Hornblendes from a Region of 

High-Grade Metamorphism, 

Front Range, Colorado 

8y DOLORES J. GABLE and VERTIE C. SMITH 

GEOLOGICAL SURVEY BULLETIN 1392 

Chemically analyzed Precambrian igneous 

2nd metamorphic hornblendes in genera{ do 

'lot conform to the same patterns in a region 

Jj high-grade metamorphism where hornblende 

7ompositions are dependent primarily on the 

nineral assemblage 

NITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Gable, Dolores J. 192 2-

Hornblendes from a region of high-grade metamorphism, Front Range, Colorado. 

(Geological Survey Bulletin 1392) 

Bibliography: p. 

1. Hornblende-Front Range, Colo. and Wyo. 2. Metamorphism (Geology) 3. Mineralogy-Front Range, Colo. and Wyo. 

I. Smith, Vertie C., joint author. II. Title. III. Series: United States Geological Survey 

Bulletin 1392. 

QE75.B9 no. 1392 (QE391.AS3] 557.3'08s (549'.66] 

74-31211 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D. C. 20402- Price 70 cents (paper cover) 

Stock Number 024-001-02599 
CONTENTS 

Page Abstract _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 

Introduction _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 

Scope and purpose ________________________________ _ 

Sample preparation and laboratory studies _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1 

Geologic setting _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2 

Host-rock description __ _ _ ____ _ _ _ ___ _ _ _ __ _ _ _ ____ _ _ ___ _ _ _ 5 

General character ___ ______ ______ _ ____ _______ ____ _ _ 5 

Chemical composition _ ------------------------- _____ 7 

Hornblende _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 

Chemical composition of hornblende _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 10 

Physical properties of hornblende _ _ _ _ _ ___ _ _ _ __ _ _ _ _ _ __ _ _ _ _ 15 

Textural relations o(hornblende _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 21 

Petrology _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 22 

Chemical variations in hornblende in relation to host-rock 

composition, mineralogy, and metamorphism _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 22 

Chemical variations in relation to host-rock composition _ _ _ _ _ _ _ _ _ _ _ 22 

Element distribution between biotite and hornblende _ _ _ _ _ _ _ _ _ _ _ _ 23 

Front Range amphiboles as indicators of oxidation during metamorphism _ _ _ 29 

Conclusions________________________________________ 33 

References cited _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 33 

ILLUSTRATIONS 

Page 

FIGURE 1. Generalized geologic map of a part of the Front Range, Boulder, 

Gilpin, and Jefferson Counties, Colo _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 4 

2-12. Graphs showing: 

2. Chemical variations of some Front Range hornblendes _ _ _ _ 11 

3. Comparison of major oxides in hornblendes grouped according to 

the source rock-------------------------- 16 

4. Replacement of Fe+2 by Mg+2 in hornblende _ _ _ _ _ _ _ _ _ _ 17 

5. Variations in hornblende composition and its relations to 

refractive-index alpha _ ____ _ _ __ _ _ _ _ __ _ _ __ _ _ _ 20 

6. Composition of hornblendes showing major element variations 22 

7. Comparisons of modal percentage of minerals in hornblendebearing assemblages and major oxides of source rock and 

hornblende _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 24 

8. Comparison of Na/Na+Ca in host rock and in hornblende _ 25 

9. Distribution of manganese between biotite and hornblende _ 28 

10. Distribution of Fe+2 and Mg+2 in hornblende and biotite _ _ 29 

11. Plot of ferric iron in hornblende versus degree of oxidation in 

host rock __ ____ ______ _____ _____ ____ __ _ 30 

12. Plot of magnesium versus ferric iron, showing degree of oxidation 

in some hornblendes ----------------------- 31 

m 
IV CONTENTS 

TABLES 

Page 

TABLE 1. List of samples, field numbers, and locality descriptions used in 

this report _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 6 

2. Chemical and spectrographic analyses and modes of hornblendebearing rocks, central Front Range, Colorado _ _ _ _ _ _ _ _ _ 8 

3. Modes of hornblende-bearing rocks for which no chemical analyses are available ______________________ ~ _ _ _ __ 10 

4. Chemical and spectrographic analyses of hornblendes and mineral 

formulas _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 12 

5. Optical and physical properties of hornblendes - _ _ _ _ _ _ _ _ 18 

6. Chemical and spectrographic analyses of biotite coexisting with 

hornblende ____________________ --------- 26 

7. Mg, Fe+2 , Fe+3 ions for analyzed hornblendes, table 4 _____ 32 
HORNBLENDES FROM A REGION 

OF HIGH-GRADE METAMORPHISM, 

FRONT RANGE, COLORADO 

By DOLORES J. GABLE and VERTIE C. SMITH 

ABSTRACT 

Calcic hornblendes from Precambrian metamorphic and igneous terranes in the central 

Front Range, Colo., were subjected to a sillimanite-grade metamorphism and have compositions that are more dependent on host-rock mineral a~semblages than on host-rock 

chemical composition. The partitioning of the mafic oxides Mg, Fe+2, Fe+3, and Mn 

between biotite and hornblende is necessary in relating the total mineral assemblage to 

the composition of the host rocks. Orderly distribution of the mafic oxides between biotite 

and hornblende yields rather consistent distribution patterns, and these patterns appear 

to relate to the regional metamorphism. Fe+3 in the metamorphic host rocks controls, 

though perhaps imperfectly, the Mg content in hornblende, whereas Fe+3 in igneous host 

rocks only exerts partial control over Mg in accompanying hornblende. In general, 

oxidation-reduction ratios in the igneous hornblendes increase with increases in Fe+3 but 

decrease in metamorphic hornblendes with increasing Fe+3; 

INTRODUCTION 

SCOPE AND PURPOSE 

The crystalline core of the Front Range, which lies just west of 

Denver, Colo., contains many hornblende-bearing gneisses and syntectonically intrusive hornblende-bearing rocks, all Precambrian in age. 

Bulk rocks and the minerals hornblende and biotite separated from 

them were studied to compare the metamorphic effects on hornblende 

between rock types produced by a sillimanite-grade regional 

metamorphism. Hornblende, inasmuch as it occurred in all rocks 

chosen, was studied in the greatest detail to relate its physical and 

chemical properties to· the metamorphism. Particular attention was 

given ·to hornblende and biotite in the intrusive Boulder Creek 

Granodiorite and associated rocks emplaced near the end of the major 

regional metamorphism in order to compare their chemical behavior 

with the hornblendes from· adjacent Precambrian gneisses. 

SAMPLE PREPARATION AND LABORATORY STUDIES 

Chemical analyses were made in the Denver and Washington 

laboratories of the U.S. Geological Survey; analysts are credited in the 

appropriate tables in text. Sta~dard rock analyses were made following 

1 
2 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

the procedures described by Peck (1964); rapid rock analyses and 

colorimetric and atomic-absorption studies were done following the 

procedures outlined by Shapiro and Brannock (1962). Spectrographic 

analyses are arbitrarily reported to the nearest number in the series 1, 

0.7, 0.2, 0.15, and 0.1, whose numbers represent approximate midpoints 

of group data on a geometric scale. Precision of a reported value is approximately plus or minus one bracket at 68-percent confidence or two 

brackets at 95-percent confidence. 

Pure mineral separates of hornblende and biotite were obtained by 

use of ultrasonic cleaner, heavy liquids (bromoform and methylene 

iodide), and a Frantz isodynamic magnetic separator. Purity of some 

separates also necessitated the use of centrifuge-separation procedures 

similar to those described by Schoen and Lee (1964). All samples, by 

binocular microscope, contained no more than 1 percent impurities. 

Optical determinations were made on single grains and studied by 

modified spindle-stage procedures described by Wilcox (19159) and 

Wilcox and Izett (1968). Refractive indices were determined using the 

focal-masking techniques described in detail by Cherkasov (1955a, b; 

1957). For each sample numerous grains were first observed in an index 

oil close to /1, and the variability of index was observed. Indices were 

then run on an average of three grains for each of the samples. The 

range in indices for samples is due to zoning. 

X-ray powder-diffraction data were obtained by using a Norelco Xray diffractometer with Ni-filtered CuKa, radiation. Scans were made 

at 0.25° per minute in the range 8°-72°(20) with a chart speed of 1 inch 

per minute. An internal standard of CaF2 was added to each sample 

and thoroughly mixed. Both an aluminum-mount holder and a 

collodion-membrane mount (Gude and Hathaway, 1961) were used for 

some of the runs, however, because most samples contained ample 

hornblende and because a sharper pattern resulted with the aluminum 

mount, most hornblendes were run in this way. The holders were 

carefully packed to reduce preferred orientation of grains. Observed interplanar (d) spacings for each sample were processed through the 

"least-squares unit-cell refinement" program on an IBM-360-65 computer to determine the major cell parameters (Evans and others, 1963). 

Mineral densities were measured by two different methods. Samples 2 

and 16 were measured by sink-float in heavy liquids, range of accuracy 

±0.002; the other samples were measured by taking a weighed 5-gram 

sample, heating it in water to drive off the absorbed air, and then 

weighing the sample again, range of accuracy, ±0.01. 

GEOLOGIC SETTING 

Precambrian metamorphic rocks form the major part of the 

crystalline core of the Front Range, where they occur as a thick layered 
GEOLOGIC SETTING 3 

succession that is, in part, both migmatitic and pegmatitic. The most 

common and widespread layered rocks in the area are biotite gneisses 

(fig. 1). lnterlayered with the biotite gneiss is a thick leucocratic gneiss 

unit, granitic in appearance, that is referred to as microline gneiss. 

Hornblende gneiss and amphibolite, a third unit, is generally well 

foliated and interlayered conformably with biotite gneiss and 

microcline gneiss units. Hornblende -gneiss, biotite gneiss, and 

microcline gneiss are all finely interlayered with lesser units, including 

those of calc-silicate gneiss, garnet-quartz gneiss, and quartzite. 

Amphibolite, as small concordant lenses, is associated with _all major 

rock units. It is more massive than hornblende gneiss, generally finer 

grained, and may have a well-developed foliation and lineation. 

The intrusive rocks of the area are Precambrian and CretaceousTertiary in age. Intrusives of Precambrian age are dominantly of two 

groups: (1) granodiorite, from the Boulder Creek Granodiorite (Boulder 

Creek Granite of Lovering and Goddard, 1950, pl. 2), and (2) biotitemuscovite-quartz monzonite which belongs to either Silver Plume 

Quartz Monzonite (Silver Plume Granite of Ball, 1906, p. 383) or quartz 

monzonite which is intimately associated with the Boulder Creek 

Granodiorite. Intrusive Precambrian rocks not named and in part 

associated with the Boulder Creek Granodiorite include quartz diorite, 

diorite, hornblende, and some gabbro. These rocks were 

metamorphosed to varying degrees during a 1,450- to 1,740-m.y. 

(million year) major regional metamorphism (Hedge and others, 1967). 

Intrusives not affected by the regional metamorphism and of no further 

concern to this report include Silver Plume Quartz Monzonite, 1,450 

m.y. in age, and numerous Cretaceous-Tertiary igneous rocks that cut 

diagonally across the northern part of the study area. 

The framework of the area developed during a series of regional 

metamorphic events, all Precambrian in age. At least three periods of 

deformation have been recognized. The first two periods of deformation 

were accompanied by a pervasive high degree of metamorphism. The 

third period was largely cataclastic in character and local in extent and 

was superimposed on the two earlier plastic deformations. Migmatization and emplacement of the Boulder Creek Granodiorite took place 

late in the first period of deformation and continued into the second 

period, as indicated by forcible syntectonic emplacement of these rocks. 

Some quartz diorite, diorite, and hornblendite are intrusive in Boulder 

Creek Granodiorite and, in turn, are invaded by Silver Plume Quartz 

Monzonite; thus, an intermediate position in the intrusive sequence is 

reasonably well established for at least some of these mafic rocks. 

Metamorphism throughout the area does not appear to have been 

particularly uniform but, in general, was of amphibolite to lowgranulite grade. The highest grade of metamorphism appears to have 

been centered in the Central City-Morrison areas where muscovite, 
4 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

105°37'30" 30' 22'30" 105°15' 

Goble Gable Wells 

(1969) (1972) (1967) 

SimS Sheridan 

and Not and 

Goble published others 

(1967) (1967) 

INDEX SHOWING 

SOURCE OF DATA 

40°0 "B_.f.?l 

··~ 

52'3 

~ LOCATION OF QUADRANGLES 

Gable 0 

(1968) 

39° 

37' 

3d' 

0 

6 MILES 

6 KILOMETRES 

EXPLANATION 

~ Alluvium, colluvium, and glacial deposits (Quaternary) 

INTRUSIVE ROCKS 

g Intrusive rocks (Tertiary and Cretaceous) 

Silver Plume Quartz Monzonite (Precambrian Y) 

Quartz monzonite of Boulder Creek offmity (Precambnon X) 

Boulder Creek GranodiOrite (Precombnon X) 

~ Dionte, gabbro, quartz diorite, and hornblendite 

undifferentiated (Precambrian) 

~ Gneissic granodiorite (Precambrian) 

METASEDIMENTARY ROCKS (PRECAMBRIAN X) 

hi~!'d Microclme-quortz-plogioclose-biotite gneiss± 

.......... hornblende± garnet 

jiiil)!i! Migmotitic microcline gneiss 

nb Predom1nontly biotite-quartz-plagioclase gneiss 

gnbs Sillimonitic biotite-quartz-plagioclase gne~s- Both 

biot1te and S1llimon1tic biotite -quartz-plagioclase 

gneiss may contain layers that ore garnet, cordierite, gednte, or hornblende bearing 

--Contact 

--Fault 

Q Mica schist m Predommonlly hornblende gneiss-lnterloyered with 

amphibolite, colc-s1hcote gneiss, and biotite-quartzplagiOClase gneiss 

I Quartzite 

Schistose quartzite 

Cole -silicate gneiss 

ell Hornblende sample locality and number 
HOST -ROCK DESCRIPTION 5 

sillimanite, and potassic feldspar appear to be in equilibrium (Gable 

and Sims, 1969, p. 3) in rocks of appropriate composition. At Ralston 

Buttes the lack of microline associated with sillimanite and muscovite 

may be due to lower grade or to rock composition. Sheridan, Maxwell, 

and Albee (1967, p. 37-38) believed that it is due to a lesser intensity of 

metamorphism. 

HOST-ROCK DESCRIPTION 

GENERAL CHARACTER 

Hornblendes were separated from mafic layers in microcline gneiss, 

hornblende gneiss, and in amphibolite, intrusive Boulder Creek 

Granodiorite, hornblende diorite, and hornblendite, tables 1-3. 

Microline gneiss layers contain only sparse, widely scattered, 

hornblende-bearing lenses (sample 10, table 3), some of which are mafic 

enough to be called amphibolite. These amphibolite lenses are interlayered with the gneiss generally near or adjacent to the contact of 

microcline gneiss with biotite gneiss. Microcline gneiss with singular 

scattered hornblende crystals occurs adjacent to nearly all hornblendebearing gneiss contacts, whether the contacts are gradational or 

sharply defined. The hornblendes from microcline gneiss form ragged, 

generally subhedral grains, intergrown with or partly altered to biotite. 

Hornblende gneiss (samples 1-7, table 2) is a fine- to mediumgrained, nearly equigranular rock that is weakly to strongly foliated and 

possesses good mineral alinement, primarily of hornblende. The gneiss 

consists predominantly of hornblende and plagioclase and widely varying amounts of quartz, biotite, and pyroxene. Hornblende-plagioclase 

biotite and hornblende-plagioclase-pyroxene gneiss (samples 1, 5, 6, 

table 2) are common variants in hornblende gneiss. Interlayered within 

the hornblende-plagioclase gneiss are lenses of amphibolite, biotite 

gneiss, microcline gneiss, calc-silicate gneiss, and lesser amounts of 

quartz gneiss. Amphibolites in the hornblende gneiss unit are 

represented by samples 2, 3, 4, 7, and 8, table 2. 

Hornblendes from the hornblende gneiss unit are 0.2-0.6 mm in size, 

rarely twinned, and only occasionally zoned. Biotite, where present, is 

subordinate in size to hornblende and in many places is interstitial. 

Plagioclase, the second most abundant mineral, ranges in composition 

from oligoclase to calcic andesine. Quartz is generally subordinate in 

size to hornblende and feldspar and is xenoblastic. It becomes 

elongated in strongly foliated rocks and shows a slight tendency to be 

segregated in the lighter layers. 

Microcline gneiss and hornblende gneiss are believed to have been 

mostly of sedimentary origin, but some may have been of volcanic 

FIGURE 1 (facing page). - Generalized geologic map of a part of the Front Range, 

Boulder, Gilpin, and Jefferson Counties, Colo. 
6 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

TABLE 1. - List of samples, field numbers, and locality descriptions used in this report 

[All localities in the Tungsten, Central City, and Ralston Buttes quadrangles are taken from the 1942 editions of the 

topographic maps; all localities in the Morrison quadrangle are from the 1957 edition) 

Sample 

No. 

6 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

Field No. Description of sample locality 

Amphibolite and hornblende plagioclase gneiss in the rock unit hornblende gneiss 

(Amphibolite consists of hornblende and plagioclase and <15 percent of other minerals) 

N-322A 

GG-1 

GG-2 

GG-3 

GG-4 

M-276a 

M-279a 

M532-C2 

S257-52X 

St-5 

St-3 

T-477-67 

T-483-67 

T415-67 

T-467-67 

T-170--68 

N-72-65 

T-219 

D-245b 

8376--1-53 

Outcrop in hornblende gneiss layer, 1,150 feet southwest of Rollinsville Ranger Station 

warehouse on southeast slope of knob, sec. 35, T. 1 S., R. 73 W., Nederland quadrangle. 

Roadcut in hornblende gneiss layer, 1,250 feet west of sedimentary Precambrian contact on 

Golden Gate Canyon road, sec. 20, T. 3 S., R. 70 W., Golden quadrangle. 

Roadcut in hornblende gneiss layer, 1,450 feet west of elevation 6,618 feet on Golden Gate 

Canyon road, sec. 25, T. 3 S., R. 71 W., Ralston Buttes quadrangle. 

Roadcut in hornblende gneiss layer, 800 feet west of elevation 7,184 feet on Golden Gate 

Canyon road, sec. 23, T. 3 S., R. 71 W., Ralston Buttes quadrangle. 

Roadcut in hornblende gneiss layer on Robinson Hill road, 1,500 feet east of horseshoe curve 

and 1/4 mile southwest of intersection of Robinson Hill road with Golden Gate Canyon 

road, sec. 17, T. 3 S., R. 71 W., Ralston Buttes quadrangle. 

Roadcut in hornblende gneiss layer, 1,900 feet west of Mother Cabrini Shrine road on old 

U.S. Highway 40, Mount Vernon Canyon, sec. 16, T. 4 S., R. 70 W., Morrison quadrangle. 

Rcadcut in hornblende gneiss layer, 1,300 feet east of BM7053, old U.S. Highway 40, Mount 

Vernon Canyon, sec. 17, T. 4 S., R. 70 W., Morrison quadrangle. 

Amphibolite lens in biotite-cordierite gneiss 

Hornblende from amphibolite layer in garnet-cordierite-sillimanite-biotite gneiss; roadcut, 

1,150 feet west of sedimentary Precambrian contact on Colorado Highway 74, Bear Creek 

Canyon, sec. 34, T. 4 S., R. 70 W., Morrison quadrangle. 

Hornblende from amphibolite layer in cordierite-gedrite-biotite gneiss, 800 feet south of the 

Glory Hole, Quartz Hill area, Central City quadrangle. 

Microcline gneiss 

Microcline gneiss, outcrop in Buttermilk Gulch, southeast corner of Central City quadrangle. 

Boulder Creek Granodiorite and associated rocks 

Boulder Creek Granodiorite on Ute Creek, Idaho Springs quadrangle, sec. 7, T. 3 S., R. 73 W. 

Boulder Creek Granodiorite from cut on aqueduct 1,800 feet southwest ofBM7608, sec. 3, T. 

1 S., R. 72 W., Tungsten quadrangle. 

Quartz diorite phase of Boulder Creek Granodiorite, from cut on aqueduct, 1, 750 feet 

northwest of quarter corner 8220, sees. 2 and 11, T. 1 S., R. 72 W., Tungsten quadrangle. 

Boulder Creek Granodiorite from roadcut 900 feet east ofWiniger Ranch, sec. 27, T. 1 S., R. 

72 W., Tungsten quadrangle. 

Boulder Creek Granodiorite from cut along aqueduct 2,200 feet east of Castle Rock, sec. 9, T. 

1 S., R. 72 W., Tungsten quadrangle. 

Mafic phase of Boulder Creek Granodiorite, outcrop located approximately at 7,400-foot 

contour on ridge northwest of Calhoun Gulch, sec. 6, T. 1 S., R. 71 W., Tungsten 

quadrangle. 

Hornblende diorite 

Hornblende diorite lens, 1,000 feet west of Colorado Highway 160 and 3,200 feet due north of 

Nederland Post Office, sec. 13, T. 1 S., R. 73 W., Nederland quadrangle. 

Biotitic hornblende diorite lens in roadcut, 500 feet due northeast of BM8592 on Colorado 

Highway 119, sec. 6, T. 2 S., R. 72 W., Tungsten quadrangle. 

Homblendite 

Hornblendite lens located approximately 400 feet south-southwest of section corner 8,904 

feet between Chase Gulch and North Clear Creek, Central City quadrangle. 

Hornblendite lens on Pisgah road, 1/2 mile due east of Pisgah Lake, sec. 4, T. 3 S., R. 73 W., 

Central City quadrangle. 
HOST-ROCK DESCRIPTION 7 

ongm. The interlayered biotite gneisses are considered to be of 

sedimentary origin. Fine interlayering and conformity of the biotite, 

calc-silicate, quartzite, and hornblende gneiss, with no crosscutting 

relationships, lend support to all being of predominantly sedimentary 

origin. 

Boulder Creek Granodiorite ranges in composition from mafic quartz 

diorite to quartz monzonite, but its average composition is granodiorite 

(samples 12-16, table 2). The rock is medium to very coarse grained 

with a predominantly hypautomorphic-granular texture. Leucocratic 

minerals are as much as 4 em long. Biotite, hornblende, and iron oxides 

commonly cluster together, and accessory minerals, sphene, allanite, 

epidote, and some apatite are generally associated with them. 

Hornblende forms euhedral to anhedral crystals, as much as 1.5 em 

long, which may be embayed by biotite. Rare local hornblende crystals 

have altered pyroxene centers. Traces of iron oxides occur along fractures and grain boundaries of hornblende. Plagioclase is oligoclaseandesine; potassic feldspar is microcline. 

Hornblendite (samples 19, 20, tables 2, 4) occurs in intrusive pods 

and lenses; it is generally fine to medium grained, weakly foliated to 

nonfoliated, and equigranular and consists mainly of hornblende and 

subordinate plagioclase. The hornblende is in euhedral to subhedral 

grains. In sample 19 anhedral pyroxene is subordinate in size to 

hornblende and is partly altered to chlorite. The hornblende in sample 

20 is of two generations; the larger crystals display a clustering of 

opaque oxides along the cleavage. The centers of a few of these crystals 

have colorless patches which are probably remnants of pyroxene. The 

second-generation hornblende crystals are subhedral, are smaller in 

size, and have no inclusions. 

Hornblende diorite, a medium-grained to rarely coarse grained intrusive rock, is found in elongated lenses as much as 2 km long and less 

than 1 km wide, although most lenses are considerably smaller. The 

diorite is generally massive and equigranular; however, biotitic phases 

are somewhat foliated but generally lack the strong foliation that 

characterizes the amphibolites of the area. Hornblende and plagioclase 

with variable amounts of pyroxene and biotite make up the bulk of the 

rock (table 2, samples 17, 18). The hornblende is euhedral to anhedral, 

nearly free of inclusions, and equigranular with the plagioclase. Locally, 

hornblende alters to epidote. The plagioclase is andesine to labradorite 

and well twinned. The iron oxides are magnetite and hematite that occur in single and intergrown crystals. 

CHEMICAL COMPOSITION 

Because of the different rock types involved, the chemical and spectrographic analyses (table 2) cannot be compared exactly, but there are 
TABLE 2. - Chemical and spectrographic analyses and modes of hornblende-bearing rocks, central Front Range, Colo. CXl 

[Samples 1, 8, 14 by standard rock analyses; E. S. Daniels analyzed sample 1; G. 0. Riddle, sample 8; E. Engleman, sam~le 14. Samples 2-7, 12, 13, 15-19, rapid rock analyses; analysts, Paul Elmore, Sam Botts, Gillison Chloe, Lowell Artis, H. mith, J. Kelsey, J. Glenn, Joseph Budinsky. nd, not determined; Tr., trace; L, detected but below limit of determination; __ _, :X not looked for. Sample localities are listed in table 1] 0 

Sample No. __ ----------_ 1 2 3 4 5 6 7 8 12 13 14 15 16 17 18 J9 t:Jj 

~ 

Rock name ------------- Amphibolite and hornblende glagioclase gneiss Amphibolite lens Boulder Creek Granodiorite and Hornblende Hornblendite &j in the rock unit horn lende gneiss 1 in biotite- associated rocks diorite z cordierite gneiss tj 

t,%:1 

Chemical analyses, in weight percent 00 

~ Si02 ----------------- 44.75 52.3 53.1 61.4 51.2 48.9 49.6 49.34 64.9 61.8 62.25 63.5 52.2 55.1 47.4 51.2 0 

AbO, ---------------- 14.34 13.4 14.5 11.3 13.7 13.8 15.9 14.52 16.8 16.8 16.88 16.7 19 13.4 14.4 12 F~O, ________________ s= 3.70 3.7 2.7 2.5 2.7 2.7 2.1 3.89 1.8 1.5 1.67 1.9 2.6 3.8 3.5 2.6 

FeO MgO ----------------~ 8.30 11.5 10.6 6.6 8.1 9.7 9.5 9.00 2.4 3 3.01 3 4.9 6.1 7.2 6.6 :X _________________ 8.50 4.4 3.7 3.9 7:7 7.1 5.6 7.43 2.1 2.5 2.55 2.6 5 7.7 9.7 10.5 ........ 

CaO ----------------- 13.63 8.7 8.5 9 9.6 11 10.9 9.68 3.6 5.4 4.30 4.3 7.4 8.6 9.3 11.1 0 Na20 _______ --------- 1.74 2.6 3.1 2.2 3 2.2 2.4 2.04 3 3.4 3.43 3.2 3.9 1.9 2 .90 :X 

K20 ----------------- 1.24 .28 .45 .59 .91 1.4 .49 .65 3.6 3.1 3.84 3 2.4 1.1 2.2 1.1 6 H20+ ---------------- 1.78 .87 .81 .69 .77 1 1 1.70 .73 .88 .65 .70 1.2 .51 1.1 1.3 

~ H20- ---------------- .07 .13 .09 .09 .07 .08 .09 .06 .05 .06 .07 .03 .14 .10 .16 .13 

TI~----------------- .95 1.4 1.6 .82 1 .92 .95 1.17 .49 .75 .59 .62 .82 .55 1.2 .97 

P20n ----------------- .20 .17 .24 .52 .18 .12 .15 .12 .36 .53 .29 .28 .23 .09 .73 .48 t,%:1 MnO .22 .27 .26 .18 .22 .26 .23 .25 .09 .09 .07 .09 .15 .24 .26 .21 

s= C02 -================ .40 <.05 <.05 .30 .06 .08 .40 .04 <.05 .11 .14 .05 <.05 <.05 <.05 <.05 Cl F ___________________ _______ ----- __ ---- .02 nd nd nd nd nd nd .04 nd nd .04 nd .04 .03 .10 .02 t,%:1 

.12 nd nd nd nd nd nd .06 nd nd .15 nd .19 .13 .39 .25 s ___________________ t-3 nd nd nd nd nd nd nd nd nd nd nd nd 0 nd nd 0 > Subtotal ___________ 99.96 100 100 100 99 99 99 99.99 100 100 99.93 100 100 99 99 99 s= Less 0------------ .05 nd nd nd nd nd nd .04 nd nd .07 nd nd nd nd nd Total 0 __ ----- ______ . 99.91 100 100 100 99 99 99 99.95 100 100 99.86 100 100 99 99 99 Powder density ___________ nd ~ 23.08 23.08 22.98 23.05 23.10 23.05 3.03 22.95 22.95 nd 22.95 23 2 3.08 •a.os 22.92 Oxidation ratios :X 

........ 

00 

mol 2(Fe.O,)X100 s= ---- =(percent) 29 23 19 26 23 20 17 30 40 31 32 36 32 39 31 27 ~ 

Z(Fe20,) + FeO 0 

0 

Spectrographic analyses, in parts per million t"' 

9 Ag __________________ Ba __________________ 0 <1 <1 <1 <1 <1 <1 0 1 0 0 0 0 L L 0 500 500 200 700 300 200 150 70 1,000 1,000 1,500 1,000 500 150 700 100 

Be ------------------ 0 0 0 2 1 0 0 0 3 3 2 3 1 0 0 0 Ce ____________ ------ 0 0 0 0 0 0 0 0 0 300 L 300 500 0 500 100 

Co ------------------ 50 30 30 30 50 30 50 30 20 20 15 20 30 50 50 70 
Cr __________________ 150 0 10 150 300 70 50 150 30 70 70 50 150 500 300 150 Cu __________________ Ga __________________ 70 50 2 15 20 20 30 70 7 30 7 20 200 10 5 150 15 20 20 10 15 15 15 20 10 10 30 15 15 10 15 10 

La ------------------ Mo 0 0 0 100 0 0 0 0 0 70 50 70 70 30 150 0 __________________ Nb 0 5 3 3 3 3 3 0 5 0 0 7 5 5 5 5 __________________ Nd 0 0 0 3 0 0 0 0 3 3 L 3 10 0 3 7 __________________ 100 0 70 0 0 0 0 0 

Ni ------------------ 150 10 15 50 70 50 30 50 50 50 30 50 100 150 200 200 

Ph ------------------ 15 0 0 0 0 0 0 10 7 7 15 7 7 5 1 0 Sc ______ ---------- __ 30 20 30 20 50 50 50 100 7 10 15 10 20 70 70 30 

Sr ------------------ 1,500 150 200 500 300 300 300 300 700 1,000 1,500 1,000 1,000 300 700 300 

v------~------------ 200 150 200 150 200 150 300 200 70 70 70 70 150 150 150 200 y ___________________ 20 30 50 30 30 20 30 30 30 50 30 30 50 30 50 30 

Yb ------------------ 2 3 5 2 3 2 3 5 3 5 3 3 5 3 5 3 Zr __________________ 30 50 70 150 70 30 50 50 15 30 150 10 150 150 100 30 

Modes, in volume percent ::X:: 

Potassic feldspar __________ 19 2.5 18.1 16.6 6.2 Tr. 0 

Plagioclase _______ -12.6 00 ---- __ 4.4 26.8 35.4 34.9 35.6 40.7 21.5 34.3 46.7 43.9 40.7 53 28.5 29.2 1 ~ Quartz --------------- .2 10.9 9.9 44 3.1 Ti. .9 2.7 27.6 14.1 23.9 18.7 3 5.3 3.8 6 Biotite ______ ----~----- .5 .1 .4 2.5 6.3 14.4 17.9 14.7 14.6 12.8 20.8 Tr. ::0 Muscovite-sericite _________ 15.5 8.2 .1 0 Hornblende------------- 65.1 60.6 52.3 30.8 56.9 56.6 56.5 68.2 .2 14.1 1.4 7.8 22.6 56.5 42.8 79 0 

Clinopyroxene _____ ----- _ 14.4 --- --- 2 --- 7.4 Orthopyroxene ___________ --- --- --- --- ~ 

--.2 --.5 10 t:l Apati~ --------------- .4 .4 . 4 .7 .1 Tr . .1 .7 .5 Tr . .1 1.9 2 tz:j Iron oxides _____________ Tr. .8 1.2 Tr. .3 Tr. . 1 1.2 .7 .4 .9 .6 .2 .6 .3 Tr . 00 Allanite _______________ Tr. .3 1.1 .4 Tr. 1 0 Sphene ________ ---- ___ --- --- .7 1.9 1.1 .1 .7 "T-;.- .6 1.1 .4 .1 .7 --- .1 Tr. 

~ Zircon ___ ;:.. _______ ---- _ ___ 'r-;.- "T-;.- Tr. Tr. Tr. Tr. Tr. Calcite _______________ Tr. 4.1 1.1 .1 .8 .5 .4 .2 Tr. --- Chlorite ---- ____ ------- -'r-;. --- .1 .1 .1 Epidote _________ ------ 2 .7 .3 1.7 1.6 .5 .4 1.4 .8 1 0 Total ______ ------- 100 100 100 100 100 100 100 -loO 100 100 100 100 100 100 100 100 z Composition of plagioclasea _________ --' An4, Ana a Ana4 Ana4 Ansa-34 An4s Anso An4a An,s Ana4 An,. Ana, Ana, Ans1 An4o nd 

'Amphibolite consists of hornblende and plagioclase and less than 15 percent of other minerals. 'Powder density by sink/float- estimated accuracy±0.002. 3Chayes (1952). 

Sample No. Lab. No. Sample No. Lab. No. 

1 -------------- 0100280 12 ------------- W170105 2 ______________ W170057 13 ------------- W170106 3 ------------ __ W170058 14 ------------- 0101974 4 -------------- W170059 ffi _____________ WTiillM 

5 -------------- W170060 16 ------------- W171834 6 -------------- W170054 17 _______ ------ W173309 

7 -------------- W170055 18 _____________ W173310 

8 -------------- 0100505 19 ------------- W171836 co 
10 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

TABLE 3. - Modes of hornblende-bearing rocks for which no chemical analyses are available 

[Sample localities are listed in table 1. Tr., trace; nd, not determined] 

Sample No. 

Potassium feldspar _____ _ Plagioclase _ _ _______ _ .5 Quartz __________ _ 

Clinopyroxene _______ _ Hornblende ________ _ 99.5 Biotite __________ _ Apatite __________ _ Tr. 

Iron oxides _ _ _ _ _ _ _ _ __ Tr. Sphene __________ _ Epidote __________ _ Zircon ___________ _ Allanite _ _ _ _ _ _ _ _ _ _ _ 

Total ________ _ 100.0 

Composition of 

plagioclase _ _ _ _ _ _ _ _ _ nd 

1

Approximate mode by visual estimates. 

1

10 

15 

30 

35 

5 

15 

Tr. 

Tr. 

100 

Ani4-I6 

4 

40 

35 

1 

14 

1 

1 

3 

1 

Tr. 

Tr. 

100 

An30-32 

20 

Tr. 

Tr. 

100 

Tr. 

Tr. 

100 

nd 

some similarities. Samples 12, 13, and 15, Boulder Creek Granodiorite, 

are very similar, whereas sample 16, also Boulder Creek Granodiorite 

but a diorite in composition, is more mafic and intermediate to 

hornblende diorite of samples 17 and 18. The highest MgO, CaO, and 

lowest Ab03 values belong to homblendite sample 19. Hornblendeplagioclase gneiss and amphibolite (samples 1-8) have a composition· 

similar to hornblende diorites. Spectrographic analyses of trace 

elements are very similar between rock types except for vanadium, 

which dominantly is found in hornblende gneiss and amphibolite, and 

nickel, which is predominantly in hornblende diorite and hornblendite. 

Barium is variable between rock types but is highest in hornblende 

gneiss, amphibolite, and Boulder Creek Granodiorite. 

HORNBLENDE 

CHEMICAL COMPOSITION OF HORNBLENDE 

Chemical analyses of 20 hornblende samples are presented in table 4; 

only the analysis of sample 9 has been published earlier (Gable and 

Sims, 1969, p. 54). Structural formulas were calculated on the basis of 

24 (0, OH, F, Cl), using a computer program and assuming a structural 

formula of Ao-IXzYsZsOzz. The current trend is to calculate hornblende 

formulas on the basis of 23 oxygens, but 24 was used here inasmuch as 

Phillips (1963, p. 701) stated that a possible deficiency in hydrogen in 

OH sites can be overlooked if the anhydrous method of calculating a 

formula is followed. Although Dodge, Papike, and Mays (1968, p. 400) 

found that hydrogen deficiency is unimportant in the calculation of a 

structural formula for common hornblende, they did find that specific 

gravities calculated on "conventionally" analyzed hornblendes, on the 
HORNBLENDE 11 

basis of 24 oxygens, agreed better with observed specific gravities. 

Three of the hornblendes in table 4 have less than 1.70 weight percent 

H20, but these hornblendes were still computed on the basis of 24 oxygens because the hydrogen deficiency appeared real for the 

hornblendes from the igneous rocks in comparison with those from 

metamorphic gneisses (table 4). 

All hornblendes are calcic and plot between the two end-member 

series tremolite-ferrotremolite and pargasite + magnesiohastingsite-ferropargasite + hastingsite (fig. 2). 

Major element variation between hornblendes of the various rock 

types of table 4 is shown in figure 3. CaO, Na20 and K20 vary little in 

hornblendes, whereas Fe203 is somewhat more variable, with the 

highest Fe203 values represented by Boulder Creek and hornblende 

diorite analyses. FeO and Ab03, in general, increase as Si02 and MgO 

Pargasite + 

Magnesiohastingsite 

Fe rropargasite 

+ Hastingsite 

I() 

+ 

Ql 

lL 

+ v 

<[ 

+ 

0 

z 

1.0 r-------"T,----...-,--:---r,-----r---1-----, 

0.8-

0.6 -

I 

I 

I 

I I 2 

14 o o'll OJ 15~~ ~I 0 4 °1o 

18 0 13 06 ~ 16 1 

17• o5 I 

I 

-

-

9 I -----o----T-----------

0.4-

0.2""'" 

•'9 I 

I 

I 

I 

I 

I 

I 

I 

I 

-

-

o~--~J ____ ~~---~~~~~-----~'--~~ 0 0.2 0.4 0.6 0.8 1.0 

Tremolite Fe +2;Mg + Fe+ 2 Ferrotremol ite 

FIGURE 2.- Chemical variations of some Front Range hornblendes expressed as the 

numbers of (Fe+2/Mg+ Fe+2) and Na +AI+ Fe+3/4 atoms per formula unit. From 

Colville, Ernst, and Gilbert (1966, p. 1734): D, Boulder Creek Granodiorite; & , 

hornblende diorite; •, hornblendite; 0, hornblende gneiss;8, microcline gneiss. 
12 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

TABLE 4. - Chemical and spectrographic 

[Sample numbers 1-8 and 12-20 by standard rock analysis; Vertie C. Smith analyzed samples 1, 8, 16-19; George 0. atomic absorption, Na20 and K20 by flame _photometer, analyst Wayne Mountjoy; Si02 and Ah03 determined Gillison Chloe, Lowell Artis, and H. Smith. Spectrographic analyses for samples 1, 8, 20, Harriet Neiman; 2-7, 

not found. Sample localities are listed in table 1] 

Rock name ---------- ______________ _ Amphibolite and hornblende-plagioclase gneiss in the rock unit hornblende gneiss 

Sample No. ____________ ------------- 6 

Chemical analyses, in weight percent 

SiO, ------------ AbO" ____ ----_--

Fe,O:~ ----------- FeO MgO ------------

___________ _ 

CaO ------------ Na,O __________ _ 

K,O ------------ H,O+ ----------- H,O- ----------- TiO, ------------ P,O., ___________ _ 

MnO ----------- Cl ____________ _ 

F --------------

41.18 

12 

5 

12.53 

10.05 

12.12 

1.36 

1.37 

1.86 

.01 

1.81 

.07 

.28 

.04 

.18 

41.86 

11.93 

5.31 

17.30 

6.74 

11.24 

1.32 

.81 

1.97 

.00 

1.14 

.05 

.35 

.01 

.05 

41.13 

11.55 

4.91 

18.04 

6.68 

11.10 

1.27 

.72 

1.92 

.00 

2.12 

.10 

.39 

.01 

.06 

42.99 

11.03 

5.37 

15.56 

7.78 

11.85 

1.14 

.81 

1.87 

.01 

.85 

.15 

.34 

.01 

.08 

45.57 

9.73 

3.94 

12.25 

11.44 

11.92 

1.05 

.82 

1.94 

.00 

.91 

.06 

.30 

.01 

.11 

42.90 

11.65 

3.90 

13.99 

9.46 

12.10 

1.08 

1.29 

1.84 

.00 

1.27 

.03 

.33 

.05 

.12 

44.14 

10.61 

3.73 

15.15 

9.06 

11.92 

1.04 

.66 

1.90 

.01 

1.20 

.02 

.33 

.03 

.06 

Subtotal _____ _ 99.86 100.08 100.00 99.84 100.05 100.01 99.86 

LessO ----------- ~9 ~2 ~3 ~3 ~5 .00 ~4 

Total _ _ _ _ _ _ _ _ 99.77 100.06 99.97 99.81 100.00 99.95 99.82 

Semiquantitative spectrographic analyses, in parts per million 

(Looked for but not found- Ag, As, Au, B, Bi, Cd, Ge, Hf, In, Li, Pd, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, Zn) 

Ba _____________ 700 

Be ------------- 0 Co _ _ _ _ _ _ _ _ _ _ _ _ _ 50 

Cr ------------- 150 

Cu ------------- 50 Ga _ _ _ _ _ _ _ _ _ _ _ _ _ 20 La _____________ 0 

Mo_____________ 0 Nb_____________ 0 Nd_____________ 0 

Ni ------------- 150 

Ph ------------- 0 Sc ------------- 50 

Sr ------------- 500 

v -------------- 500 y -------------- 30 

Yb ------------- 3 

~ ---------- w 

100 

0 

50 

5 

20 

20 

0 

7 

0 

0 

10 

0 

70 

50 

500 

100 

10 

50 

100 200 0 0 50 50 10 200 

5 5 

20 20 

0 0 

7 7 

0 0 

0 0 15 50 

0 0 70 50 

50 70 

500 300 100 20 

10 5 70 100 

70 

0 

50 

300 

15 

20 

0 

7 

0 

0 100 

0 

70 

20 

300 

50 

5 

70 

Mineral formulas, number of ions on the basis of 24 oxygens (0, OH, F, Cl) 

100 

0 

50 

50 

10 

20 

0 

7 

0 

0 

50 

0 

70 

50 

500 

50 

5 

70 

70 

0 

50 

50 

20 

20 

0 

0 

0 

0 

50 

0 

70 

50 

300 

50 

5 

50 

(Z) 

Tetrahedral 

Si ____________ _ 6.203 6.366 6.293 6.515 6.723 6.435 6.616 

AliV -----------

AliV __________ _ 

(Y) Ti ____________ _ Fe+3 ___________ _ 

Octahedral Mg ____________ _ Fe+' ___________ _ Mn ___________ _ 

Na ------------- X = Ca + Na "" 2.0 Ca ____________ _ 

A = RemainingNa + K K -------------

See footnotes at end of table. 

1.797 1.634 1.707 1.485 1.277 1.565 1.384 

8.00 

.333 

.205 

.567 

2.256 

1.578 

.036 

4.98 

.397 

1.956 

.263 

2.62 

8.00 

.504 

.130 

.608 

1.528 

2.200 

.045 

5.02 

.389 

1.831 

.157 

2.38 

8.00 

.375 

.244 

.565 

1.523 

2.308 

.051 

5.07 

.377 

1.820 

.140 

2.34 

8.00 

.485 

.097 

.612 

1.757 

1.972 

.044 

4.97 

.335 

1.924 

.157 

2.42 

8.00 

.415 

.101 

.437 

2.516 

1.511 

.038 

5.02 

.300 

1.884 

.154 

2.34 

8.00 

.495 

.143 

.440 

2.115 

1.755 

.042 

4.99 

.314 

1.945 

.247 

2.51 

8.00 

.490 

.135 

.421 

2.024 

1.899 

.042 

5.01 

.302 

1.914 

.126 

2.34 
HORNBLENDE 13 

analyses of hornblende and mineral formulas 

Riddle analyzed samples 2-7, 13-15; Elaine L. Brandt analyzed sample 20. Samples 1Q-12, Fe, MgO• and CaO by colorimetrically by G.T. Burrow. Sample 9 is by rapid rock analysis, analysts include Paul Elmore, Sam Botts, 

13-15, Leon A. Bradley; 9, John C. Hamilton; 16-19, Bruce W. Lauthom. Nd, not determined; 0, looked for but 

Amphibolite in bio- Microcline Boulder Creek Granodiorite tite cordierite gneiss gneiss and associated rocks Hornblende diorite Homblendite 

8 9 *10 *11 *12 13 14 15 16 17 18 19 20 

Chemical analyses, in weight percent 

44.47 48.6 44.1 46.1 45.2 43.43 42.84 43.13 43.85 46.42 44.48 49.38 52.89 

11.58 8.4 11.7 12.3 11.4 10.29 10.44 10.57 10.12 10.14 10.27 7.32 5.05 

4.85 3 1

25.5 1

21.5 118.6 5.93 6.19 5.99 5.80 4.09 4.83 2.94 2.01 

12.51 9.6 11.80 11.71 11.62 11.61 10.09 10.22 8.19 5.40 

10.36 15.7 5.9 9 10 10.79 10.37 10.66 10.92 13.40 12.34 15.52 18.67 

11.47 10.4 10.7 11.4 11.6 11.85 11.83 11.83 11.98 11.26 12.10 12.49 12.69 

1.07 1 1.30 1.11 1.16 1.20 1.19 1.09 1.10 1.16 1.17 .67 .44 

.38 .22 1.37 1.26 1.34 1.17 1.34 1.31 1.14 .53 1.07 .41 .27 

1.89 2

2.3 1.70 1.64 1.57 1.69 1.84 1.62 1.54 2.16 

.03 Nd .02 .03 .08 .05 .00 .02 .01 .00 

.78 .36 1.13 1.20 1.24 .96 .83 1.06 .74 .29 

.01 .12 .02 .31 .06 .08 .01 .11 .04 .00 

.34 .31 .37 .46 .40 .31 .30 .30 .21 .18 

.05 Nd .14 .13 .14 .10 .06 .17 .04 Nd .07 Nd .25 .35 .27 .32 .17 .30 .44 .06 

99.86 100.01 100.09 100.03 99.96 100.03 100.30 100.06 99.94 100.11 

.04 .14 .18 .14 .15 .08 .17 .20 .03 

99.82 100.01 99.95 99.85 99.82 99.88 100.22 99.89 99.74 100.08 

Semiquantitative spectrographic analyses, in parts per million 

(Looked for but not found- Ag, As, Au, B, Bi, Cd, Ge, Hf, In, Li, Pd, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, Zn) 

.30 30 50 50 50 70 50 150 200 50 

0 0 3 5 2 2 0 2 0 0 

50 50 50 50 50 70 70 50 30 30 

300 500 70 100 100 150 300 200 700 150 

30 30 20 2 10 30 20 15 20 15 

20 <20 20 20 20 30 20 30 15 7 

0 30 0 0 70 0 0 70 0 0 

0 0 0 0 7 0 0 0 0 0 

0 0 10 10 10 10 10 10 0 0 

0 0 0 0 200 0 0 200 0 0 

70 500 100 100 100 150 300 300 500 500 

0 70 0 0 0 0 0 0 0 20 

70 50 70 150 150 100 70 100 20 70 

20 50 70 100 100 100 50 200 500 100 

500 500 200 300 300 500 300 200 200 150 

30 15 150 300 300 30 30 100 0 30 

3 0 10 20 20 3 3 7 0 3 

20 15 50 50 100 70 30 70 0 30 

Mineral formulas, number of ions on the basis of 24 oxygens (0, OH, F, Cl) 

6.581 6.960 6.603 6.624 6.645 6.497 6.455 6.486 6.549 6.741 6.583 7.120 7.406 

1.419 1.040 1.397 1.376 1.355 1.503 1.545 1.514 1.451 1.259 1.417 .880 .594 

8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 

.601 .377 .668 .708 .620 .311 .309 .359 .331 .476 .375 .364 .239 

.087 .039 .135 .130 .133 .127 .136 .140 .108 .091 .118 .080 .031 

.540 .323 '.552 3.643 3 .658 .668 .702 .678 .652 .447 .538 .319 .212 

2.285 3.351 1.317 1.928 2.191 2.406 2.329 2.389 2.431 2.900 2.722 3.336 3.897 

1.548 1.150 32.317 31.682 31.402 1.476 1.476 1.461 1.450 1.225 1.265 .988 .632 

.043 .038 .047 .059 .051 .039 .037 .038 .026 .021 

5.10 5.28 4.99 5.09 5.00 5.04 5.01 5.08 5.01 5.18 5.06 5.11 5.03 

.307 .278 .377 .309 .331 .348 .348 .318 .318 .327 .336 .187 .120 

1.819 1.596 1.717 1.755 1.827 1.899 1.910 1.906 1.917 1.752 1.919 1.930 1.904 

.072 .040 .262 .231 .251 .223 .258 .251 .217 .098 .202 .075 .048 

2.20 1.91 2.36 2.30 2.41 2.47 2.52 2.48 2.45 2.18 2.46 2.19 2.07 
14 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

TABLE 4. - Chemical and spectrographic analyses 

Rock name ________________________ _ Amphibolite and hornblende-plagioclase gneiss in the rock unit hornblende gneiss 

Sample No. ________________________ _ 

Mineral formulas, number of ions on the basis of 24 oxygena (0, OH, F, Cl)- Con. 

OH ------------ F --------------

Cl -------------

100Mg/(Mg+Fe+'+Fe+3 +Mn) ______________ 

*Partial analysis only. 1

Total iron reported as FeO. 'Total H,O reported only. 

1.869 

.086 

.010 

1.97 

35.6 

1.998 1.959 .024 .029 .003 .003 

2.03 1.99 

34.9 34.3 

1.880 1.909 1.841 1.890 

.038 .051 .057 .028 

.003 .003 .013 .008 

1.92 1.96 1.91 1.93 

40.1 55.9 48.5 46.1 

3Estimated, using similar hornblende composition for breakdown of FeO and Fe203 from total reported FeO. 

Sample No. Lab. No. 

1 - ------------ 0100966 2 ------------- 0102173 3 ------------- 0102174 4 ------------- 0102175 

5 ------------- 0102176 6 ------------- 0102177 7 ------------- 0102178 

Sample No. Lab. No. 

8 ------------- 0100968 9 -------------- 161528 

304835 

10 ------------- 0132492 11 ------------- 0132491 12 ------------- 0132489 13 ------------- 0102179 

Sample No. Lab. No. 

14 ------------- 0102180 15 ------------- 0102181 16 ------------- 0102510 17 ------------- 0102509 18 ------------- 0102512 19 ------------- 0100967 20 ------------- 0100504 

decrease, however the fluctuations in Alz03 are not as great as in the oxides FeO and MgO. 

Trace elements in hornblendes (table 4) vary in relation to rock type. 

Barium, chromium, nickel, strontium, and vanadium are consistently 

high in all hornblendes. Nickel and niobium are more abundant in the 

igneous hornblendes with one exception- sample 15. In sample 9, the 

trace-element content is more characteristic of the igneous 

hornblendes. Lanthanum and lead found in igneous hornblendes occur 

in sample 9 as well. 

In a perusal of the hornblende mineral formulas (table 4) certain 

characteristics become evident: 

"Z" site. - The amount of tetrahedrally coordinated aluminum 

varies widely due to the high silica content of some hornblendes. In the 

igneous hornblendes alumina values for granodiorite range from 1.38 to 

1.54; in diorite, from 1.23 to 1.42; and in homblendite, from 0.59 to 0.88. 

Low alumina-high silica values in the hornblendites are probably due 

to the presence of the end-member tremolite. In the metamorphic 

gneisses, alumina values range from 1.04 to 1.80. Samples 9, 19, and 20, 

because of their low iron and alumina and high silica and magnesia content (table 4), probably belong to the "rare" group described by Dodge, 

Papike.,and Mays (1968, p. 407) and Nockolds (1941, p. 457). Nockolds 

proposed that the hornblende may have formed at the expense of preexisting hornblende and pyroxene by the action of later fluids. 

"Y" site. - The magnesium and iron cation substitutions in the octahedral site are gross replacements of Fe+2 by Mg (fig. 4). In the plot of 

Fe+2:Mg, metamorphic hornblendes plot below a Mg of 2.5 and above 
HORNBLENDE 15 

of hornblende and mineral formulas - Continued 

Amphibolite in bio- Microcline tite cordierite gneiss gneiss Boulder Creek Granodiorite and associated rocks Hornblende diorite Hornblendite 

------------------------ 9 *10 *11 *12 13 14 15 16 17 18 19 20 

Mineral formulas, number of ions on the basis of 24 oxygens (0, OH, F, Cl) -Con. 

1.836 2.197 1.676 1.618 1.495 1.684 1.782 1.599 1.481 2.017 .033 .118 .167 .128 .151 .078 .140 .201 .027 .013 .036 .033 .036 .025 .015 .043 .010 

1.88 2.20 1.83 1.82 1.66 1.86 1.88 1.78 1.69 2.04 

51.7 68.9 31.4 45.4 51.5 52.3 51 52.2 53.2 62.9 59.6 71.4 81.8 

an Fe of 1.5; most igneous hornblendes plot at Mg 2.0 or above and 

below an Fe+2 of 1.5. Titanium and manganese influences on the 

hornblende structure are not readily understood, but [here] titanium in 

the unit cell in hornblende from gneiss appears to be dependent on the 

mineral assemblage and is lower in the hornblendes associated with 

biotite and sphene. 

"X" site. -In all samples but No. 9, calcium plus sodium equals two 

positions in the hornblende structure. In sample 9, there is excess 

hydrogen and, because the "A" site, as believed by some, can accommodate hydrogen ions (two hydrogen ions are counted as one in 

calculating the cation numbers), the excess hydrogen values are 

probably in the "A" site as H30+ (hydronium ion) (?) thus balancing 

the "X" position (2) and the OH, F, Cl position. 

OH, F, Cl sites.- There are two hydroxyl positions and, according to 

Phillips (1963, p. 701), if there are not enough hydrogen atoms in the 

OH position and if there is no error in the water analysis, the balance is 

made up with oxygen atoms. Ferric ions equal to this number of oxygens are required in the "Y" site to balance the substitution (accomplished by the oxidation of ferrous ions to ferric iron). In table 4 

there is an inverse relationship between OH- and Fe+3 values (the 

higher the OH - 1, the smaller the Fe+3 values) except in sample 19. The 

water analysis in sample 19 may be low inasmuch as there is no appreciable increase in Fe+3 even though OH- values are low. 

PHYSICAL PROPERTIES OF HORNBLENDE 

Physical properties- refractive indices, birefringence, pleochroism, 

unit-cell parameters, and density- for hornblendes measured are compared in table 5. In a general way the refractive indices of the 

hornblendes vary systematically with compositional changes (fig. 

5A -C). A plot of indices of refraction against the replacement of Mg by 

Fe+2

, expressed as the ratio 100 Mg:Mg+ Fe+2+ Fe+3+ Mn (fig. 5D), 

shows a well-defined correlation which is similar to the magnesium-iron 

substitution observed by Binns (1965, p. 314). The scatter of points in 

figure 5D indicates the importance of other ionic substitutions. All but 
16 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

1-

z 

w 

u 

0::: 

GROUP 2 4 5 

~ 5r-------------------------~-+---------------r----+---~--1 

II 

(!) 

5L---~-L~--L--L~--L--L~~~~~--L-~~--~~~~--~~ 

2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 

SAMPLE NUMBER 

FIGURE 3. - Comparison of major oxides in hornblendes, grouped according to the source 

rocks. Group 1, amphibolite and hornblende gneiss; 2, microcline gneiss; 3, Boulder 

Creek Granodiorite; 4, hornblende diorite; 5, hornblendite. For chemical analyses of 

numbered samples, see table 4. 

two plots on each of the graphs in figure 5 fall into either an igneous or a 

metamorphic field. Sample 9, an amphibolite, generally plots in the ig-
3.5 

:J 

.....J 

~ 3.0 

t: 

z 

:::> 

a:: 

~ 2.5 

en 

~ 

~ <( 

-2 0 N . 

+ 

0' 

~ 

1.5 

HORNBLENDE 17 

0.5 1.0 1.5 2.0 2.5 3.0 

Fe+2 (ATOMS PER UNIT CELL) 

FIGURE 4. - Replacement of Fe+2 by Mg+2 in hornblende. 0, Boulder Creek 

Granodiorite; A, hornblende diorite; •, hornblendite;Q, hornblende gneiss;0, 

microcline gneiss. 

neous field while sample 1 plots in both the igneous and the 

metamorphic fields. 

Pleochroism was observed in oils and recorded in table 5. The green 

or blue-green color of hornblende in metamorphic gneiss is commonly 

referred to as indicative of high-grade metamorphism, and the browner 

colors are reserved for the highest grades of metamorphism. With rising 

temperatures, titanium (Engel and Engel, 1962) and, to a certain extent, ferric iron (Henderson, 1968) produce the browner shades. In the 

rocks of this area many of the hornblendes must share titanium with 

biotite and sphene; therefore, color as a temperature indicator must be 

used with caution. Correlation of birefringence, 2V, and extinction 

angles in hornblende with rock types is not possible. These physical 

properties vary between homblendes with no particular regard to composition. 
Sample No. 

a I 

(31 

Indices 

'Yl -------------- Birefringence1 

2Va(-) 

Z/\c 

ll' 

{3 

Pleochroism 2 

(in oil) 

'Y--------------- Density (calculated) ___ _ Density (measured) ___ _ 

a, 1\ 

b, 1\ c, 1\ 

./3° ,-degree a sin {3 ___________ _ 

Cell volume (A") _____ _ 

1.669 

1.678 

1.685 

.016 74°±1° 16° 

po 

dyg 

, grg 

3.35 

Nd 

9.868±0.004 

18.107± .009 5.30± .002 104°32'±1.1' 9.552 915.6± .37 

TABLE 5. - Optical and physical properties of hornblendes 

[Nd, no data] 

1.668 

1.684 

1.691 

.023 62°±1 ° 18° 

lo 

dyg 

mg 

3.30 

3.30 

9.862±0.002 

18.121± .003 5.324± .001 104°34' ± .8' 

9.545 919.2± .26 

1.662 

1.669 

1.688 

.026 680±40 

16° 

dy 

dylg 

grbg 

3.32 

3.32 

9.862±0.004 

18.102± .007 5.335± .002 104°35'±1.1 9.544 920.2± .39 

4 

Optical properties 

1.664 and 1.672 1.674 and 1.684 1.682 and 1.692 .018-.020 

58°-79° 

15°-16° 

mgny 

~b~ 3.28 

3.29 

1.652 

1.661 

1.673 

.021 70°±2° 13° 

pyg 

grg 

grbg 

3.20 

3.21 

Unit-cell parameters 

9.874±0.002 

18.o75± .004 5.292± .001 104°31' ± .09' 

9.559 912.9± .28 

9.863±0.002 

Hl.096± .004 5.319± .002 105°05' ± 1.2' 

9.523 917± .34 

6 

1.656 and 1.660 1.668 and 1.671 1.676 and 1.679 .019- .021 65°-75° 

11°-13° 

po 

pg 

grg 

3.25 

3.27 

Nd 

Nd 

Nd 

Nd 

Nd 

Nd 

1.651 

1.668 

1.676 

.025 74°±3° 21° 

myeg 

db~ 

3.25 

3.25 

9.858±0.004 

18.080± .007 5.339± .001 104°31'± .9' 

9.541 919.7± .32 

8 

1.634 and 1.654 1.649 and 1.668 1.657 and 1.676 .022-.025 

74°-86° 

16°-18° 

gry 

dylg 

bg 

3.29 

Nd 

9.816±0.002 

18.044± .007 5.334± .002 104°36'± .8' 

9.499 912.6± .41 

9 

1.639 and 1.648 1.651 and 1.660 1.654 and 1.668 

.018 78°±1° 20° 

grg 

pyeg bg 

3.19 

Nd 

9.834±0.002 

18.039± .. 006 5.304± .002 104°33'±1' 9.518 902.2± .38 

1-4 

00 

:I: 

0 

~ &j 

~ t:.:rj 

r:n 

~ a:: 

= 0 

:I: 

6 

~ t:.:rj 

a:: 

~ 0 

~ -

r:n 

~a:: 

(") 

0 

~ 

9 
SampleNo --------- 10 11 13 14 15 16 17 18 

Optical properties 

Indices a' -------------- 1.678 1.669 1.665 Nd 1.664 1.658 1.653 1.653 

{3' -------------- 1.692 L683 1.677 Nd 1.675 1.667 1.663 1.666 

~ -------------- 1.700 1.688 1.683 Nd 1.683 1.675 1.672 1.675 irefringence1 _______ .022 .019 .018 Nd .019 .023 .019 .022 

2Va(-) ----------- 78°±3° 66°±2° 66°±3° Nd 66o±2o 69°±3° 75°±4° 66°±2° 

ZAC------------- 23° 15°-17° 25° Nd 14° 22°-26° 17°-19° 14° 

Pleochroism" (in oil) a _______________ lo pg myeg Nd pyeg lo gry dy-lohr 

{3 --------------- dylg dyg dyg Nd dyg dyg grg dyg-po 

Y--------------- mg mg mg Nd mg grbg pg grg-lo Density (calculated) ____ 3.30 3.24 3.24 3.25 3.25 3.32 3.24 3.29 Density (measured) ____ Nd Nd 3.24 3.23 3.26 3.30 Nd Nd 

Unit-cell parameters 

~: i ============= 

9.880±0.005 Nd Nd 9.872±0.006 9.874±0.003 Nd 9.827 ±0.003 9.873±0.002 18.057± .012 Nd Nd 18.044± .002 18.079± .006 Nd 18.072± .004 18.089± .004 5.292± .004 Nd Nd 5.327± .006 5.328± .003 Nd 5.312± .003 5.320± .002 {3°,-degree __________ 104°34' ±1.5' Nd Nd 104°16' ±3.4' 105°04'±1.5' Nd 104°32' ±1.1' 105°01' ± .8' 

.a sin {3 ----------- 9.563 Nd Nd 9.568 9.534 Nd 9.51;1 9.536 Cell volume (A') ______ 912.2± .48 Nd Nd 918.8± .95 918.2± .46 Nd 911.6± .42 917.5± .34 

1

Accuracy of determination estimated as ±0.001. 2br, brown; b, blue; d, dusky; dyl, dark yellowish; g, green; gn, greenish; gr, grayish; I, light; m, moderate; o, olive; p, pale; y, yellow; ye, yellowish. 

19 

1.648 

1.656 

1.668 

.020 80°±1 ° 10°-17° 

pg 

grg 

po 

3.20 

Nd 

9.845±0.004 

18.074± .008 5.305± .004 104°34'±1.1' 

9.529 912.1± .56 

20 

1.625 

1.637 

1.645 

.020 80°±2° 

15°-17° 

grg 

pyeg bg 

3.12 

Nd 

9.839±0.002 

18.057 ± .002 5.283± .002 104°27'±1.4 9.528 907.6± .44 

::I: 

0 

~ tJ:j 

t"" 

t.%.1 

z 

~ 

t.%.1 

1--' 

<0 
20 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

1.675 

Metomorphlc 

hornblendn 

(' 

1.675 

l.&&e 

a 

)( 

l!tl.&ee 

~ 

... 

> 

j::: 

~ 1.645 a: 

... 

... 

a: 

1.635 

1.625 

1.615 0 

10 • 

lllftll- hornblende• 

a 

)( 

1.685 

1.675 

l!f I.M5 

~ 

~ j::: 

~ 1.645 

s 

a: 

1 ~· Met-phlc 

17~ oal L honllllendel 

T I 07 1

'. I I 

~· 6' 

I 

I 

I 

1 

B 

30 

.zo 

D 

FIGURE 5. -Variations in hornblende composition and its relations to refractive index 

alpha. A, Variations of ferrous iron; B, total iron (FeO+Fe20a); C, replacement of 

magnesium by iron expressed as the ratio 100 Mg : (Mg+Fe+2+Fe+3+Mn); D, 

magnesium and silicon in the unit cell in relation to refractive-index a.O, amphibolite 

and hornblende-plagioclase gneiss; 0, microcline gneiss; 0, Boulder Creek 

Granodiorite, •, hornblende diorite; •, hornblendite. Vertical dashed lines indicate 

range of refractive indices. 

Unit-cell parameters (table 5) correlate in a general way with the 

chemistry of the hornblendes. The interrelationship of iron and 

magnesium is broadly reflected in the parameters a, A and cell volume 

(A3) which are both generally reduced in size with an increase in 

magnesium over iron. No apparent correlation of b or c A, or 13 with 
HORNBLENDE 21 

composition is readily indicated. The hornblendes do have a similar 

crystal structure relationship to four of the six hornblendes investigated 

by Heritsch, Paulitsch, and Walitzi (1957), Heritsch, Bertoldi, and 

Walitzi (1959), Heritsch and Kahler (1959), and Heritsch and Reichert 

(1959) and tabulated by Deer, Howie, and Zussman (1963, p. 267). 

Densities, both measured and calculated (table 5), agree fairly well 

and are within 0.02 of one another. The lowest hornblende densities 

belong to those in the hornblendites and the highest to those in the 

hornblende gneiss. Densities are a crude measurement of the substitution of Mg for Fe+2 in hornblende. 

TEXTURAL RELATIONS OF HORNBLENDE 

Textural relations between minerals can be related more or less to the 

environment in which the minerals developed. In metamorphic rocks, 

growth of hornblende and biotite is related to the major Precambrian 

thermal event, and, in samples containing both biotite and hornblende, 

the hornblende is clearly older or the same age as the biotite. 

In hornblende gneiss, biotite is interstitial or forms overgrowths on 

hornblende. Iron oxides and (or) sphene ma.y occur along the grain 

boundaries or fracturesin hornblende (samples 2, 7), especially where 

biotite is sparse or not observed at all. In samples 1 and 6, hornblende 

has been partly replaced by pyroxene, and wide reaction zones occur in 

the hornblende at the contact with pyroxene. 

Hornblende from Boulder Creek Granodiorite, diorite, and 

hornblendite, where modified, contain new biotite as overgrowths on 

hornblende or along fractures or cleavage in hornblende. Sphene 

overgrowths on magnetite in these sam.e rocks are common and appear, 

also, to be related to the regional metamorphism. In general the 

hornblende in the diorite and hornblendite (samples 17-20) shows the 

least replacement. In sample 18 the biotite and hornblende are in 

hemihedral to euhedral crystals, free of inclusions; almost no reaction 

between mafic minerals is observed in thin section; Exsolution is finely 

developed in the hornblendes from sample 9 where oriented exsolution 

lamallae (on 100) is observed. These lamallae are best observed in 

grains immersed in oil; they are extremely narrow and appear to be exsolved cummingtonite. Binns (1965, p. 314) recognized exsolved cummingtonite in hornblendes where the "Y" group ofcations ranges from 

5.15 to 5.30 andcalcium is less than 1.75 cations per formula unit; sample 9 lies within this range. With exsolution cun:imingtonite in 

hornblendes of sample 9, it seems reasonable that the systematic 

anomalous position of sample 9 in every plot of chemical and optical 

properties may be due to a compositional change in hornblende and, 

therefore, the hornblende is related to an igneous origin rather than a 

metamorphic one. 
22 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

100 I I 

80 - -

• c -. • ~ 60 - ---.- 0'1 • • -

~+ • r 0 Q) 

0 LL. • + 40 - •• 0'1 

~ 0 -

20 r- -

0 I I 

6.0 6.5 7.0 7.5 

Si PER FORMULA UNIT 

FIGURE 6. - Composition of hornblende showing major element variations. The number 

of silicon atoms per formula unit is a direct function of the amount of aluminum in 

tetrahedral coordination.-, Boulder Creek Granodiorite; +, diorite; A, hornblendite; 

e, hornblende gneiss and amphibolite; 0, microcline gneiss. 

PETROLOGY 

Previous workers have demonstrated that the bulk chemistry of the 

host rocks, the mineral assemblage, and the metamorphic history are 

important factors in determining the composition of hornblende. Many 

of the hornblende-bearing rocks of this report have been subjected in 

varying degrees to a regional metamorphism; therefore, it is understandable that the major-element variation in hornblende should 

cluster (fig. 6). The homblendes from the Boulder Creek Granodiorite 

and associated rocks and the diorite plot much closer to the main 

cluster of metamorphic hornblendes than do those from homblendite. 

This may be due to composition, but habit of the hornblende and 

freshness of rock indicate that the homblendites were intrusive at a 

somewhat later time and were not subjected to the brunt of the main 

regional metamorphism 

CHEMICAL VARIATIONS IN HORNBLENDE 

IN RELATION TO HOST-ROCK COMPOSITION, 

MINERALOGY, AND METAMORPHISM 

CHEMICAL VARIATIONS IN RELATION TO HOST-ROCK COMPOSITION 

Compositions of Front Range hornblendes are related more to the 

mineral assemblage than to the bulk composition of the host rock. The 
CHEMICAL VARIATIONS IN HORNBLENDE 23 

oxides in hornblendes, however, have a tendency to mimic bulk compositions of host rocks in which biotite is lacking from the assemblage. 

These relations are shown diagrammatically in figure 7. Figure 7 shows 

the modal percentage of the minerals in the hornblende assemblages 

and the percentage of the oxides Si02, FeO (total iron), MgO, CaO, and 

Ab03 in both the hornblende and its source rock. The interrelationship 

of the mineral assemblage and host-rock composition is clearly evident 

for Si02, Ab03, and CaO in the respective minerals quartz, plagioclase, 

calcite, biotite, and pyroxene. Modal quartz and hornblende reflect the 

fluctuations of Si02 very well; Ab03 and CaO, however, reflect a partitioning of these oxides between mineral pairs other than pairs including hornblende. Ah03 content is fairly constant in hornblende; 

therefore, fluctuations in Ab03 in host rock are expressed generally in 

the mineral pair plagioclase-biotite. As the percentage of hornblende 

decreases, the Ab03 enters biotite and some plagioclase. This is typical 

of the Boulder Creek Granodiorite. No relationship exists between 

Ab03 and modal quartz or between Ah03 and normative quartz in 

metamorphic rocks, but in Boulder Creek Granodiorite, as normative 

quartz increases, so does Ab03. CaO is constant in hornblende, and increased amounts of CaO in the host rock produce some pyroxene; with a 

decrease in hornblende and plagioclase, calcite develops. Hornblende 

percentages drop drastically with a decrease in CaO and, at the same 

time, while there is an increase in plagioclase due to excess Ab03, the 

anorthite content of plagioclase decreases. Interrelation between the 

iron-magnesium minerals and host rock is not easily visualized from 

figure 7; this is probably due to the presence of the two iron-magnesium 

minerals hornblende and biotite in the mineral assemblage. In figure 10 

a near 1:1 relationship exists between FeO and MgO in biotite and 

hornblende, indicating that biotite and hornblende compositions 

should be consiqered together in relating mineral compositions to hostrock compositions. 

Na/Na+Ca values in host rock and hornblende (fig. S) also strongly 

indicate that the mineral assemblage determines composition of 

hornblende; anorthite content in plagioclase shows a straight-line 

relationship in figure SA for hornblende which is not evident in figure 

SB for host rock. Metamorphic hornblendes tend to reflect higher 

anorthite (fig. SA) values except for sample 17. In this sample the 

biotite content is considerable, and the plagioclase has absorbed the excess Ca normally absorbed by hornblende. In figure SB the Na/Na+Ca 

values clearly divide into two provinces: (1) granodiorite (samples 

12-16) and (2) the more mafic rocks (samples 1-S, 17, 1S). 

ELEMENT DISTRIBUTION BETWEEN BIOTITE AND HORNBLENDE 

Distributions of Mg, Fe, Mn, and V between coexisting mineral pairs, 

biotite (table 6), and hornblende (table 4) are fairly ordered. 
24 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

30 (f) 

~ 

20 0 

1-

1- <( 

z 

w 10 

u 1- a: z w 

Cl. w 

__J u <( 0 a: 0 w 

~ 20 Cl. 

w 

0 

10 X 

0 

0 

(f) 

0 

Q) 

LL. 

(f) 

<t 

z 

0 

a: 

__J 

<( 

1-

0 

1-

0 

0'1 

:E 

0 

0 

u 

r<'l 

0 

C\1 

<i 

-· Source [·~ ........ 1 

: : rock : ! 

r•! I I r+l 

•• I - : I 

I 

I 

I 

I 

I 

I 

I I ,.r .. .,....,, 

2 3 4 56 7 81213141516171819 

SAMPLE NUMBER 

FIGURE 7. - Comparisons of modal percentage of minerals in hornblende-bearing 

assemblages and major oxides of source rock and hornblende. Group 1, amphibolite and 

hornblende-pl~gioclase gneiss; 3, Boulder Creek Granodiorite; 4, hornblende diorite; 5, 

hornblendite. 
w 

0 

z ..... w z 

...J.w 

CD U z 0::: 0::: w 0 Q_ 

I 1-

z I 

<.9 

zo 

0.2 r0.1 rCHEMICAL VARIATIONS IN HORNBLENDE 

I I 

.. _,. 

I I 

• •••• 

25 

1 

-

-

• 01~ ~ A 

z o~----~~--~~-----4~-----+1-----+'----~ I I I I 1 

- 0.4 r-

~ ~ u w 0 u 0::: 0::: 

w z Q_ 0.3-

I 

0 ..... 

UI 

0 + <.9 z - 0 w z~ 

-0.2-

0.1 

10 

I 

20 

.3 

2.5 

e

4 

18. 

I 

30 40 

.a •6 •

7 

I I 

50 60 

ANORTHITE PERCENTAGE OF PLAGIOCLASE 

-

-

-

B 

70 

FIGURE 8. - Comparison of Na/Na +Cain host rocks and in the hornblende in relation to 

anorthite percentage of plagioclase. e, metamorphic rock and hornblende; •, igneous 

rock and hornblende. 

The distribution of tetrahedral aluminum in hornblende (table 4) is 

fairly well ordered except in hornblende samples 10-12 but these are not 

complete analyses and AlzOa may be in error. Titanium appears to be 

evenly distributed in the biotite and hornblende of the Boulder Creek 

Granodiorite only. The concentration of calcium in hornblende is high 

and reasonably consistent (table 4; fig. 8) and the small variations 

should have no appreciable effect on the iron-magnesium distribution. 

Other than manganese, magnesium, and iron, which will be discussed 

in more detail, none of the other elements are considered here, as they 

show no significant trends. 

In those samples for which analyses for both minerals have been 

made, manganese is fairly well distributed between biotite and 
t-:l 

0) 

TABLE 6. - Standard rock chemical and spectrographic analyses of biotite coexisting with hornblende 

:X 

[Nd, not determined; *,apatite included in weight percent of Alz03; N, not detected; Lj detected but below limit of deter- 0 mination. Sample localities are listed in table 1 

~ Sample No.------------------ 5 10 11 12 13 14 15 16 18 tJ:j 

t"'4 

l%.2 

Chemical composition in weight percent z 

Analyst: Vertie C. Smith t:1 

l%.2 SiOz ______________________ 37.43 35.96 37.02 37.67 37.50 37.88 37.34 37.43 36.97 00 

AbO, --------------------- *15.77 14.90 15.56 15.53 15.32 15.08 15.55 15.51 15.09 

FezO, --------------------- 2.88 3.10 1.65 3.16 2.21 2.78 2.86 3.81 3.30 ~ FeO ---------------------- MgQ ______________________ 14.98 21.06 17.87 14.34 15.21 14.51 14.52 13.33 12.69 0 12.46 8.34 11.56 13.05 13.21 12.83 12.67 13.60 14.10 ~ 

CaO ---------------------- 1.33 .09 .20 .25 .52 1.34 .47 .98 .41 :X 

K~ 

NazO ------------------------------------------- 8.01 

.34 9.33 9.36 

.23 .25 9.43 .28 9.04 .29 

8.79 

.24 9.29 

.22 8.58 .28 8.87 

.13 -

Q HzO+ _____________________ 3.88 2.89 2.61 3.02 3.41 2.85 2.98 3.55 3.31 :X HzO- --------------------- .22 .05 .03 .10 .11 .15 .04 .17 .23 

TIOz----------------------

PzOo 

MnO ----------------------

2.13 2.73 

Nd 

.20 .03 

.26 

2.08 

.04 

.44 

1.90 

.14 

.02 1.95 

.26 

.01 2.23 .05 

.30 

2.84 

.07 

.26 

1.58 

.07 

.22 

3.34 

.10 

.17 ~ l%.2 Cl __ ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Nd .10 .17 .12 .14 .13 .15 .09 .15 

F---~-------------------- .29 .80 .78 .58 .56 .81 .61 .75 .67 ~ 

99.92 99.87 99.62 99.79 99.74 99.97 99.87 99.95 99.53 l%.2 

.12 .36 '.37 .27 .27 .37 .29 .34 .31 ~ Total ______ --------- ___ 99.80 99.51 99.25 99.52 99.47 99.60 99.58 99.61 99.22 ~ 

0 

Density (calculated _____________ 2.92 3.04 2.99 2.94 2.93 2.95 2.95 

Density (measured) ------------- Nd Nd Nd Nd Nd Nd Nd 3.02 -3:-o! ~ :X Percent biotite in rock ________ - - - 2.5 15 14 14.4 17.9 14.7 14.6 12.8 20.8 -

00 

Number of ions on the basis of 24 oxygen& (0, OH, F, Cl) ~ 

0 

ffi ----------------------- 5.615 5.633 5.714 5.700 5.669 5.751 5.655 5.599 5.560 0 

M ----------------------- 2.385 2.367 2.286 2.300 2.331 2.249 2.345 2.401 2.440 t"'4 

9 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 

M ----------------------- 0.403 0.383 0.544 0.470 0.339 0.449 0.431 .333 .235 

TI ----------------------- .240 .322 .241 .216 .222 .255 .323 .178 .378 

Fe~ ---------------------- .325 .365 .192 .360 .251 .318 .326 .429 .374 
1.879 2.759 2.307 1.815 1.923 1.842 1.839 1.667 1.596 .025 .035 .057 .044 .033 .039 .033 .028 .022 2.786 1.947 2.659 2.943 2.977 2.903 2.860 3.032 3.161 

5.66 5.81 6.00 5.85 5.81 5.81 5.81 5.67 5.77 

Ca ______________ ~- _______ Na _______________________ .214 .015 .033 .041 .084 .218 .076 .157 .066 .099 .070 .075 .082 .085 .071 .065 .081 .038 K ----------------------- 1.533 1.864 1.843 1.820 1.743 1.702 1.795 1.637 1.702 

1.85 1.95 1.95 1.94 1.91 1.99 1.94 1.88 1.81 

F ________________________ .138 .396 .381 .278 .26M .389 .292 .355 .319 (') Cl _______________________ Nd .027 .045 .031 .036 .033 .039 .023 .038 ::t: 

OH ---------------------- 3.662 2.967 2.656 3.048 3.328 2.734 2.970 3.542 3.320 t.%j 

3.80 3.39 3.08 3.36 3.63 3.16 3.30 3.92 3.68 :;:: 

a 

100Mg: Mg+Fe+2+Mn ----------- 45.1 28.1 38.7 47.0 46.1 46.1 49.8 50.0 52.3 > t"l 

Semiquantitative spectrographic analyses, in parts per million < Analyst: Leon A. Bradley 

~ Ha ----------------------- 1,500 1,000 2,000 1,000 1,000 1,000 1,000 1,000 5,000 

Co ----------------------- 100 70 50 100 100 100 100 100 70 

Cr ----------------------- 500 50 50 100 200 150 150 200 30 ro .. 200 10 7 20 150 15 100 300 200 0 

G~======================= 50 100 50 50 50 50 50 50 50 z 

00 

u ----------------------- N N N N N N N 300 N ....... Nb Mo-----------~----------- _______________________ 7 7 10 5 20 5 5 7 7 z N 20 N N N N 10 N 20 Ni _____ ------------- _____ 70 15 30 100 100 100 100 300 500 ::t: 

Pb ----------------------- N 20 N N N N N 10 N 0 

Sc ______________ --------- Sr _________________ ------ 10 20 20 20 10 30 20 10 7 ~ 20 15 20 20 15 20 20 50 70 t:J:j v ________________________ 300 150 300 300 300 300 300 300 300 y ________________________ N 50 N N N 20 N N L ~ Yb _______________________ 

Zn __________ -------- _____ 3 2 3 1 2 2 N N N ~ N N N N N N N N 300 t.%j Zr ________________ ------- 70 50 50 50 30 100 70 20 N 

Sample No. Lab. No. Sample No. Lab. No. Sample No. Lab No. 

5 -------------- OWn« 12 -------------- 0102150 15 -------------- 0102151 

10 -------------- 0102143 13 -------------- 0102148 16 -------------- 0102521 11 -------------- 0102145 14 -------------- 0102149 18 -------------- 0102518 

l-...,:) 

-3 
28 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

-C/) 

~ 0.16 

0 

1-

<{ 

-

w 

a 

z 

w 0.14 

_j 

CD 

z 

0: 

0 

I 

~ 0.12 

~ 

+ 

c: 

~ 

+ 

0' 0.10 ~ 

+ 

N 

+ Q) 

lL.. 

~ 0.08 

~ 

0.04 0.06 0.08 1.0 \ 

Mn/Fe+2 + Mg +Mn+Ti IN BIOTITE (ATOMS) 

FIGURE 9. -Distribution of manganese between biotite and hornblende. 0, 

amphibolite and hornblende-plagioclase gneiss; 0, Boulder Creek 

Granodiorite; • , hornblende diorite. 

hornblende (fig. 9). Manganese, like titanium, displays a greater degree 

of equilibrium in the Boulder Creek Granodiorite samples. 

The iron-magnesium distribution in hornblende and biotite, summarized in figure 10, shows interesting trends. The homblendes of 

known igneous origin occupy the range: Mg,· 2.3 to 4.0:Fe+2, 0.5 to 1.5; 

the metamorphic hornblendes occupy the range: Mg, 1.3 to 2.5:Fe+2, 1.5 

to 2.4. Boulder Creek hornblendes lie midway between the above division which is probably indicative of the metamorphic imprint made on 
FRONT RANGE AMPHIBOLES 29 

4.0 

Igneous 

~\ hornblendes 

3.5 -;d\ 

~\ ·o-~19 (0 • 0 9 \~Igneous biotite 

:J ~\ 18•\ _J 

w ~~ 16o\12 0 13 u 3.0 0' \•17 ._ 15{{:114 

z ·1~- Q{5 011 ::> 

a:: ~\ ..... o.,..)\ ~ 2.5 16 

~ 3 /I': 

en I I o8 ~;~Metamorphic ~ 14 ,o .... ~.., X'biotite ._ 

0 0 Q',- 12 'C(6 <( ......... 

N-2.Q 7 ........... II 'O 1~0 

0 ' 

+ Cl 

~ 4o'2 3 "QO 

Metamorphic hornblendes~ ........_ _ 

1.5 

- 0.5 1.0 1.5' 2.0 2.5 

Fe+2 (ATOMS PER UNIT CELL) 

FIGURE 10. -:-· Distribution· of Fe+2 versus Mg+2 between hornblende and 

biotite. Sample-10 hornblende, F'e+2 estimated. 0, amphibolite .and 

hornblende-plagioclase gneiss; 0, microcline gneiss; D, Boulder Creek 

Granodiorite; • , hornblende diorite; •, hornblendite. 

3.0 

the rocks. Biotites a'ssociated· 'with the above hornblendes mimic 

hornblende trends (fig. 10). . , 

FRONT-RANGE AMPHIBOLES AS INniCATORS 

OF OXIDATION DURING. METAMORPHISM ,· 

The thermal environment in which hornblende developed is in 

general characteristic of the sillimanitic-potassic feldspar grade of 

regional metamorphism. This metamorphism is related principally to 

the major deformation, and because emplacement of the Boulder Creek 

Granodiorite, occurred late in ·the major.regional metamorphic event, 

hornblendes from the granodiorite were modified in part .by: ,the 

metamorphism. Hornblendes least affected by metamorphism are 

those from some of the diorites and hornblendites. In some outcrops, 
30 HORNBLENDES FROM HIGH-GRADE METAMORPIDSM, COLO. 

0.80 

w 19. 

0 

z 

w 

....J 

CD 

z 17. 0.70 a: 

0 186. 

J: 

z 

016 -CJ) 015 

014 z 

0 0.60 ..... 

<{ 

u 

C\J - CITY, 

+ NEDERLAND Q) 

u.. 0.50 + 

0' 

~ 

........ 

0' 

~ BUTTES 

0.40 

0.10 0.20 0.30 0.40 0.50 

Fe+3/ Fe+3+ Fe+2 (CATIONS) IN ROCK 

FIGURE 11. - Magnesium (Mg/Mg+ Fe+2) in hornblende versus degree of oxidation 

(Fe+3

fFe+3 + Fe+2) in host rock. 0, amphibolite and hornblende-plagioclase gneiss; D, 

Boulder Creek Granodiorite; & , hornblende diorite; •, hornblendite. 

diorite and hornblendite bodies exhibit few metamorphic structures, 

though in others they are clearly evident, perhaps indicating their 

emplacement at different times; certainly, some are younger than the 

major metamorphism. 

One control of metamorphism is the degree. of oxidation that has occurred in a host rock and its contained amphibole (table 7; figs. 11, 12). 

Plots of Mg (Mg/Mg+ Fe+2) in hornblende versus degree of oxidation 

(Fe+3(Fe+3+ Fe+2) in rock indicate that, with increasing Mg in 

hornblende from metamorphic rocks, the Fe+3, or the oxidation degree, 

in host rock increases (Bard, 1970, p. 127), but Front Range 

metamorphic hornblendes separate into two areas, possibly due to 

different degrees of metamorphism. In figure 11, a break at 0.60 cations 

of Mg/Mg+ Fe+2 places all the metamorphic hornblendes except sample 

5 below 0.60 cations and all igneous homblendes above this value. With 
3.8 

3.4 

-CJ) 

2 0 3.0 

g 

w 

0 

z 

w 2.6 

...J 

CD 

z 

a: 

0 

~ 

~ 2.2 

0' 

~ 

1.8 

1.4 

FRONT RANGE AMPHIBOLES 31 

I. 0 ......___.....___ _ _.___-L.-_ __.__ __ ---~.-_----L._---L._--'--oi._----JL-----J 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

Fe+3 HORNBLENDE (ATOMS) 

FIGURE 12. - Plot of magnesium versus ferric iron showing degree of oxidation in some 

hornblendes. 0, amphibolite and hornblende-plagioclase gneiss; 0, Boulder Creek 

Granodiorite; A , hornblende diorite; •. hornblendite. m, magnetite; h, hematite; i, ilmenite. 
32 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

increasing Mg in igneous homblendes, there is erratic distribution of 

Fe+3 in host rock, indicating Mg in hornblende is not completely controlled by the Fe+3 content of host rock, but there is a closer correlation 

between Fe+3 and Mg in hornblende and host rog_k within igneous rock 

types. -

Bard (1970) indicated that with Mg enrichment and Fe+2 impoverishment in hornblende there is a rise in Fe+3 which holds true for Front 

Range metamorphic hornblendes but is not true for known igneous 

homblendes (table 7); in fact, the reverse is true, with increasing Mg 

and decreasing Fe+2, Fe+3 decreases (f1g. 11). This reversal in ~e+ 3 for 

the igneous hornblendes no doubt is an- indication of their igneous 

origin. 

TABLE 7. - Mg, Fe+2, Fe+3 ions for analyzed hornblendes, table 4 

[Arrow indicates direction of enrichment] 

Sample No. Mg+2 Fe~- Fe+3 Fe total/Fe total+ Mg oxidation-reduction ratios 

IGNEOUS HORNBLENDES 

20 3.897 . 0.632 0.212 0.319 

19 3.326 .988 .319 .418 

17 2.900 1.225 .447 .514 

18 2.722 1.265 .538 548 

16 2.431 1.450 .652 .620 

13 2.406 1.476 .668 .622 

15 2.389 1.461 .678 .623 

14 2.329 1.456 .702 .633 

METAMORPHIC HORNBLENDES Ralston Buttes Area 

4 1.757 t 1.972+ 0.612t 0.728 

3 1.528 2.200 .608 .774 

2 1.523 2.308 .565 .770 

Morrison and Nederland-Central City Area 

8 2.285t 1.548 + 0.540t 0.626 

6 2.115 1.755 .440 .654 

7 2.024- 1.899 .421 .676 

In figure 12, igneous hornblendes have a definite trend while 

metamorphic hornblendes cluster as to areas (fig. 11); samples 1, 6-8, 

Morrison, Central City, Nederland; and 2-4, the Ralston Buttes area. 

The orderly relationship along the igneous trend (fig 12) is- not displayed by the metamorphic hornblendes; with increasing Fe+3 in igneous hornblendes, there is a regular decrease in Mg, and the ratios 

Fe total /Fe total + Mg for these horriblendes decrease. The trend is 

somewhat similartQ that .observeqbyCzamanske and Wones (1970) for 

hornblende Fe/Fe+ Mg ratios to monitor oxidation-reduction reactions 

in monzonite, .granodiorite, .. arici granite during magmatic processes. 

This trend-in the Boulder Creek hornblendes appears relic and is indicative of their igneous origin. This trend also indicates that under 

regional metamorphic conditions the Boulder Creek Granodiorite and 
.REFERENCES CITED 33 

associated rocks reacted as a single unit to the partial pressures·ofoxygen. Hornbiende gneiss and amphibolite, because of varyingFe*3 '(fig. 

12), seem to have behayed as much more narrowly defined units closed 

to oxygen. Relationships between Fe+3 and Mg or Fetotal /Fe total + Mg in 

biotite are not nearly as well defined and probably reflect the . partitioning of iron, not used by the hornblendes, between biotite and the 

iron oxides. 

Iron oxides, as observed in. polished sections and as shown in figure 

12, are predominantly ilmenite below 2.5 .atoms of Mg+2 and 0,60 atoms 

of Fe+3, followed by lesser amounts of hematite and some magnetite. 

Above 2.2 atoms of Mg+2 and above 0.60 atoms of Fe+3

, the iron oxides 

are dominantly magnetite: In table 7, samples 14-20, the lower 

oxidation-reduction ratios belong to samples· that are · dominantly 

magnetite-bearing. 

CONCLUSIONS 

Regional metamorphism has left a definite and orderly imprint on 

the hornblE~ndes in the Boulder Creek Granodiorite. Decided trends indicate thathornblende composition is dependent primarily on mineral 

assemblage and secondarily on host rock composition in a potassic 

feldspar-sillimanite grade of regional .. metamorphism. Regional 

metamorphism is responsible for some element variation in hornblende, 

principally due to the activity of oxygen fugacity. Fe+3 appears to 

regulate the iron-magnesium content in hornblende and, where it coexists with biotite, in biotite as well .. 

REFERENCES CITED 

Ball, S. H., 1906, Pre-Cambrian rocks of the Georgetown quadrangle, Colorado: Am. 

Jour. Sci.,· 4th Ser., v. 21, p. '371-389. 

Bard, J. P., 1970, Composition :of hornblendes formed .during the Hercynian progressive 

metamorphism of the Aracena metamorphic belt [SW Spain]: Contr. Mineralogy 

and Petrology, v. 28, no. 2, .p. 117-134. 

Binns, R. A., 1965, Mineralogy of metamorphosed basic rocks from the Willyama 

Complex, Broken Hill district, New South Wales- [Pt.] 1, Hornblendes: Mineralog·. 

Mag., v~ 35, no .. 270, p: 306-326. 

Chayes.-· F., 1952, Relations: between .composition and indices of refraction in natural 

plagioclase: Am. Jour. Sci., Bowen Volume, P•'85-105. 

Cherkasov, Yu. A., 1955a, Dispersion method for measuring the index of refraction: Sbornik Nauchno-Tekh. Inf., Ministerstvo Geologii Okhrany. Nedr., v; 1, p. 140-142 (in 

Russian). 

_1955b, A new variant of the immersion method: Vses. Nauchno-lssled. Inst. Mineral. 

Syr'ya, p. 52-57 (in Russian). 

_1957, Oprimenenii "fokal 'nogo ekranirovaniia" pri izmereniiakh pokazatelei 

prelomleniia immersionnym metodom [Application of "focal screening" to measurement of indices of refraction by immersion method], in Rozhkova, E. V., ed:, 

Sovremennye metody mineralogicheskogo issledovaniia gornykh porod, rud, i 

mineralov [Modern methods of mineralogic investigation of rocks, ores, and 
34 HORNBLENDES FROM HIGH-GRADE METAMORPHISM, COLO. 

minerals]: Gosudarstvennoe Nauchno-Tekh. lzdatel'stvo Literatury po Geologii 

Okhrane Nedr. Moscow, p. 184-207. Translation by Ivan Mittin in Internat. Geology 

Rev., 1960, v. 2, p. 218-235. 

Colville, P. A., Ernst, W. G., and Gilbert, M. C., 1966, Relationships between cell 

parameters and chemical compositions of monoclinic amphiboles: Am. Mineralogist, 

v. 51, nos. 11-12, p. 1727-1754. 

Czamanske, G. K., and Wones, D. R., 1970, Amphiboles as indicators of oxidation during 

magmatic differentiation: Geol. Soc. America Abs. with Programs, v. 2, no. 7, p. 531. 

Deer, W. A., Howie, R. A., and Zussman, J., 1963, Chain silicates, v. 2 of Rock-forming 

minerals: New York, John Wiley & Sons, 379 p. 

Dodge, F. C. W., Papike, J. J., and Mays, R. E., 1968, Hornblendes from granitic rocks of 

the central Sierra Nevada batholith, California: Jour. Petrology, v. 9, pt. 3, p. 

378-410. 

Engel, A. E. J., and Engel, C. G., 1962, Hornblendes formed during progressive 

metamorphism of amphibolites, northwest Adirondack Mountains, New York: Geol. 

Soc. America Bull., v. 73, no. 12, p. 1499-1514. 

Evans, H. T., Jr., Appleman, D. E., and Handwerker, D. S., 1963, The least squares 

refinement of crystal unit cells with powder diffraction data by an automatic computer indexing method [abs.]: Am. Crystallographic Assoc., Ann. Mtg., Cambridge, 

Mass., 1963, Program and Abstracts, E-10, p. 42-43. 

Gable, D. J., 1968, Geology of the crystalline rocks in the western part of the Morrison 

quadrangle, Jefferson County, Colorado: U.S. Geol. Survey Bull. 1251-E, 45 p. 

_1969, Geologic map of the Nederland quadrangle, Boulder and Gilpin· Counties, 

Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-833 [1970]. 

_1972, Geologic map of the Tungsten quadrangle, Boulder, Gilpin, and Jefferson Counties, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-978. 

Gable, D. J., and Sims, P. K., 1969, Geology and regional metamorphism of some highgrade cordierite gneisses, Front Range, Colorado: Geol. Soc. America Spec. Paper 

128, 87 p. 

Gude, A. J., 3d, and Hathaway, J. C., 1961, A diffractometer mount for small samples: 

Am. Mineralogist, v. 46, nos. 7-8, p. 993-998. 

Hedge, C. E., Peterman, Z. E., and Braddock, W. A., 1967, Age of the major Precambrian 

regional metamorphism in the northern Front Range, Colorado: Geol. Soc. America 

Bull., v. 78, no. 4, p. 551-558. 

Henderson, C. M. B., 1968, Chemistry of hastingsitic amphiboles from the Marangudzi 

igneous complex, Southern Rhodesia: lnternat. Mineralog. Assoc., 5th, Cambridge, 

England, 1966, Papers and proceedings, p. 291-304. 

Heritsch, H., Bertoldi, G., Walitzi, E. M., 1959, Strukturuntersuchung an einer 

basaltischen Hornblende vom Kuruzzenkogel siidlich Fehring, Steiermark: Min. 

Petr. Mitt. (Tschermak), v. 7, p. 210. 

Heritsch, H., and Kahler, E., 1959, Strukturuntersuchung an zwei Kluftkarin thinen. Ein 

Beitrag zur Karinthinfrage: Min. Petr. Mitt. (Tschermak), v. 7, p. 218. 

Heritsch, H., and Reichert, L.. 1959, Strukturuntersuchung an einer basaltischen 

Hornblende von Cernosin, CSR: Min. Petr. Mitt. (Tschermak), v. 7, p. 235. 

Heritsch, H., Paulitsch, P., and Walitzi, E. M., 1957, Die Struktur von Karinthin und 

einer barroisitschen Hornblende: Min. Petr. Mitt. (Tschermak), v. 6, p. 215. 

Lovering, T. S., and Goddard, E. N., 1950, Geology and ore deposits of the Front Range, 

Colorado: U.S. Geol. Survey Prof. Paper 223, 319 p. 

Nockolds, S. R., 1941, The Garabal Hill-Glen Fyne igneous complex: Geol. Soc. London 

Quart. Jour., v. 96, pt. 4, p. 451-511. 

Peck, L. C., 1964, Systematic analysis of silicates: U.S. Geol. Survey Bull. 1170, 89 p. 

Phillips, R., 1963, The recalculation of amphibole analyses: Mineralog. Mag., v. 33, no. 

263, p. 701-711. 
REFERENCES CITED 35 

Schoen, Robert, and Lee, D. E., 1964, Successful separation of silt-size minerals in heavy 

liquids, in Geological Survey research 1964: U.S. Geol. Survey Prof. Paper 501-B, p. 

B154-B157. 

Shapiro, Leonard, and Brannock, W. W., 1962, Rapid analysis of silicate, carbonate, and 

phosphate rocks: U.S. Geol. Survey Bull. 1144-A, 56 p. 

Sheridan, D. M., Maxwell, C. H., and Albee, A. L., 1967, Geology and uranium deposits 

of the Ralston Buttes district, Jefferson County, Colorado, with sectioni on Paleozoic 

and younger sedimentary rocks, by Richard Van Horn: U.S. Geol. Survey Prof. Paper 

520, 121 p. 

Sims, P. K., and Gable, D. J., 1967, Petrology and structure of Precambrian rocks, Central City quadrangle, Colorado: U.S. Geol. Survey Prof. Paper 554-E, 56 p. 

Wells, J. D., 1967, Geology of the Eldorado Springs quadrangle, Boulder and Jefferson 

Counties, Colorado; U.S. Geol. Survey Bull. 1221-D, 85 p. 

Wilcox, R. E., 1959, Use of the spindle stage for determination of principal indices of 

refraction of crystal fragments: Am. Mineralogists, v. 44, nos. 11-12, p. 1272-1293. 

Wilcox, R. E., and Izett, G. A., 1968, Optic angle determined conoscopically on the spindle stage- [Pt. 1], Micrometer ocular method: Am. Mineralogist, v. 53, nos. 1-2, p. 

269-277. 

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