 

same type which contain abundant interstitial material 

in other parts of the Ultramafic zone, and the mosaic 

texture is best developed in the stratigraphically thinnest areas of the zone. 

The anhedral nature of the constituent grains is attributed to the nearly complete secondary enlargement 

of originally euhedral settled crystals to the point 

where all grains are mutually interfering. In rocks 

with xenomorphic texture, almost the entire original 

pore space in the crystal mush is composed of sedondarily enlarged material and little or no interprecipitate 

magma was trapped to forn1 interstitial minerals. 

This process is believed to have been favored by relatively slow rates of accumulation and burial, so that 

the settled crystals had ample opportunity to continue 

growth after deposition, either in contact with the 

main magma or by diffusion through the slowly accumulating crystal pile. 

In the Ultramafic zone of the Stillwater complex, 

the large compositional and textural differences between rocks with automorphic-poikilitic textures and 

those with xenomorphic textures are believed to be 

caused by local conditions of deposition and burial, 

and these differences are not related to any fundamental variation in composition, or crystallization sequence of the parent magma. Thus, poikilitic harzburgite is genetically closely related to dunite, for both 

formed from accumulations of olivine crystals. Poikilitic harzburgite, on the other hand, differs in origin 

to a greater degree from granular harzburgite, even 

though it may have the same mineralogic composition, because the latter contains settled bronzite and 

the former does not. Similarly, automorphic bronzitite with ·substantial amounts of interstitial feldspar 

and augite are closely related to monomineralic bronzitite with xenomorphic textures, because both rocks 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 81 

formed from a bronzite crystal mush; their textural 

and compositional differences are attributed to their 

postdepositional history, and especially to their rate 

o:f burial. 

DEVELOPMENT OF THE ULTRAMAFIC ZONE 

In a study of Ultramafic rocks, the problem of origin 

invites speculation at all stages of the investigation. 

Some tentative conclusions regarding the development 

o:t: the Ultramafic zone can be drawn from the preceding data on texture and mineral associations in the 

rocks. Even though such conclusions must necessarily 

be incomplete without consideration of detailed data 

on stratigraphy and chemistry, and will no doubt require subsequent revision, it appears worthwhile to 

summarize them in this report. 

CONDITIONS OF CRYSTALLIZATION AND DEPOSITION 

The layered rocks composing the Ultramafic zone 

of the complex are believed to have formed during 

crystallization of a single magma, by accumulation of 

crystal precipitates that fell to the floor of the magma 

chamber and were enlarged or cemented after deposition. The various magmatic processes influencing the 

final rock products can be divided into three groups: 

( 1) those that affect the character and growth of the 

primary precipitate crystals prior to deposition; (2) 

those that affect the distribution of the primary precipitate during descent and arrival at the floor of the 

magma chamber; and ( 3) those that largely affect the 

crystallization of the interprecipitate material after 

deposition. At any instant of time, of course, free 

growth, deposition, and consolidation went on simultaneously at different horizons in the intrusion, and 

the level of all three rose gradually as the pile of crystals accumulated. 

PREDEPOSITION AL CONDITIONS 

Some tentative conclusions regarding the properties 

of the Stillwater magma a1~d its early free crystallization products can be assembled, partly from the present investigation, and partly from the .result of earlier 

investigations. 

THE MAGMA 

The composition of the Stillwater magma, on the 

hasis . of a chill-zone ana.lysis reported by Peoples 6 

( 1932, p. 45) and on more recent analyses by the U.S. 

Geological Survey, most nearly resembles a saturated 

normal basalt. 1\iagma of this composition therefore 

apparently furnished the crystals composing the lower 

0 Peoples, .T. W., 1932, Geology of the Sttllwater complex: unpublisllCd doctoral thesis, Princeton University, 180 p. 

part of the Ultramafic zone. During crystallization 

of the zone the magma became progressively enriched 

in silica owing to .the subtraction of olivine and chromite, and the effects of this change in composition are 

apparent even within the Ultramafic zone by the increased abundance of bronzite at the expense of olivine 

upward in the section. From exsolution phenomena in 

the pyroxenes and from experimental work by Bowen 

and Schairer ( 1935, p. 200-201), Hess ( f91:1, p. 582-

583) estimates that the initial temperature of the Stillwater magma was about 1120°C, and that after about 

60 percent of the volume of the magma had crystallized, the temperature was about 1100° C. These figures are based on dry measurements at one atmosphere 

of pressure. Recent studies of the crystallization behavior of a natural tholeiitic basalt by Yoder and 

Tilley (1956, p. 169-171) showed that at 1090° C and 

5000 bars water pressure, pyroxene disappeared. 

Brown (1957, p. 541) assumes from this data that the 

Skaergaard magma began to crystallize at about 1090° 

C. The apparent agreement between these figures 

arises because increasing pressures tend to increase the 

melting point, whereas water dissolved in the magma 

tends . to depress it. · The absolute ten1perature value 

should be considered subject to further experimental 

work. ..Comparison of known compositiQnS of. bronzites in the Ultramafic zone with Brown's (1957, fig. 

5) crystallization curve for calcium-poor pyroxenes 

indicates that the magma temperature in the Stillwater complex decreased no more than about 20°C 

during the crystallization of the entire Ultramafic 

zone. This is in accord with the estimates of Hess · 

(1941, p. 583) and with the experimental .results of 

Yoder and Tilley (1957, p. 158-159). 

The density of the magma throughout the crystallization and accumulation of the Ultramafic zone was 

less. than that of olivine and bronzite, which readily 

sank in it. By the time the Ultramafic zone had accumulated and calcic plagioclase first appeared as a 

primary precipitate, the magma density was below 

that of the plagioclase, which was deposited in the 

norites immediately above the Bronzitite member. 

Very little information is available on which to base 

estimates of the viscosity of the Stillwater magma. In 

a recent discussion of the viscosity of dolerite sills 

with compositions similar to that of the Stillwater 

chilled margin, Jaeger and. Joplin (1956, p~ 445) suggest limits between 40,00Q and 280 poises. Hess ( 1956, 

p. 448) gives a preferred value of 3,000 poises. As a 

first approximation, the viscosity of the Stillwater 

magma probably lay within .an order of magnitude 

above or below 3,000 poises. 
82 STILLWATER COMPLEX, MONTANA 

PRIMARY CRYSTALLIZATION PRODUCTS 

The primary crystallization products of the magma 

that were formed during the entire course of crystallization of the Ultramafic zone are olivine, chromite, 

and .bronzite. Augite was not a primary precipitate, 

for It has about the same density as bronzite, and, if 

present, would have been recorded as a settled mineral 

iri the rocks. Plagioclase either . was not a primary 

precipitate or it floated. The probability is strong 

that plagioclase did not crystallize from the free 

magma for the following reasons : ( 1) Not a single 

settled crystal of plagioclase has been observed in the 

Ultramafic zone, and even if plagioclase had floated, 

a considerable number of grains would have been 

forced to the floor by settling olivine and bronzite. 

(2) Plagioclase does not occur as inclusions in the settled minerals of the Ultramafic zone, except in one 

specimen of bronzitite collected a few feet below the 

layered norites. ( 3) No evidence of rafting of plagioclase has been observed either in the Ultramafic zone 

or in the no rite and gabbro above it. ( 4) There is no 

settled plagioclase in the upper part of the Bronzitite 

member, even though the magma density could not 

have been significantly different than in the settled 

plagioclase rocks immediately above it. The contact 

between the Bronzitite member and the overlying 

norite is not caused. by density change in the magma, 

inasmuch as density change must be gradual and the 

bronzitite-norite cqntac~ is so sharp that it iRvolves 

only one layer of crystals. Furthermore, some settled 

plagiocla~e is present in local 1;10rite layers in the Basal 

zone, indicating that plagioclase was cap~ble of settling in the original magma. 

The euhedral shapes, equidimensional habits, absence 

of zoning, and paucity of inclusions in the three settled 

minerals indicate that they grew freely while falling 

slowly i~ saturated solution, and that the magma had 

no rapid fluctuations in equilibrium. The crystals 

themselves 'vere apparently present in the magma in 

logarithmic size distributions, and they apparently 

continued to grow as they settled toward the floor. 

The generally smooth grain-size changes of settled 

n1inerals within the cyclic units suggest that crystallization ·and deposition were continuous during their 

accumulation. The sharp grain-size breaks accompanying compositional changes at the bases and tops 

of the cyclic units, on the other hand, SJJggest hiatuses 

of crystallization. 

DEPOSITIONAL CONDITIONS 

During deposition, conditions were such as to produce layering. The extensive laleral continuity of 

layers, the gravity structures within the complex, and 

the generally conformable relations between the complex and overlying Paleozoic sedimentary rocks indicate that the layering was originally formed in a 

nearly horizontal position. In the following section, 

certain conclusions about the forces responsible for the 

transport and deposition of the primary precipitate, 

and about the origin of the various types of layers 

·are summarized. 

AGENTS OF TRANSPORTATION AND DEPOSITION 

Textures and structures in the Ultramafic zone suggest that the magma above the gradually rising floor 

of the magma chamber was essentially stagnant 

throughout the period of accumulation of the entire 

Ultramafic zone,. and only very locally did relatively 

weak currents disturb its quiescence. Local areas 

where currents were strong enough to sort crystals are 

marked by scour, local unconformities, and abnonnal 

sorting characteristics in the settled grains. Still 

weaker currents are indicated in some rocks by crude 

lineations of elongate grains. The general absence of 

currents at the site of deposition is indicated by the 

extreme scarcity of the above features throughout the 

Ultramafic zone, and by the absence of hydraulic 

equivalence between coexisting settled crystals in 

nearly all the rocks in the zone that contain two or 

n1ore settled minerals. Convection currents, therefore, 

do not appear to have been active during deposition 

of settled crystals. Furthermore, magma currents do 

not appear capable of accounting for the distribution 

of settled crystals. They cannot have caused the 

rhythmic layering, most of which is hydraulically upside down. The characteristic thick-bedded, predominantly monomineralic compositional layering in the 

Ultramafic zone also does not appear to be a result of 

current sorting of a mixture of floating crystals; ourrents or pulsatory variations in current velocity could 

not have sorted olivine from bronzite of nearly the 

same size and specific gravity, to form the thick poikilitic harzburgite and bronzitite layers;- Currents also 

cannot have brought about the repetitive order of the 

compo~itional layers. In particular; current sorting 

seems incapable of producing the Bronzitite member, 

a unit 2,000-feet thick, containing no settled mineral 

other than bronzite. 

Hess ( 1956, p. 449) has pointed out that· crystals 

fall through relatively viscous magn1a at very slow 

rates, and that, in the absence of curr~nts, crystals 

cannot be expected to sink appreciable distances in 

moderate lengths of time. Calculations of absolute 

settling rates are subject to considerably more error 

than consideration of relative settling rates (p. 30-

35), but, with certain assumptions, orders of magni-
DEVELOPMENT OF THE ULTRAMAFIC ZONE 83 

tude can be obtained. Assuming that the falling crystals obey Stokes' law (as their Reynolds numbers indicate), that the magma density is 2.7, and the viscosity 

of the magma is 3,000 poises, then olivine and bronzite 

crystals 2 mm in diameter would fall about 140 meters 

per year, and 0.2-·mm chromite crystals would fall 

about 4 meters per year. If it were assumed that crystaJl.izabon occurred at the top of the intrusion, then 

crystnJs falling at such rates would not reach the bottom of the intrusion during the largest part of the 

cooling history. If, however, crystallization occurred 

a,t the bottom of the intrusion rather than at the top, 

then gravity settling of the crystallization products 

could explain the depositional nature of the textures 

without recourse to magma currents. The physical 

aspects of bottom crystallization will be discussed in 

a subsequent section. 

ORIGIN OF THE LAYERING 

The genesis of the ~rarious types of layering- in the 

Ultramafic zone will be discussed in greater detail in 

a subsequent publication, but those aspects of origin 

that are in some respect illuminated by the textures 

of the rocks will be briefiy discussed here. All the 

varieties of layering in the Ultramafic zone of the complex are fundamentally related to the action of gravity 

on dispersed crystals of the three primary precipitate 

minerals. Thus, all layers are parallel to the floor of 

the intrusion; layers never· crosscut; sedimentary structures such as scour and slump occur locally; individual 

grains lie with their long dimensions in the layering 

plane. That these particles we1.·e discrete is shown by 

the general absence of clumping. . 

The plamtr orientation of settled 1ninerals is believed to be partly a result ·of simple gravitational 

forces, which caused settled crystals to assume their 

most stable positions, and partly a result of postdepositional compaction. Such layering is developed 

only where settled crystals have elongate or platy 

habits, and planar structure is most pronounced where 

platy or elongated crystals are most prominent. Lineation o:f long crystal axes within the layering plane 

is conunonly lacking. Planar dimensional distribution o:f crystals, where present, is similar to tha.t observed :in the plagioclases of the Skaergaard intrusion, 

called "igneous lamination" by "'\Vager and Deer ( 1939, 

p. 37-38). These authors note, however, that where 

Skaergaard p·lagiocl~tse crystals are elongate, a lineation o:f long axes· occurs in the layering plane, and 

they ascribe the layering to the action of convection 

cutTen ts sweeping across the floor of the intrusion. 

Although such currents undoubtedly would have intensified the platy parallelism of settled crystals in the 

Ultramafic zone of the Stillwater complex, it seems 

likely that they would also have imparted a strong 

lineation to the fabric. 

If the rocks of the Ultramafic zone were formed by 

the successive accumulation of crystals, then the compositional layers are a result of a changing supply of 

primary precipitate 1ninerals available for deposition 

at the floor of the intrusion. The character of this 

changing supply was such that contacts between layers 

are generally knife sharp (fig. 88) and ·that about 85 

percent of the rocks of the zone are composed essentially of only one of the three settled minerals. Furthermore, these layers-defined by the abrupt appearance and disappearance of the settled minerals-are 

repeated many times throughout the section, and in a 

regular order. The size ranges and densities of olivine 

and bronzite in the rocks are identical, yet for the most 

part they occur in mutually exclusive· layers. As 

stated above, this type of layering could not have been 

produced by gravity sorting or convection currents. 

One must conclude, therefore, that the rhythmically 

repeated compositional layers reflect repeated clul.nges 

in the supply of crystallization products of the Stillwater magma, and that each layer represents the crystallization products available for deposition at that 

time. As a corollary, the seque1~ce of the layers represents th~ order of crystallization of the settled minerals. . Not only does current sorting fail to provide ai1 

explanation for the distribution of settled minerals in 

the Ultramafic zone, but current transportation of 

crystallization products :from the roof to the. floor of 

the intrusion seems incompatible with lack of current 

structures, lineation, and hyqraulic equivalence in the 

rocks. As Hess (1956, p. 449) has pointed out, convection cu~rents in the Stillwater m.agma must have 

existed. The present evidence, however, indicates that 

they were not active where the settled minerals were 

deposited at the floor of the intrusion during the accumulation of the Ultramafic zone. · 

The layering caused by changing proportions of 

settled minerals in the Ultramafic zone is believed to 

have several modes of origin. In a few places, thin 

layers composed of gradually changing proportions of 

olivine and chromite are developed with chromite increasing upward. These layers, \Vhich are hydraulically similar to the rhythmic layering of "'\Vager and 

Deer ( 1939, p. 37) may be caused by differences in relative settling rates of suspended mixtures of large olivine crystals a.nd small chromite crystals, as proposed by 

I-Iess (1938a, p. 266) and "'\Vager and Deer (1939, p. 

272-273) for similar layering involving pyroxene and 

plagioclase. The normal sequence of such layering in 

the Ultramafic zone, however, is from predomin~ntly 
FIGURE 87.-Photomicrograph of chilled gabbro at base of the Stillwater complex. 

Texture is ophitic; plagioclase laths make up about one-half of the specimen, 

ophitic hypersthene and augite the remainder. Crossed nicols. 

FIGURE 88.-Photograph of contact between poikilitlc har1lburgite (at left) and 

granular harzburgite (at right). The rocks are nearly identical in grain size. 

Olivine is the only constituent in the poikilitic harzburgite; settled bronzite 

and olivine occur in about equal amounts in the granular harzburgite. 

EXAMPLES OF OPHITIC TEXTURE IN THE CHILLED GABBRO AND LAYERING IN THE ULTRAMAFIC ZONE 

00 

1-t>--

Ul 

:j 

~ 

~ t<:1 

~ 

(") 

0 

~ t-< 

t<:1 

:>< 

~ 0 

z 

~ 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 85 

small chromite crystals at the base to predominantly 

larger olivine crystals at the top. Layers of this type 

are hydraulically upside-down, and could not have 

been produced by differential settling. The most probable explanation is that they form as a result of changing proportions in the supply of settled olivine and 

chromite to the deposition surface, owing to a gradual 

cessation of chromite crystallization and concomitant 

inauguration of olivine crystallization. A third type 

of layering caused by gradually changing proportions 

of settled minerals up,vard in the rocks of the Ultramafic zone is developed on a larger scale in most granular lutrzburgite layers. In these rocks, the bronzite 

typically increases in amount stratigraphically upward 

at the expense of settled olivine (fig. 11), and this 

change is accompanied by an increase in bronzite grain 

size and a decrease in olivine grain size (fig. 19). 

Again, this layering is not easily explained in terms 

of differential settling rates, because the two minerals 

involved have nearly equal specific gravities and are 

in hydraulic equivalence only at those horizons within 

the layers where the grain sizes of the two minerals 

are equivalent. Jn view of their consistent stratigraphic position between poikilitic harzburgite and 

bronzitite, the granular hatzburgite layers seem best 

explained as crystal accumulations that reflect a gradual decrease in the amount of olivine crystallized by 

the magma, as bronzite becomes the dominant crystallization product. 

The several varieties of size layering described earlier in this paper are also, for the most part, probably 

a result of changing crystallization conditions in the 

magma, rather than a result of mechanical sorting; 

but the uncommon current-bedded layers described on 

page 31 a.re an exception to this general hypothesis. 

The sharp size breaks accompanying mineralogical 

changes at the bottoms of the poikilitic harzburgite 

layers n.re believed to represent hiatuses of deposition. 

The fact that there is no 1nixing of fine bronzite with 

the coarser olivine overlying it at these contacts is 

taken to indicate that all the bronzite .had an opportunity to fall to the floor before olivine crystals were 

available for deposition. The gradual size increases 

and decreases of settled minerals within layers and 

within cyclic units suggest that deposition was continuous throughout their accumulation. 

In summary, the types of layering and the textural 

features of these layers indicate that: ( 1) simple crystal settling occurred from magma near the floor of the 

:intrusion; (2) the magma from which the settling occurred was essentially stagnant during crystal deposition, and magma curre.nts did not materially effect 

either the transportation or deposition of such crystals; ( 3) single layers contain all the settled minerals 

being crystallized at the time of their deposition, and 

the stratigraphic sequence, therefore, parallels the order of crystallization in the magma;· and ( 4) deposition and, therefore, crystallization were continuous 

during the deposition of the individual layers and cyclic units, but discontinuous between cyclic units. 

POSTDEPOSITIONAL. CONDITIONS 

The primary precipitate formed a crystal mush on 

the floor of the intrusion, and the interstices of this 

mush were filled with the basaltic n1agma from which 

the primary precipitate had crystallized. The textures 

of the rocks provide some evidence as to the character 

of the mush, and the composition and crystallization 

course of the interprecipitate magma. 

CHARACTER OF THE CRYSTAL MUSH 

The porosity of the crystal mush on the floor of the 

intrusion during the accumulation of the rocks of the 

Ultramafic zone is estimated to have ranged from 

about 20 to 50 percent and to have averaged about 35 

percent. Differences in initial porosity can be assigned 

to the physical properties of the crystal aggregatesize, sorting, shape-and to the amount of compaction 

prior to cementation. The permeability of the mush 

presumably was influenced by the packing and by the 

grain size of the individual layers. The thickness of 

unconsolidated mush at any qne ti1ne on the floor of 

the intrusion apparently ranged from about 6 inches 

to about 10 feet, because beds of this range of thickness 

are involved in slump structures. Interprecipitate 

magma, however, ·probably existed at much greater 

depths in the 1nush, since it would take only a small 

amount of secondary enlargement to weld the settled 

crystals and impart a rigidity to the crystal pile. 

INTERPRECIPITATE ~AGMA 

The composition and physical properties of the interprecipitate magma within the unconsolidated mush 

on the floor of the intrusion must be initially identical 

with those of the main n1agma immediately ab.ove it. 

At greater depths in the mush, however, two processes 

operate to change its character: diffusion and crystallization -differentiation. At shallow or intermediate 

depths in the musli·, w·here interconnecting pore spaces 

are available, diffusion between the interprecipitate 

magina and the main magma is possible. The effect 

· of such diffusion, as pointed out by Hess ( 1939, p. 431) 

would· be to crystallize interprecipitate 1ninerals of the 

same phase and composition as were being generated 

in the main n1agma above the floor of the intrusion. 

If only one phase was being precipitated by the main 
86 STILLWATER COMPLEX, MONTANA 

magma, and if accumulation rates were slow enough 

for diffusion to operate at maximum efficiency, completely n1onomineralic rocks would probably result. 

At deeper levels in the mush, or where continuous 

pore spaces were blocked by welding or impermeable 

layers, diffusion between the main and interprecipitate 

magma would not be operative, and the interprecipitate magma would be trapped. Textural and- mineralogic relations indicate that the order of crystallization in this trapped magma is such that successive 

residual fractions are enriched in sodium, silicon, iron, 

and volatiles. 

The relatively lower temperature crystallization 

products, the absence of exsolution lamellae in the latest crystallized parts of interstHial augite and bronzite, 

and the zoning-all indicate falling temperature of 

crystallization as the trapped interprecipitate magma 

solidifies. An inference as to the final consolidation 

temperatures in those rocks which contain grossularitepyrope as a very late crystallization product may be 

drawn from Yoder's (1950, p. 250) conclusion that the 

assemblage quartz-grossularite-anorthite is incompatible at temperatures above about 650°C. 

CRYSTALLIZATION OF THE INTERPRECIPITATE MATERIAL 

Results of the present investigation substantiate 

Hess' ( 1939, p. 431) hypothesis that the course of crystallization in the interstices of the crystal mush is 

largely controlled by the rate of accumulation of that 

mush. Where the rate of accumulation and burial was 

rapid, the interprecipitate magma was effectively 

trapped and crystallized in place, with the formation 

of new mineral phases not encountered as primary 

precipitates until much higher in the section. Where 

the rate of accumulation and burial was slow, diffusion 

between the main ·and interprecipitate magma was 

relatively effective and all or much of the pore space 

between settled crystals was filled by material of the 

same composition as the primary precipitate. 

MINERAL ASSOCIATIONS IN THE ULTRAMAFIC ZONE 

Because the constituent minerals of Ultramafic zone 

rocks can be readily divided into primary precipitate 

and interprecipitate material, it is possible to examine 

separately the mineral assemblages produced during 

the early stages of crystallization of the main Stillwater inagma, and the mineral assemblages produced 

by the same magma where differentiation was spatially 

restricted by entrapment in the crystal mush. 

PRIMARY PRECIPITATE 

The most striking feature of the primary precipitate 

assemblage is the tendency for the three settled minerals to occur singly rather than in combination with 

one another. In the case of olivine-bronzite associations, for instance, poikilitic harzburgite, which contains no settled bronzite, and bronzitite, which contains 

no olivine, are considerably more abundant rock types 

than granular harzburgite, which contains a mixture 

of the two minerals. In order to derive a quantitative 

expression of this tendency, stratigraphic thicknesses 

of each of these three rock types were measured, totaled, and recalculated to 100 percent in three complete 

sections of the Ultramafic zone; results are presented 

in table 10. Settled olivine and bronzite occur together in about 12 percent of the rocks of the zone; 

they are mutually exclusive in about 88 percent. 

TABLE 10.-Relative stratigraphic thicknesses of harzburgit~ and 

bronzitite in three complete sections of the Ultramafic zone 

West Benbow Mountain View West Fork 

section section section 

AverRock type age 

Total Total Total ratio 

thick- Ratio thick- Ratio thick- Ratio ness ness ness (feet) (feet) (feet) 

------------- Poikilitic harzburgite. _ 1, 370 27.4 1, 675 23.8 7.50 20.8 24.0 

Granular harzburgite.- 840 16.8 860 12.2 280 7. 8 12.3 Bronzitite ______________ 2, 785 55.8 4, 515 64.0 2, 580 71.4 63.7 

--------- ----- TotaL----------- 4, 995 100;0 7,050 100.0 3,6lb 100.0 100.0 

The tendency toward monomineralism of settled 

minerals is also evident in olivine-chromite mixtures. 

Although the two minerals apparently occur in all 

proportions, much of the chromite in thb Ultramafic 

zone is concentrated into massive layers containing no 

other settled mineral. 

Where settled minerals do occur as mixtures, there 

is a strong tendency for bi~ineralic associations. In 

rocks composed of mixtures of olivine and chromite, 

settled bronzite does not occur; in rocks composed of 

mixtures of olivine and bronzite, chromite occurs only 

in amounts less than 3 percent of the rock or 5 percent 

of the settled constituents. 

The rocks of the Ultramafic zone contain only three 

settled minerals, so that it is possible to outline their 

fields of coexistence on a triangular plot. In order to 

establish their true proportions, it is necessary to subtract the amount of interprecipitate material in the 

individual specimens and recalculate to 100 percent, 

but in view of the practically universal occurrence of 

secondary enlargement and reaction replacement a certain care must be exercised in making the calculations. 

In most chromitite and bronzitite, only one settled 

mineral is present, and the recalculation may be made 

directly. In bronzitites which contain chromite, and in 

poikilitic harzburgites, it is necessary to 1nake modal 

counts in areas where plagioclase is the interstitial 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 87 

mineral, in order to avoid the effects of reaction replacement. In granular harzburgites, the same procedure is generally followed, but in cases where this 

is impractical it is necessary to compute the original 

proportions of the settled minerals fron1 the mode of 

the entire rock. In olivine chromitites and poikilitic 

chromite harzburgites, the original boundaries of olivine crystals are outlined by the closely packed chromite around them, and it is possible to measure the 

proportions directly. In rocks which contain two or 

more settled crystals, the assumption is made that the 

amount of secondary enlargement of each mineral is 

the same. This assumption is not invariably valid, 

but; the error involved is not believed to be large 

enough to change the boundaries of the fields of occurrence. 

~1odes of 141 specimens of rocks of the Ultramafic 

zone, measured and recalculated by the methods .described above, were plotted individually in figure 89. 

Chromitite, olivine chromitite, and poikilitic chromite 

harzburgite are disproportionally represented in order 

to outl1ne their fields of occurrence. Nevertheless, the 

monomineralic tendencies of the settled minerals are 

obvious from the crowding at the apices of the diagram. ~1ost bronzitite, which contains no olivine by 

definition, also contains no chromite. Nine of the 38 

bronzitite specimens measured contain chromite in 

amounts less than 0.5 percent. Three bronzitite specimens contain between 1 and 4 percent settled chromite. 

Each of these three specimens occurs within 100 feet 

of the top of the Bronzitite member, and the significance of this abnormal chromite concentration at this 

particular horizon is discussed in a subsequent section 

of this report. - _ 

~fixtures of olivine and bronzite apparently occur 

in all pr.oportions between hr1ooOlo and br2oOlso· 

About two-thirds of the olivine bronzitite and granular harzburgite specimens contain measurable chromite, but the total amount in any one specimen does 

not exceed 5 percent. In 3: general way, the chromite 

content increases with increasing amounts of olivine. 

Specimens containing proportions of olivine and 

bronzite between bt20ol8o and brool1oo were looked for 

but not found, and it is believed that a gap in the field 

occurs in this range. 

Olivine and chromite apparently occur in all proportions over the range ol1 0ochr0 .to olochr10o. Th~ 

CHROMITE 

FIGURE 89.-Ratios of settled minerals in rocks of the Ultramafic zone. 
88 STILLWATER COMPLEX, MONTANA 

line of variation between chromitite and poikilitic 

harzburgite contains no settled bronzite. 

The fields of settled mineral associations shown in 

figure 89 indicate the minerals which accumulated 

simultaneously, and ·not necessarily those which crystallized together. The restriction of associations does 

indicate, however, which minerals typically could not 

have been crystallizing together. Thus, during the 

accumulation of the chromite mushes now represented 

by the massive chromitites, no olivine or bronzite was 

crystallizing; had they been, their size and density 

would have caused their ineorporation into the mush. 

Similarly, during the accumulation of most bronzitite, 

no mineral other than bronzite was crystallizing within 

settling distance of the floor of the intrusion. Such 

monomineralic crystallization could take place over 

considerable periods of time: during the accumulation 

of the 1,500- ·to 3,000-foot thickness of the Bronzitite 

member, the only mineral crystallizing from the Stillwater magm·a was bronzite. 

If the presence of thick layers containing only one 

settled mineral connotes monomineralic crystallization 

from the magma, the presence of layers containing 

mixtures of settled minerals does not necessarily connote cotectic relationships between them. There appear to be three meehanisms whereby the observed distributions of two or more settled minerals might occur: 

( 1) by cotectic crystallization, ( 2) by mechanical mixture of minerals crystallized at different times or 

plaee~, and ( 3) by simultaneous crystallization at different horizons in the magma. 

The distribution of chromite inclusions in olivine 

in a few rocks suggests a cotectic relation between 

these two minerals. In the great majority of such 

rocks, however, inclusions do not occur, and it is not 

possible to estimate the quantity of rocks or the relative proportions of the two minerals through which 

a cotectic relationship existed. The constant assoeiation of a small amount of chro:mite with all rocks 

formed from olivine crystal mushes, and the localization of all chromitite layers within poikilitic harzburgite layers certainly suggests that the crystallization fields of olivine and chromite are adjoining. The 

general absence of rocks containing mixtures of chromite and bronzite, on the other hand, suggests that 

the crystallization fields did not adjoin in the part of 

the field where crystallization occurred, at least until 

nearly the entire Ultramafic zone had crystallized, and 

the original magma had been considerably modified by 

differentiation, including depletion of the magma in 

chromium. 

It is obvious from inspection of figure 89 that the 

efficacy of mechanical mixing of settled crystals eannot be large. Current mixing has, however, changed 

proportions of settled minerals in a few currentbedded olivine chromitite and granular harzburgite 

layers, and differences in settling velocity no doubt 

account for some of the olivine-chromite graded beds. 

Neither current deposition nor suspensimi, however, 

appear to be capable of producing the thick, internally 

homogeneous olivine chromitite and poikilitic chromite 

harzburgite layers, nor do these mechanisms readily 

explain the· olivine-bronzite size-proportion relations 

in granular harzburgite layers. 

The association of olivine and bronzite in the granular harzburgite and olivine bronzitite layers, in fact, 

demands a special explanation. It is abundantly demonstrated in the Stillwater rocks that olivine and 

bronzite have a reaction relationship and consequently 

eannot crystallize at the same time and place in the 

magma. It is proposed, therefore, that during certain 

conditions of crystallization olivine and bronzite crystallized simultaneously at slightly different horizons 

above the floor of the intrusion. The abrupt appearance of bronzite in relatively small quantities at the 

bases of the granular harzburgite layers suggests that 

bronzite began crystallizing at a higher level than the 

olivine, fell through the zone of olivine crystallization, 

and became incorporated in the erystal mush. Upward 

in the granular harzburgite layers, olivine becomes 

smaller and less abundant, which suggests that olivine 

crystallization was restricted to increasingly narrow 

zones near the floor of the intrusion. Such a mechanism 'vould not only provide an explanation for the 

regular grain-size and abundance variations of the two 

minerals, but would explain the absence of bronzite 

reaction shells on the olivine in the rocks. A similar 

process may account for some olivine chromitite and 

poikilitic chromite harzburgite layers, because many 

of these layers gradually increase in olivine at the expense of chromite upward in the section; however, 

the relative effect of cotectic crystallization in these 

rocks cannot be evaluated. 

INTERPREC.IPIT ATE MATERIAL 

If the settled crystals represent the early crystallization products, or solidus, of the Stillwater magma, 

then the interprecipitate material should in a broad 

sense represent the liquidus, or the n1agma fron1 which 

the settled crystals were precipitated. It would seem 

reasonable, therefore, that one eould determine magma 

compositions layer by layer, stratigraphically upward 

in the section, by measurement of the volume and composition of the interprecipitate material in the various 

layers. Unfortunately, it can be shown that the interprecipitate material does not necessarily represent the 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 89 

composition of the liquids trapped in the interstices 

of the crystal mush, and, furthermore, that the trapped 

magma in individual layers is not necessarily of the 

same composition as the main magma from which the 

settled crystals in that layer were precipitated. It 

would be possible, however, to obtain an approximate 

overall composition for the trapped magma if that 

portion of the interprecipitate material which crystallized from it could be recognized. 

The interprecipitate material may be divided into 

( 1) the secondary enlargement material, ( 2) the reaction replacement material, and (3) the interstitial minerals. Of these, the secondary enlargement n1aterial 

properly belongs, mineralogically and chemically, with 

the primary precipitate, because it grew in equilibrium 

with the main magma. Reaction replacement material 

and interstitial minerals, for the most part, represent 

new phases caused by entrapment and later crystallization of the magma. Those portions of the reaction 

replacement minerals which formed from previously 

existing settled crystals, however, were not entirely 

constituted from the trapped magma; where bronzite 

replaced olivine, for instance, only Si02 was added. 

The interSLitial material, including those portions of 

reaction minerals which fill interstices, is entirely a 

product of the trapped magma. The interstitial material alone should furnish an approximation of the 

composition of the trapped magma, but this approximation does not include elements provided in reaction 

replacement, nor does it include that small portion of 

the trapped magma that crystallized to form minerals 

already present as settled constituents. 

A serious limitation in obtaining approximations of 

the composition of trapped magma is the problem of 

obtaining significant measurements of interstitial minerals. Reproducible proportions of the relatively finegrained and evenly distributed settled minerals may 

be obtained from small slabs or thin sections, but the 

extreme grain-size ranges in the interstitial material 

make equivalent accuracy impossible to obtain, even 

where large slabs were used. Furthermore, the interstitial material makes up only a small part of the 

rocks, and errors are considerably magnified by recalculation to 100 percent. 

Although signi·ficant measurements of interstitial 

material in individual specimens is generally impractical, an average value for a large ~lumber of specimens 

is probably reasonably representative. Qualitatively, 

the variatjon in proportions of interstitial constituents 

is greater between rock types than. within them. In 

order to obtain quantitative data, the amounts of interstitial minerals in 94 specimens containing greater 

than 10 percent total interstitial material were measured, recalculated to 100 percent, averaged by rock 

type, and plotted in figure 90. In most rocks, more 

than 95 percent of the total interstitial material is 

composed of the three minerals-bronzite, chromian 

augite, and plagioclase-so that averages are plotted 

for simplicity on a triangular diagram. The only major exception occurs in the chromitite specimens, in 

which olivine constitutes an average of about 20 percent of the interstitial material. This was combined 

directly with bronzite for purposes of plotting on the 

diagram. 

The ratios of interstitial minerals in the three major 

rock types of the Ultramafic zone-poikilitic harzburgite, granular harzburgite, and bronzitite-fall 

nearly on a straight line in figure 90, and are arranged 

in the same sequence as their appearance in the cycles. 

Chromian augite makes up about 10 percent of the 

average interstitial material in all three rock types, 

and plagioclase replaces bronzite as an interstitial constituent as settled olivine gives way to settled bronzite. 

The ratios of interstitial 1ninerals in chromitite and 

olivine chromitite layers, which lie stratigraphically 

within the poikilitic harzburgite layers, do not fall on 

this curve. Both contain relatively greater amounts 

of chromian augite and relatively less plagioclase than 

the averages for the 1najor rock types. 

Assuming that the plotted data are representative 

of the average proportions of interstitial minerals in 

the various rock types, and that the interstitial minerals are reasonably representative of the composition 

of the trapped magma, the gross changes in interstitial 

mineralogy indicate the broader chemical trends in the 

liquid once present in the crystal mush. The trapped 

magma in the average chromitite apparently contained 

slightly larger amounts of iron and magnesium, and 

less aluminum, than the trapped 1nagma in the average 

poikilitic harzburgite. The compositional trend of the 

three major rock types is toward strong calcium and 

aluminum enrichment in passing from poikilitic harzburgite through granular harzburgite to bronzitite. 

Because the major rock types occur in a regular order 

of succession, this variation coincides with the variation within the cyclic units and is repeated some 15 

times throughout the Ultramafic zone. 

If it is further assumed that the composition of the 

trapped magma in some measure reflects the composition of the main magma-an assumption that cannot 

a.t present be verified-then there appear to be at least 

two possible explanations for this variation: (1) The 

composition of the trapped magma reflects the composition of the main magma. from which the settled crystals 

precipitated, and its compositional change within the 

cycle reflects the changing composition of the main 
90 STILLWATER COMPLEX, MONTANA 

PLAGIOCLASE 

BRONZITE CHROMIAN AUGITE 

FIGURE 90.-Ratios of principal interstitial minerals in rocks of the Ultramafic zone. 

magma as it precipitates new phases of settled crystals. 

(2) The composition of the trapped magma in the entire cycle represents the average composition of the 

main magma, but the trapped magma has differentiated 

in place, first forn1ing bronzite in the lower olivine-rich 

layers and leaving a plagioclase-rich residuum to crystallize in the uppr bronzite-rich layers. A choice between these hypotheses cannot be made until additional 

data on the chemical composition of the· interstitial 

minerals are available. 

CLASSIFICATION OF ROCK PRODUCT'S 

The rocks of the Ultramafic zone are generally made 

up of about 65 percent settled minerals and 35 percent 

interprecipitate minerals. The interprecipitate material may be all or in part of the same composition as 

the settled minerals, the remainder being made up of 

new phases precipitated from magma trapped in the 

interstices of the crystal mush. 

The mineralogic composition of individual sp~cimens can be expressed as modes, and the specimens 

can be assigned rock names on the basis of their total 

mineral constituents. This normal basis of classification, which suffices to distinguish many igneous rocks, 

fails to provide a breakdown suitable for field distinctions in the Stillwater complex for two reasons: (1) 

A standard mode does not differentiate between settled and interstitial minerals, so that many granular 

and poikilitic harzburgite specimens, for instance, have 

identical modes. (2) The variability of interstitial 

constituents in the rocks complicates the classification 

and leads to a multiplicity of rock na.mes based on 

minor constituents that are not consistent along the 

strike of the layers. 

The problem of classification in Stillwater rocks is 

therefore similar to that in sedimentary, rather than 

igneous rocks. The bulk composition of the rocks does 

not represent the composition of the magma from 

which they crystallized and is not, therefore, of direct 

genetic importance. As with sediments, the allogenic 

minerals of the Ultramafic zone are spread out in mappable beds, and the interstitial constituents are dependent on postdepositional factors that may vary from 

place to place along the beds. The fundamental basis 

for a field classification of Stillwater rocks must, therefore, be the presence, absence, and proportions of the 

settled crystals in the rocks. 

Ideally, new terms expressing the various fields of 

settled mineral associations should be coined, or rock 

names should be avoided altogether in the manner of 

Shand (1942, p. 409-428), to avoid confusion with 

existing compositional terminology: Standard ultra-
DEVELOPMENT OF THE ULTRAMAFIC ZONE 91 

mafic rock names have, however, been applied to Stillwater rocks in previous publications, and precedence 

for their use has been set. Rather than burden the 

literature with new names, therefore, the existing terminology fot; rocks of the IDtramafic zone of the complex is here more rigidly defined. 

The fields of the various rock types are based entirely on presence, absence, and proportions of settled 

minerals; these are shown diagrammatically in figure 

89. The sequence of rocks made up of mixtures of 

settled olivine and chromite are arbitrarily divided on 

the basis of ratios of the two minerals in the rocks. 

Chromitites range from chr1oo to ol5chro5, olivine chromitites from ol5chro5 to ol5ochr50, and poikilitic duomite harzburgites from ol50chr50 to olo5chr5. The 

poikilitic harzburgite field ranges from olo5chr5 to 

ol1oo, although no rocks are at present known that contain less than 0.5 percent chromite. Rocks composed 

of mixtures of olivine and bronzite commonly contain 

some chromite, so that the fields are two dimensional. 

Granular harzburgites occur within the field bounded 

by broolloo, brooOllo, brs50l1ochr5, and olo5chr5, although 

specimens containing less than 20 percent settled bronzite have not been observed. Olivine bronzitites occur 

within the field bounded by hr90ol10, br1oo, bro5chr5 and 

brs50l1ochr5, and bronzitites range from br100 to br95 

chrr.. Other fields, such as chromite bronzitites or granular chromite harzburgites, could be added to the 

general classification if specimens with corresponding 

ratios of settled minerals are subsequently observed. 

The term dunite is a useful one, and it has been used 

in previous publications to describe certain layered 

rocks of the Ultramafic zone in the Stillwater complex. 

It is retained as a term to describe rocks composed of 

95 or more percent olivine. Dunite is, therefore, a 

particular variety of poikilitic harzburgite, whose settled constituents are olivine and accessory chromite, 

and which is characterized by extreme secondary enlargement of olivine. 

This system of nomenclature leads to some unconventional classifications-for instance, one specimen 

containing only 50 percent modal bronzite must be 

placed in the bronzitite field. In addition, some rocks 

that in terms of strict mineralogic nomenclature are 

websterite, lherzolite, gabbro, or norite are included as 

bronzitite or harzburgite. Wherever the total min~ralogic composition of the rocks is pertinent to the discussion, however, individual modes will be presented. 

SIGNIFICANCE OF THE DEPOSITIONAL CYCLES 

The cyclic units are the building blocks of the Ultramafic zone, and any attempts at reconstructing the 

541905 0--'61--5 

genesis of the zone must primarily be concerned with 

their origin. 

SUMMARY OF SALIENT FEATURES 

The cyclic units are internally continuous, repetitive 

stratigraphic layers in which the settled minerals appear and disappear in a regular order. Minerals were 

continuously being deposited during the cycles, but the 

boundaries between cyclic units represent hiatuses. 

Based on the law of superposition, the stratigraphic 

sequence of settled minerals represents their order of 

accumulation within the cyclical unit. Further, in the 

absence of rafting and current sorting, the order of 

accumulation within the cycles represents the order of 

crystallization of the settled crystals from the magma 

above the floor of the intrusion. 

vVithin the Peridotite member, the order of crystallization of primary· precipitate minerals during accumulation of the cyclic unit ·was olivine, chromite, 

olivine, bron:zite, as represented by the rock types 

poikilitic harzburgite, chromitite, olivine chromitite, 

poikilitic harzburgite, granular harzburgite, and bronzitite. The order of disappearance within the cycle was 

chromite, olivine. Although the order of succession 

of the rock types is consistent, not all cyclic units 

within the Peridotite member are con1plete: some are 

terminated within the poikilitic harzburgite layer and 

some within the granular harzburgite, so that bronzitite is not developed. The character of the cyclic units 

changes gradually upward in the Peridotite member: 

stratigraphically lower cyclic units contain more poikilitic harzburgite relative to bronzitite, and higher 

cyclic units are proportionally enriched in bronzitite. 

In a few cyclic units near the top of the Peridotite 

member, the rock type at the base of the unit is granu:-

lar harzburgite. 

vVithin the Bronzitite member there appear to be 

two possibilities regarding cyclic units: ( 1) the Bronzitite member represents a single continuously deposited 

upper portion of the highest cyclic unit of the Ultramafic zone, and differs from bronzitite layers within 

the Peridotite member only in its thickness; or (2) an 

unknown number of cyclic units consisting only of settled bronzite occur within the men1ber, concealed by 

its monotonous lithology. The apparently continuous 

size changes of bronzite crystals within the member 

seem to favor the first possibility, but the samples are 

widely spaced, and sharp small-scale size changes are 

known to exist. The presence of two thin, apparently 

lenticular layers of granular harzburgite in the Bronzitite member in the Mountain View section, where it 

reaches its n1aximum development, favor the second 

possibility. In either case, virtually the only primary 
92 STILLWATER COMPLEX, MONTANA 

precipitate of the Stillwater magma during the accumulation of the member was bronzite. 

Near the top of the Bronzitite member, a new cyclic 

unit consisting of a lower layer of coarse bronzitite 

with accessory chromite, and an upper layer of norite, 

can be recognized. The order of crystallization within 

this unit was therefore essentially bronzite, plagioclase. 

CHEMICAL ASPECTS 

Assuming that deductions as to the order of crystallization of the primary precipitate within the cyclic 

units are valid, certain phase boundary relationships 

can be deduced. The investigated chemical field most 

nearly resembling the composition of the chilled gabbro of the Stillwater complex is the system diopsideforsterite-anorthite-silica (Osborn and Tait, 1952, p. 

425-432). If iron is assigned to magnesium, the chilled 

gabbro composition plots at about di2ofOaoan4oq10, 

near the anorthite-forsterite-silica face of the tetrahedron. This point is within the forsterite field near 

the spinel boundary. The primary crystallization 

product would therefore be forsterite, and this would 

next be joined by spinel. Following forsterite--spinel 

crystallization, however, the magma would precipitate 

forsterite-anorthite and dissolve spinel. If the natural 

system resembles the investigated one, then the chromite field of the former must be larger than the spinel 

field of the latter and must extend as far as pyroxene, 

for no settled feldspar is found in the cyclic units of 

the· Ultramafic zone. In the investigated system the 

forsterite-clinoenstatite join is a reaction boundary, 

and the same situation is indicated in the cyclic units 

by the early disappearance of olivine. The system 

anorthite-forsterite-silica (Andersen, 1915, p. 407-454) 

accurately predicts the order of crystallization in the 

final cyclic unit of the Ultramafic zone. Assuming 

that the Stillwater magma had been enriched in Si02 

by the precipitation and settling of considerable quantities of olivine and chromite, the diagram predicts 

that bronzite will then crystallize, followed by bronzite 

plus plagioclase, and this is the observed relation. It 

is of interest to note that in preliminary investigations 

of the crystallization behavior of Kilauean tholeiite 

basalts compositionally similar to the chilled gabbro_ 

of the Stillwater complex, Yoder and Tilley ( 1956, p. 

169; 1957, p. 157-158) found the order of crystallization to be olivine, bronzite, plagioclase. 

Several features of the cyclic units cannot, however, 

be explained in terms oi the investigated systems. If 

the magma began crystallization in the olivine field 

during the formation of the Peridotite member units, 

then no simple explanation for the massive chromitite 

layers indicating slightly later monomineralic crystallization of chromite is apparent. Nor does the system 

explain the ubiquitous small quantities of chromite 

that occur with olivine in the poikilitic harzburgite 

layers above and below the chromitite layers. Further, the vertical disappearance of chromite before the 

appearance of a third solid phase, and the absence of 

resorption textures involving chromite, do not correspond with the experimental system, but this may be 

a result of the early exhaustion of the small chromium 

content of the magma. 

The text11res of the granular harzburgites-the rocks 

which lie between the layers representing the crystallization of olivine alone and bronzite alone-are also 

not what one would expect to find at a reaction boundary, in that euhedral olivine and bronzite crystals 

exist side by side throughout the rock. Bowen ( 1928, 

p. 58-59), however, emphasizes that: 

There is nothing in the existence of a reaction relation between two crystal species to prevent the occurrence of both as 

perfectly formed crystals with no evidence of a resorption of 

the earlier member of the reaction pair ... under certain 

conditions of rapid cooling the liquid may ignore the olivine 

crystals and pass immediately into the pyroxene field with 

formation of crystals of that phase. 

Still, several granular harzburgite layers exceed one 

hundred feet in thickness, and it seems probable that 

olivine continued to crystallize after bronzite had begun to precipitate. For this reason, it has been proposed that two different crystallization layers existed 

briefly in the magma above the floor of the accumulating crystal mush. 

HISTORY OF THE CYCLE 

In view of the chemical complexity of the natural 

magma compared to the inv.estigated systems, the correspondence between crystal products and their order 

of appearance and disappearance is surprisingly good; 

this correspondence, combined with textural evidence, 

perhaps justifies a preliminary reconstruction of the 

history of accumulation of a cycle. The many evidences, assumptions, and inferences involved have been 

stated elsewhere in the text and need not be repeated 

here. 

The cycle began with the precipitation of olivine 

crystals from the n1agma near the floor of the intrusion, and these immediately began to sink. Crystals 

that formed near the floor had less opportunity for 

growth, and thus the grain size of the olivine in the 

mush was finer near the floor. After 5 to 100 feet of 

mush had formed, olivine crystallization ceased 

abruptly and chromite was temporarily the only crystallization product of the magma, perhaps as a result 

of supersaturation and excursion of the liquid into 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 93 

the chromite field. A thin, relatively impermeable 

layer of fine-grained chromitite was then formed in 

the crystal mush-a layer that later acted as a cap 

rock to interstitial solutions attempting to rise as the 

mush compressed slightly-resulting locally in the 

formation of gabbroid pegmatites along its base. As 

with olivine, the growth of chromite crystals forming 

nearest the floor was arrested by deposition, .so that 

the basal chromite crystals were slightly finer· grained 

than those near the center of the layer. Olivine then 

joined chromite as a crystallization product and the 

two crystallized together, olivine increasing in amount 

and chromite decreasing until the chromium content 

of the magma was virtually exhausted. 

Olivine then continued to crystallize alone, its crystal diameters increasing upward as crystals arrived 

from increasingly greater distances above the floor. 

Bronzite began to crystallize at a higher .level in the 

magma, fell through the zone of olivine crystallization, and, at a sharp boundary in the mush, the two 

minerals began to accumulate simultaneously. Olivine 

crystallization became restricted to narrower and narrower zones n.bove the floor and thus began to decrease 

in size and total amount, whereas bronzite increased 

in both. Along n, gradational contact olivine finally 

ceased to be a crystallization product, and bronzite 

continued to precipitate alone. Bronzite crystals gradually decreased in size to the top of the cycli~ unit, 

perhaps due. to a combination· of gradual cessation 

of crystallization and differential settling of the finer 

crystallizn.tion products. The crystallization had ended 

and all crystal products had reached the floor before 

olivine of the succeeding cycle began to precipitate. 

During the entire time of accumulation and primary 

crystallization of the cyclic unit the magma was stagnant, or nearly so. 

Once collected, most settled crystals continued to 

grow somewhat into the interstices of the mush and 

against their neighbors, the amount of secondary enlargement being largely a function of the rate of accumulation. This enlargement tended to produce a 

rigidity in the 1nush by welding grains together and, 

for the most part, prevented the squeezing out of the 

magma trapped in the remaining interstices of the 

mush. I-Iowever, the mush was not so rigid as entirely 

to prevent loss of porosity by compressional weight of 

overburden, inasmuch as voids caused by contraction 

during change from liquid to solid state have not been 

observed. The crystallization behavior of this trapped 

1iquid on cooling depended a great deal on the settled 

crystals with which it was choked: where it surrounded olivine, the magma attempted to make olivine 

over into bronzite, but it lacked the volume to accomplish this entirely and finally crystallized plagioclase 

and augite. Similarly in bronzitite, the·. magma on 

cooling reacted with bronzite to form augite and finally 

crystallized plagioclase, accompanied by a small 

amount of quartz in a few rocks. 

Two major problems remain in establishing the possibility of this simplified mode of origin: ( 1) to establish that bottom crystallization is physically reasonable, and (2) to derive a mechanism capable of causing 

repetitive cyclic·al deposition. 

BOTTOM CRYSTALLIZATION 

The nature of the layering suggests that crystallization of the primary precipitate of the Ultramafic zone 

occurred at the bottom, rather than the: top of the 

intrusion. Had the settled minerals formed at the top 

and settled directly through 5 or 6 miles. of magma, 

the rocks would exhibit features of extreme differential 

settling. Furthermore, Hess ( 1956, p. 449).. has shown 

that, at reasonable magma viscosities, the time of fall 

from roof· to floor is large, considering the probable 

solidification rate of the intrusion. Transportation of 

high-level crystallization products to the floor of the 

magma chamber by magma currents fails to explain 

the mineralogic, rather than size-density, sorting that 

occurs in individual layers and cycles. The most reasonable explanation for the observed textures and 

structures, therefore, would seem to be that the primary precipitate crystallized near the bo~tom of the 

intrusion and accumulated by simple cry~tal settling 

on the floor as rapidly as it formed. 

Many authors have postulated nonconvective cooling 

and crystal settling as probable or possible mechanisms 

for differentiation and layering in mafic sill-like intrusions (Tyrrell, 1916, p. 123-128; Bowen, 1919, p. 411-

423; Walker, 1940, p. 1083-1089; Edwards, 1942, p. 

603-604; and Cornwall, 1951, p. 190-193). Subsequent 

to initial chilling effects, cooling in nonconvecting, 

moderately thick horizontal bodies not in an infinite 

medium must be predominantly from the upper surface, where thermal gradients are increased by the 

proximity of the earth's surface, which is ·maintained 

at constant temperature by atmospheric processes. 

Lovering ( 1935, p. 86) has demonstrated· that, after 

an appreciable time, temperatures in the country rock 

below a tabular body may actually exceed those in the 

body itself. The major site of crystalliza~ion in such 

bodies depends on the relative magnitudes o·f the actual thermal gradient and the melting-poip.t gradient 

in the magma. Disregarding the early chill effects, 

crystallization in a stagnant magma may: ( 1) proceed 

inward from both the upper and lower surfaces at 

essentially equal rates providing the body is relatively 
94 STILLWATER GOMP'LEX, MONTANA 

thin or deeply buried (Jaeger, 1957, p. 311-312); (2) 

proceed in all parts of the magma simultaneously if 

the body has its upper surface at shallow depths and 

a compositional gradient is established (Cornwall, 

1951, p. 192); or (3) proceed predominantly downward from the top if the body is extremely thick. 7 

The heat relations and site of major crystallization 

subsequent to early chill effects are, however, considerably different in convecting magmas (Jaeger, 1957, 

p. 317). Wager and Deer (1939, p. 266-270) proposed 

a theory of convection circulation in the Skaergaard 

intrusion in which crystallization began in the upper 

part of the intrusion, owing to cooling, and continued 

during the descent in the convection current, owing to 

increase in pressure. The growing crystals contributed 

to the mean density of the magma, and, at the bottom 

of the continuous cycle, they spread out over the floor 

of the intrusion. This process has been illustrated 

diagrammatically by Wager ( 1953, p. 346) . Hess 

(1956, p. 449) has stated that he favors the same mechanism to explain the crystallization process in the 

Stillwater complex. 

Wager and Deer base their ideas of crystallization 

at depth in the magma on the assumption that, in a~ 

convecting magma, the melting point increases with 

depth faster than the actual temperature of the magma, 

and these authors ( 1939, p. 268-269) state: 

If the upper part of the Skaergaard magma is regarded as on 

the point of precipitating crystals of the primary phases, 

plagioclase, olivine and pyroxene, then the increase in pressure 

as the magma is carried down by the convective circulation 

should cause some crystallization, since the thermal gradient 

is about 0.3° centigrade per kilometre and the raising of the 

melting point for these minerals lies between 1 o and 5° centigrade per kilometre of descent. 

Wager and Deer's figure for the adiabatic gradient is 

derived from Jeffreys (1929, p. 138-139), and their 

melting-point gradient from measured or calculated 

data for the minerals forsterite, fayalite, diopside, and 

anorthite. 

The relative values of the adiabatic and meltingpoint gradients are extremely important in determining the site of crystallization in a convecting magma, 

and a review of their nature seems warranted here. 

The equation for the adiabatic gradient in a perfectly 

convecting magma may be derived from the first law 

of thermodynamics, and the solution requires a knowledge of the temperature, specific heat at constant pressure, and qoefficient of volume expansion of the magma. 

From this equation, Adams ( 1924, p. 462) determined 

the adiabatic gradient in a postulated molten mantle 

7 Hunter (1958, p. 2,0-24) has recently proposed· a mechanism 

whereby very hydrous magmas cooling by conduction would crystallize 

from the bottom upward. 

to be "something less than 1 °C. per kilometer of 

depth." Jeffreys (1929, p. 138), using in the same· 

equation the parameters: temperature 1400°K, specific 

heat 0.2 calories per gram -degree, and coefficient of 

volume expansion 2X10-5 per degree, determined the 

thermal gradient of the mantle to be about 0.3°C. per 

kilometer.. The application of Adams' and Jeffreys' 

values to the mantle as a whole have been criticized 

(Verhoogen, 1956, p. 30, 33-34), but the equation for 

adiabatic gradient and even the parameters used by 

Jeffreys would seem to express very adequately the 

thermal distribution in a convecting 1nagma in the 

upper levels of the crust. An expression for the melting-point gradient may be derived from the Gibbs free 

energy and Clausius-Clapeyron equations, and Jeffreys 

( 1952, p. 272), using fairly typical values for igneous 

rocks, calculated the gradient to be about 3°C per 

kilometer. ~{ore recently, Yoder ( 1952, p. 364-37 4) 

experimentally determined the change in melting point 

in diopside with pressure, and he, on the basis of this 

experimental work, estimated the gradient for basalt 

to be 10°C per 1000 bars, or about 3°C per kilometer. 

Thus, it would appear that in a convecting basalt sill, 

the increase of melting point of the basalt at depth 

would be about 10 times the increase in temperature. 

If these gradients are of the right order of magnitude, then Adams' (1924, p. 459-472) argument on 

solidification of the earth may be applied to solidification of the Stillwater magma; crystallization n1ust 

have begun at the bottom of the relatively thick sill, 

once the initial chill effects were overcome, and convection established on an adiabatic gradient. This 

relation is illustrated in figure 91, a diagrammatic 

representation of the heat relations in an initially 

molten sill 8 kilometers thick (the assumed initial 

thickness of the Stillwater magma). At t 1 it is assumed that the magma has established its adiabatic 

gradient above the melting-point gradient, and no 

crystallization occurs. As heat is lost from the top of 

the sill, the temperature of the magma decreases uniformly, and the adiabatic gradient is maintained. At 

t2 the temperature curve in the cooling magma intersects the melting-point temperature curve at the bottom of the sill, and crystallization begins, while the 

temperature at the top is still more than 20 degrees 

above the melting point. At t 3 , after considerable convective cooling and consolidation have taken place, the 

zone of crystallization has moved upward although 

remaining near the floor of the intrusion. In relatively thin sills, or in thick ones which have reduced 

their thickness by precipitation and building at the 

floor, these conditions may not obtain. At the chosen 

gradients, a sill 2 kilometers thick beginning to crys-
DEVELOPMENT OF THE ULTRAMAFIC ZONE 95 

TEMPERATURE, IN DEGREES CENTIGRADE 

1100 1105 1110 1115 1120 1125 1130 

0~------------------------------~~-----

rn 

ct: 

LIJ 

~6 

:E 

0 

...J 

;: 

z4 

rJ) 

rn 

LIJ 

~2 u 

i 

..... 

t, 

TEMPERATURE, IN DEGREES CENTIGRADE 

1100 1105 1110 1115 1120 1125 1130 

0 .• · :-:Lcini'OT crYST"alfizatro~. · ~ -. .-. · . -.. -. . · . . --=-:-=-. :-: 

f2 

TEMPERATURE, IN DEGREES CENTIGRADE 

1100 1105 1110 11 5 1120 1125 1130 

Fxounm 91.-Hypothettcal crystalltzatton behavior in a thick 

convecting sill. 

tallize would be only about 5 degrees above its melting 

point at the top, and if the gradients near the top are 

slightly superadiabatic, the cooling might cause crystallizations like those pictured by Wager and Deer, 

and IIess. Through the Ultramafic zone of the Stillwater complex, at least, the available evidence suggests that crystallization of the primary precipitate 

occurred entirely at the bottom if the magma was 

thoroughly mixed. 

The question remains whether convection did, in 

fact, exist in the Stillwater magma during crystallization and accumulation. The abundant current structures cited by Wager and Deer (1939, p. 262-266) as 

evidence for current deposition in the Skaergaard intrusion are for the most part lacking in the Ultramafic 

zone of the complex. A theoretical discussion of convection is hampered by lack of know ledge concerning 

'original shape and dimensions of the Stillwater 

magma chamber, but, ideally, a Newtonian fluid cooling predominantly from the top is unstable by reason 

of increased density of the surface layers and will 

convect, provided the viscosity is not so great as to 

prevent it. Scheidegger (1953, .P· 144) uses the product of the Grashoff and Prandtl numbers to predict 

convection in the mantle, and although there is some 

doubt as to the significance of this product in highly 

viscous materials, it should at least be more applicable 

to liquid magmas than to solid substances. Calculation of the Grashoff-Prandtl product for the Stillwater 

magma, assuming a cell diameter of 15 miles and a 

temperature difference of only one degree, shows that 

convection would not be damped unless the viscosity 

were about 3 X 1016 poises, a figure some 13 orders of 

magnitude greater than the expected viscosity. Inasmuch as any crystallization at the top of the intrusion 

would serve to increase the probability of .overturn, 

convection appears to be theoretically compelling. 

INTERMITTENT CRYSTALLIZATION 

An attempt has been made to. show· that the cyclic 

units are the fundamental building blocks of the 

Ultramafic zone, and that each cyclic unit represents · 

a single crystallization sequence that occurred near the 

bottom of the intrusion chamber. A brief and tentative discussion of mechanisms capable of producing the 

repetition of these crystallization sequences is necessary in order to complete a preliminary outline of 

origin for the lower portion of the complex. Two 

such mechanisms will be considered. The first, multiple intermittent injections of magma during crystal:-

lization, which was proposed by Lombaard ( 1934, p. 

40-52) to explain certain oscillations in the stratigraphy of the Bushveld complex, appears to have serious limitations in application to the Stillwater. The 

second, a process ·which involves continuous convection 

at the top and intermittent crystal~ization at the bottom during the consolidation of a single intrusion, is 

proposed by n1e, for it appears, at this stage of the 

investigation, to be the most probable mechanism to 

account for the cyclical distribution of compositional 

layers in the Ultramafic zone. 

INTERMITTENT MAGMA I~JECTION 

Lombaard's multiple injection hypothesis, which was 

later adopted by Kuschke (1939, p. 71-73) and Van 
96 STILLWATER COMPLEX, MONTANA 

der Walt ( 1941, p. 111), involved differentiation and 

remelting of successive crystal products in a subcrustal 

magma chamber, and injection of these liquids in 

pulses to form the Bushveld complex.8 This process 

was criticized by Turner and Verhoogen (1951, p. 223), 

who objected to the coincidence of preintrusive differentiation in an elusive, hidden chamber. Cooper (1936, 

p. 44-45) postulated repeated injections of original 

magma to account for repetitive stratigraphy in the 

Bay of Islands complex, and this seems a more reasonable process. . 

A superficially plausible explanation for cyclic variation in the Ultramafic zone of the Stillwater complex 

can be constructed using a mechanism similar to 

Cooper's. A portion of the magma was injected, 

formed the chill zone, began to convect, and crystallized olivine and then pyroxene at the base of the 

magma chamber, forming cyclic unit 1. A second 

pulse of the same magma was then injected into the· 

upper part of the chamber. The fresh magma supplied heat, stopped convection, and terminated crystallization ~t the base. After mixing, convection was reinaugurated, crystallization began once more, and cyclic unit 2 was deposited. This process was then repeated some 15 times during the accumulation of the 

Peridotite member. The injections thereupon ceased 

and allowed the remaining magma to crystallize bronzite alone until, with continued cooling, the mineral was 

joined by plagioclase. 

Hess' (1940, p. 377; 1941, p. 584) mineral variation 

curves for the Stillwater complex indicate that no 

fresh magma was injected into the Stillwater system 

subsequent to the first appearance of settled plagioclase at the top of the Ultramafic zone;--and the simple 

mineralogy of the Bronzitite member indic;ates that a 

similar situation prevailed. New influxes of magma, 

if such occurred at all, must therefore have ceased 

early in the history of the complex. The possibility 

that magma injection occurred during the accumulation of the Peridotite member cannot be eliminated, 

but several strong objections to the theory exist: ( 1) 

The initial injection of the complex was accompanied 

by emplacement of considerable amounts of iron-copper-nickel sulfides along the base, and there is no reason to suppose that subsequent injections would be 

lacking in similar amounts of sulfur, but no sulfide 

concentrations at the bases of the cyclic units have 

been observed. (2) Magma injections forming stratigraphically higher cyclic units would be emplaced 

after consolidation of the lower cyclic units, but no 

8 M(}re recently, Worst (1958, p. 334-345) has elaborated on Lorn· 

baard's theory and adopted it to ex·plain the. formation of the Great 

Dyke. 

intrusive disturbances of layers of the illtramafic zone 

have been observed. (3) The total olivine-bronzite 

ratio in the cyclic units of the Peridotite member 

changes slightly and gradually from bottom to top, 

so that Lombaard's coincidental theory of simultaneous differentiation in a hidden chamber must, to some 

extent, still be called upon. Brown's ( 1956, -p.-44=4.5) --

recent "reflux" theory, which involves not only successive emplacement of magma but concomitant extrusion, seems to account very satisfactorily for the 

rhythmic units at Rhum, but, when applied to. the 

cyclic units of the Stillwater complex, it is also subject 

to the three objections listed above. 

V ARIABLE·DEPTH CONVECTION 

In consideration of the objections to multiple injection as an explanation, it seems preferable to turn to 

a hypothesis based on single magma injection to account for the repetitive distribution of the cyclic units 

in the Peridotite member. A mechanism involving 

variable-depth convection and periodic refreshment of 

stagnant bottom magma undergoing crystallization 

seems to fit the observed facts, at least within the 

Ultramafic zone of the complex. A tentative outline 

of this mechanism will be made here, and a more detailed treatment .will be presented in a subsequent 

publication. 

It is presumed that the magma was injected within 

a relatively short period of time, and essentially as a 

single pulse, into relatively cool wall rocks. Conduction of heat by the wall rocks resulted in an initial 

depression of temperature in the magma at both top 

and bottom of the intrusion, producing fine-grained 

chilled gabbro at the margins. During this initial 

stage of crystallization, the magma presumably was 

relatively stagnant, and hotter in the central portion 

than at either top or bottom. At this stage, the rates 

of heat loss from the top and bottom were much the 

same, magma temperatures decreased nearly symmetrically, and supercooling resulted near both margins. 

At both the top and bottom of the chamber the magma 

was dense, not only because of the relatively lower 

temperatures obtaining, but because clouds of early 

precipitates began to form in the magma near the margins, which substantially increased the effective density. At the top of the intrusion, the magma was unstable-that is, its temperature gradient was superadiabatic-and convection commenced almost immediately. Free crystals that might have formed near 

the upper contact were re-fused when carried into the 

hot interior by convection, because the amount of 

magma in the sill was very large compared to the 

amount of crystallization at the top-at least at this 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 97 

C/) 

C/) 

LIJ 

z 

::.: 

u 

5: 

tChilled gabbro 

TEMPERATURE~ Chilled gabbro 

""'~!!'/. -...-.:/~" ~:0. o/;, 

Magma 

temperatures 

at t, 

Magma 

temperatures 

~<i'r~ at tJ 

0(? 

~ Continued accumulation 

'-.... ot bronzite crystals to 

"'- form Basal zone I bronzitite 

~:::=:==~·:~.~,E:E/ .. E:.~ ..... :~ ... :.· ... :·.~· .. ·=~····=} Chilled gabbro:; Pile of accumulated crystals) -

13 

C/) 

C/) 

LIJ 

z 

::.: 

u 

i 

tC/) 

C/) 

LIJ 

z 

::.: 

~ 

J: 

tTEMPERATURE __,_. Chilled gabbro 

""'~I!'/ ,-~., 

"-'<i':o. ....,;:/.;, Magma 

temperatures "<i'r~ at t, "-.o;C?_,, " Precipitation of bronz· 

'-.... ite crystals and 

"'-, accumulation as Basal 

"'- zone bronzitite 7 ~·. ~ · ... zo~e· cit ~rystaliiz~tion ~.nd settli~g· .·: · ~· .~} .. : . ·H: : . . : ~ : . ~ . : , • ~ . . 6. . . . ... . . . . . . . . .. I 

C~iUed. gabbro> . : ~ile of accumulated crystals/

2 

• 

""'~1!1/1.. ~<i':o. 

TEMPERATURE._,__. 

Chilled gabbro 

Magma 

temperatures 

at '• 

o/.; 

~ ... 

~$. 

~ Precipitation of olivine 

"'-"' crystals and accumu· 

"'- lation as lower part of 

"'- cyclic unit 1, Peridotite 

member 

-:-. -.-.... -... -.-.. ~· :-:-=-: '· .. "--;-:--: -. -.. -. -... -.-.. 1 · ·. · ·. : : : · .. · · ·. Zone of ~rys!al~1za~1<?n. a~d-·~ettl1ng : :: :. · .. •. : : :: · · .. J ........ ·. . . . . \ .>, .... - ...... } 

~d gabbroj Pile of accumulated crystals/ 

40 

Basal zone7 

t, 

FIGURE 92.-D~agrammatlc 1llustrations of proposed mechanism for crystallization in the lower part of the Stillwater complex. 

early stage of the history of the intrusion. Near the 

base of the intrusion, however, the situation was considerably different. Relatively cool, crystal-charged 

magma near the bottom was stable, and its crystal 

products were preserved by settling to the floor of the 

chamber. Furthermore, the amount of supercooling 

and the amount of crystal precipitation near the bot-· 

tom was greater than that near the top, by reason of 

the slope of the melting-point curve. A schematic diagram of temperature relations hypothetically existing 

at this time ( t1 ) is shown on fig~1re 92. 

On the basis of the foregoing discussion it is therefore presumed: (1) that convective overturn occurred 

in the upper portion of the intrusion; (2) that the 

convective cycle extended to considerable depths in 

the intrusion; and ( 3) that magma near the bottom 

of the chamber did not join in this circulation. 

Convection in the upper part of the magma would 

have established a true adiabatic temperature gradient there. Convective overturn thus would have 

raised the temperature of the magma near the upper 

contact and maintained it at high values. High temperatures resulted in ( 1) a high rate of conductive 

heat loss at the top and ( 2) virtual cessation of crystallization there, because the adiabatic magma temperature exceeded the melting-point temperature for 

pressures at that depth (fig. 92, t 2 ). During this period, heat lost through the upper contact was supplied 

by a uniform decrease in the temperature of the convecting portion of the magma. 

Crystallization in the lower stagnant magma would 

continue at gradually increasing temperatures (fig. 

92, t 2 ) as the originally supercooled magma was 

warmed by latent heat of crystallization until a twophase crystal-liquid equilibrium condition was reached 

(fig. 92, t3 ). The crystal products of this process settled to form the Basal zone bronzitites.9 

During this period, heat loss through the lower contact was supplied locally by latent heat of crystallization in the stable bottom layer. Presumably, an equilibrium state would have been approached in the bot-· 

tom layer, the temperature would have followed the 

pressure-melting relation, and the rate of crystallization would have been just sufficient to supply the conductive heat lost through the bottom. As the rate of 

such conductive heat loss through the floor of the intrusion diminished, and as latent heat supply produced 

by crystallization of the interstitial magma in the accumulating crystal mush became appreciable, the 

o Preliminary Investigations of the composition of these layered 

bronzltltes show the predicted relation : higher temperature. phases 

occur upward In the section. 
98 STILLWATER COMPLEX, MONTANA 

·amount of crystallization of primary precipitate :from 

the stable bottom layer would have· decreased, with 

attendant decrease in density::. of the layer. At the 

same time, the temperatures of the magma in the bottom layer would have risen to and have been maintained at the melting-point curve by supply of latent 

heat of crystallization. The original temperature gradient in the lower stagnant layer would at this time 

have been reversed (fig. 92, t3 ) • 

Hypothetically then, at the stage in the cooling history numbered t 3 in figure 92, temperatures in the upper convecting portion of the magma were determined 

by the true adiabatic gradient and the progressively 

decreasing upper-contact temperatures, and were still 

greater than melting-point temperatures in that part 

of the sill. Temperatures in the lower, stagnant portion, on the other hand, had been raised to or slightly 

above the melting point at depth, and ',crystallization 

had virtually stopped. The melting-point gradient 

is superadiabatic, and conditions were there:fore favorable for the lower, stagnant magma to join in the convective overturn; but owing to the high density of the 

crystal-laden magma, this could not be accomplished 

until the residual suspended crystals in the lower layer 

had settled out. As this residual settling progressed, 

the circulation of the upper convecting portion of the 

magma would penetrate to progressively increasing 

depths, until the bottom layer of magma, partially 

fractionated by its crystallization of early mineral 

phases, was completely incorporated into the convective system. 

When the heretofore stagnant lower layer did, eventually, gradually join the upper convecting system, 

fresh, relatively cool magma would have been brought 

to the bottom as an adiabatic gradient was established 

there (fig. 92, t 4 ). This would reinitiate crystallization at the bottom in that portion of the fresh magma 

brought below the melting-point temperatures. A 

cloud of crystalline precipitate would form, again increasing the density of the bottom layer and causing 

it to stagnate while convective circulation was maintained in the upper part of the intrusion. Due to the 

decreased thermal gradient in the rocks below the floor 

of the sill, this second stagnant magma was not supercooled to the same extent as the magma that produced 

the bronzitites of the Basal zone, and the initial depression of temperatures below the melting point was 

caused by overshoot rather than rapid conduction. 

First olivine, then chromite, then bronzite would have 

been precipitated and successively dropped to form 

cyclic unit 1 of the Ultramafic zone. Crystallization 

would virtually stop- when the temperature of the bottom layer was raised uniformly to the melting point 

by latent heat of crystallization, but the bottom layer 

'vould not again join the circulation until its density 

was reduced by crystal settling. This process is presumed to have been repeated, with the crystallization 

and deposition of the stratigraphically higher cyclic 

units from successive stagnant magma layers, each 

considerably thicker than the pile of crystals precipitated from it. The transition to the stagnant state of 

the bottom layer would not be expected to take place 

under conditions of thermodynamic equilibrium, owing 

to inertia of the deeply convecting system and supercooling prior to nucleation. Thus, layers of finite 

thickness, as postulated, might be expected to have 

been brought below the melting-point temperature 

before the increased density due to crystal precipitation brought about stagnation. 

A variation of the above hypothesis, which appears 

less probable at this time, concerns intermittent rather 

than variable-depth convection. It seems possible that 

crystallization in the lower layer might have stagnated 

the entire magma column during precipitation and settling. In such a case, adiabatic temperature gradients 

would not have been maintained in the upper portion 

of the intrusion, and magma near the top would have 

been conductively cooled to a considerable extent. 

After cessation of crystallization and settling in the 

bottom layer, the unstable column would have overturned one-half cycle, at which time the cool magma 

from the top would have reached the region of higher 

melting point at the bottom and crystallization would 

have once more stabilized the magma. Intermittent 

convection would require a much longer cooling time 

for the intrusion as a whole than would variable-depth 

convection, and a considerably thicker upper chill zone 

would be expected. 

In the favored hypothesis, the cyclic units are pictured as depositional products of periodically refreshed stagnant magma which became stabilized by 

bottom . crystallization. Because the bottom magma 

during each crystallization cycle was stagnant, it became enriched in silica as crystallization proceeded, 

and the early chromite and olivine were followed by 

bronzite. Each overturn brought a fresh supply of 

magma to the bottom and the cycle was repeated; but 

after such selective crystallization of a number of 

overturns, the composition of the entire magma was 

gradually but appreciably changed, so that higher 

.cycles in the Peridotite member systematically contained smaller proportions of olivine. In this manner 

olivine· and chromite finally ceased altogether to be 

crystallization products, and the magma crystallized 

bronzite alone in the Bronzitite member, and bronzite 

plus plagioclase at the base of the norite section above. 
DEVELOPMENT OF THE ULTRAMAFIC ZONE 99 

An important assumption made in developing the 

foregoing theory is that crystallization at the top of 

the Stillwater magma was minor during the formation 

of the Ultramafic and Basal zones, and that crystal 

phases forming at the top did not reach the floor of 

the intrusion and did not become incorporated in the 

layered rocks. Two points may be made in support of 

this assumption: ( 1) it seems theoretically probable, 

and (2) it supports the field relations. Theoretically, 

it must be admitted that the maintainance of singlephase adiabatic temperatures up to the roof of the 

intrusion as illustrated in figure 92-tz, ts, and t4-

seems very unlikely. Not only would extremely rapid 

convection be required, but the marginal rocks would 

begin to melt. No doubt magma temperatures near 

the roof decreased markedly to the melting point, and 

some free crystals were produced there while the rocks 

of the Basal and Ultramafic zones were accumulating 

at the bottom. The fate of such crystals would be remelting, as long as magma above the melting-point 

gradient remained in the center of the intrusion. Preliminary considerations of the probable heat-flow relations indicate that considerably more crystallization 

took place at the bottom of the intrusion than at the 

top during the early cooling history. After all of the 

Ultramafic zone rocks had aecumulated, some 6 to 7 

km of magma still remained in the intrusion; and, 

even if the assumption is made that equal amounts of 

crystals were produced at both margins, it seems un-:-

likely that the magma temperatures would have been 

driven down to the melting-point gradient by this 

time. 

It must be emphasized, however, that as the crystal 

pile increased in thickness above the Ultramafic zone 

and as magma temperatures decreased during the continued fractionation and cooling of the complex, crystallization from the top must have become increasingly 

important. The amount of specific heat available for 

remelting crystals would progressively decrease. The 

magma temperatures would approach and maintain 

the melting-point gradient for whatever mineral phases 

were precipitating at that time, and a two-phase crystal-liquid equilibrium would result. Under these conditions, the temperature gradient would be superadiabatic, and the whole magma would have been able to 

overturn, precipitating crystals throughout. Crystals 

which began to grow at all levels in the magma would 

be carried to the bottom and deposited. This is essentially the mechanism proposed by Wager and Deer 

(1939, p. 266-270) to account for the observed relations 

in the Skaergaard intrusion, and adopted by Hess 

(1956, p. 449) to explain the origin of the Stillwater 

complex. 

The theory of variable-depth convection and bottom 

crystallization as outlined here appears capable of accounting for the repetitious stratigraphy, general absence of current features, compositional rather than 

size-density layering, absence of hydraulic equivalence, 

and size distributions and other previously described 

features of the Ultramafic zone. The theory of continuous convection and crystallization appears capable 

of explaining layered igneous rocks characterized by 

current structures, size-density layering, lineation of 

primary precipitate mineral axes, and continuous differentiation toward lower temperature mineral phases. 

The two theories are not mutually exclusive, and it 

seems likely that conditions of variable-depth convection, bottom crystallization, and magma refreshment 

in the lower part of the Stillwater complex gave way 

to continuous convection and deposition in the upper 

part as the magma volume and, particularly, the 

magma thickness diminished. 

SIMILARITY TO CHEMICAL SEDIMENTS 

Several authors, particularly Wager and Deer ( 1939, 

p. 271) and Turner and Verhoogen (1951, p. 236) have 

compared the origin of rocks of layered intrusions to 

that of detrital sediments. Thus, Wager and Deer, 

who offer an explanation of origin of layers by what 

might be called· igneous density currents, state: 

There is a close analogy between the structural features of the 

layered series and of sedimentary rocks. The igneous .lamination of the layered series is similar to the lamination. of a 

micaceous silt; in both cases it is due to the lamellar form of 

the material deposited. The unlaminated material of melano~ 

cratic bands in the layered series, like sandstone is composed 

of fragments with a general rounded shape and in both there 

is no fissility .... In other ways the rhythmic layering is comparable with rhythmic variation of sandstones and shales, the 

light bands of the layered series being comparable with the 

shale because they represent material carried by feeble currents, while the dark bands are comparable with sandstone 

composed of heavy grains only transported by vigorous 

currents. 

Brown (1956, p. 44-45) has, however, suggested that 

the layered ultrabasic rocks of Rhum originated in a 

·manner analagous to evaporites, and I propose the 

same comparison for the rocks of the Ultramafic zone. 

The Stillwater ultramafics and evaporites appear 

to have the following common features: (1) both have 

a bedded distribution acquired by crystallization and 

settling of primary precipitates from a saturated solution; (2) both are constructed of compositional layers, 

commonly monomineralic or bimineralic, which are derived by fractional crystallization 10 (Pettijohn, 1957, 

10 To ~ knowledge, no one has as yet proposed halite or anhydrite 

magmas. 
100 STILLWATER COMPLEX, MONTANA 

p. 428, 484); (3) in both, the compositional layers are 

commonly repeated in a cyclic fashion (Stewart, 1951, 

p. 446, 460; Schaller and Henderson, 1932, p. 8) ; and 

( 4) primary settled grains within the layers are single 

crystals, although more material of the same composition may be added as crystallographically oriented 

overgrowths after deposition (Dellwig, 1955, p. 88-

93). To account for saline deposits and also the lower 

part of the Stillwater complex, appeal must be made 

to physico-chemical rather than mechanical processes 

to explain the composition and distribution of the 

rocks. 

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~Jones, W. H.., Peoples, J. W., and Howland, A. L., 1960, Igneous 

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Krumbein, W. C., 1935, Thin-section mechnnical analysis of indurnted sediments: Jour. Geology, v. 43, p. 482-496. 

---1938, Size frequency distributions of sediments and the 

normal phi curve: Jour. Sed. Petrology, v. 8, p. 84-90. 

---1H39, Preferred orientation of pebbles_ in sedimentary 

deposits: Jour. Geology, v. 47, p. 673-706. 

---1942, Flood deposits of Arroyo Seco, Los Angeles County, 

California: Geol. Soc. America Bull., v. 53, p. 1355-1402. 

Kuenen, P. H., 1951, Properties of turbidity currents of high 

density : Soc. Econ. Paleontologists and Mineralogists, Special Pub. 2, p. 14-33. 

Kuschke, G. S. J., 1939, The critical zone of the Bushveld 

igneous comple;x, Lydenburg District: Geol. Soc. South 

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Lombaard, B. V., 1934, On the differentiation and relationships 

of the rocl{S of the Bushveld complex : Geol. Soc. South 

Africa 'l'rans., v. 37, p. 5-52. 

Loveland, R. P., and Trivelli, A. P. H., 1927, Mathematical 

methods of frequency analysis of size of particles; pt. 1-

Methods: Franldin Inst. Jour., v. 204, p. 193-217. 

J..~overing, T. S., 1935, Theory of heat conduction applied to 

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69-94. 

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/ H.ept., v. 1, p. 353-360. 

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371-416. 

I Peoples, J. W., Howland, A. L., Jones, W. R., and Flint, Delos, 

1954, Geologic map, sections, and map of underground 

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1780. 

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drill cores from the potash field of New Mexico and Texas: 

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in peridotites, with special reference to New Zealand 

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Meddelelser, Bd. 12, p. 335-349. 

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in East Greenland ; pt. 3--The petrology of the Skaergaard 

intrusion, Kangerdlugssuaq, East Greenland: Meddelelser 

om Gryjnland, v. 105, no. 4, 335 p. 

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1059-1106. 
102 STILLWATER COMPLEX, MONTANlA 

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Walt, C. F. J. van der, 1941, Chrome ores of the western 

Bushveld complex: Geol. Soc. South Africa Trans., v. 44, 

p. 79-112. 

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Mineralog. Mag., v. 29, p. 913-924. 

v!'Wimmler, N. L., 1948, Investigation of chromite deposits of 

the Stillwater complex, Stillwater and Sweetgrass Counties, Montana: U.S. Bur. Mines Rept. Inv. 4368, 41 p. 

Winchell, Horace, 1958, The composition and physical properties of garnet : Am. Mineralogist, v. 43, p. 595--600. 

Worst, B. G., 1958, The differentiation and structure of the 

Great Dyke of Southern Rhodesia: Geol. Soc. South-Africa 

Trans. and Proc., v. 61, p. 283-354. 

Yoder, H. S., Jr., 1950, Stability relations of grossularite: 

Jour. Geology, v. 58, p. 221-253. 

---1952, Change of melting point of diopside with pressure: 

Jour. Geology, v. 60, p. 364-374. 

Yoder, H. S., Jr., and Tilley, C. E., 1956, Natural tholeiite 

basalt-water system: Carnegie Inst. Wash. Year Book 55, 

p. 169-171. 

--- 1957, Basalt magmas: Carnegie Inst. Wash. Year Book 

56, p. 156-161. 

NoTE: While this publication was in press, the following important paper was published: "The Stillwater Igneous 

Complex, Montana, A Quantitative Mineralogical Study": Geol. Soc. America Memoir 80, 230 p., 1960, by H. H. Hess. 

Although it is unfortunate that referenre to Hess's memoir could not be made in this publication, many of his ideas had 

been outlined in earlier papers that are heavily ctted here. 
INDEX 

A .h~ 

Anorthosite of Banded zone. ___ ------------------------------- __ -----------__ 4 

Augite, as Interstitial constituent.-------------------------- 11, 66, 69, 71, 72, 73, 76-78 

as settled constituent.-------------------------------------------_________ 82 

composition In Ultramafic zone.------------------------------------------ 5 

oriented Intergrowths ln. _____ ------------------------------- _____ -------_ 76 

porcontago of Interstitial materiaL---------------------------------------- 89 

reaction textures with bronzite and olivine .... ---------------------------- 47-52 

zoning or.---------------------------------------------------------------- 76-77 

Augite olkocrysts, shape •. ---~------------------------------------------------ 66-67 

Automorphic toxture, development ... ---- _______ ----------------- ____________ 79-80 

B 

Banded zone, anorthosite .•• __ ----- __ ------------------------------ __________ _ gabbro .••.• ____ --_ •. _ ..• __________ ._______________________________________ 4 

norlte •.•.. -----.-------•. -.----.--•. --------------------------------------

textures._ ... -.• -•• -.-.-.-.. ------.-------.-------.------- ~--------------- thickness .• __ . __ ._ •.•.• ___ . ____ . __ ._ •• ___ . ____________________________ .___ 4 

Basal zone, chilled gabbrO.------------------------------------------ 14, 17-18,21, 79 

distribution of settled minerals. ___ --------------------------------------- 14, 82 

general geology __ ---------------------------------·-------·- __ .. __________ 2-4 

interstitial constituents .•.• ---------------· __ -----.... ------- ______ ------_ 73 

rate of accumulation .. ----------------------- _______ ---------_____________ 62 

secondary enlargement in _______ --------------------- __ ------- __ ---------_ 61 

textures .... -·--.•. --•. ------·---------------------------- __ ----- _________ 14, 79 thickness ...•••••. _ .• --_._._. ___ • __ •.. ________ ._._________________________ 2 

Bay of Islands complex _____________ ------------.. -------------------------___ 96 

Bedding, size-graded .... -------------------------------------------------. ____ 21-23 

Biotite, as Interstitial constituent .. ---------------------------------- 66, 69, 71, 72, 73 

Biotite olkocrysts, shape .... -----------·------- _____ ---------------- __________ 66, 67 

Bottom crystallization, as condition of deposition.---------------------------- 93 

relation to convective and nonconvective cooling _____ ----------------- 83, 93-94 

Bowen, N. L., quoted.-------------·-------·--------------------------------- 92 

Bronzite, as Interstitial constituent ... ----·-·---------------·----------- 66,69, 71-78 

as settled constituent of Basal zone._---------------------------·--------- 14 

as settled constituent of Ultramafic zone---------------------------- 14-15,20-42 

compositional range in Ultramafic zone.---------------------------------- 4 

interprecipltate, oriented intergrowths in ... ------------------------------ 74-76 

percentage of Interstitial materiaL------------------------------------ 89 

reaction textures with olivine.---------------------------------------- 47,52 

zoning ln .• ---------------------------------------------- __ ----------- 76-77 

settled crystals, enlargement .. -------------------------------------------- 54 

Inclusions in ________ -------------------------- _____ ----------_________ 17 

oriented Intergrowths in.------------------------------·----------- 18,74-76 

petrofabric analyses ________ --------_------------- ___ --------- _________ 39-42 

reaction textures with augite ... ---·-------------------------------- 47, 50,52 

shape ... -------.------------------------------------------------------ 13 

size-density relations ......• __ ----------------------- __ ---------------_ 31-34 

size variations, cyclic ________ -------------- ___ ------- ________ -------__ 23 

size variations, overalL ... -----------------------____________________ 26 

Bronzlto oikocrysts, orientation ______________ .. _____ ------- ______ ---------____ 79 shape .• __ .. ____________________ .__________________________________________ 66-67 

stratigraphic size distribution .... ------ ___ -------------------------------- 77 

Bronzitite, coexistence or settled minerals in ... ------------------------------- 86-88 definition .... ---------------- __ ------- _________ ·___________________________ 5, 91 

development of texture._------------------------------------------------- 79-81 

dist•·ibutlon of interprecipitate materiaL.-------------------------------- 71-72 

distribution of settled minerals. _________ ------~-------------"--__________ 14 

initial porosity------------------- __ --------------- ________________________ 62-63 

intcrprecipitate material in __ -------- __ ------ _____ ------------- ___________ 88-89 

of Basal zone._ .. __ ----------------------------- ___ -------------- __ ------_ 14 

of Bronzitite member, Ultramafic zone ... --------------------------------- 14 

of Peridotite member, Ultramafic zone .. ---------------------------------- 14 

orientation of settled crystals in .... ------------------------------------ 39,41-42 

reaction replacement ln.-----------------------·--------------------------- 50,52 

relative abundance in Ultramafic zone------------------------------------ 86 

secondary enlargement in ... ---------------------------·-------------_____ 54 

Bronzitite mom ber, composition ___________________ -------------------------__ 4 

cyclic units in .. _------···-------------·---------------------------------- 91-92 

Bronzitite member-Continued Page defined.---. __ -____ . __ -._-- ___________________________ .. ______________ .___ 4 

distribution of interstitial constituents in.-------------------------------- 73 

distribution of settled crystals in .... -------------~------------------------ 14 

grain-size variation in cyclic units ..... ------------------------------------ 26 

secondary enlargement in ________________ --·---------- __ ------------------ 6Q-61 

textures ____ --------------------------------------------- _______________ 4, 79-81 

Bushveld complex, chain texture in ... ---------------------------------------- 34 

dimensional fabrics oL ... ------- ___________ ------------------ _____ ------- 46 

Merensky Reer. _________ --------------------------- __________ -----·-- _ _ _ _ 46 

multiple injection of magma.-----------------·----·-------·-------··----- 95-96 

pseudoporphyritic poikilltic texture in .... -------------------------·------ 54 

c 

Chain texture, origin. __ --· __ ------------- ___ -----·---_ ..••. ----·--------- _____ 34-35 

Chemical sediments, similarity to layered intrusive rocks.-----·-------------- 99 

Chilled gabbro, as part of Basal zone ..... ------------------------------------ 14 grain size ... _. __ .. __ • __ • __ ._._. __ •. _. __ ..•.. _._ ..•. __ . __ . __ ...• _._ •. _ .• _.. 21 

hypersthene oC..----------- __ -------------"· -------- _____ ··-·-·-----·-·--- 17-18 

ophitic texture oL ___________________ ------ ___ --··------··-.•• _ .•••.• ---- 79 

Chromian augite. See Augite. 

Chromite, as settled constituent of Ultramafic zone ... ------------------ 14-15, 20-42 

compositional range in Ultramafic zone.-------------------------------·-· 5 

settled crystals, enlargement ..•. _--------------- ___ ----------------------- 54, 58 inclusions in ... ____ •.•. _. ____ •. _-•. ---.. _ ••..•• ___ ••.• __ ._. __ . __ .•• _._ 15 

shape ___ ... _ .. _____ -._- ____ ._ •••• __ .•• ______ ••• _. __ .• -. _______ . ___ ... _ 13 

size-density relations .. ___ ... ----------------------------------------_ 3Q-35 

size variation, in layers .. ---------------------------------------------- 23,26 overalL .••.•.. _. ____ •.•.. --~- ...•.. ____ • __ ..•... -- _______ • _______ • 26 

Chromitite, coexistence of settled minerals··in. _ -·--·-------------------------- 86-88 

definition .. ----·------------·----------·----·------------·- ____ ---_----.. 5, 8, 91 

development of texture. _____ ----------·---_----------------------_----- __ 79, 80 

distribution of interprecipitate materiaL.-------------------------------· 72-73 

distribution of settled minerals ____ -------------------"·--·-·-·--·--·--·-. 14 

initial porosity ____ ------------·---------------· ___ ··-·--·--···.--···----- 63 

interprecipitate material in ____ -------------------·--------------------. __ 88-90 

secondary enlargement in ... ---------------------------------------·-----. 54, 58 

textural types. ___ ----------------------------- ___ ---------------_________ 54 

Clinopyroxene, as oriented intergrowths in bronzite __________________________ 18,74 

Classification of Stillwater rocks by proportions of settled minerals. ____ . __ ._.__ 9Q-91 

Compression, postdepositionaL ... ------- __ .--------------------------------- 46-47 · 

Convection currents, evidence of. ___ -------------------------- 29-30,35,46,82-83,95 

mechanism for Skaergaard layering_---------------------------··--------- 35,94 

role in origin of Ultramafic zone. ___ ---------------------------------·-·-- 95-99 

Crystal accumulation, mechanism at Skaergaard complex. ____ --------------- 35,94 

mechanism at Stillwater complex _______________________ 14-15,35-37,45-47,82-85 

relation of rate to secondary enlargement._------------------------------- 62 

Crystal growth, deductions from shapes .... ---------------------------------- 13 

relation to lognormal size distributions .... --------------·-·--------------- 35,37 

Crystal mush, initial porosity ____ ----------------·------------------- 58, 60,62-65,85 

laminar flow, as mechanism of dimensional fabric.------------------------ 46 

planar structure. __ -------------- __ .--------------------------------- __ 11, 45-47 

relation of reaction replacement to porosity ... ---------------------------- 52 

self-supporting character _______ .. ------------·----------._________________ 11 

Crystal shape, modified by reaction replacement. __ .---------------------- 47, 50, 52 

modified by secondary enlargement ... ------------------------------------ 52,54 

settled crystals ____________ -----------------------------------·.---------__ 13 

Crystallization, in closed system .. -------------------------------·------------ 15 

Crystallization and deposition conditions, depositionaL---------------------- 82-85 

postdepositionaL_----- __ ----------·-----------------.-------------------- 85-86 predepositionaL ______ . __ . __________ ...... _. __ .• _. _. __ ..• ____ •. _. __ .. _. _. _ 81-82 

Current structures ... ------ __ .---------·- __ ----·--------------------_ 29-30, 35, 82-83 

Cusp texture, as evidence against laminar mush flow ... ---------------------- 46 

proposal of term._-------.-----------... -------- ___ --------_-------------- 23 

Cyclic units, description oL _ -----------------------------------------------·- 14 

grain-size variation .. __________ -------.. ---------- ___ ----- ______ -----_____ 23 

interstitial constituents .•. --------------------·----- _______ .. ___ ---·------ 73 

order of accumulation. _____ .-------·-··----- _______ .•• __ ·--·------ ___ .___ 15 

103 
104 INDEX 

Cyclic units-Continued Page 

origin of grain-size variation. __ -----------------------------------------__ 37 

proposed theory of bottom crystallization and variable-depth convection .. 96-99 

reaction relations._-------------------------------------------------- __ :___ 92 

significance and origin_--------------------------------------------------- 91-93 

D 

Deer, W. A., quoted .... ------------------------------------------------ 11, 61, 94,99 

Density, Stillwater magma.----------------------------------------------- ___ 31, 81 

Deposition, conditions ___ ---------------------~---- __ ------------------------_ 81 

settling rates ____ ------------------------_-------------------------------- 82-83 

Depositional conditions, agency _______ --------------------------------------_ 82-83 

bottom crystallization._----------------------------------------------- __ . 83 

crystallization near chamber floor ... -------------------------------------- 85 

current-free magma •.• --._ .. _-----.--._-._--.--.--_.--.---.. _ •. __________ ._ 82-85 

lateral continuity--------------------------------------------------------- 82 

supply-------------------------------------------------------------------_ 83 

textural, evidence of. _____ ------------------------------- __ ----------- ____ 82-85 

Depositional cycles, chemical aspects ... -------------------------------------- 92 

history of the cycle ...... -------------------------------------------------- 92 significance._._. ___________ • __ ._.-.• -_ .. __ .-_ .• _ .• _.- __ • __ .. __ . _______ .___ 91 

summary of salient features_______________________________________________ 91 

Differentiation and settling in Ultramafic zone, mechanisms ________________ 93-100 

Diffusion mechanism, of Hess .. _--------------------------------------- 61, 62, 80,85 

Dunite, compositwnal zoning of constituents._. ___ --------------------------- 76 

definition ____ ----- __ --------------_-----------.-------------------------- 5. 8, 91 

development of texture .. ------------------------------------------------. 80 

of Peridotite member----------------------------------------------------- 18 

secondary ____ ---------~----------------------------------------------- •• 5, 8, 18 

secondary enlargement in, textural evidence------------------------------ 54 

E 

Exsolutlon, of clinopyroxene tn interprecipitate bronzite_ --------------------- 74,76 

of clinopyroxene in settled bronzite_______________________________________ 18 

of orthopyroxene in interprecipitate augite.------------------------------- 74,76 

F 

Fabric analysis ___ --------------------_------------------------------------- __ 20--47 Fieldwork. _______________ ----. __ --- __ ---- __________________ • ____ • ______ ._____ 2 

Fluid movement, relation to crystal distribution _____________________________ _ 

Fractures, in settled crystals----------~---------------------------------------

G 

35 

18 

Gabbro, of Banded zone •• --------~-----------------~------------------------- 4 

Gabbro pegmatites. ______ ---------------------------------------------------- 8 

General geology of the complex ..• ------------------------------------------__ 2--4 

Grain size, poikilitlc crystals-------------------------------------------------- 77 

settled crystals. __ - __ ------------------------------------- __ ------------ __ 20-37 

lateral variation._---------------------------------------- ___ ------- __ 26-28 

sorting characteristics ... _--------------------------------------------- 28-30 

vertical variation.-------------------------------------------------- __ 26-28 

Grain size characteristics, comparison with those of sediments._-------------- 20-21 

relation to crystal accumulation. ___ -----------------------_------ ________ 35-37 

Grain size distribution, poikilitic crystals------------------------------------- 77-79 

settled crystals ... ____ ------------------------------------------------ ____ 28-30 Granular harzburgite, coexistence of settled minerals in _______________________ 86-88 . defin1tion ..• _ _ _ __ __ _ _ _ __ _ _ _ __ _ _ _ _ _ _ _ _ __ _ __ _ _ _ _ _ _ _ ___ _ __ __ __ _ __ __ _ _ _ ___ _ _ __ 5, 91 

development of texture. __ ------------------------------------------------ 80 

distribution of interstitial materiaL·------------------------------------- 71 

distribution of settled minerals .. ----------------------------------------- 14 

initial porosity_ ... _------ __ .------------------------------------------- __ 63-64 

interprccipitate material in.-------- ______ ------ ____ ------------------ ____ 88-90 

reaction replacement in _______ ----------------- _________________ ---------- 47, 50 

relative abundance in Ultramafic zone. ____ . _______ ---------------------- 86 

secondary enlargement in ____ --------------------------------------------- 58 Ora vity stratification ____ . _________ ._----_____________________________________ 35 

Grossularite-pyrope, as interstitial constituent of bronzitite. ________________ 5, 71-74 

significance. ___ .•. _____________ . _____ . __ . __ . ______ ._______________________ 86 

H 

Harzburgite, definition. _________ ------------ ______ ---------__________________ 5, 8 

See also Granular harzburgite; Poikilitic harzburgite. 

Heat relations, during crystallization of ultramafic zone.--------------------- 93-99 

Hydraulic equivalence, relation to crystal accumulation ______________ ~------- 3o-35 

significance ____________ ------------------------------------------------ 82, 83, 85 

Hypautomorphic texture, development·-------------------------------------- 80 

Hypersthene crystals, oriented intergrowths in ___ ---------------------------- 17-18 

Igneous lamination, of Wager and Deer--------------------------------------- 46,83 

Inclusions, in interprecipitate materiaL-------------------------------------- 74 

in settled minerals ______________________________ -------------------------- 15-17 

Page 

Initial porosity, causes of variation. __ ---------------------------------------- 64-65 

for rock types----------------------------------------------------------- __ 62-64 

Intermittent crystallization, as mechanism for cyclic deposition __ ------------- 95 

Interprecipitate magma, differentation .. ---- __ --- _ ------ __ --- _ -~-- _ ----. ____ • __ 85-86 

final consolidation temperature. ______________ ---------------------------- 86 

order of crystallization ________________________ -------------------------- __ 73-74 

Intermittent magma injection __ -------------------~-------------------------- 95-96 

Interprecipltate material, chemical composition--.------------------------- 11,88-90 

crystallization. ___ ---------_--- ___ ----- ____ -- _____________________ --- __ 86, 88-90 

definition. ________ ---_---- ____ --_----------- __ --------_----_---- ________ ._ 11, 65 

distribution. __ ----------------------------------------------------------- 67-74 

in bronzitite ______ ---------------------------------------------------~ 71-72 

in chromitite. _ ----------------~------------------------------ -------- 72 

in granular harzburgite. ---------------------------------------- ____ __ 71 

in olivine chromitite ________ ------------------------------------------ 72-73 

in poikilitic chromite harzburgite·--------------------------------- 69,72-73 

stratigraphic .. ~-_--- _________ •• ____ ----_. _____ • _______ -----___________ 73 

internal character ________ ------------.--- __________________ • ____ -----_____ 74-77 

mineral associations_-___ -------------------------------------------------- 88-89 

poikilttic crystal size and size distribution ___ ----------------------------- 77-79 

proportion in layered rocks------------------------------------------ 4, 11,62-65 

shape ..•• _________ • __ • ______________ .----- ____ .----_-- __ • _____ --.---______ 65-67 

volume. ____________________________ ---- ___ ---- _______ ----_. __ .________ 11, 62-65 

genetic implications. __ -------------------------------~---------------- 11, 85-86 

Interstitial, definition of term·------------------------------------------------ 11,65 

Interstitial grains, of interprecipitate materiaL-------------------------------- 11, 65 

Interstitial material, amount in correlative subunits.------------------------- 61 

anhedral grains __________ -------------------------------------------- __ 11, 65-67 

frequency distribution of volume----------------------------------------- 58,60 

relation to stratigraphic thickness----------------------------------------- 62 

shape of interstices ... --------------------------------------------------- __ 65-66 

Interstitial minerals._-------------------------------------------------------- 11 

distribution._.--------------------------------------------------------- __ 67-73 

ratios in rocks of zone·---------------------------------------------------- 89-90 

relation to character of settled minerals----------------------------------- 73 

stratigraphic distribution. ___ ----------. ____________ ------________________ 73 

See also Augite; Biotite; Bronzite; Grossularite-pyrope; Plagioclase; Quartz. 

K 

Kilauean basalts .. ~--. __ •.. _________ --- ____ • ____ -- __ ._ ____ ••. ---••. ----.---_-- 92 

L 

Lamellae. See Oriented intergrowths. 

Laminar mush flow, as mechanism of dimensional fabric______________________ 46 

Layered rocks, compositional mixtures--------------------------------------- 2, 8-11 

constitution. __________ ._--_--_--_.-_---.--------------------------.------ 4 

origin .. _______ ------ ______ ----- _____________ --- _______ • _________ • _________ 81-99 

similarity to bedded sedimentary rocks.---------------------------------- 4, 99 

textures. _________ ----- ___ .----_-------_---------------------------.---.-- 13 

use of igneous and sedimentary terminology •. ---------------------------- 13 

Layering, by proportions of settled crystals _______________ 4, 13-15, 23, 35, 82, 83, 85,88 

compositionaL·---------------------------------- 2, 4, 8, 13-15,82,83,85, 88,92-93 

current stratification--------------------------------------------- 29-30, 35,82-83 

dimensional orientation of settled crystals------------------- 4, 11,37--47, 82, 83,85 

evidence of origin-----------~--------------------------------------------- 83, 85 grain-size stratification __________________________________ 21, 23, 26, 28, 35, 37, 85,88 

gravity stratification.----- ______ • __ • _______ --------------.----.-- ___ .---._ 35 

original horizontality _ ---------------------------------------------------- 82 

similarity to bedding_---------------------------------------------------- 4, 99 types. ________________ .----- ______________________ --- _______ --- ___ ._--____ 4 

Lineation·---------------------------------------~------------- 11, 35,37-38,46--47,83 

Lognormal distributions, of settled minerals.--------------------------------- 28,29 

significance ____________________ ---------------------------------------- 35-37, 82 

Lugar sill .. __________________ • ____________________ --- _______ -----.----. __ - __ -_ 46 

M 

Magma, agents of transportation and deposition·----------------------------- 82 

basaltic, composition._--------------------------------------------------- 81 

origin of the complex. __ ---------------------------------------------- 4 cooling and crystallization ____________________________________________ ---- 93-99 

density ________________________________________________ --- ______ -----. __ ._ 31, 81 

depositional conditions. ____ ---------------------------------------------- 82-85 interprecipitate .• _. __________________________________ • __ _ _ _ _ ___ _ _ _ _ _ 73-74, 85-86 

order of crystallization .... ------------------------------------------------ 15,83 

origin of layering ___ .----------------------------------------------------- 83, 85 predepositional conditions. __________ • ____________________ • ___ --_. __ --_--_ 81-82 

primary crystallization products ______________ ---------------------------- 82 

temperatures.-------------------------------------------- _______ 18, 76, 81, 85-86 

trapped, chemical variation within cyclic units.-------------------------- 8~90 
INDEX 105 

Magma-Continued Page 

trapped-continued 

composition •..••••••.••..•.•••••••• ···-·-----------·-. ___ ··---·-··--· 88-90 

differentiation ln •. -----------------------------------------------.... 86 

order of crystallization .••••••• ------------·-------------- ___ -------- __ 73-74 viscosity __ ---. __ • ____ .• __ --- ___ • __ ••• ________ •. ____ • ___________________ 31, 81, 95 

Magma currents, as mechanism of dimensional fabric. ______ ------------·----- 46 

evidence against origin of layering ~Y----------··------------------------- 88 

relation to layering._--------·-·--·---·-···--·--------------·------------- 82,83 

structures produced by--------------------------------------------- 29-30,35,46 Mlnernl associations, intcrprecipltato materiaL _______________________________ 88-89 

primary precipitate.--. ___ ••• _. ___ • ___ . ______ •• __ ._._. ____________ • ____ . __ 86-88 

rock products classification_.-------------·-···--------- ______ -------- ____ 90-91 Stillwater com plox. ______ •• _ •• _____________________ • _____ • _ _ _ _ __ __ __ _ __ _ _ _ 4-5 

Modal clnsslftcntlon, Inadequacy In complex._________________________________ 90 

N 

Nor! to, Banded zone. ___ .--------··-·----------------------------------______ 2, 92 Dasal zone ________________________________________ -------------·------- __ 3-4, 82 

Norito pegmntltos. ---·- _ -------------·----------------------------·---- ____ __ '8 

0 

Oikocrysts. See Poikilitlc crystals. Olivine, as Interstitial constituent _________________________________________ 66-67,72 

as settled constituent of :Ultramafic zone •.• --------.-------- 14-15,20-37,38, 42-45' 

compositional range In Ultramafic zone_·-----------------------.---------- 4 

deformation of.--··----------------------------------------------------- __ 18.:.20 

intcrpreclpitate, percentage in chromitite. ---------------------------·--·- 89 · zoning in •• __ ••••• ______ •• ---- ___ •. _______ •• __________________________ ·18, 76 

settled crystals, enlargement.-----------------------------·-··c·------- 52, 54, 58 

inclusions in .••••••••••• __ ---.• _._._. ____ •. _. ___ • __ •• _____ •• _____ .____ 15, 17 

intcrgrowt11s in·------------------------·········-···------····--·-··- 17 

petrofabric analysis·-··-·--·-···--·········--·-----------···---------- 42-45 

reaction textures with bronzlte. __ • -------· ------------------------ 47, 50, 52 shape ..•. _____________ •• ______ •. _________________________________ .____ 13 

size-density relations .. _.--------------------·---------------------. __ 31-34 

size variation, overalL----------- __ ----------·-· .• -------------------. 26 

size variations, cyclic.·-------------------·---···-----·--------------- 23 

undulatory banding in_. __ -----·--·----------------------··-----_·" 18-20, 42-45 

Olivino-+bronztte reaction replacement.-------------------·-------------- ____ 47, 88 

--' Olivine bronzitite, coexistence of settled minerals in-------------------------· 86-88 

definition .•.•••••••• .: •.•.•.•.•. ·····----- ___ ------------------------------- 5, 91 

distribution ofinterprecipitate materiaL. __ ---------·-···-··-------------- 71-72 

distribution of settled minerals. ___ ---------------------------·-·--·______ 14 

orientation of settled crystals in-----------------------··-----------·------ 39,41 

secondary enlargement in---------···------------------------------------- 58 

Ollvino-+chromian augite reaction replacement.------------------------------ 47,50 

Olivine chromitlte, coexistence of settled minerals in •. -----------·------------ 86-88 

definition •..•. ------·-------------------------------------------·----_____ 8, 91 

distribution of lntorpreclpitate materiaL.--------------------··-·--····-·· 72-73 

distribution of settled minerals._ .. _________ --------~----------------_____ 14 

initial porosity .•.. --·-··-------------------··-··-·--··------------------. 64 

intorpreclpltate material in •. ---------------·------------·- ___ ------- _____ 88-90 

reaction replacement in .• __ -------------------_-------- ___ --------------__ 47 

secondary enlargement in _______________________ -------------------------- 58 

Olivine chromltltes, size-density relations In__________________________________ 30 

sorting characteristics ...•.••.•••.•... ------- ____ --·····-·--------· ________ 28-30 

Olivine oikocrysts, shape.····-·-·-·-------------------·--·------·-·----------~ 66 

Orientation, of polkllltlc crystals .... ----··--·-··-·-··----··-··--·-··-·--·----- 79 

of settled crystals .. -~---- __ ·-··--·-·-------------·-·--·-·------_---------- 37-47 

Oriented Intergrowths, In intorprecipltate materiaL_.------------------------ 74, 76 

In settled crystals ________ --------------· __ ·---------- ___ -----------_------ 17-18 

Orthopyroxene, as oriented intergrowths-------------------------------------- 76 

p 

Packing density, In crystal mush.----------------------------------------- 62,64-65 

Palisades diabase .. ------------------------------------· __ -----_______________ 76 

Paragenesis of settled minerals, inference from Inclusions .. -------------------- 17 

Peridot! to member, composition. __ --------------------- _______________ -------

cyclic units ln. __ --·-------------------- __ --------------- ________ --------- 14, 91 defined_. ______ • __ • _____________ . __________ ._. ____________ ._______________ 4 

distribution of Interstitial constituents in.-------------------------------- 73 

distribution of settled crystals in .•.. ------------·----------------~-------- 14 

grain-size variation In cyclic units. __ ------------------------------------- 23 

secondary enlargement ln ...••.•. ----- __ -------------------- _____ --------- 6G-61 

succession of rock types.--------- ___ ------------------------------- __ .•.•. 91 

textures •.. ------ __ -----------------------------------·-- ___ ------------ 4, 79-81 

Permeability, crystal mush.------------------------------------------·------- 85 

Potrofnbric studies, dimensional orientation. __ ------------------------------- 37-47 

Plagloclnso, as interstitial constituent.---------------------------------- 66,69, 71-78 

as settled constituent In Banded zone ... ---------------------------------- 14,92 

as settled constltutent In Basal zone. __ ----------------------------------- 14,82 

Plagioclase-Continued Page 

compositional range in Ultramafic zone.---------------------------------- 4 

interprecipitate, percentage of interstitial materiaL _____ ------------------ 89 

interprecipitate, zoning in _____ ---------------------- __ ------------------- 76-77 

Plagioclase oikocrysts, orientation ... --------------------------------- ______ ._ 79 

shape ___ ---.--. __ .• __ ---------.------------.--.--.---.-.-_-._--_-•.••. -... 66-67 

Planar structures, in layered rocks .. ---------------------------- 4, 11,37-47,82,83,85 

Poikilitic chromite harzburgite, coexistence of settled minerals in. __ • _____ .___ 86-88 

definition ... _ ••. ----•. _.-.• --.-~·-.---.-----.- .• --.-.. -..• _-._- __ -_ .. _ •..• _ 8, 91 

distribution of interprecipitate materiaL.---------"·--------------------- 72-73 

initial porosity------------------------,----------------------- _________ --"· 64 

interprecipitate material in._------------------------------------ __ ----".. 89 

secondary enlargment in ..... -------------------------------------------__ 58 

Poikilitic chromite harzburgltes, sorting characteristics .. --------------------- 28-30 

Poikilitic crystals, distribution._ .. ----·----------_-----------------------... _ 67, 69 

orientation. ___ ----------------------------------------------·---------- __ 11, 79 ' 

relation to layering plane ... ----·--·--·--·-------------------------------- 11 

relation to reaction replacement _________________________ --------------·- __ 47, 50 

shape .•. -••.•. _ .•.• --.--.---.---•. -•• -.------•• ----•• ----•• -••• -.•• ---.--. 66-67 

size •..• --.--.• _--.---•• ----•• --•• --•. -.• -.--•. ------.----.---.--••.•• -.--- 77 

relation to size of settled crystals .• ---··----------~-------·-··-···-·--: 77-79 

size ~istribution. _. --••• ---••• -.------.---.-•.• -.--•• ----.--- -~-.-- •• -.-.. 77-78 

Poikilitic ~arzburgite, coexistence of settled minerals in.------------------·--· 86-88 

~:::::~;;t ~-f i~;i.;~~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: ~ ~~ ~ ~: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: ~ ~: ~ ~: 5

' :~ distribution of interprecipitate materials·-----~----------·-··-·--·-------- 69 

distribution of settled minerals._. _____ --------------------------- __ --·--· 14 

initial. porosity-·-··-···-····-----------------------··----···--·--·-------· 63, 64 

interprecipitate material in ... ___ ---•---------·······-----------------··-. 69 

olivine.-+bronzite reaction replacement .. ------------------------·-········ 47 

orientation of settled crystals in .... ------------------------------------ 38,42-43 

reaction replacement in--------------------------------------------------- 47,50 

·relative abundance in Ultramafic zone.·--·-----------------------···----- 86 

secondary enlargement, textural evidence.-------------------------------- 54 

Poikilitic harzburgites, volume percent settled minerals wlthl,n and between 

olkocrysts •..... ~ "-------··-··--·--------···-··-·--------· -------· 50 

Poro~ity, crystal mush .... --------------····---------------------------- 52,62-65,85 

Postdepositional compaction, relation to grain orientation._--------------- 46-47,85 

Postdepositional conditions, character of the crystal mush.------------------- 85 

crystallization of the interprecipitate materiaL.----------··--·-···------- 86 

interprecipitate magma •.• __ ••. -.--•. --.--. __ •• _-•.•. _ .. _ •• _-••.•• -.• -_-._ 85-86 

Postdepositional reaction replacement._.--------------·--···-··----.• _....... 47 

Primary crystallization, depositional conditions. ____ -------···--···-··-·----- 82-85 

predepositional conditions.-···-···--·-·--·---------··-····---·-····----·- 81 products ____ .•. _ ••.•• _-•••• _ ••• --•. --.-•. -. __ • __ . __ ._ •• _ •. __ ._ .. _. __ • __ .__ 82 

Primary precipitate, definition of term ... _-----------------------------·-·--- 11 

See Settled crystals; Settled minerals. 

Purpose of report.---·-··-·---·------·-···--·_-- ___ -·--.. ···--·-------_-·-··-. 

Pyroxene, crystallization temperatures ... ---····-·-------------------------· 18,76,81 

Q 

Quartz, as interstitial constltutent of bronzitite.----------·----------- 5,66, 71-74,86 

Quartz oikocrysts, shape ___________ -----------------····------------__________ 67 

R 

Rate of crystal accumulation .. __ -------·---------------·-··------------------- 61-62 

Reaction replacement, discussion of data .. _.--------------------------------- 52 

relation to porosity of crystal mush.-------------------------------------- 52,93 

volume relations.-----.------------------------··-·---------·------------- 50, 52 

Reaction replacement of primary precipitate, bronzite.-+chromian augite .....• 47,50 

olivine.-+bronzite .•.•..•...• __ --------------- __ ---------------------- _ ---- 47 

olivine.-+chromian augite._-------------·--·-------------------·--·-- _____ 47, 50 

Rhum, ultrabasic rocks of .•.. ------------·-······-··------------------------· 14,99 

Rhythmic layering, distinction from size-graded layering_____________________ 23 

hydraulic relations. ___ .--------·--··-- __ -------------------- ______ -··- ___ 35, 83 

Rhythmic units, of Brown. ___ ----------------------------------------------_ 14' 

Rock types, average modes ... ------------------------------------------------ 5 

composition. __ -·--···-··-··---_--------------------·--------------------- 4-11 

initial porosity·------------------------------.• _____ ·----------- ___ ------- 62-64 

order of cryst3llization .. -----····----------------------·-·. ----- __________ 73-74 

ratios of interstitial minerals .•.. --------- __ ------------------------ _______ 89-90 

ratios of settled minerals ...... ---------------------.---------------·----_. 86-88 

secondary enlargement .. ___ -------·----------------------·------------··- 52-58 

settled and interstitial constituents ..... ---------------------------------- 67-73. 

Rocks of Ultramafic zone, lithologic types.---·-·--··---------------·--------- 5-8 

modes.--····-···········--·······-·-·-·-··--·-----------··--··-·-·--·---- 5, 30 

similarities and contr~:;ts. _ --------·-··--··------------------------·--·--· 8, 13 

s 

Scope of report .. __ ··-··-·····-·-··--··-····-··-··--· .. -------------------·-_.. 4 

f:lecondary enlargement, amount by rock type._-------------------------···-· 58,60 
106 INDEX 

Secondary enlargement-Continued 

amount in correlative subunits ___ .---------------------------------------

definition ... --------------------------------------------------------------

Page 

61 

52 

discussion of data __________ ----------------------------------------------- 61-62 

intracycle distribution ______ ---------------------------------------------- 60 

mechanism oL .... ------------------------------------------------- 61-62, 85-86 

postdepositionaL------------------------------------------------------ 11, 85-86 

relation to rate of crystal accumulation ..... ------------------------------ 62,86 

relation to rock type .. ---------------------------------------------------- 54 

stratigraphic and areal distribution.-------------------------------------- 6G-61 

textural evidence._-------- __ --------------------------------------------- 52, 54 

volume relations _______ -----------------~--------------------------------- 58-60 

Settled crystals, definition- of term.----------------------------------------_-- 11 

deformation. ___ --------------.---------------------------------------. 18-20, 52 

distribution within rock types-------------------------------------------- 13-14 

euhedral grains.------ ___ ------------------------------------------------- 11, 13 

fractures ... ----------------------------------------------.---------------- 18 

genetic interpretation of shape.-------------~----------------------------- 13,83 

grain size, measurements. ___ --------------------------------------------- 2Q-21 

relation to size of poikilitic crystals ..... ------------------------------ 78-79 

inclusions ________ ------------------------------------------------.-------- 15-17 

internal character _____ ---------_------------------------------------------ 15-20 

lineation ... ------------------------------------------------ 37-38,41,42,46-47,82 

mineral associations. ___ -------------------------------------------------- 86-88 

order of crystallization ___ ------------------------------------------------. 15, 92 

orientation, discussion of data. ____ --------------------------------------_ 45-46 

petrofabric studies. ___ ----------------------------------------------- 37-38 

postdepositional compression. ____ ------------------------------------ 46-47 

orientation of elongate forms .. -------------------------------------------- 38,46 

oriented intergrowths ... -------------------------------------------------- 17-18 

overall size variation _____ ------------------------------------------------- 26 

partial resorption ___ ------------------------------------------------------ 13 

planar orientation. ________ ----------------------------------------------- 37 

proportion in layered rocks .. -------------------------------------- 4, 8, 11,62-65 

relation of interference to amount of matriX.------------------------------ 52,54 

relations to interprecipitate materiaL------------------------------------ ~7-65 

secondary enlargement. ___ ----------------------------------------------- 52-62 

amount_._--------------- __ ---------------------------------------------- 61 

shape ___________________ -_---------------------------------------------~-- 13 

size-density relations ____________ ------------------------------------------ 3Q-35 

size distribution, lognormaL--------------------------------------------- 35,37 

size-graded bedding, large-scale._----------------------------------------- 21-23 

small-scale ... ___ ._--.---------------.-------.-----------.------------- 23-26 

size variation and distribution, comparison with sediments.----------- 2Q-21, 99 

size variation, in cyclic units----------------"·---------------------------- 23 sorting _____________ -·--- _______________ ----. ___ -- ____ ---_-- ________ -·- 21, 28-30 

undulatory extinction. __ -----------------------------------------------·- 18-19 

See also Primary precipitate. 

Settled minerals, accumulation oL.------------------------------------ 35-37,82-85 

associations _____ • ___ • ______ -.---.----------------------------------------- 86-90 

basis of classification ______ ------------------------------------------------ 90 

coexisting, hydraulic equivalents oL------------------------------- 3Q-35,82-85 

depositional conditions .. _------------------------------------------------- 82-85 distribution and interrelations in Ultramafic zone _____________________ 4, 8, 86-88 

genetic implications. __ --------------------------------------------- 4, 82-85 

order of accumulation _____ ------------------------------------------------ 14-15 

orientation, bronzite ______ ------------------------------------------------ 39-42 

olivine ________________________ -- __ ---_--_--------------.---.-------.-- 42-45 

paragenesis inferred from inclusions ... ------------------------------------ 17 

shape __________________ ---_------.---------------------------------------- 13 

zoning in. ____ ----------------_------------------------------------------- 18 

Settling by gravity, mechanism of dimensional fabric.------------------------ 46,83 

Settling of primary crystals, relation to layering plane._-------------------- 4, 11,83 

Settling rate, absolute calculation oL ... -------------------------------------- 82-83 

relative, coexisting minerals ____________ ----------------------------------- 3Q-37 

Shape, interstices of primary precipitate .. ------------------------------------ 65-66 

poikilitic crystals ... ------------------------------------------------------ 66 

settled crystals. ___ ------------------------------------------------------- 13 

Size-density relations, settled minerals .. --------------------------------------- 30-35 

Size distribution, of poikilitic crystals ... -------------------------------------- 77-78 

of settled crystals ____ ------ ___ -------------------------------------------- 21-37 

Size-frequency distributions, layered rocks .. ---------------------------------- 28-30 

Skaergaard intrusion, igneous lamination in. __ ------------------------------- 83 

inverted pigeonites oL .. ------------------------------------------------- 76 

0 

Skaergaard intrusion-Continued Pa~e 

magma currents in _______ -------- ___________________________ . ___________ ._ 46 

magma temperature oL _____ ------------------ _____________ ----- _____ _ ___ 81 

mechanism of crystal accumulation .. ---------------------------------- 35,94-95 

rhythmic layering in .... -------------------------------------------------- 2:3, !l3 

secondary enlargment in ___ -- ___ ------- _________ . ___ -----------___________ 61 

textures of rocks. __ ---------- __ ----------_---------_----- ____ ~ ____ . __ ----- 11 

Slump structure _________ -------_---------_--- __________________ . ________ .. 14, 62, 85 

Solid-solution minerals, of Ultramafic zone ..... ------------------------------ 4-5,92 

Sorting of settled crystals ___ ------ __ -__ ---.-----.. __ ----------_--- _____ ---_ 21, 28-30 

Stillwater complex, general geologY------------------------------------------- 2-4 

thickness._------- ____ --- ___ ------ __________ . _____ -------------------.____ 2 

Stillwater magma, characteristics.-----------------_-------------- ___ -----____ 81 

convection in _________ -------------------------------------------------___ 95 

crystallization behavior ____ ------ ______ -----. _____ ------__________________ 81 

density_. ___ . __ .. __ .• __ .. _._._._._._. ___ • ___ • ___ ._________________________ 81 

temperature gradients oL. _. ___ --- ________________ ------------------- ____ 93-99 

Synncusis texture, of Vogt .. -------------------------------------------------- 34 

T 

'l'emperature, final consolidation of interprecipitate magma ___________________ 85-86 

interprecipitate magma ________ ------------------------------------------- 76 

Stillwater magma __________ ------------------. __ ----.--------.----. ______ 18, 81 

Temperature gradients, during crystallization. __ ----------------------------- 93-99 

Textures, automorphic-poikilitic .. -------- _____ -------------------------- _____ 79-80 

developement of.. _______ .------------------- __ .. ------------------------- 79-81 

hypautomorphic ... ____ . _____ .-------.--.. --. _ .. ---.---•.. -.---.--.. ----.. 80 

of interprecipitate materiaL ___ -----------.---------.-------------------.. 74-75 

rock __ .-.. -.. _ ... _ .. __ .-.---.. -.----.. -•.. -.. --.. -.... -•... __ -- ___ •... -.__ 11-81 

terminology ___ ---------------------------------------------------------.. 11, 13 

xenomorphic. ___ -----------------------------------------------------·- __ 80-81 

Trapped magma, chemical variation .. ---------------------------------------- 89-90 

composition. ____ --------------------------------------------------------- 88-90 

differentiation in. ___ ----------- _____ --------------------_---------- __ ---- 86 

order of crystallization ___ ----------------------------_-------_--------- ___ 73-74 

Troctolite pegmatites .. -------------------------------------- ____ ___ _ _ ____ ____ 8 

u 

Ultra basic zone, of Peoples. _____ --------------------------------------------- 2 

Ultramafic zone, development_ _______ ----------------------------- __ ------ __ 81-100 

development of textures. __ ------------------------ ________ ------ ____ ----- 79-81 

distribution of settled minerals ______ ------------------- ____ -------_----- __ 14-15 

general geology~ _____ --------------------------------------------.-------- 2-4 

grain size and distribution------------------------------------------------ 2Q-36 

interstitial constituents _________________ -----------------------.------- 11, 67-73 

mineral associations .. __ -------------------.------------------------------ 86-91 

rate of accumulation _____ ------------------------------------------------- 62 

rock types--------------------------------------------------------- ___ 5, 8, 9Q-91 

settled constituents ___ -------------------------.---------------.---------- 11 

textures. ___ -~-------- ______ ------------------------------------_------ 11, 79-81 

thickness._---------------------------------------_-------- _____ ---_______ 4 

See also Bronzitite member; Peridotite member. 

Undulatory extinction, settled crystals .... --------·--------------------- 18-20,43,45 

Undeformed rocks, dimensional fabric ... _____ -------------------------------- 45-46 

,dimensional fabric, mechanism .. ------------·--------'-------------------- 46-47 

v 

Variable-depth convection _______________ ------------------.---------------.-- 96-99 

Viscosity of Stillwater magma._------------------------------------------- 31, 81,95 

Volume relations, reaction replacement.-------------------------------------- 50,52 

secondary enlargment. __ ------------------------------------------------- 58-60 

w 

Wager,,L. R., quoted ... ------------------------------------------------ 11, 61, 94,99 

X 

Xenomorphic texture, development .. -------------------------------------- 18, 8D-81 

z 

Zones of the complex, of Peoples----------------------------------------------

usage of this report __ ----------------------------------------------------- 2-4 

Zoning, of interprecipitate materiaL. __ --------------------------------------- 76-77 

of settled crystals .. ------------------.------------------------------------ 18 

-----