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UNITED STATES DEPARTMENT OF THE INTERIOR

MINERALS OF THE 

MONTMORILLONITE GROUP

THEIR ORIGIN AND RELATION 

TO SOILS AND CLAYS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 205-B

UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

Professional Paper 205-B

MINERALS OF THE 

MONTMORILLONITE GROUP

THEIR ORIGIN AND RELATION 

TO SOILS AND CLAYS

BY 

CLARENCE S. ROSS AND STERLING B. HENDRICKS

Shorter contributions to general geology, 1943-44

(Pages 23-79)

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1945

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

Washington 25, D. C. - Price 55 cents

CONTENTS

Abstract.............

Introduction..........

Methods of study.

Previous work....

Acknowledgments. 

Nomenclature........

Montmorillonite..

Beidellite........

Page

........................... 23

............................ 23

........................... 24

........................... 24

........................... 25

........................... 25

........................... 25

........................... 26

Saponite........................................ 26

Hectorite....................................... 27

Nontronite...................................... 27

Volchonskoite................................... 28

Kaolin minerals.................................. 28

Hydrous micas (illite)............................ 28

Mixed-layer minerals................................. 29

Crystal structure..................................... 29

Location and description of analyzed-clays.............. 30

Montmorillonite-beidellite series................... 30

Nontronites..................................... 32

Samples from soils and chromium-bearing samples... 32

Saponite-hectorite series .......................... 32

Samples containing silica or an iron-bearing impurity 32

Chemical analyses.................................... 34

Impurities in clays................................... 36

Minor constituents................................... 37

Base exchange....................................... 37

Physical chemistry of base exchange................ 37

Determinations of exchangeable bases.............. 38

Mineralogy of the montmorillonite group................ 39

General relations................................. 39

Methods of calculation of mineral formulas.......... 41

Derivation of formulas............................ 44

Montmorillonite-beidellite series............... 44

Nontronite.................................. 45

Montmorillonite-beidellite-nontronite series...... 46

Page

Mineralogy of the montmorillonite group—Continued. 

Derivation of formulas—Continued.

Saponite-hectorite series...................... 46

Suggested formulas........................... 47

Thermal studies...................................... 48

Essential hydroxyl............................... 48

Interlayer water................................. 52

Optical properties.................................... 54

Methods of determination......................... 54

Determinations of indices of refraction.............. 55

Pleochroism..................................... 57

Electron-microscope photographs....................... 57

Synthesis........................................... 58

..... 60

..... 60

..... 61

..... 61

..... 62

..... 62

..... 63

..... 63

..... 64

..... 64

..... 64

Physical properties........................... 65

Definition........................... ..... : . 65

Mode of occurrence.......................... 65

Ordovician bentonite......................... 67

Volcanic materials at Magnet Cove, Ark............ 67

Clays of the geyser region in Yellowstone Park...... 68

Vein and gouge clays............................. 69

Pyrophyllite and talc................................. 70

Bibliography........................................ 72

Index............................................... 79

Origin and mode of occurrence................

General features........................

Soils and rock weathering................

Discussion.........................

Australian soils.....................

Piedmont soils......................

Java soils..........................

Glacial soils........................

Summary of soils and rock weathering. 

Bentonites.............................

Structure of parent glass.............

Ill
IV CONTENTS

ILLUSTRATIONS

Page

PLATE 1. Alteration of volcanic materials to montmorillonite and kaolinite............................................ 58

2. A, B, and C, Alteration of volcanic materials to montmorillonite and kaolinite; D, Varved sedimentary clay...... 58

3. A, Bentonite showing volcanic-ash structure; B, Vein clay with clay-filled joint cracks; C, Granite gneiss with plagi-v

clase altered to beidellite; D, Plagioclase phenocryst in rhyolite partly altered to montmorillonite.............. 58

4. Bentonite showing volcanic-ash structure................................................................. 58

5. Alteration of various rock types to montmorillonite and associated minerals.................................. 58

6. Magnesium and iron-rich clays.......................................................................... 58

7. A, Beidellite replacement of molluscan shell; B, Montmorillonite from pegmatite; C and D, Electron micrograpl ""

of clays............................................................................................ 58

8. Electron micrographs of hectorite and nontronite.......................................................... 58

FIGURE 5, 6, and 7. Dehydration curves of montmorillonite clays..................................................... 49-51

8. Curves showing relation of vapor pressure to cleavage spacing in montmorillonite............................. 53

9. Curves showing relation of indices of refraction to ferric iron in minerals of the montmorillonite-nontronite'series 56

TABLES

Page

TABLE 1. Analyses of montmorillonite-beidellite series.............................................................. 34

2. Analyses of nontronite................................................................................. 35

3. Analyses of samples from soils and chromium-bearing specimens from Russia................................. 35

4. Analyses of hectorite and saponite....................................................................... 35

5. Analyses of samples containing excess silica............................................................... 35

6. Analyses of nontronites containing an iron-bearing impurity................................................ 35

7. Analyses of specimens containing chlorite, vermiculite, or other iron-bearing impurity.......................... 35

8. Analyses of hectorite and a clay from Amargosa Valley before and after treatment with ammonium acetate...... 38

9. Base-exchange capacities of some montmorillonite clays.................................................... 39

10. Relation of ions to crystal structure..................................................................... 42

11. Calculations of impurities in representative clays.......................................................... 43

12. Proportions of end members in selected samples........................................................... 46

13. Indices of refraction and birefringence as affected by the presence of ferric iron in the montmorillonite-nontroni^e

series.............................................................................................. 55

14. Indices of refraction of clays determined on films.......................................................... 57

15. Indices of refraction determined on clay grains............................................................ 57

16. Indices of refraction of minerals related to the montmorillonite group........................................ 57

17. Indices of refraction of clays dehydrated at 210° C......................................................... 57

18. Conditions of synthesis for hydrous aluminum silicates..................................................... 59

19. Analyses of bentonite and related rocks, Santa Monica Mountains, Calif...................................... 66

20. Analyses of pyrophyllite and talc...................................................................... 70
MINERALS OF THE MONTMORILLONITE GROUP

THEIR ORIGIN AND RELATION TO SOILS AND CLAYS

By CLARENCE S. Ross, Geological Survey, United States Department of the Interior, and STERLING B. HENDRICKS, Bureau of Plant Industry, Soils, and Agricultural Engineering, United States Department of Agriculture.

ABSTRACT

A detailed study has been made of the montmorillonite group 

of clay minerals, one of the most widely occurring of all mineral 

materials. Minerals of this group are exceedingly fine grained 

crystalline aggregates, and they present an unusually wide range 

in composition. Essential information on the group was obtained 

only after study, by all available mineralogical methods, of more 

than 100 carefully selected specimens. This information has 

made possible the determination of the limits of variation in composition and the function of various ions in the crystal lattice. 

The underlying causes for the important property of base exchange have been established. This property plays a prominent 

part in determining the physical properties of minerals of the 

montmorillonite group, and in particular their behavior towards 

water.

Interpretations of chemical analyses are based on knowledge 

of the crystal lattice of montmorillonite and related minerals. 

This lattice is of a layer type in which silicon and aluminum ions 

have tetrahedral coordination with respect to oxygen. Aluminum, 

ferric and ferrous iron, magnesium, lithium, chromium, manganic 

and manganous manganese^ and some other ions occupy positions 

having octahedral coordination with respect to oxygen. Exchangeable cations are on the surfaces of the silicate layers, and 

their amounts are determined by the excess negative charge 

within the layer. These cations usually are sodium and calcium

Exchangeable ions

T

but potassium, magnesium, hydrogen, and aluminum may be 

exchangeable.

The composition and mineral relations of the clay minerals are 

intimately related to the conditions under which they form. In 

general, alkalic feldspars and the micas tend to alter to kaolin 

minerals, whereas ferromagnesian minerals, calcic feldspar?, and 

volcanic glasses commonly alter to members of the montmorillonite group. Presence of magnesium ions in the altering system 

is favorable for the formation of montmorillonite-type minerals, 

in many of which it is characteristically present. The formation 

of montmorillonite-type minerals is favored by alkaline conditions, and that of kaolinite by acid conditions. These relations 

are modified by climatic factors, such as the length of time during 

which weathering continues. Montmorillonite-type minerals form 

as weathering products in soils and are formed at moderate!7 high 

temperatures up to at least 200°C., in association with many 

types of mineral deposits.

Formulas of a special type are used to express the composition 

of the members of the montmorillonite group. In these formulas 

the name and the ions forming the crystal lattice are given on one 

line, and the exchangeable ions on the line above. Arroys are 

placed over the groups having a deficiency of charge, whi«h require the presence of additional base external to a silicate layer.

Name [Ions with octahedral coordination] [Ions with tetrahedral coordination]Oio[OH] 2

Na.33

Montmorillonite [AU.erM Si* 0JO [OH] 2

Beidellite AUi 7 [A1.8iSi,.i 7 ] Oi 0 [OH] 2

Na.ss

or Al 2. 22 [Ali.ooSi3.oo] Oio[OH] 2

.

+ 3 T

Nontronite Fe 2. 0 o[Al. 33Si3.67] Oi 0 [OH] 2

Nontronite (aluminian) Fe 2.i 7 [A1.83

NaT ' 3 s 

Hectorite [Mg2. 67Li. 3 3] Si4 010 [OH] 2

Saponite

Saponite (aluminian) Oio[OHj 2

INTRODUCTION

This paper on the clay minerals of the montmorillonite group presents the results of a detailed systematic 

study of an important group of minerals, whose wide 

occurrence and varied uses give them unusual interest.

The clay minerals are the dominant materials of 

shales and surficial rocks and so are among the most important structural materials of the earth's surface. They 

are the end products of weathering of a wide variety of 

rocks. Clays are the characteristic materials of soils and 

play a fundamental part in the physical and chemical 

properties of soils and soil fertility; thus, the very existence of life, both plant and animal, is related to clay 

mineralogy. The physical and chemical properties .of

clays, which constitute the most abundant raw material 

of the ceramic industry, are related to their mineral 

identity. Clay mineralogy in its relation to the physical 

properties of soils, shales, and other poorly consolidated 

rocks is an essential factor in a wide variety of engineering problems.

Progress in the science of clay mineralogy has aided 

not only in an understanding of the clay minerals, but 

also in an understanding of the large group of minerals 

characterized by platy structure—the group that includes the clay minerals. No mineral group, therefore, 

has a more fundamental bearing on so wide a variety of 

human interests.

23
24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

METHODS OF STUDY

Mineralogic studies of the clay minerals present unusual difficulties and their systematic investigation has 

therefore been long delayed. All the members of the 

montmorillonite group are exceedingly fine-grained; 

they vary in chemical composition within very wide 

limits; and they are commonly admixed with other finegrained mineral materials. Progress in methods of mineral research has tended to overcome some of these difficulties and to encourage a study of the clay minerals. 

The information furnished by chemical analyses', including base-exchange experiments, and by X-ray, 

petrologic, and thermal methods of mineralogic research 

has been brought to bear on the minerals of the montmorillonite group and has provided a means of determining the limits of variation. It also permits the presentation of chemical formulas that express relations 

within the group.

The development of a technique for studying small 

mineral grains in immersion oils improved the means for 

direct study of finely crystalline materials like the clays 

and facilitated the recognition of impurities. The perfection of X-ray methods for determining crystal structure and for studying crystalline materials in minute 

grains, however, provided the most fundamental mode 

of attack on clay problems. About the time that these 

methods were being newly applied a renewed demand 

for more definite information developed. Many ceramic 

engineers were not content with scanty knowledge of 

their most essential raw material, and students of soils 

came to see the inadequacy of purely chemical methods 

unrelated to mineral identity. The complex composition 

of most soils introduced so many variables that the preliminary study of pure minerals of the types present in 

soils seemed necessary. The marked base-exchange 

properties of bentonite and the evidence that its characteristic mineral, montmorillonite, was a widespread 

component of soils suggested its usefulness in the study 

of the fundamental property of base exchange. The 

properties and possible uses of bentonite attracted widespread interest in this material, whose mode of origin 

(alteration of glassy volcanic ash) gave a clay mineral 

that was commonly very pure or contained only fairly 

large volcanic-rock phenocrysts, thus providing ample 

supplies of an essentially pure clay mineral for experimental purposes. All these factors seemed to make a 

renewed study of the clay minerals opportune, and 

about 1924 an intensive study was started in the laboratories of the Geological Survey, United States Department of the Interior, initiating the modern attack on the 

problem of clay mineralogy. The fundamental bearing 

of clay mineralogy on many soil problems led to the 

study of clay minerals in the Bureau of Plant Industry, 

Soils, and Agricultural Engineering, United States 

Department of Agriculture. The two lines of research 

developed an interdependence that led to the present 

cooperative study.

The montmorillonite group contains minerals of widely varying chemical composition, but the aluminous 

members are most widespread in occurrence. Less 

abundant members of the group are characterized by 

ferric iron, magnesium, chromium, or nickel. These 

elements as well as ferrous iron, manganese, lithium, and 

perhaps other elements may occur as minor components 

proxying aluminum. Calcium, sodium, and potassium 

also are present in varying proportions, and a determination of their role has presented a distinct problem.

Water is present in several forms, and its function also 

has been a problem.

Modern X-ray technique has given a means of determining the role of the various ions in the crystal structure. Aluminum may play two roles, one as the characteristic ion having octahedral coordination with respect 

to oxygen, as in montmorillonite, the type mineral of the 

group, and the other as partly proxying silicon in 

tetrahedral positions, as in beidellite. A large group of 

ions may occupy octahedral coordination positions, and 

other ions, exchangeable bases, occupy positions between the lattice sheets. The proportion of these exchangeable bases varies within certain limits and thereby introduces another variable, in that the unsatisfied 

valencies within the crystal lattice are not constart.

PREVIOUS WORK

The present paper on the minerals of the montmorillonite group is the result of studies that have beer under 

way since about 1924. Minerals of bentonite 2 nd related clays were first investigated, but they pre^nted 

so many complex problems that attention was turned 

to the kaolin minerals, which afforded simpler relationships, because they showed little isomorphism between 

distinct end members and because some of them occurred as well-crystallized material. However, work on 

members of the montmorillonite group has continued 

from the initiation of the program, and many workers 

have been making essential contributions.

The investigation of the montmorillonite group of 

^ay minerals began with a study of bentonites. These 

exhibit certain interesting properties and uses, but their 

detailed study has resulted largely from the eas^ with 

which pure clay material of this type can be soured 

from bentonites. Another source of pure clay minerals 

is the clay developed in certain pegmatites. Only rarely 

do approximately pure sedimentary clays occur naturally; they commonly contain detrital rock grains, of which 

quartz is in general the most abundant. These admixed 

materials are so fine-grained that it is always difficult 

and commonly impossible to separate impuritie-' completely from the clay. Soils also are of limited usefulness 

for this investigation because they are commonly made 

up of exceedingly fine-grained mixtures of several clay 

minerals. Gouge and vein clays, including soim nontronites and saponite, occasionally occur in pure 

condition.

About a hundred years ago there was a narked 

interest in clays but the work was largely chemical, and 

on this basis numbers of names were proposed. Some 

of this work was good, but for much of it there hs s been 

no way of determining its character or the purity of the 

material analyzed. Later, the limitations of purely 

chemical studies came to be realized, and the likrMhopd 

of mixtures of minerals, the exceeding fineness of grain, 

and the difficulty of microscopic study led to a lessened 

interest in clays. On the basis of accumulated information, Edward S. Dana (1892), in "The System of 

Mineralogy," 6th edition, assigned species rank to some 

of the large number of names that had been proposed 

and listed others as probably synonyms. Considering 

the difficulties presented, his judgment has been shown 

to have been remarkably good and his conclusions valid.

Minerals of the montmorillonite-beidellite grouo have 

been qualitatively identified as components of tH clay 

fractions of soils by many workers. A number of attempts have been made to carry out approximately
MINERALS OF THE MORTMORILLONITE GROUP 25

quantitative estimates of the amounts of these minerals 

in selected soil samples. This has been accomplished 

chiefly by a coordination of chemical analyses, differential thermal analyses, and X-ray diffraction methods. 

The identity, character, and methods of identification 

of the minerals of soils have more than usual significance, 

and some of the authors who have contributed papers 

illustrating these studies are listed below. (For titles, 

see Bibliography, pp. 72-77.)

1930 

Hendricks and Fry.

1931 

Kelley, Dore, and Brown.

1935

Bray, Grim, and Kerr. 

Jacob, Hofmann, and Loofman.

1936

Antipov-Karataev and Brunowsky. 

Correns and Schliinz. 

Kawamura and Funabiki.

1937

Clark, Riecken, and Reynolds. 

Kelley and Dore. 

Engelhardt.

1939

Alexander, Hendricks, and Nelson. 

Edelman, van Baren, and Favejee. 

Favejee.

Harden and Favejee. 

Kelley, Dore, Woodford, and Brown. 

Kelley, Woodford, Dore, and Brown. 

Sedletsky.

1940

Hendricks and Alexander. 

Maegdefrau and Hofmann. 

Russel and Haddock.

The first of a series of papers on the clay minerals 

were written by Ross and Shannon (1925, 1926) in a 

cooperative study made by the Geological Survey and 

the United States National Museum. Further work, 

done by Ross and Kerr (1931a, 1931b, 1934) in cooperation with Columbia University, included studies of the 

kaolin minerals as well as a preliminary paper on the 

minerals of the montmorillonite group. Kerr also has 

published papers independently, which will be referred 

to on later pages of this report. He rendered great help 

in determining relationships within the group, showing 

the identity of the different numbers by means of their 

X-ray diffraction patterns.

Later stages of the work were carried on by Ross in 

cooperation with Sterling B. Hendricks, of the Bureau 

of Plant Industry, Soils, and Agricultural Engineering, 

United States Department of Agriculture, who has been 

interested in the bearing of clay mineralogy on soil 

problems. Progress of this work has been recorded in a 

number of papers by Hendricks (1936,1938,1939,1941, 

1942), Hendricks and Alexander (1939, 1940a, 1940b), 

Hendricks and Fry (1930), Hendricks and Jefferson 

(1938, 1939), Hendricks, Nelson, and Alexander (1940), 

Hendricks and Ross (1938), and Ross (1943).

ACKNOWLEDGMENTS

Aid in the study of the clay minerals has come from 

many sources. D. F. Hewett (1917), of the Geological 

Survey, had recognized the character and mode of origin 

of bentonite before the present study began and thus 

had laid the foundation for further work. In particular 

we wish to acknowledge the help of Waldemar T. 

Schaller, of the Geological Survey, who has aided by his 

advice throughout the study and also has critically read 

the manuscript. W. F. Foshag, of the National Museum, 

has generously placed the collections of the Museum at

our disposal, and his advice has been helpful. Edgar T. 

Wherry, of the University of Pennsylvania, has long 

been interested in problems of clay mineralogy and hajs 

been ever ready with advice. W. P. Kelley, of the California Citrus Fruit Experiment Station and the Uriversity of California, who has a deep interest in the more 

fundamental soil problems, has been generous with his 

helpful advice. C. E. Marshall, first with the University 

of Leeds, England, but now at the University of Missouri, has made fundamental contributions to the clay 

studies and has added valuable advice. Those who have 

aided by the contribution of materials or in other essential ways are too numerous to list individually.

The study of the clay minerals was for some time 

under the auspices of the National Research Council; 

and David White, Waldemar Lindgren, and W. H. 

Twenhofel, as chairmen of the Section of Geology and 

Physiography, were deeply interested in the progress of 

the work. Many of the chemical analyses used in the 

work were made by Forrest A. Gonyer under a fund 

granted by the National Research Council. Later the 

chemical work was carried forward by members cf the 

Section of Chemistry of the Geological Survey. P. G. 

Nutting, of the Geological Survey, has been engaged in 

a study of the oil clarification properties of clays and has 

made a large number of dehydration tests, which have 

been most helpful to the mineralogical studies.

In the course of the studies on which this paper is 

based the selection of material and the optical examinations were made by C. S. Ross, most of the dehydration 

tests by P. G. Nutting, and the earlier X-ray studies by 

P. F. Kerr. The base-exchange experiments, the study 

of the relation between exchangeable bases and interlayer water, many of the X-ray experiments, and the 

calculations of mineral composition were made by S. B. 

Hendricks in the laboratories of the Bureau of Plant 

Industry, Soils, and Agricultural Engineering. Many 

other independent contributions will be referred to in 

the body of this paper. Wide cooperation has been one 

of the most pleasant factors in the work, and thi? has 

greatly contributed to its successful culmination.

NOMENCLATURE

The clay minerals present unusually difficult problems in nomenclature. Some of the names were proposed 

long ago, before an accurate knowledge of chemical composition was available; for some the identity o* the 

original material is in doubt; and for others the parity 

of the sample and, therefore, the accuracy of the cl ^mical analysis, is questionable. Many of the old names 

have fallen into disuse and are met with only in -^he 

early literature.

Parts of the discussions of nomenclature in previous 

papers by Ross and Kerr (1931b, 1934) and by Kerr 

(1932) apply to the minerals included in the present 

paper and need not be repeated in detail, but others 

require consideration.

MONTMORILLONITE

The name montmorillonite was proposed by Damour 

and Salvetat (1847) for a clay material for which they 

gave the following chemical analysis:

Analysis of montmorillonite from Montmorillon, Franc?

SiO.................. 49.40

Al,Oi................ 19.70

Fe2 03 ................ .80

MgO................. .27

1.50 

, Kn 1 - 60

CaO. 

Na20 

K2 0

HjO.'................ 25.67

98.84
26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

Although the sample is unusually low in MgO and 

anomalous in other ways, it is essentially montmorillonite.

Breithaupt (1832, p. 99) described galapectite from 

Angleur, France. An analysis of material from Angleur 

by Le Chatelier (1887, p. 210) was that of a typical halloysite, although optical properties of another sample 

from the same locality, studied by Ross and Kerr (1934), 

proved to be those of montmorillonite. Thus, some 

galapectite is definitely montmorillonite, but in general 

it is probably to be correlated with halloysite rather 

than montmorillonite.

Other names of minerals that were considered by 

Dana (1892) to be at least in part montmorillonite but 

proposed since 1847, the date for montmorillonite, are 

confolensite, described by Dufrenoy (1856, p. 583), from 

Confolens, at St. Jean-de-Cole near Thiviers, France, 

and delanonite, described by Kenngott (1853, p. 633), 

from Millac near Nontron, France.

Le Chatelier (1887, p. 210) presented the formula 

4Si02 .Al 203.H20+Aq for montmorillonite. Dana (1892, 

p. 690) quotes Le Chatelier and gives the composition 

of montmorillonite as probably KUA^Si^^+nAq, or 

Al 203 .4Si02.H20+nAq. Ross and Shannon (1926) proposed the formula (Mg,Ca).Al203 .5Si02 .nH20 to explain the MgO and CaO which were found to characterize all of the many new analyses presented. Later 

X-ray studies showed the different role of Mg and Ca 

and the inadequacy of this formula.

Wherry, Ross, and Kerr (1930) stated: "Chemically 

the original mineral so-called [montmorillonite] was a 

hydrous aluminum silicate with the ratio Si02 :Al 203 = 

4:1, and perhaps the majority of other occurrences have 

this ratio. Many vary from it, however, some being 

higher in silica, up to at least 5:1, while others are lower, 

reaching 3:1 or even 2:1, without change in structure, 

although with a slight alteration of the unit-cell dimensions as shown by X-rays. In addition, MgO is frequently present, apparently as an essential constituent."

Ross and Kerr (1931) published a preliminary paper 

in which the several members of the montmorillonite 

group of clay minerals were listed. These were montmorillonite; beidellite, with an assumed Al203 :Si02 

ration of 1:3; an unnamed member with a ratio of 1:2; 

nontronite with a Fe203 :Si0 2 ratio of 1:3; and saponite, 

a magnesian member containing a little A1 203 .

The only name that antedates montmorillonite and 

appears to have included at least in part clay materials 

of the montmorillonite type is smectite. This name was 

at one time widely used in the mineralogical texts of the 

18th century and appears to have been, in general, 

synonymous with "fuller's earth." Cronstedt, in an 

original paper (1788, p. 98) and in a later translation by 

Magellan, mentions "smectis," but lists it among magnesian earths, which seems to correlate it with saponite 

rather than montmorillonite. Kirwan (1794, p. 184) 

used the terms "walkerda" and "smectis" in addition to 

fuller's earth. Haiiy (1801, p. 444) used the terms 

smectite clay and fuller's earth. The general impression 

seemed to have prevailed, as indicated by Breithaupt 

(1841, p. 544), that smectite was a hydrous aluminum 

silicate containing magnesium, calcium, and other constituents.

Kerr (1932) has shown that certain clays that have 

been described as smectite from Cilly, Lower Styria, 

and Woburn Sands of England are typical bentonites— 

derived from glassy volcanic ash—and are composed of

the mineral that has commonly been known as montmorillonite.

Since the time of its proposal, the name montmorillonite has been commonly used and has been accepted 

by Dana (1892, p. 690), while smectite has faller into 

disuse, and where used at all, apparently has impUed a 

fuller's earth rather than a definite mineral. Kerr (1932, 

p. 198) concludes:

Since the X-ray diffraction patterns, chemical analyse?, and 

refractive indices show such close relationship, it seems hardly 

worth while to retain the two names smectite and montmorillonite 

for what appears to be the same mineral. In view of the large 

amount of modern literature on montmorillonite it seems in the 

best interests of science to continue the use of montmorillonite 

and to drop that of smectite.

BEIDELLITE

Larsen and Wherry (1917) described the clay mineral 

that occurred as a gouge clay in a mine at Beidell, Colo., 

under the name leverrierite. Leverrierite had been first 

described by Termier (1890), after a study of material 

from Beaubrun, Rive-de-Gier, near St. Etienne, France. 

The identity of the original leverrierite with kaolinite 

was recognized by Dana (1892, p. 687) and later by 

Cayeux (1916). Optical and X-ray studies by Ror^ and 

Kerr (1931b) of type material from France, kindly supplied by the Museum of Natural History, Paris confirmed Dana's conclusion that the original material was 

kaolinite. The Colorado material was a distinct mineral, therefore, Larsen and Wherry (1925) proposed the 

name beidellite for the material previously described by 

them as leverrierite, and Ross and Shannon (1925) 

described the properties of beidellite.

Even since the withdrawal of the name leverrierte by 

Larsen and Wherry, however, that name has been applied to a mineral of low index of refraction and comparatively high birefringence, which is distinct from the 

type material. De Lapparent (1934) has restudied type 

material and reports that it is composed of alterrating 

plates of kaolinite and muscovite. The identity of the 

type leverrierite with kaolinite, or a mixture of kaolinite 

and muscovite, seems, therefore to be generally accepted, and the name leverrierite should be discarded.

SAPONITE

An early reference to claylike materials that se^m to 

have included saponite was made by Cronstedt (1758, 

pp. 98, 236, 237) under such names as "smectis"; "walkerda," that is, fuller's earth; "kessikel," used as a substitute for soap; and "terra lemnia," which the translator remarks is used as a fuller's earth in England. 

Saponite appears to have been included under "steatite" 

by Kirwan (1794, p. 152). Svanberg (1840, p. 153) 

described saponite, and his paper, which was republished later (1842), contained the following chemical 

analysis:

Analysis of saponite from Svardsjo, Sweden

Si02 . A1 2 03 

Fe2 03 

MgO.

50.8

9.4

2.0

26.5

CaO. 

H,0.

.7 

10.5

99.9

In the meantime the same material was described 

under the name "piotin," by Svanberg (1841). NicUes 

(1859) described a mineral under the name saponite, 

but, as the combined K20, MgO, and FeO amounted to 

only 0.63 percent, the material was clearly not saponite.
MINERALS OF THE MONTMORILLONITE GROUP 27

Hilgard (1916) described under the name "lucianite" 

a claylike material from Hacienda Santa Lucia near 

Mexico City. The analysis given below shows that it is 

essentially similar to saponite and differs only in being 

a little higher in calcium oxide and lower in magnesia 

than most saponites. The high calcium content strongly 

suggests impurities.

Analysis of lucianite from Mexico

SiO2 . ................ 43.00

A12O3 .. .............. 3.48

Fe2O3 ... ............. 1.76

MgO................. 17.11

CaO.... ............. 9.06

Insoluble.............. 1.83

Loss on ignition....... 19.60

Soluble salts.......... 2.60

98.44

Thalite was described by Owen (1852), who concluded 

that it contained a new rare earth, which he proposed to 

call thalium. Bolingite was described by Hannay 

(1877), but his analysis contained approximately 5 percent of calcium carbonate and obviously was impure.

Analyses quoted by Dana (1892, p. 682) all contained 

alumina, and he made no attempt to assign a formula 

to the mineral. Ross and Kerr (1931) published a preliminary paper on the clay minerals in which they 

identified saponite as a member of the montmorillonite 

group.

HECTORITE

Foshag and Wqodford (1936) described an unusually 

interesting clay mineral from Hector, Calif. This unique 

mineral has provided a key in understanding some of the 

relationships in the montmorillonite group. It is essentially the magnesian end member of the group (see 

p. 47), and the almost total absence of A1 2 03 , the presence of essential Li 20, and the dominance of F over OH 

seem to establish the Hector clay as a distinct mineral 

species. For this species the name hectorite has been 

proposed by Strese and Hofmann (1941). -

The name hectorite was applied by Cox (1882) to a 

"hydrated pyroxene mineral" from the serpentine rocks 

of the Dun Mountains of New Zealand, and it is listed 

by Dana (1892, p. 364) under the pyroxenes, as one of 

the "more or less well defined alteration products of 

various kinds of pyroxenes," although the analysis suggests that it is an amphibole related to cummingtonite. 

Its identity, however, was never established, the name 

as proposed by Cox never gained acceptance, and in 

this meaning it has fallen into complete disuse. As Cox's 

proposal was made more than 50 years ago, the reassignment of the name will not conflict with the rules of 

nomenclature as given by Dana (1892, p. xliii).

Marshall (1937b, p. 938) states: "An end member of 

the series is probably Foshag and Woodf ord's magnesian 

bentonite, in which practically all the aluminum has 

been replaced by magnesium." Hofmann, Endell, and 

Bilke (1935, p. 469) have examined a synthetic magnesium silicate, which they conclude is this same end 

member.

The paper by Foshag and Woodford (1936) presented 

two chemical analyses, one by Dr. Foshag and the other 

by R. E. Stevens, of the Geological Survey. These are 

as follows:

Analyses of hectorite

[1, Foshag, analyst; 2, Stevens, analyst) 

1 2

SiO2 .......... 53.68 51.26

A12O3 .. ....... .60 .36

Fe2O3 ......... ..... .09

FeO........... ..... .06

MgO........ .. 25.34 23.25

CaO........ .52 2.60

SrO......... .. ..... Tr.

NasO......... 3.00 3.47

K2 0

Li«O

H O +

H2O-....

Cl........

CO2 .......

F.........

1 

.07

.... 1.12

.... 8.24

.... 7 . 28

.31

100.16

2 

.10

.60

5.14

11.56

1.35

None

99.84

2.73 

F................... 5.35

Loss on ignition...... 4. C 0

For some unknown reason the standard qualitative 

test for determining fluorine gives a negative reaction 

when applied to hectorite; consequently, these analyse 

failed to show fluorine, although Stevens specifically 

looked for it. Thus the presence of fluorine in hectorite 

escaped detection until reported by Harold R. Eyrich, 

who has been engaged in the commercial production of 

the Hector clay. Mr. Eyrich submitted the following 

analysis made by the Sharp, Schurtz Co., of Lancaster, 

Ohio:

Analysis of hectorite from Hector, Calif. 

Si02 ................ 59.77

Fe2 O3 ............... .026

AljOj............... .49

CaO................ .45

MgO................ 27.46 100.2.76

The hectorite was reanalyzed by Stevens and the 

results are given in table 4 (p. 35). Some samples of 

Hector clay contain varying amounts of calcium caiv 

bonate, but the material supplied by Mr. Eyrich is 

seemingly free from this and other impurities.

The low atomic weight of lithium makes the 1 percent 

of that element an important constituent of hectorite. 

The studies given on pages 37 to 39 show that this element provides the correct substitution of a monovalent 

base for the bivalent magnesium required to give a 

member of the montmorillonite group its distinctive 

physical and base-exchange properties. A distinctive 

name for the Hector clay, therefore, seems fully justified.

NONTRONITE

The name nontronite was proposed by Berthi^r 

(1827), who stated that the mineral was discovered by 

Lanoue and was associated with the manganese ore of 

Perigueux in the Arrondissement of Nontron, near the 

village of Saint Pardoux. It forms onion-shaped masses 

commonly smaller than a man's fist. Associated impurities are quartz sand, manganese oxides, and clay, 

but pure material is easily obtained for analysis. Tre 

color is canary yellow, slightly greenish. The following 

chemical analysis was given:

Analysis of nontronite from Nontron, France

SiO2 . A1 2 03 

Fe2 O3 

MgO.

44.0

3.6

29.0

2.1

H2O.

Clay.

18.7 

1.2

98.6

Breithaupt (1829) described pinguite from Volkenstein Erzegebirge. No chemical analysis was given, frit 

the mineral was described as being siskin to oil green in 

color.

Krantz (1857) reported a mineral from the vicinity 

of Menzenberg in the Siebengebirge for which he proposed the name gramenite. No chemical analysis was 

given and no reason for differentiating it from pinguite 

or nontronite, which it was described as resembling. 

About the same time Bergmann (1857) published an 

analysis of gramenite not essentially different from 

nontronite.

Nontronite, together with faratsihite, a material 

described by Laroix (1914), has been assigned to the 

kaolin group by Mugge (Rosenbusch, 1914), following 

the conclusion of Weinschenk (1897). An optical examination of faratsihite shows that much of the material 

has too high a birefringence to be purely a member of the 

kaolin group. Recently a study of faratsihite by Hendricks (1939) has shown that it is a mixture of kaolinite 

and nontronite. He concludes that it is very improbable 

that the kaolinite lattice will accommodate appreciate
28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

>$>}................traces

H,0............ ..... 20.00

amounts of iron in solid solution and that nontronite 

and kaolinite, therefore, belong to distinct groups of 

minerals.

The first to recognize the relationship between nontronite and montmorillonite appears to have been Collins (1877) who stated that chloropal, nontronite, 

gramenite, and montmorillonite must be regarded as 

belonging to the same series. Larsen and Steiger (1928) 

published a study of nontronite and compiled the 

chemical, dehydration, and optical properties of this 

and related minerals. They pointed out that nontronite 

and beidellite form an isomorphous series.

Chloropal was described from Unghwar, Hungary, by 

Bernhardi and Brandes (1822). One sample was listed 

as being an opaline mineral, but an earthy form also was 

described, and chemical analyses of both were given. 

That of the earthy variety is given below. The color 

is siskin, or light yellowish, green.

Analysis of earthy chloropal from Unghwar, Hungary 

SiOi................. 45.00

AliO................. .75

Fe*0( ................ 32.00

MgO................. 2.00 9975

Glocker (1839) described material from the same 

locality under the name unghwarit.

The name chloropal has a slight priority over nontronite. However, the material has been shown to be 

definitely crystalline by both optical and X-ray methods, 

and the name chloropal, suggesting a deeply colored 

opaline mineral, is unsuitable. Dana (1892, p. xliii) has 

laid down the rule that a name having priority may 

properly be set aside when its signification is glaringly 

false. Mineralogists for sometime have used the name 

nontronite as the most suitable for this mineral.

VOLCHONSKOITE

Kammerer (1831) described a claylike mineral containing chromium which was obtained from Okhansk, 

Russia, and a description and analysis were published 

by Kersten (1839). Since then, several other analyses of 

similar material have been presented, and these are 

quoted in table 6, page 35. Dana (1892, p. 696) made 

no attempt to determine its relationships and listed it 

under the heading, "appendix, to clays."

Angel (1913) described "wolkonskoit" in some detail 

and presented new analyses, but likewise refrained from 

suggesting relationships. Larsen and Berman (1934, 

p. 164) list volkonskoite as a chromium kaolinite.

Serdiuchenko (1933, table 3) described a number of 

chrome-bearing beidellites and concluded that volchpnskoite was a member of the group including beidellite 

and nontronite. From Serdiuchenko's discussion, however, it is not clear that he had an opportunity to study 

the original material directly.

Through the courtesy of W. F. Foshag, material from 

the type locality of Okhansk was made available for 

study from the collections of the United States National 

Museum. Studies of volchonskoite under the microscope indicate that the material has optical properties 

similar to those of nontronite except for the color, which 

is "dark green" by Ridgway's color standard. An examination of the material by X-ray methods showed a 

typical nontronite pattern.

KAOLIN MINERALS

The mineral relationships discussed in th;s paper, 

particularly the mineral paragenesis, involve references 

to several clay minerals that do not belong to the montmorillonite group. These should be briefly mentioned.

The kaolinite group includes five minerals; of these, 

kaolinite, dickite, halloysite, and nacrite (Poss and 

Kerr, 1930) have the same chemical corrDOsitipn, 

Al2Si205[OH]4, and endellite has the composition 

Al2Si205 [OH] 8.

Kaolinite is the most important member of the group, 

being a widespread component of soils, where it is the 

result of weathering processes. Rarely it is the result of 

low-temperature hydrothermal processes. Dickite is a 

widely occurring mineral in certain types of mineral 

veins, where it is normally the result of hydrothermal 

processes, as is also the very rare mineral nacrite. Halloysite may occur in some soils but the X-ray diffraction 

patterns of halloysite and kaolinite are so similar that 

they are difficult to recognize where associated with 

other clay minerals, as in soils. The relations between 

halloysite and endellite have been described by Alexander, Faust, Hendricks, Insley, and McMurd:<* (1943), 

who have shown that halloysite is an alteration product 

of endellite. Endellite, which loses two molecules of 

water and inverts to halloysite at 50° C., is very unstable. The wide occurrence of halloysite, however, 

indicates that endellite is an important, although 

evanescent, material.

The name halloysite was proposed by Berthier (1826) 

and this usage established by him has been followed for 

more than a century by mineralogists throughout the 

world. Notwithstanding this, a shift of names was 

proposed by Hofmann, Endell, and Wilm (1934), calling 

halloysite metahalloysite, and applying halloysite to 

the new mineral characterized by 8(OH). Sudi a shift 

of names is precluded by the long-established rules of 

scientific nomenclature and cannot be accepted. For 

this reason Alexander, Faust, Hendricks, IiHey, and 

McMurdie (1943) have applied the name endellite to 

the form with the higher content of OH and 1 ave used 

the name halloysite as originally proposed by Berthier.

HYDROUS MICAS (ILLITE)

Minerals related to the micas, but containing less 

potassium, are commonly in association with other clay 

minerals and therefore will be briefly mentioned. These 

minerals, in general, have not been distinguished from 

the true micas, but have commonly been grouped together under such descriptive terms as hydro-micas 

(Galpin, 1912), hydrous micas (Bayley, 1920), glimmerton (Endell, Hofmann, and Maegdefrau, 1035), and 

illite (Grim, Bray, and Bradley 1937). The us^ of these 

descriptive terms has been due to the realization that 

specific names should await a better understanding of 

the group. The only specific mineral name that has 

been proposed for minerals of this group is bravaisite 

(Mallard, 1878), used for a mineral of the muscovite 

type. Grim's analyses of illite (1942) are similar to that 

of bravaisite, and Nagelschmidt (1943) has published 

a number of analyses of similar material.

The relationships of the potassium-bearing soil materials are only partly known, and a final decision about 

grouping is not yet possible; however, a large number of 

chemical analyses are available, which show tho presence 

of about 6 percent of potassium, and the variation from 

this value generally is small. Bravaisite has precedence
MINERALS OF THE MONTMORILLONITE GROUP

as the name for this micalike mineral with about half the 

potassium of muscovite. Other clays contain much less 

potassium—quite a number show about 1 percent of 

that element, others 2 or 3 percent. (See tables 1-3, 

analyses 7, 30, 46, 47, 54, 62, 69, and 70.) The studies 

of mixed-layer minerals (see below and Grim and Rowland, 1942, p. 803) show that these materials are made 

up of mixed layers of muscovite or bravaisite and 

beidellite or, less commonly, mixed layers with montmorillonite; that is, minerals of the montmorillonite 

group are commonly dominant in these materials. The 

evidence, therefore, seems to indicate that these mixtures are not specific minerals and do not merit a 

specific mineral name, at least not until more adequately 

understood.

Bravaisite and those materials of problematic relationship to it are important soil-forming materials, 

which characterize many mineral deposits. They will 

be discussed briefly; detailed study is desirable but 

aside from the scope of this paper.

The presence in soils of minerals related to the micas 

was first recognized by Hendricks and Fry (1930). Ross 

and Shannon (1926) gave an analysis of potash-bearing 

bentonites of Ordovician age and of a similar clay frorri 

China. In a subsequent paper Ross and Kerr (1931) 

state that the potash clays resembled sericite. Maegdefrau and Hpfmann (1937) published a detailed study of 

micalike minerals in c?ay fractions of soils. The most 

thorough study of this group of clay minerals belonging 

to the mica group has been made by Grim, Bray, and 

Bradley (1937), who show the wide distribution of a lowpotash mica mineral among the clay minerals. Another 

paper in which the micalike minerals in soils are discussed is that by Hendricks and Alexander (1939), from 

which the following is quoted:

"The amount of K2O is very variable, the total range being from 

the order of 1 percent to that required for muscovite (11.8 percent 

K2O). Many of the analyzed samples, however, contain approximately 5 percent K2O, which can be taken as the average value. 

* * * Perhaps * * * the gradation in composition is so 

continuous as hardly to justify the naming of intermediate 

minerals. For this reason the entire series will simply be referred 

to as "hydrous micas." * * * This type of mixed minerals is 

difficult to recognize, but is probably a common constituent of 

soils. In particular, montmorillonites and hydrous micas probably occur together."

There is widespread geologic evidence of a strong 

tendency for the fixation of K20 at the expense of Na20 

in the rocks of the earth's surface. Spencer and Murata, 

in an unpublished paper, have shown that, although 

there is a preferential adsorption of K20 by the normal 

clay minerals, adsorption does not afford an adequate 

explanation of this fixation. Seemingly, the only other 

explanation is that micas or micalike minerals tend to 

form and thus fix K2 0.

Students of soils have long recognized that potassium 

added to soils tends to become fixed and becomes unavailable or difficultly available as plant food. Tmog 

and Jones (1938) have shown that the tendency of 

potash to become fixed in an unavailable form is increased by wetting and drying, and that clays of the 

bentonite type have this property more strongly than 

other minerals that were tried.

Thus several lines of evidence indicate the wide occurrence of mixed layers of micas and minerals of the 

montmorillonite group. This makes the petrologic estimation of these minerals very difficult in the clay fractions of soils. It also results in some question about 

selection of mineralogic specimens of montmorillonite 

from soils for analysis and study.

When the amount of K20 present in a specimen is or** 

percent or more, then the presence of mica-type mine"'- 

als or of micalike layers in the montmorillonite is probable. About ten of the samples examined in this worV 

contained such amounts of K20. These are retainH 

since it was not considered entirely justifiable to eliminate them on the basis of analyses alone.

MIXED-LAYER MINERALS

The importance of mixed-layer minerals among thow 

with a platy structure is gaining recognition, and tH 

effect of such intermixtures, with minerals of the montmorillonite group must be considered. Gmner (1934, 

p. 561) in describing yermiculite, a mineral closely r> 

lated to montmorillonite, states that potassium-bearir^ 

minerals of this group are made up of interstratifie-i 

layers of mica and vermiculite.

Hendricks and Jefferson (1938), who also have studied 

vermiculite, state, "It is not surprising to find that ve~- 

miculites form mixed structures with the chlorites and 

micas. In fact, any mixture of mica, vermiculite, pyrophyllite * * * and talc layers would not be unreasonable." In the same way, the clays of the montmorillonite group may contain mixed layers of mica'", 

chlorite, talc, or brucite, and possibly other minerals of 

similar character.

CRYSTAL STRUCTURE

Studies of the micas made by Maugin (1927, 192P , 

1930), followed by that of Pauling (1930) on micas and 

related minerals, laid the foundation for an understanding of minerals with platy structure. Pauling proposed 

structures for gibbsite, A1 2[OH] 6 ; brucite, Mg3[OH] 6 ; 

talc, 3Mg0.4Si02.H20; pyrophyllite, Al203.4Si02.H2O; 

the micas; and the chlorites. He considered these minerals to be made up of layer lattices, the silicates bar** 

formed by combination of gibbsite or brucite layers with 

silicate layers.

This work and that of W. L. Bragg and his associates 

(1932, 1937, 1938) showed that A1+3, Mg+2, FC+2, Fe+», 

Mn+2, Mn+3, TJ+4, M+2, and Li+1 could play similar roles 

in the crystal structures. These ions can all be in t\ ^ 

lattice positions surrounded at the corners of an a]> 

proximately regular octahedron by oxygen and hydrox7l 

or fluorine ions; that is, they have octahedral coordination. Silicon, Si*4, can undergo substitution, in par*, 

only by A1+3, since it has tetrahedral coordination with 

respect to oxygen.

Pauling's work was significant also in its recognition 

of the similarity in structure of pyrophyllite and tal?.. 

These minerals, as shown below, have the sequence of 

atomic planes along the pseudohexagonal axes, which 

are normal to the micaceous cleavage. N in the table js 

a large factor giving the number of units of structure in 

a layer.

Atomic planes

PYROI*HYLLITB TALC

NX NX 

Silicate layer /6 O^2 Silicate layer /6 O~z

(tetrahedral) \4 Si+4 \

Gibbsite layer 

(octahedral)

Silicate layer 

(tetrahedral)

4 O~2 

4 Al+; 

4 O-*.2[OH]-i

4Si+* 6O~2

(tetfahedral) \4 Si+4

Brucite layer 

(octahedral)

Silicate layer 

(tetrahedral)

6

4

4Si+4 

6 O~2

This succession of atomic planes forms a neutrH 

structure, which can be considered as one layer. Crystals
30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

are formed by superposition of these extended layers 

and, as each layer is neutral, there is only a small attractive force between them, resulting in excellent 

cleavage.

In the micas, as shown by the work of Pauling (1930) 

and of Jackson and West (1931,1933), the succession of 

atomic planes within a layer is approximately as follows:

Atomic planes

MUSCOVITE PHLOGOPITE

NX NX

Tetrahedral (6 O~2 Tetrahedral (6 O~2

layer \3 Si+4 .lAl+» layer \3 Si+*.lAl+1

Octahedral

layer

Tetrahedral /3 Si +4 .lAl' + 

layer \6 O~2

Interlayer K+1

4 O-2 .2[OH,F]-' 

6 Mg+2 

4 O-'

Octahedral 

layer

Tetrahedral J3 Si +4 .lAl+3 

layer \6 O~ 2

Interlayer K+1

Here the layer as a whole has an excess negative 

charge resulting from the substitution of A1+3 for Si+4. 

This is compensated for by K+1 situated between successive layers on the surface along which the excellent 

cleavage occurs. Each of the K+1 ions has 12 neighboring oxygen ions, 6 being at the corners of a hexagon in 

the top of one layer and 6 similarly placed in the bottom 

of another layer. Potassium, K+1, in the micas can be 

replaced by Cs+1, Rb+1 , Ca+2 , and Na+1. The appreciably smaller radius of Li+ apparently prevents its substitution for K+, as has been substantiated by recent 

work on lithium micas by Miser and Stevens(1938) and 

by Stevens (1938).

Hofmann, Endell, and Wilm (1933, 1934) proposed a 

structure for montmorillonite similar to that of the 

silicates with layer lattices. The lattice was the same as 

that of pyrophyllite, the composition being taken as 

Al 203.4Si02.H2 0. In the structure proposed, the water 

absorbed bore a definite relation to the spacing of the 

plane parallel to the layer surface. (See p. 54.)

In 1935, several papers were published that added to 

the knowledge of the relationships of the various elements occurring in minerals of the montmorillonite 

group. The conclusion of Collins (1877) about the relationship of nontronite and montmorillonite, and those 

of Larsen and Steiger (1928) and Ross and Shannon 

(1925) about the relationships between beidellite and 

nontronite were confirmed by Gruner (1935), who 

concluded:

An X-ray examination of nontronite, morencite, pinguite, 

faratsihite, chloropal, and stilpnochloran shows that these socalled species are structurally identical. * * * The structure 

is practically identical with that of montmorillonite (bentonite). 

Therefore, montmorillonite, beidellite, and nontronite form an 

isomorphous series.

Importance of isomorphous replacements in the montmorillonite lattice similar to that in the micas was first 

emphasized by Marshall (1935b, 1935c, 1936b) and 

Holzner (1935). Marshall particularly called attention 

to the fact that the silicate layers in montmorillonite 

carry a negative charge, in the same manner as do the 

micas, which has to be balanced by ions external to the 

layers. These ions are the ones that take part in the 

base exchange shown by montmorillonites. One of 

Marshall's conclusions was:

Montmorillonite, beidellite, and some of the nontronites are 

members of a single series. In montmorillonites the chief lattice 

replacement is that of magnesium for aluminum; in the beidellites 

that of aluminum for silicon predominates; in many nontronites 

each of these is overshadowed by the substitution of iron for 

aluminum.

In recent years Gruner (1934b) and Hendricks and 

Jefferson (1938) have studied the manner in which the 

silicate layers of minerals, with layer lattices are superimposed. However, absence of any pronounced regularity in montmorillonite makes a detailed discussion 

unnecessary.

Typical X-ray powder-diffraction data from montmorillonite are to be found in several revie^ articles 

that have appeared and in various original publications 

by Englehardt (1937), Nagelschmidt (1938), Edelmann, 

van Baren, and Favejee (1939), and MehnH (1939).

LOCATION AND DESCRIPTION OF ANALYZED

CLAYS

The following section gives the location and mode of 

occurrence, so far as known, of the clays whose analyses 

and mineral composition are considered in this paper. 

The color was determined wherever samples could be 

directly compared with the Ridgeway color standards, 

and those color descriptions are indicated by (R) after 

the name of the color. The physical properties will be 

described in a later section. (See pp. 48-57.) Where 

analyses are copied from earlier studies, the author's 

name and the date of publication are given. (See bibliography, pp. 72-77.) Some of the analyses irade in the 

course of this study were published previously in a 

report by Ross and Shannon (1926, p. 90) and in a compilation of chemical analyses made in the laboratories 

of the Geological Survey (Wells, 1937).

MONTMORILLONITE-BEIDELLITE SERIES

1. Santa Cruz, N. Mex., sec. 6, T. 20 N., R. 9 E. Bentonite 

Very pure cream-buff clay. Wells (1937, p. 108). J. G. Fair 

child, analyst. .

2. Cilly, Styria. "Smectite" (montmorillonite). Described 

by Kerr (1932). A. M. Smoot of Ledoux & Co., analyst. The 

United States National Museum collection of minerals contains 

a sample of type material from Cilly (U. S. Nat. Mus. No. 13098).

3. Amargosa Valley, Calif. Bentonite. An unusually friable* 

light-gray clay. Wells (1937, p. 107). J. G. Fairchild, analyst.

4. Tehachapi, Calif. Bentonitic clay forming a 1 ?d about 8 

feet thick, light-grayish white to greenish white; d?scribed by 

Kerr and Cameron (1936). A. M. Smoot of Ledoux & Co., 

analyst.

5. Dixon, N. Mex. Bentonite. A compact pinkish creamcolored clay. Wells (1937, p. 107). J. J. Fahey, aralyst.

6. Tatatila, Veracruz, Mexico. A very pure bentonite varying 

in color from almost white to La France pink (R). Transmitted 

to the United States National Museum by ManueJ Santillan, 

Director, Institute Geologica, Mexico. U. S. Nat. Mus. No. 

101,836. Unpublished analysis, K. J. Murata, analyst.

7. Maricopa, Kern County, Calif. Bentonite. Ross and 

Shannon (1926, p. 89). Earl V. Shannon, analyst.

8. Conejos quadrangle, Colo. Bentonite, buff pink (R). 

From between lava flows. Washed to free clay from included 

igneous rock phenocrysts. Collected by Ross. Ross and 

Shannon (1926, p. 89). Earl V. Shannon, analyst.

9. San Diego County, Calif. Bentonite. Well" (1937, p. 

107). J. G. Fairchild, analyst.

10. Near Landshut, Mainburg, Nieder-Bayern, Germany. 

Pinkish-buff (R) bentonite, so-called "Tonsil" clay. Unpublished analysis, J. G. Fairchild, analyst.

11. Near Rideout, Utah (exact locality unknown). So-called 

saponite. Unpublished analysis, F. A. Gonyer, analyst.

12. Ardmore, S. Dak. Bentonite, gray, used in making 

"refinite" (water softener). Wells (1937, p. 69). W. T. Schaller, 

analyst.

13. Irish Creek, Rockbridge County, Va. Gouge clay formed 

in shear zone in granodiorite, Irish Creek tin mines. Friable, 

cream-colored. Collected by Ross. Unpublished aralysis, J. G. 

Fairchild, analyst.

14. Hector, San Bernardino County, Calif. (3 miles south of 

Hector). Pale-gray bentonite associated with hectcrite, sample 

76. Collected by Bernard Moore. Unpublished analysis, R. E. 

Stevens, analyst.
MINERALS OF THE MONTMORILLONITE GROUP

15. Otay, San Diego County, Calif. Bentonite, selected white 

material associated with pink material (sample 38). J. E. Whitfield, analyst.

16. Polkville, Smith County, Miss. Glaze farm. Bentonite, 

deep olive buff (R). Wells (1937, p. 107). R. C. Wells, analyst.

17. Anglar, France. So-called galapectite, U. S. Nat. Mus. 

No. 4777. Pinkish buff (R). Wells (1937, p. 107). J. G. Fairchild, analyst.

18. Montmorillon, France. Montmorillonite from the type 

locality was described by Damour and Salvetat (1847), who 

describe the occurrence as follows: "The mineral occurs in nests 

penetrating a brownish shale in the vicinity of Montmorillon 

(Vienne)." Lacroix (1910, p. 749) states that the rose-colored 

montmorillonite of Vienne "was encountered 25 meters below 

the surface in the course of digging a well at Bordes in La Trimouille. It forms a bed a meter in thickness in the recent gravels."

The present study is based on type material obtained from 

Professor Lacroix of the National Museum of Natural History, 

Paris. The color is La France .pink (R) and greatly resembles 

that of some of the pegmatite clays. Examination under the 

microscope reveals no clear evidence as to its mode of origin. It 

is apparent that the material is not quite homogeneous. No nonclay materials are present, but the material appears to contain 

two slightly different types of clay. This is confirmed by a 

dehydration curve that shows slight irregularities. The X-ray 

diffraction pattern, however, indicates that the material is 

essentially montmorillonite of the same type as that in bentonites 

and pegmatite clays. Unpublished analysis, Roger C. Wells, 

analyst.

19. Upton, Wyo. Bentonite, ivory color. Wells (1937, p. 

71). J. G. Fairchild, analyst.

20. Lemon, Smith County, Miss. (Husband farm). Bentonite, olive buff (R). Nos. 20 and 23 are parts of the same sample. 

No. 20 forms irregular, paler, more opaque patches associated 

with the denser, more translucent material of No. 23. As this 

bentonite contains marine fossils, principally echinoids, the ash 

from which it formed was evidently deposited in the ocean, but 

alteration may have occurred later. Wells (1937, p. 107). R. E. 

Stevens, analyst. (See electron micrograph pi. 7, £>.)

21. Sppkane, Wash. Salmon buff (R). Fillings of vesicular 

cavities in basalt. Collected by Edward Sampson. Unpublished analysis, J. G. Fairchild, analyst.

22. Atzcapozalco, Mexico, on the southern slope of Cerro de 

Malinche. Very pure clay associated with basalt fragments below a basalt flow. Buff pink, mottled carrot red (R). Collected 

by W. F. Foshag. U. S. Nat. Mus. No. R-7591. Wells (1937, 

p. 107). E. T. Erickson, analyst.

23. Lemon, Smith County, Miss. Bentonite. (See sample 

20.) R. E. Stevens, analyst.

24. Mexico, exact locality unknown. Pure white and shows 

the lowest iron content of the bentonites examined. Wells (1937, 

p. 107). J. G. Fairchild, analyst.

2.5. Rosedale, Alberta, Canada. (See pi. 5, A.) Pale olivegray (R) bentonite, received from H. S. Spence, Mines Branch, 

Department of Mines, Canada. It is described as follows by 

Spence (1924, p. 14): "At Rosedale, the bentonite seen in the 

workings of the Rosedale Coal Co. measures 6- to 10 inches in 

thickness, and the material compares favorably in point of colloidality with the type Wyoming Clay." This is a sodium bentonite that forms unusually perfect gel-like masses, which swell 

to nearly eight times the original volume without breaking down 

or losing shape. A. Sadler, Mines Branch, analyst.

26. Santa Monica Mountains (north slope), Los Angeles 

County, Calif. Bentonite. Collected by M. N. Bramlette. 

Wells (1937, p. 66). Charles Milton, analyst.

27. Near San Antonio, Tex. (16 miles west of San Antonio on 

the Castroville road). Ivory-yellow (R) bentonite. Ross and 

Shannon (1926, p. 89). Earl V. Shannon, analyst.

28. Greenwood, Maine. Pale Hermosa pink (R) pegmatite 

clay, reported to be an alteration product of pollucite. Wells 

(1937, p. 107). J. G. Fairchild, analyst.

29. Fort Steel, Wyo. Bentonite, pale yellow. Purified by 

washing to free from igneous-rock phenocrysts. Ross and Shannon (1926, p. 89). Earl V. Shannon, analyst.

30. Osage, Wyo. Bentonite from the lower part of the Belle 

Fourche shale member. A sodium bentonite with tendency to 

disperse and remain in permanent suspension. Collected by 

W. W. Rubey. Wells (1937, p. 71). J. G. Fairchild, analyst.

31. Lemon, Smith County, Miss. Husband farm. Buff pink 

(R) bentonite. Wells (1937, p. 107). J. J. Fahey, analyst.

32. Otay, Calif. Bentonite associated with sample 14. LigM 

grayish vinacous (R). Ross and Shannon (1926, p. 88). Er^l 

V. Shannon, analyst.

33. Branchville, Conn. Pegmatite clay described by Bnnh 

and Dana (1880, p. 283), rose red in color. Possibly contains a 

phosphate-bearing impurity. G. J. Brush, analyst.

34. Near Leakey, Real County, Tex. (6 miles west of Leakej'), 

in the Edwards Plateau. Nearly white bentonitic clay received 

by Dr. R. F. Ryan, Vacuum Oil Co., Houston, Tex. "Wells (19S7, 

p. 108). J. J. Fahey, analyst.

35. Twin Falls, Idaho. Clay encountered in tunnels of Twin 

Falls irrigation project. Tawny olive with a little buff-pink (R) 

material. Forms masses 2 to 14 inches, rarely several feet, in 

thickness. Commonly cut by minute fractures in which a 

darker-colored but equally pure clay has been deposited. (S°e 

pi. 3, B.) Submitted by Lynn Crandall, district engineer. We'ls 

(1937, p. 98). J. J. Fahey, analyst.

36. Pala, Calif. Pegmatite clay, rose pink to deep rose pink 

(R). Associated with pink tourmaline and lepidolite in gembearing pegmatites. Collected by W. T. Schaller. (See pi. 

7, B.) Wells (1937, p. 107). J. G. Fairchild, analyst.

37. Montmorillon, France. (See also sample 18.) Ross and 

Shannon (1926, p. 90). Earl V. Shannon, analyst.

38. Booneville, Miss. (2 miles east). Bentonite from Eutrw 

formation. Upper Cretaceous. Buff pink (R). Collected by 

Ross and Nutting. Unpublished analysis, J. G. Fairchild, 

analyst.

39. Maniquipi, Chihuahua, Mexico. Princessa mine. Nearly 

white. Occurs in fault zone in quartz latite. . Collected by W. F. 

Foshag. Wells (1937, p. 98). E. T. Erickson, analyst.

40. Clairmont (2 miles north), Los Angeles County, Calif. 

Pegmatite clay, white to pale buff. Forms mass from 1 to 2 

inches thick in a pegmatite. Described by Loudermilk and 

Woodford (1934, p. 263). J. D. Loudermilk, analyst.

41. Glen Riddle, Pa. Clay filling horizontal joint seams in 

gabbro described by Tomlinson and Meier (1937). Nearly white 

in color. Adolph E. Meier, analyst.

42. Pala, Calif. Pegmatite clay, similar to No. 26. Collected 

by W. T. Schaller. Unpublished analysis, F. A. Gonyer, analyst.

43. Clay from Master Products Corp., Los Angeles, Calif. 

Ross and Shannon (1926, p. 89). Earl V. Shannon, analyst.

44. Beidell, Colo. ' Gouge clay from the Esperanza mine at 

the gold and silver mining camp of Beidell, Saguache County, 

Colo. (U. S. Nat. Mus. No. 93239), described by Larsen and 

Wherry (1917,1925) and by Ross and Shannon (1925). It forms 

masses of gouge clay as much as several feet across and occurs as 

platy masses as much as several inches across. The color is 

pecan brown (R) with a slightly reddish tint, and the material is 

very plastic. Collected by Larsen. Edgar T. Wherry, analyst.

45. Embudo, N. Mex. Pegmatite clay, collected by Frank 

Hess. Purified by washing. Ross and Shannon (1926, p. 90). 

Earl V. Shannon, analyst.

46. Fairview, Utah. Submitted by R. O. Baker, Fairview, 

Utah. Shows very fine laminations. Color, ferruginous (P). 

Wells (1937, p. 90). J. G. Fairchild, analyst.

47. Wagon Wheel Gap, Colo. "Gouge clay." Mouse gray 

(R), forms platy masses as much as a foot across in a fluorte 

mine. Collected by Ross. Unpublished analysis. Char'°s 

Milton, analyst.

48. South Bosque, McLennan County, Tex. Bentonitic clay 

collected by L. W. Stephenson. Unpublished analysis, J. J. 

Fahey, analyst.

49. Carson District, Idaho. Blackjack vein, collected by 

Shannon. Ross and Shannon (1926, p. 94). Earl V. Shannon, 

analyst.

50. Pontotoc, Miss, (railroad cut % mile southwest of Pontotoc). Color, warm buff (R). Clay material, which has replaced calcareous shells. Collected by L. W. Stephenson and 

described by Ross and Stephenson (1939). (See pi. 7, A.) J. G. 

Fairchild, analyst.

51. Nashville, Ark. Color, mineral gray (R). Bentonitic 

clay associated with volcanic rock minerals (phenocrysts of 

orthoclase), purified by washing. Collected by Ross. Ross and 

Shannon (1926, p. 94). Earl V. Shannon, analyst.

52. Nashville, Ark. Same locality as sample 51. Earl V. 

Shannon analyst.

53. Ancon, Canal Zone. Blackish-brown (R) laminated clay; 

contains organic matter. Ross and Shannon (1926, p. 9°). 

J. G. Fairchild, analyst.
32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

64. Roseland, Va. Mine, American Rutile Co. Clay filling 

joint cracks in anorthosite. Pinkish cinnamon (R). Collected 

by Ross. Unpublished analysis, R. C. Wells, analyst.

NONTRONITES

65. Stary-Krym, near Mariupol, southern Russia. Nontronite. Described by Serdiuchenko (1929) as occurring in the 

graphite mines of Stary-Krym, about 12 kilometers north of 

Mariupol in a region of the Priasov crystalline slates. It forms 

platy crusts, which are dark olive or locally yellow in color. 

Serdiuchenko, analyst.

66. Woody, Calif. Nontronite from veinlets as much as 1 inch 

across in a garnet-pyroxene rock that represents roof pendents of 

a metamorphosed limestone included in granodiorite. Larsen 

and Steiger (1928). Earl V. Shannon, analyst.

67. Petrovsk, Krivoi-Rog region, southern Russia. Nontronite described by Serdiuchenko (1929) as occurring with graphite in layers, in veins 1 to 5 mm. by 1 to 2 cm., and as single 

phenocrysts in crystalline rocks. It is dark green to yellow 

green and is plastic where fresh. Serdiuchenko, analyst.

58. Spokane, Wash. Nontronite-beidellite. Intermediate 

between nontronite and beidellite, thus showing the isomorphism 

of the iron and alumina end members. Yellowish citrine (R) 

claylike material lying between two basalt flows. Collected by 

Edward Sampson. Ross and Shannon (1926, p. 94). Shannon, 

analyst.

69. Near Spokane, Wash, (about 17 miles southeast of 

Spokane in SW. %, sec. 6, T. 23 N., R. 45 E.). Relations unknown. Color clear dull yellowish green (R). Unpublished 

analysis, J. G. Fairchild, analyst.

60. Santa Rita, N. Mex. Yellowish-green claylike material. 

Wells (1937, p. 109). R. C. Wells, analyst.

61. Chevy Chase, Md. Claylike zone in schist, associated 

with a little mica, which was removed by washing. Collected by 

P. G. Nutting. Wells (1937, p. 109). R. C. Wells, analyst.

62. Szekes-Fejevar, Hungary. "Pinguite"-like decomposition 

product with grass-green color from Rac quarry. Described by 

Erdelyi (1927). Excess Fe2O3 = 0.80. Erdelyi, analyst.

63. Stary-Krym, near Mariupol, southern Russia. Same 

locality as sample 56. .Serdiuchenko (1929), analyst.

64. Paulusbrunn, near Tachau, Germany. Same locality as 

sample 67. Rosenbusch (1927, p. 594). Gulmann, analyst.

65. Sandy Ridge, N. C. Forms veinlets 6 to 10 mm. wide in 

schist. Collected by J. B. E. Shaffer, Sandy Ridge. Color, 

clear dull green (R). (See electron micrograph, pi. 7, B.~) Wells 

(1937, p. 109). R. C. Wells, analyst.

66. Spruce Pine, N. C. Nontronite. Yellowish-green clay. 

Alteration zone in gneiss; collected by P. G. Nutting. Wells 

(1937, p. 109). L. T. Richardson, analyst.

67. Paulusbrunn, near Tachau, Germany. Described by Noll 

(1930) as light-green compact clay, which is an alteration product 

of hornblende-rich gneiss. Noll, analyst.

SAMPLES FROM SOILS AND CHROMIUMBEARING SAMPLES

68. Maniquipi, Chihuahua, Mexico. Princessa mine. Liver 

brown (R). Associated with sample 39. Collected by W. F. 

Foshag. Wells (1937, p. 98). E. T. Erickson, analyst. An 

X-ray examination shows that this clay from the Princessa mine 

contains about 20 to 25 percent of kaolin, together with a small 

proportion of ferric iron. Notwithstanding this admixture, it 

gives very definite optical properties characteristic of a member 

of the montmorillonite group and is included to illustrate the 

intimacy of the admixture of clays that may occur in some 

samples.

69. Rothamstead, England. A soil clay of the montmorillonite type described by Marshall (1936b). Marshall, analyst.

70. Missouri. Putnam clay. A very pure montmorillonite 

type of soil clay separated and analyzed by Marshall (1935b). 

The Putnam clay characterizes soils in southern Ohio, Indiana, 

and Illinois and in northeastern Missouri.

71. Emerald mines, Urals. Beidellite (chromian). Analysis 

listed by Serdiuchenko (1933).

72. Cheghet-Lakhran Valley, Urals. Beidellite (chromian). 

Serdiuchenko (1933) reports that it overlies the metamorphosed 

serpentine and underlies the ore. Occurs as a discontinuous 

lenticular bed of olive-green clay matter, 40 cm. thick. L. N. 

Venerovsky, analyst.

73. Same as sample 72, but material washed. L. N. Venerovsky, analyst.

74. Bolshia-Laba River. Beidellite (chromian). Reported 

by Serdiuchenko (1933) to form emerald-green pebbles and spots 

in the Carboniferous red conglomerate along the upper course of 

the river. N. Nikolski and E. Marklennikova, analysts.

75. Gedmayshkh Valley, in the territory of the Malka ironchrome-nickel deposits, Kabardino-Balkaria. Beidellite (chromian). Reported by Serdiuchenko (1933) to serve as s cementing 

material in a thin bed of sandstone lying on a green serpentine 

rock. S. T. Derkacheva, analyst.

SAPONITE-HECTORITE SERIES

76. Hector, San Bernardino County, Calif. Hectorite. 

Material received from Harold R. Eyrich. New analysis, R. E. 

Stevens, analyst. Occurs about 3 miles south of Hector. 

Described by Foshag and Woodford (1936) as a bentonitic 

magnesian clay mineral. It is overlain by a basaltic lava flow 

and developed at the top and in the cracks of a tvff bed. It 

seems evident that highly magnesian solutions have caused the 

formation of a magnesian bentonite rather than the normal 

aluminum mineral. The material has a marked trmslucency 

when moist and resembles very pure tallow, but it is pure white 

and opaque when dry.

77. Montreal, Canada. Saponite collected by Dr. J. A. Bancroft and described by Graham (1918), who reports as follows: 

"The saponite * * * Was * * * collected near the western 

end of the Canadian Northern Railway tunnel * * * which 

passes from east to west under Mont Royal * . * * and traverses the intrusive contact between the nepheline syenite and 

Trenton limestone. When first collected, this substance is somewhat translucent, soft, and quite plastic, with very mvdi the consistency and appearance of candle grease. * * * The dry 

material is soft, has a somewhat soapy feel when rubbed between 

the fingers, and does not adhere to the tongue. Moistened with 

water, it forms a claylike paste."

78. Svardsjo, Sweden. Orginal analysis of saponite by L. F. 

Svanberg (1842).

79. Ahmeek mine, Mich. Saponite.

80. Kearsarge mine, Mich. Saponite.

The materials from the Ahmeek and Kearsarge mines of 

Michigan have been described by Palache and Vassjr (1925) as 

follows: "Saponite was found in quantity in the Soutl Kearsarge 

mine and in less abundance but better crystallized in the Ahmeek 

mine. In the former it is one of the latest hydrothenr^l minerals 

to form, occurring in cavities left by excessive solution during 

replacement of the amygdaloid. It thus lines the vails, a last 

generation of adularia alone being later. In the Almeek mine 

it was found chiefly as solid filling of replacement vu~s in which 

it surrounded crystals of copper and quartz; it was also found 

with copper and epidote crystals in open cavities, as minute 

stalactites as much as half an inch in length but less than a tenth 

of an inch in diameter. These were composed of spherulitic 

groups of fibers and plates, radiate to axial lines. It is white to 

light-buff colored, 1 or less in hardness, with an unituous feel, 

somewhat plastic, but not adherent to the tongue." Helen E. 

Vassar, analyst.

81. San Bernardino County, Calif. (12 miles south of Needles). 

From a shaft about 20 feet deep. Obtained through Frank 

Sanborn, California State Mining Bureau. Walter L. Gibson, 

analyst.

82. Glasgow, Scotland. U. S. Nat. Mus. No. 4727. Wells 

(1937, p. 110). L. T. Richardson, analyst.

SAMPLES CONTAINING SILICA OR AN IRONBEARING IMPURITY

83. Quilchina, British Columbia. Soda bentonite. Collected 

by Hugh Spence (1924, p. 14). Described by Ross and Shannon 

(1926, p. 84). Earl V. Shannon, analyst.

84. Santa Rita, N. Mex. Bentonite (montmorillonite). 

Nearly pure white; free from visible impurities but contains a 

little cristoballite. Collected by Ross. Unpublished analysis, 

R. E. Stevens, analyst.

85. Tierra Amarilla grant (three miles east of), N. Mex. Bentonite. Pink sample freed from crystal grains by washing. Collected by Ross (1926, p. 89). Earl V. Shannon, analyst.

86. Westcliffe, Colo. Unpublished analysis, R. F. Stevens, 

analyst.
MINERALS OF THE MONTMORILLONITE GROUP

87. Baschenowski, Russia. Nontronite from an asbestos 

mine. Described by Kurnakov and Cernykh (1929). V. V. 

Cernykh, analyst.

88. From Magnitnaja Mountains, Urals. Nontronite. Described by Kurnakov and Cerynkh (1929). Excess Fe 2O3 =* 

14.63. A. N. Zavaritsch, analyst.

89. Okhansk, Siberia. Volchonskoite; material originally 

described by Kammerer (1831), redescribed by Kersten (1839). 

Correlated with nontronite by Serdiuchenko (1933). Ilimow, 

analyst.

90. Woody,' Calif. Nontronite with a somewhat different 

origin from sample 56, also described by Larsen and Steiger 

(1928). Found at the Woody locality and at many contact 

metamorphic garnet-hedenbergite zones. It is a platy to fibrous, 

citron-green to light yellowish green aggregate, derived from the 

alteration (probably hydrothermal) of hedenbergite. George 

Steiger, analyst.

91. Unghwar, Hungary. The "chloropal" described by Bernhardi and Brandes (1822). Described as occurring mixed with 

opal. Brandes, analyst. The United States National Museum 

collection contains chloropal from Unghwar, specimen 3882.

92. "Stilpnochloran" from Gobitschau, Moravia. Described 

by Holzner (1934) as being light-brown fibrous material. Gruner 

(1935) studied the material by X-ray and reported that it is structurally identical with nontronite. He also reports that Holzner 

stated in a private communication that he suspected that the 

material was identical with nontronite.

93. Morenci, Ariz. "Morencite." (U. S. Nat. Mus. No. 

86033.) Described by Lindgren and Hillebrand (1904) as follows: "In a lime shale on the intermediate level of the Arizona 

Central mine, Morenci, 200 feet below the surface, brownish or 

greenish spreading masses are found, containing brownish-yellow 

silky seams. The enclosing material consists largely of the same 

material as the seams, but impure and mixed with a little chlorite 

and pyrite. * * * The minute fibers are brownish yellow and 

slightly pleochroic." Hillebrand, analyst.

94. St. Andreasberg, Harz. Nontronite. Collected 192° 

Rose Geilmann, analyst.

95. St. Andreasberg, Harz. Nontronite. Described by Isk^l 

(1935) as occurring as fine fibers.

96. Okhansk, Siberia. Described by Kokscharow (185F). 

V. Iwanow, analyst.

97. Baltatarak, Kazakstan. Nontronite described by Serdincbenko and Iskiill (1933) as follows: "The nontronite is associated 

with graphite deposits, which are confined to metamorptfc 

schists, traversed by granitic rocks. The nontronite is the resist 

of reactions in the zone of weathering." Serdiucheako, analyst.

98. Island of Mag.ee, County Antrim. Nontronite. 

scribed by Tomkief (1934). Occurs, together with calcite, in 

cavities in a basalt agglomerate. Excess Fe 2O3 = 1.60.

99. Nontron, France. Nontronite, described by Berthier 

(1829), forms onionlike masses associated with manganese ore of 

Perigueux near the village of Saint Pardoux. Specimens containing a chlorite or yermiculite impurity, probably as mix^d 

layers, or an iron-bearing impurity.

100. Cathkin Hills, Blackburn, Scotland. Saponite (bo'wlingite). Described by Caillere (1935).

101. Webster, N. C. Nickel saponite. The dunite of tve 

Webster area is cut by hydrothermal veins in which talc and 

anthophyllite form the borders in contact with dunite and vnrmiculite commonly forms the middle portion. A few of tve 

larger veins contain discontinuous lenses of oligoclase in th^ir 

middle parts. During weathering the small proportion of nicl'el 

in the olivine (commonly about 0.25 percent NiO) is released and 

tends to become fixed in the yermiculite. Further weathering 

converts part of the vermiculite to a very plastic apple-green 

clay. The material was washed, and the portion that stayed in 

almost permanent suspension was analyzed. Unpublished 

analysis, R. C. Wells, analyst.

102. Cathkin Hills, Scotland. Saponite (bowlingite). E •*• 

scribed by Caillere (1935).
34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

CHEMICAL ANALYSES

The following tables give chemical analyses of clays belonging to the montmorillonite group. Altf^ough the 

greater part of these analyses have been made in the course of the present work, some were published in earlier 

papers. A few have been copied from recent papers on the clay minerals, especially some of the analyses of nontronites and chromium-bearing beidellites.

TABLE 1.—Analyses of montmorillonite-beidellite series

SiOz. .......

AhO3 .......

FeO. .......

MnO.......

MgO.......

CaO........

KsO........

NasO.......

Li 2O. ......

C02 ........

Ti02 . ......

Cl.........

P-Os.......

SOs........

HjO— .....

H20 + .....

Total. . . .

1

52.30

14.43

2.00

.05

6.15

2.12

.52

.50

.20

.02

i5. 78

6.55

100.62

2

52.43

15.95

1.42

.10

5.02

2.97

.08

.08

.22

.30

13.96

7.60

100.13

3

54.58

16.44

2.59

.11

4.90

.72

.81

3.02

.18

11.10

5.49

99.94

4

53.88

11.66

4.60

.18

8.61

1.56

.39

* .15

.44

.ie

8.21

9.98

99.82

5

51.37

15.41

.78

.14

6.22

Tr.

.46

.79

.29

Tr.

.23

15.76

8.20

99.65

6

52.09

18.98

.06

.06

3.80

3.28

14.75

7.46

100.48

7

50.33

16.42

2.42

4.10

1.39

.12

J23.95

^

99.73

8

54.46

16.84

3.36

4.84

3 Oft

.80

16.10

99.60

9

53.96

15.44

1.12

Tr.

6.99

.80

.54

.94

.19

.29

(14.22

\ 6.34

•100.76

10

54.28

17.92

3.88

3.76

1.80

.25

.10

.20

10.84

7.32

100.35

11

50.41

13.25

2.51

Tr.

8.27

.96

.18

.61

.24

13.11

10.24

99.78

12

51.64

18.25

1.24

1.40

3.41

2.18

.12

2.09

13.22

6.33

99.88

13

50.50

17.23

2.36

3.71

2.40

.25

.02

16.78

7.41

100.66

14

53.02

18.50

2.33

.13

4.04

.80

.16

3.80

.13

11.69

5.44

100.04

15

50.30

15.96

.86

6.53

1.24

.45

1.19

J23.61

1

100.14

16

50.20

16.19

4.13

4.12

2.18

.16

.17

.20

/15.58

\ 7.57

100 . 50

17

50.86

18.76

2.07

3.48

1.76

.37

Tr.?

}?2 . 34

99.64

18

51.14

19.76

.83

Tr.

3.22

1.62

.11

.04

/14.81

\ 7.99

199.75

SiO2 . ......

A12O3 . .....

FesOa.....'.

MnO.......

MgO.......

CaO.......

KjO.......

NaiO ......

Li 20.......

C02 . ......

TiO2 .......

Cl.........

P2O5 . ......

SO3 ........

H20— .....

H*0 + .....

Total. ...

19

55.44

20.14

3.67

.30

2.49

.50

.60

2.75

.16

J14.70

100.69

20

50.37

17.11

2.68

-.20

4.59

1.23

.09

.56

/12.29

110.93

100.05

21

49.48

16.30

3.12

.05

4.69

2.15

.36

.03

.62

Tr.

14.41

9.44

100.65

22

50.44

16.26

5.38

Tr.

3.92

.72

.42

16.00

6.30

99.44

23

50.39

17.37

2.84

Tr.

4.56

1.29

.04

.46

13.31

10.11

100.37

24

51.50

19.44

.10

5.07

.48

.20

14.50

8.04

99.33

25

54.88

19.92

4.10

.22

2.83

2.22

.26

1.75

.37

.25

.09

.16

.07

8.10

4.28

99.50

26

50.03

16.75

5.83

.47

.01

2.78

1.20

.60

.26

.65

.24

13.53

7.52

3100.03

27

52.08

18.20

2.88

4.48

2.28

J20.80

^

100.72

28

54.28

22.94

.24

2.92

3.18

. v/o

.02

.06

/ 7.82

\ 8.70

100.24

29

49.20

17.60

1.60

5.08

1.52

/......

\......

[25.52

^

100.52

30

53.50

21.57

3.28

1.89

1.25

1.04

1.94

.11

15.20

499.78

31

49.48

17.75

3.33

.15

4.42

.16

1 12 / - 12

.29

,'18.26

1 5.86

99.82

32

49.56

15.08

3.44

.01

7.84

1.08

.40

£t£» , y\>

100.37

33

51.20

22.14

.18

3.53

3.72

.56

1.42

17.08

99.83

34

51.74

22.81

.39

3.32

Tr.

f .26

I -17

Tr.

Tr.

J11.84

\ 9.19

599.83

35

49.55

17.78

6.61

3.24

.58

15.45

6.88

100.09

36

50.06

21.32

.22

Tr.

.13

4.42

1.26

.19

.33

Tr.

14.05

7.56

99.54

SiOs. ......

Al 2Os. .....

FesOi......

FeO. ......

MnO...... . MgO.......

CaO .......

K20. ......

Na 2O. ..... 

Li'0. ......

CO2....... .

TiO2 . ......

Cl........ .

P2Os.......

SO»........

Carbon .....

H2O— ..... 

H20 + .....

Total. . . .

37

48.60

20.03

1.25

.16

5.24

1.72

J21 . 52

98.52

38

50.53

19.31

7.25

.02

2.60

.72

.34

.41

.75

.15

16.66 

7.90

100.64

39

49.34

22.88

2.36

Tr.

1.67

2.66

ii.77 

6.57

100.25

40

49.70

22.10

2.12

Tr.

2.85

1.08

None

1.17

.28

None

J21.14

100.44

41

48.79

20.73

5.40

.38

} .80

23.74

99.84

42

50.72

22.14

.78

.71

.11

4.28

1.56

/11.56 

\ 8.24

100.10

43

50.28

20.00

4.00

4.60

1.08

J19.60

99.56

44

47.28

20.27

8.68

.70

/ Tr.

\ .97

19.72

100.37

45

48.24

22.32

.36

6.64

2.04

20.84

100.44

46

46.04

22.60

.25

1.85

1.26

2.25

Tr.

.40

/10.00 

\ 9.86

99.99

47

48.05

23.01

6.67

2.14

1.52

1.04

.21

6.14 

10.46

99.24

48

51.87

26.80

1.42

4.42

1.90

.09

.03

.73

.81 

12.61

100 . 68

49

45.32

27.84

.80

.16

2.76

.12

.10

J22.64

99.74

50

46.95

27.26

2.26

.32

.01

1.39

.36

.20

.03

/ii.io

\10.55

100.43

51

45.12

28.24

4.12

2.32

.88

Il8.72

99.40

52

45.52

27.52

2.80

3.00

.52

19.60

98.96

53

44.02

28.90

5.15

.33

.50

.68

.50

.17

.53

18.64

99.42

54

44.23

31.30

5.77

Tr.

.18

.45

.95

.19

1.01

f 3.97 

\12.05

100.10

—Cl=0.07. 2CuO=0.01;ZnO=0.10;_CoO=0.12;NiO=None. 3BaO=0.16; 'Soluble SiO2= 0.45. *SO3=0.11.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

Santa Cruz, N. Mex. 

Cilly, Styria. 

Amargosa Valley, Calif. 

Tehachpi, Calif. 

Dixon, N. Mex. 

Tatatila, Mexico. 

Maricopa, Calif. 

Conejos quadrangle, Colo. 

San Diego County, Calif. 

Nieder-Bayern, Germany 

Near Rideout, Utah. 

Ardmore, S. Dak. 

Irish Creek, Va. 

Hector, Calif. 

Otay, Calif. 

Polkville, Miss. 

Anglar, France. 

Montmorillon, France.

19. Upton, Wyo.

20. Lemon, Miss.

21. Spokane, Wash.

22. Atzcapozalco, Mexico.

23. Lemon, Miss.

24. Mexico.

25. Rosedale, Alberta, Canada.

26. Santa Monica, Calif.

27. West of San Antonio, Tex.

28. Greenwood, Maine.

29. Fort Steel, Wyo.

30. Osage, Wyo.

31. Lemon, Miss.

32. Otay, Calif.

33. Branchville, Conn.

34. Leakey, Tex.

35. Twin Falls, Idaho.

36. Pala, Calil.

37. Montmorillon, France.

38. Booneville, Miss.

39. Maniquipi, Mexico.

40. Clairmont, Calif.

41. Glen Riddle, Pa.

42. Pala, Calif.

43. Los Angeles, Calif.

44. Beidell, Colo.

45. Embudo, N. Mex.

46. Fairview, Utah.

47. Wagon Wheel Gap, Colo.

48. South Bosque, Tex.

49. Carson District, Nev.

50. Pontotoc, Miss.

51. Nashville, Ark.

52. Nashville, Ark.

53. Ancon, Canal Zone.

54. Roseland, Va.
MINERALS OF THE MONTMORILLONITE GROUP 35

TABLE 2.—Analyses of nontronite

SiOs. .. 

AhOa. . 

FeaOs.. 

FeO... 

MgO.. 

CaO... 

KSO. . . NajO. . 

TiOs... 

CO2 ... 

H20—.

Total.

55

49.75

10.06

22.06

.25 

2.68

9.43

6.44

100.67

56

43.51

2.94

28.62

.99

.05 

2.22

14.05

6.62

1100.02

57

49.50

16.18

13.08

.61 

2.57

7.76

9.35

99.05

58

46.06

12.22

18.54

.28

1.62

1.66

.84

.26

98.48

59

40.72

4.96

29.57

.71

.74

1.98

.02

/ 15.46 

\ 6.66

100.82

60

40.48

6.00

29.33

.26

.31

2.90

None 

None

15.39

5.65

100.32

61

46.65 

17.65 

18.39

Tr.?

Tr.? 

1.21

.55

5.55

10.81

100.81

SiOj. .........

AhOa. ........

FeO.. ........

MgO.........

CaO. .........

KiO..........

Na2O. ........ 

Ti02 . .........

CO2 . ..........

H20- ........

HsO + ........

Total.......

62

39.54

6.04

30.32

Var.

Tr.

.04

.32

.24

} 22.27

298 . 86

63

42.74

9.76

26.71

1.06

1.94

/ 10.42

\ 6.74

99.37

64

40.42

10.42

25.31

.45

.13

.38

&\. . oU

98.91

65

41.38

9.84

27.47

Tr.?

Tr.?

Tr.?

} Tr.?

/ 12.10

\ 9.25

100.04

66

41.88

11.90

26.20

Tr.?

.10

.67

.52

7.67

10.78

99.72

67

40.02

10.55

26.16

1.37

£t\. . oO

99.93

iCuO=1.02.

55. Stary-Krym, Russia.

56. Woody, Calif.

57. Petrovsk, Krivoi-Rogregion, Russia.

58. Spokane, Wash.

59. Spokane, Wash.

60. Santa Rita, N. Mex.

61. Chevy Chase, Md.

2SrO=0.09; BaO=Tr.

62. Szekes-Fejevar, Hungary.

63. Stary-Krym, Russia.

64. Tachau, Germany.

65. Sandy Ridge, N. C.

66. Spruce Pine, N. C.

67. Tachau, Germany.

TABLE 3.—Analyses of samples from soils and chromium-bearing 

specimens from Russia

SiO2 . ......

AUOa. .....

FeiOt......

Crj03 . .....

FeO. ......

MnO.......

NiO.......

MgO.......

CaO....... 

KiO.......

isia*n

TiO2 . ......

PsOs.......

HsO— ..... 

H2O + .....

Total. . . .

68

43.76

27.88

6.42

Tr.

1.23

.SO 

1.18

.30

.30

10.59

7. .67

100.23

69

45.04

24.51

11.90

.03

i.9i "iles"

.20

.53

.58

J13.20

99.55

70

49.50

21.74

8.81

.42

.02

1.99

.28 

.94

.44

.70

2.16

13.05

100.05

71

45.83

18.55

9.29

.74

.71

Tr.

.09

2.12

1.69 

.31

.32

.22

20.20

100.07

72

43.14

16.07

4.70

5.02

.61

.21

.30

3.45

1.46

.96

/16.66 

\ 7.32

>100.06

73

44.47

19.63

10.85

1.02

.47

Tr.

.10

1.86

1.50 

.15

.14

.20

J20 . 00

100.39

74

46.09

24.04

3.96

1.12

.33

.49

Tr.

6.99

1.86

1.04

/ 5.67 

\ 7.26

98.85

75

36.69

16.77

8.70

3.76

7.83

1.60

2.57

8.42 

12.90

99.24

iP=0.01;S=0.15.

68. Maniquipi, Mexico.

69. Rothamstead, England.

70. Putnam clay, Missouri, U. S. A.

71. Emerald mines, Urals.

72. Cheghet-Lakhran Valley, Urals.

73. Cheghet-Lakhran Valley, Urals.

74. Bolshia-Laba River.

75. Gedmayshkh Valley.

TABLE 4.—Analyses ofhectorite and saponite

SiOi. .......

AbOj. ......

FeiOa.......

FeO.........

MnO. .......

MgO........

CaO.........

KjO. .......

NajO. ......

LijO........

TiO>........

COi.........

F. ..........

HiO— ...... 

H,0 + . .....

Total. ....

76

55.86

.13

.03

25.03

Tr.

.10

2.68

1.05

5.96

9.90 

2.24

H02.98

77

46.45

3.32

.42

.72

25.91

14.48 

8.13

99.43

78

50.8

9.4

2.0

26.5

.7

1 10.5

99.9

79

42.99

6.26

1.83

2.57

.11

22.96

2.03

Tr.

1.04

/ 13.65 

\ 6.85

100.29

80

42.78

6.44

J. . £t t

.12

24.78

2.35

Tr.

.75

13.96 

7.90

100.35

81

44.00

10.60

/ Tr.

I.......

24.30

2.00

12.60 

6.20

99.70

82

40.16

8.03

8.50

3.83

19.40

1.91

}T,.

11.15 

7.60

100.58

»O=F, 102.98—2.51=100.47.

76. Hector, Calif.

77. Montreal, Canada.

78. SvSrdsjQ, Sweden.

79. Ahmeek mine, Mich.

80 Kearsarge mine, Mich.

81. San Bernardino County, Calif.

82. Glasgow, Scotland.

TABLE 5.—Analyses of samples containing excess silica

SiO2 . . 

AhOs. 

Fe2O3 . 

FeO.. 

MgO. 

CaO.. 

K2O.. 

NasO. 

TiOj..

Total.

83

56.20

13.20

5.08

2.92

1.60

1 20. 32

99.32

84

54.15

15.99

.59

.06

4.48

1.90

.19

.42

.11

13.29

8.99

100.17

85

51.56

13.42

3.22

4.94

2.04

.38

.24

.78

23.46

100.04

86

54.77

15.88

.58

.62

4.99

2.08

.25

1.06

.17

9.26

10.25

99.91

83. Quilchina, British Columbia.

84. Santa Rita, N. Mex.

85. Tierra Amarilla, N. Mer,

86. Westcliffe, Colo.

TABLE 6.—Analyses of nontronites containing an iron-bearing

impurity

SiOj. .......

AUO........

Fe2O3 . ......

FeO.........

MnO........

MgO ........

CaO ........

K*O. .......

NajO. ......

TiOj. .......

P*0S ........

HzO- ......

H2 0 + ......

Total.....

87

33.98

2.77

38.19

Tr.

3.98

.98

1*7 . «TO

99.83

88

31.09

5.96

40.77

Tr.

.82

2.32

iy . 10

100.14

89

36.84

3.50

17.85

18.85

1.39

22.46

100.89

90

47.51

.37

35.17

1.40

2.50

.06

.09

Tr.

/ 7.16

\ 5.90

100.16

91

45.00

.75

32.00

2.00

1 20.00

99.75

92

36.45

6.03

34.24

.04

.97

1.91

.35

.02

.31

/ 11.01

\ 9.20

100.53

93

45.74

1.98

29.68

.83

Tr.

3.99

1.61

.20

.10

Tr.

.18

xo . y£t

»98.89

SiO*...........

A1 203 . ........

Fe*Oi.........

FeO. .........

MnO. ........

MgO..........

CaO..........

KaO. .........

NajO.........

TiO2 ..........

Ps05 ..........

HS0— ........

HtO + .. .......

94

39.88

.54

36.73

.41

Tr.

.33

2.23

.02

.01

.06

14.06

6.08

100.35

95

42.02

6.18

28.65

.44

.40

2.18

.03

.01

.05

} 20.45

100.41

96

37.01

6.47

10.43

17.93

1.66

1.91

21.84

298.26

97

50.41

2.07

31.11

1.74

.27

2.14

.79

/ 2.22

\ 9.66

100.41

98

42.03

3.93

31.69

.09

.53

1.99

.09

.12

8.50

11.30

100.27

99

48.82

4.30

35.88

.63

.35

1 9.66

99.64

'FejS=0.66.

87. Baschenowski, Russia.

88. Magnitnaja Mountains, Urals.

89. Okhansk, Siberia.

90. Woody, Calif.

91. Unghwar, Hungary.

92. Gobitschau, Moravia.

93. Morenci, Ariz.

*PbO=1.01.

94. St. Andreasberg, Harz.

95. St. Andreasberg, Harz.

96. Okhansk, Siberia.

97. Baltatarak, Kazakstan.

98. Island of Magee, County Antrim

99. Nontron, France.

TABLE 7.—Analyses of samples containing chlorite, vermiculite, or 

other iron-bearing impurity

Si02 . ...........

AUOs. ..................................

FesOa. ..................................

FeO............ .........................

NiO.....................................

MnO........... .........................

MgO....................................

CaO ....................................

K.O....................................

NasO....................................

TiOj. ...................................

HsO— ..................................

H*0 + ..................................

Total. ................................

100

40.04

6.76

9.29

2.38

19.13

2.27

.41

11.91

7.84

100.03

101

39.14

7.00

1.81

11.30

.02

22.15

.02

.08

.07

.07

6.59

11.45

99.70

102

39.37

8.11

5.94

7.36

21.47

.69

.07

7.69

9.23

100. Cathkin Hills, Scotland.

101. Webster, N. C.

102. Cathkin Hills, Scotland.
36 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

IMPURITIES IN CLAYS

All the analyses made in the course of this study and 

used in mineral interpretations have been on material 

whose purity and suitability for analysis have been 

studied under the petrographic microscope and much 

of it checked by X-ray. Other analyses whose descriptions indicated careful selection of material have been 

compiled from the literature. Nevertheless, in a material that is as fine-grained as the clays, minor impurities will escape detection in some of the materials. The 

most probable impurities are silica, which may be 

present in an amorphous condition or as cristobalite, 

and materials that are recorded as excess A1 203 and 

Fe203 in the analysis and may be present as chlorite or 

more rarely as iron and aluminum oxides or hydrous 

oxides. Ferric oxide and hydroxide have so strong a 

pigmenting effect that any appreciable proportion of 

these impurities would be revealed by the color in most 

clays, but disseminated magnetite might not be so 

recorded. Aluminum hydroxide would not be so revealed and may be present to the extent of several 

percent in some specimens. Similar amounts of sulfides 

and sulfates were observed in several specimens. Other 

samples from soils and from some veins contain kaolin 

or excessive amounts of micalike minerals, and are 

unsuitable as a basis for conclusions, but they have been 

included in the tables of analyses for purposes of illustration. The same is true of a group of analyses high in 

silica or in ferric iron.

Some of the specimens undoubtedly contained excess 

alkali salts adsorbed as such. Thus, a number of bentonites from arid regions have a marked saline taste. 

A few of the darker-colored ones contained small 

amounts of organic matter, and carbonates were undoubtedly present in a few samples. These impurities, 

however, are not expected to exceed a few tenths of a 

percent in most of the specimens.

Titanium may be present as part of the crystal lattice 

(see p. 37), but it probably is more commonly present 

as an impurity—rutile, anatase, or so-called leucoxene. 

Rutile is a common detrital mineral in sedimentary 

materials, and ilmenite commonly alters to one of the 

materials known as leucoxene. Leucoxene has been 

studied by Tyler and Marsden (1938), who found that 

it was a macrocrystalline form of rutile, anatase, or perhaps brookite. Tests in the laboratories of the Geological Survey showed that leucoxene from Roseland, Va., 

was sphene, whereas samples from Magnet Cove, Ark., 

were anatase. Leucoxene occurs in clays as irregular 

areas that are almost opaque to transmitted light but 

are white or rusty brown by reflected light. Areas of 

leucoxene in clay materials from Magnet Cove are 

shown in plates 1 and 2, and similar material is commonly present in residual clays, especially those from 

ferromagnesian rocks. In bentonites the titanium may 

have been present as part of the original glass or as 

minute microlites, but this titanium would not normally 

exceed a fraction of one percent.

Some specimens no doubt contain micalike minerals 

as mixed-layer minerals with montmorillonite (see 

p. 29), and finely disseminated detrital mica commonly 

is present in sedimentary clay materials. These are to 

be suspected when the amount of K2 0 is as much as one 

percent. (See analyses 46, 47, 54, 68, 69.) Kaolinite and 

other clay minerals of the kaolin type are associated 

with montmorillonite in many soils but probably are not 

even present as minor impurities in the nonsoil clays 

used in this study. None was detected in samples

examined by differential thermal methods, but a kaolinbearing clay from Maniquipi, Mexico, is represented by 

analysis 68.

A consideration of the relation between the various 

components indicates that the total amount of impurities, other than admixtures of mixed-layer minerals 

in the montmorillonites used in mineral interpretation, 

might be as great as 5 percent in a few o£,the specimens. 

However, in all but a few of the analyses impririties 

probably do not exceed one percent. The presence of 

even greater amounts of impurities does not seriously 

impair the calculations to be made and does not interfere 

at all with the drawing of logical conclusions about 

chemical relationships, since these depend upon results 

from many samples.

Several chemical analyses show an excess of silica over 

that required for the montmorillonite formula when only 

Si+4 is assumed to be present in tetrahedral coordination. 

Though some of these undoubtedly contain cristobalite, 

as has been noted by Gruner (1940a, 1490b), some 

probably contain no crystalline nonclay mineral. Reference to pages 43, 44 and tables 5 and 14 shows that 

the excess of Si02 commonly is small and in tH materials here considered does not approach the large 

amount observed by Gruner.

Some of the specimens, the analyses of which can be 

interpreted in terms of a montmorillonite formula, 

nevertheless, probably contain small amounts of silica 

as an impurity. The significance of this will H discussed later.

Of the high-silica clays, the material from Quilchina, 

British Columbia, contained an unusually large excess 

of Si0 2 , although careful examination under the microscope revealed no recognizable impurity that would give 

such an excess. This material and a check sample from 

Otay, Calif., were tested for soluble silica. A one-gram 

sample of each clay was added to 100 cc. of water, together with 5 grams of sodium carbonate, and boiled for 

5 minutes. The filtrate was tested for Si02 and A1 203 . 

The Quilchina sample gave 17.50 percent of soluble Si02 

and 0.81 percent of A1203 . The Otay sample gave 0.16 

percent of soluble Si0 2 and 0.07 percent of A1203 . Crystalline forms of Si02 are not appreciably soluble under 

such treatments. This test indicated that a rormal 

bentonite like that from Otay was not appreciably 

broken down by the sodium-carbonate treatment. On 

the other hand, the Quilchina sample contained 17.5 percent of soluble Si02 , but with only a small proportion of 

A1 2 03 . This indicates that the silica was not released by 

breaking down the clay mineral itself and suggests very 

strongly that the excess silica of the Quilchina sample 

was an opaline form of Si02, perhaps adsorbed between 

the crystal sheets. Sodium clays, a type that includes 

the Quilchina clay, appear to be commonly characterized by an excess of Si02.

Another moderately high-silica material is that from. - 

Santa Rita, N. Mex., which contains an excess of 4 percent Si02 after all the A1 203 and MgO have been combined with Si02 in the ratio of 2 to 4. This clay showed 

no visible crystalline impurities under the microscope, 

either in its original form or after a delicate test devised 

by Nutting. This test involves a 24-hour treatment on 

the steam bath with dilute HC1, which breaks montmorillonite down completely into an amorphous hyc1 rated 

form of Si02 with a very low index of refraction. Any 

anisotropic crystalline impurities, even though unusually minute, tend to stand out as birefracting material with an index of refraction much higher than that
MINERALS OF THE MONTMORILLONITE GROUP 37

of the silica residue; even isotropic materials are revealed by their higher index of refraction. The Santa 

Rita sample seemed unusually pure even under this test. 

This material was exarr.ined by X-ray diffraction and 

found to contain appreciable amounts of cristobalite. 

This study has made use of a large number of analyses 

of montmorillonite derived from bentonites, and this is 

due largely to the geologic relations under which bentonite forms. These relations go far toward assuring 

materials of much more than ordinary chemical purity. 

The volcanic glass from which bentonites are derived 

was uniform in chemical composition throughout any 

one deposit. The alteration of this glass to clay material 

occurred under conditions that favored the attainment 

of equilibrium. These two factors taken together tend to 

produce a single uniform mineral product. Where the 

exchangeable base is calcium, the excess silica and bases 

seem to have been effectively removed from the system. 

On the other hand, sodium clays commonly retain excess 

silica. The light color, the limpid character, and the low 

indices of refraction in small grains or in thin section 

greatly facilitate the detection of minute impurities. 

Most of the possible impurities have a much higher index 

of refraction and tend to stand out in relief or appear as 

opaque areas because of dispersal of light. Only amorphous silica, or the most finely disseminated cristobalite, 

will in general be beyond microscopic detection.

MINOR CONSTITUENTS

Several minor constituents that commonly occur in 

clays also may be considered. Marshall (1935) observes, 

"Phosphorous, in virtue of its small size and great tendency to assume a coordination number of 4,'would 

appear well fitted to enter the Si layer, but this would 

require a positive charge of one unit per atom of phosphorous." This proxying of Si by P tends to be confirmed by the work of McConnell (1937) on the apatite 

group, where he shows that 17.31 percent of Si02 proxies 

P205 in ellestadite. The mineral wilkeite, which was 

described by Eakle and Rogers (1914) and belongs to the 

same group, contains 9.62 percent of Si02 .

Phosphorus was not determined in many of the 

analyses. Of the twelve specimens in which P205 is reported, the quantity exceeds 0.30 percent in only three 

—Putnam clay, Mo. (2.16 percent P205 ), Rothamstead, 

England (0.53 percent P205 ), and Branchville, Conn. 

(1.42 percent P2 05 ). The first two were materials that 

C. E. Marshall separated from soils and analyzed. They 

were not intended to be mineralogical specimens and the 

phosphorus probably was not a constituent of the montmorillonite or the mixed-layer minerals associated with 

those materials. The unusually high P205 in the Branchville clay strongly suggests the presence of a phosphorus 

mineral.

The form in which titanium may be present in clay 

materials is related to the mode of occurrence in related 

minerals and to the materials from which the clays are 

derived. Titanium is commonly present in the silicates 

having layer lattices. Since the work of W. H. Bragg, 

Ti+* is generally considered to proxy for A1+3 or Mg+2, 

having octahedral coordination with respect to oxygen, 

rather than for Si"*-4, as had previously been the accepted 

interpretation. Thus Ti02 may possibly be present as 

part of the crystal lattice of the clay minerals, but this 

does not necessarily follow, especially for clay minerals 

formed under cool, strongly hydrating conditions.

Some micas contain titanium that is clearly a part of

the crystal lattice, as in an unpublished analysis of biotite showing 5.26 percent of Ti02 . On the other hand, 

micas commonly contain rutile needles, which tend to 

be arranged parallel to the basal cleavage of the mica 

and are the cause of the commonly observed asterrm. 

Thus titanium may or may not be present as part of the 

crystal lattice of micas, which also is probably true of 

clay materials, especially as the discussion on page 

36 indicates the common presence of leucoxene.

Constituents such as Mn+2, Mn+3, Ni+2, Fe+2, Li+', 

which can proxy for A1+3, Fe+3, Cr43, or Mg+2 in octahedral coordination, and Ba+2 or Sr+2 , which probs bly 

are situated outside the silicate layer, are to be considered as unusual rather than minor constituents. 

Under favorable geological environments there is no 

obvious reason why they should not become appreciable 

in amount.

Sulfides, sulfates, carbonates, and organic matter reported in some of the analyses are more likely present in 

impurities. Their quantities are invariably small.

BASE EXCHANGE

PHYSICAL CHEMISTRY OF BASE EXCHANGE

When a clay is treated with a solution of a salt a 

stoichiometric exchange of cations takes place. rnhis 

process of cationic exchange, or base exchange a-* it 

generally is called, is a very important factor influencing 

the properties of clays and soils. The modern study of 

the subject was initiated by J. T. Way (1850, 1852, 

1855) and H. S. Thompson (1850). They were concerned with the possible loss of soluble fertilizers f~om 

soil by leaching. Way demonstrated the equivalence of 

the exchange and further showed that it was due to 

organic matter and to hydrous aluminum silicates that 

lost the property upon ignition. His work came very 

close to being a discovery of the law of mass action and 

was so thorough that for half a century it was followed 

by none other of comparable value.

Natural zeolites and artificial permutites have pronounced base exchange. The name permutite is derived 

from permutare, to change. As these materials v^ere 

formerly considered to be better-defined than the clays, 

they have in the past, unfortunately, been studied as 

prototypes; in fact, the literature prior to about 1930 is 

confused by a rather free usage of the term zeolite to 

include zeolites and permutites as well as clay materials 

that show base exchange. It failed to recognize that 

clays are neither. (See the reviews by Wiegner, 1831.)

The study of bentonites and related clays by Ross and 

Shannon (1926) had indicated that montmorillonite was 

present in soils, and by 1930 it had became generally 

recognized that montmorillonite was a common constituent of many soils. Kelley, Dore, and Brown (1931) 

and Kelley and Jenny (1936) shortly afterward set 

about determination of base-exchange reactions of bentonites. Their work was followed by a significant contribution of Marshall and Gupta (1933), who studied 

the dissociation of the montmorillonite salts in derail. 

This period in the development of concepts about the 

base exchange of montmorillonite has been reviewed by 

Kelley (1939).

Numerous studies have shown that various cations 

differ greatly in ease of replacement in montmorillonite. 

Thus if the mineral is treated with an equimolar mixture 

of a potassium and sodium salt much more K+l than
38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

will be taken up. These "orders of replacement" 

are discussed by Marshall and Gupta (1933) and more 

recently by Schachtschabel (1940). A typical series is:

Li+I < Na+1 

Ba+2 <Rb+1

< Mg+2 < Ca+2 = Sr+2 =

Bivalent cations such as Ca+2 are preferentially held 

with respect to uniyalent ions. Partly for this reason, 

the naturally occurring montmorillonites generally have 

Ca+2 as the dominant exchangeable base, although some 

are sodium salts, others are magnesium salts, some are 

free acids, and some possibly are basic aluminum salts.

Although lithium and magnesium salts of montmorillonite can be prepared, these ions are essentially 

not exchangeable as found in the mineral. This is also 

true to a lesser extent for K+1 . In general Na+1 is exchangeable, but it is not always completely so. The exchangeable ions are usually Ca+2, Na+1, Mg+2 , and some 

K+1. A few of the specimens examined were found to 

be acid clays; that is, the exchangeable ion is in part H+1 .

Base-exchange capacities as determined for 22 of the 

analyzed specimens are listed in table 9. The clay used 

in these determinations was first dispersed by grinding 

in a slightly ammoniacal solution for a few minutes and 

then was placed on a shaking machine for 30 minutes. 

The dispersed clay was separated by centrifuging and 

then was treated with three successive lots of dilute 

(1 to 50) hydrochloric acid solutions in tenfold excess. 

After centrifuging, the acid clay was washed three times 

by resuspending in water and then twice in alcohol. It 

was dried in air at room temperature and then in 

vacuum over phosphorus pentoxide.

The acid equivalent of the treated clay was determined by titration of a weighed amount with sodiumhydroxide solution to pH 8.3, phenolphthalein being 

used as an indicator. In four samples H+1 was displaced 

by shaking with several lots of neutral barium-chloride 

solution in large excess and titrated after removal of 

clay by centrifuging. The two methods gave close to the 

same result.

The numbers of milliequivalents per gram of Ca+2, 

Na+1, and K+1 present in the various specimens also are 

listed in the table. The summation of these amounts 

agrees with the observed base exchange for seven of the 

clays — approximately one-third. It is too low for nine 

of them and too high for five. Three of the five that are 

high, contain large amounts of Na+1 and are alkaline to 

phenolphthalein. This excess sodium in part is probably 

absorbed as a salt, such as the carbonate or chloride. 

It also is probable that in some of the samples the summation is too high because of the presence of nonexchangeable Na+1 or K+1 or the presence of calcareous 

impurities.

Alkalis were not determined on four of the samples for 

which the summations of the number of milliequivalents 

per gram, of Na+1 , Ca+2 , and K+ were lower than the 

observed base exchanges. The amount of H+1 in the 

nontronite sample from Sandy Ridge, N. C., was determined by direct titration and may be slightly low in 

comparison with the observed base exchange because 

of difficulty in thoroughly dispersing the clay. However, 

the possibility exists, in this and other clays with low 

summations, that other exchangeable ions, such as 

aluminum, iron, and magnesium, are present after the 

manner described by Paver and Marshall (1934). 

Samples 3, 22, and 24 were tested qualitatively for replaceable aluminum by shaking them with a large excess 

of 0.5N KCl solution having a pH equal to 4.5. Each

SiOj. ...............

RjOs................

MgO ................

CaO................

NajO. ..............

K2 0. ...............

LisO................

H2O— .......... .....

COj.................

MnO................

Total. ............

76

57.54

.74

25.90

1.07

2.21

.16

.80

4.28

6.69

Present2

.01

99.40

76a

56.31

.77

25.70

.50

.52

.14

.82

.01

76a 

(recalculated)

57.54

.79

26.27

.57

.53

.14

.84

.01

3

56.43

20.53

5.13

.10

2.87

.31

.07

9.25

5.27

.02

99.98

3a

58.1?

20.85

5.31

.2f

.1C

.05

.or

3a 

(recalculated)

56.43

20.25

5.16

.27

.10

.05

.02

gave as much as 3 milligrams of A1 2 03 per gram of clay 

in the first extraction and markedly less on successive 

extractions. It is thus apparent that appreciable 

aluminum, in terms of exchangeable base, can be present 

external to the lattice.

The nature of the replaceable ions in two clr.ys containing a large amount of sodium was determined by 

K. J. Murata of the Geological Survey. One, the magnesium clay, hectorite, was from Hector, Calif., and the 

other was from Amargosa Valley, Calif. The clays were 

analyzed after washing with water to remove adsorbed 

salts and after treatment with ammonium-acetate 

solution in large excess. The results are given in table 8.

TABLE 8.—Analyses of hectorite and a clay from Amargo^n Valley 

before and after treatment with ammonium acetate

iWater at 160° C.

2About 1 percent calcite visible under the microscope.

76. Hectorite, Hector, Calif., clay, untreated.

76a. Hectorite, Hector, Calif., clay, after treatment with ammoniuir acetate.

3. Amargosa Valley clay, untreated.

3a. Amargosa Valley clay, treated with ammonium acetate.

Possible exchange of lithium for magnesium was also 

studied by treating a sample of hectorite on r. steam 

bath with normal lithium-acetate solution. After 

several weeks, the preparation was approximate1 y freed 

of the solution and then was treated with fresh Hhiumacetate, the treatment being continued for a total time 

of about 4 months. The preparation then was treated 

with two successive lots of normal ammonium-acetate 

solution and thoroughly washed to remove any Li+1 held 

by ordinary base exchange. Before treatment the 

sample contained 0.80 percent of Li20 and afte~ treatment, 1.05 percent of Li 20. This means that replacement of Mg+2 by Li+1 at most was very slight, and it 

confirms the view that these ions are within the crystal 

lattice and hence nonreplaceable.

Summation of the number of milliequivalents of replaced base per gram in the sample from Amargosa is 

0.93 which is only slightly less than the value fcrnid by 

titration of the acid clay—1.00 milliequivalents per 

gram. The summation for the magnesium clay is 0.75 

milliequivalents per gram if calcium is included. It 

further is to be noted that neither magnesium or lithium 

is replaced.

DETERMINATIONS OF EXCHANGEAPLE 

BASES

Correlation between observed base-exchange capacities and analytical results might appear poor and confusing. The significant observation, however, is the 

directly observed value. It is surprisingly constant for 

the 22 clays examined, the range being between 0.66 and 

1.00 milliequivalent per gram and the mean value being 

0.84. This apparently is one of the most significant 

characteristics of montmorillonite and affords a key to 

the interpretation of the analytical results.
MINERALS OF THE MONTMORILLONITE GROUP 39

TABLE 9. — Base-exchange capacities of some montmorillonile clays

6 

2

0) "5.

1

3 

6 

10 

11 

12 

13 

14 

15 

19 

20 

22 

24 

27 

28 

29 

32 

36 

50 

52 

58 

65 

84

Locality

Amargosa Valley, Calif.. . . .

Nieder-Bayern, Germany. . . 

Rideout, Utah.. ........... 

Ardmore, S. Dak. .......... 

Irish Creek, Va. ........... 

Hector, Calif. ............. 

Otay, Calif. ............... 

Uptown, Wyo. ............

W. of San Antonio, Tex. . . . 

Greenwood, Maine. ........ 

Fort Steel, Wyo. ...........

Otay, Calif..... ........... 

Pala, Calif. ................

Santa Rita, N. Mex. ....... 

Belle Fourche, S. Dak.. ....

in Silicon tetrahedral coordination.

3.90 

3.94

3.92 

3.92 

3.91 

3.91 

3.90 

3.90 

3.88 

3.87

3.87

3.85

3.84 

3.83 

3.83

3.80 

3.75 

3.46

3.29

3.63

3.42

Milliequivalents of exchangeable base per — 

gram.

1.00 

.81

.70 

.85 

.66 

.97 

.84 

.85 

.85 

.88

.70

.70

.89 

.83 

1.00

.87 

.80 

.67

.78

.88

.90

.86

.77

Total milliequivalents of Ca+2, +1KiH+, Naper,

(from analvses). gram 

1.04 

1.17

.72 

.58 

1.47 

.91 

1.54 

.92 

1.20 

.62

.25

.23

.81 

1.16 

.93

.39 

.60 

.40

.19

.60

.74 

.84

Milliequivalents 

per gram of

Ca+s

0.04 

1.17

.64 

.34 

.78 

.86 

.28 

.44 

.18 

.46

.25

.17

.81 

1.13

.54

.39

.45

.60

0

.67

Na+1

0.93

.03 

.20

.67

'i'.22 

.38 

.89 

.16

.06

'".03

'"ii

.07

0 

.13

K+i

0.07

.05 

.04 

.02 

.05 

.04 

.10 

.13 

.01

" [64 

.09

Tr.

.04

H + i

6^39 

'".24

.74

The base exchange of montmorillonite is generally 

held to be due to an excess negative charge of the silicate 

layers, but the possible location of the exchangeable ions 

with respect to the lattice has been a subject for much 

speculation. Hofmann, Endell, and Wilm (1934) suggested that these ions were around the edges of the 

cleavage plates and were he'd by lack of neutrality at 

those places arising from termination of the lattice. 

Marshall (1935) considered the exchangeable cations 

chiefly to be between the silicate layers and, to a lesser 

extent, within the layers. He indicated that K+1 , Na+1 , 

and Ca+2 , particularly when nonexchangeable, might be 

in positions within the lattice layers. Van der Meulen 

(1935) raised the question whether films of mixed 

A1(OH) 3 and Si02 .XH2 0 jellies originating from surface 

hydrolysis of mineral particles could be a site of base 

exchange.

The work of Gieseking (1938) and of Hendricks (1941) 

on the salts of montmorillonite with organic cations 

shows that such ions give characteristic values to the 

cleavage spacing d(0oi), depending upon the size of the 

organic cations. The cleavage spacing of these salts, 

moreover, does not vary with water content, and the 

total sorption of water at a particular relative humidity 

is small compared with that found when the exchangeable ions are inorganic. Both of these properties indicate that the exchangeable ions are located between the 

silicate layers of the minerals.

Observations on the hydration of montmorillonites 

having various replaceable inorganic ions lead to a 

similar conclusion by Hendricks, Nelson, and Alexander 

(1940). Typical results are those obtained when the 

replaceable cation is Ca+2 . The first step in the sorption 

of interlayer water is the hydration of each cation with 

six water molecules, which takes place at a relative 

humidity of about 5 percent. This is followed at higher 

relative humidities by further water sorption without 

increase in the cleavage spacing. The total amount of 

water taken up in these two steps corresponds to that 

required for the formation of a single layer of water 

molecules on the surfaces of the silicate layers.

The only reasonable positions for the exchangeable 

ions on the surfaces of the silicate layers are those occupied by K+1 in the micas and Ca+2 in the brittle micas. 

Ions such as Na+1 , Ca+2, Mg+2 , K+1 , probably occupy 

these positions. Absence of hydration for the proton, 

H+1 in acid clays and its small size suggest that it is 

deeply buried within the silicate layer. These replaceable ions, however, do not tie the layers together as they 

do in micas unless the montmorillonite is heated to 

about 400° C.

MINERALOGY OF THE MONTMORILLONITE 

GROUP

GENERAL RELATIONS

The minerals of the montmorillonite group have been 

recognized as an allied series, but the chemical forrm^as 

either have not been established or, where giver, are 

inadequate to express chemical composition or show 

relations within the group. The commonly accepted 

formula for montmorillonite neglects essential IlgO; 

those for beidellite and nontronlte fail to show the correct regions between A1 203 and Si02 ; and no generally 

accepted formula for saponite is available.

A study of crystal structure by X-ray method 71 has 

now made available a knowledge of the ionic sub^titutions occurring in silicates of the clay mineral type; 

without this knowledge no adequate development of 

mineral formulas is possible. X-ray diffraction studies 

have indicated the mineral that should be assigned to 

the montmorillonite group, and an adequate number of 

chemical analyses is now available to show the probable 

limits of variation.

Typical montmorillonite has been assumed to be composed only of A1 203, Si02, and H20, and the chemical 

composition has been given by Dana (1892, p. 6^0) as 

Al 203 .4Si02 .H2 0, a formula widely accepted as representing the aluminous end member. However, preliminary studies by Ross and Shannon have indicated 

the almost invariable presence of essential amounts of 

MgO and CaO, or more rarely of Na20. The large number of additional analyses presented in this paper confirm the essential character of MgO and of replaceable 

bases. The above formula is that of pyrophyllite, a 

mineral without interlayer water or replaceable bases, 

and it seems evident that the differences in physical 

properties of montmorillonite and pyrophyllite are a 

result of the bases other than A1 203 that characterize 

members of the montmorillonite group but are absent 

in pyrophyllite. (See pp. 70-71.) The crystal lattices 

of the two minerals are similar (see pp. 29-30); therefore, the bases, essentially MgO, constitute the only apparent foundation for their marked differences.

The bases A1+ 3, Fe+ 3 , Cr+3 , Fe+ 2, Mn+3 , Mn+2, Mg+2, 

Ni+ 2, and Li+1, as naturally present in clays of the montmorillonite group, are essentially nonreplaceablo and 

clearly form part of the crystal lattice; Ca and Na, on 

the other hand, and perhaps minor amounts of other 

bases are, in general, replaceable. The researcl discussed on pages 37-39 shows that these replaceable 

bases occupy a position between the silicate layers where 

they are associated with the interlayer water.

X-ray studies, discussed in detail on pages 29-30 

show that like the micas the minerals of the montmorillonite group are made up of silicate layers formed 

from double sheets of linked silicon-oxygen tetral^dral 

groups joined by aluminum-oxygen-hydroxyl octal edral 

groups. The essential feature of such a lattice, in addition to its layer characteristic, is the constancy of the
40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

oxygen-hydroxyl frame work. In this structure, for 

every twelve (oxygen plus hydroxyl) ions, that is, 

10 0~2 + 2 (OH" 1 ), there are four tetrahedral positions. These positions must be completely filled, predominantly by silicon ions, which in part can be replaced 

by aluminum ions. Correspondingly, there are three 

positions having octahedral coordination, of which two 

to three may be occupied by A1+ 3 , Fe+3, Fe+ 2, Mg+ 2, 

Ni+2, and Li+2. If, in a neutral framework, an ion of 

lower valence takes the place of one of higher valence 

[Si+ 4-Al+3 , Al+3-Mg+2 , or Mg+ 2-+Li+ 2] a negative 

charge will be developed. This charge can be neutralized 

either (1) by further addition of positive ions into vacant 

octahedral position, (2) by presence of positive ions external to the silicate layer, or (3) by substitution of 0~2 

by [OH]" 1 . The last, if present, would be expected only 

for those oxygen ions that are present in tetrahedral 

coordination about aluminum and are not shared with 

other tetrahedral groups. As it apparently is not operative it will not be discussed further until the section on 

thermal studies (p. 48).

The experiments described on pages 38-39 show 

that base-exchange value is surprisingly constant and 

is about one-third of the K+1 in the equivalent muscovite formula. In table 10 samples that approximate the 

value of one-third of the K+1 of a normal muscovite are 

designated by a dagger (f). This constancy means that 

the amount of the negative charge is neutralized by ions 

between the layers; however, the substitutions of A1+3 

for Si+4, and Mg+ 2 for A1+3 are commonly so great that 

the resulting number of external ions (exchangeable 

bases) would be much larger unless the first factor, the 

additional ions with octahedral coordination, was also 

important. Seemingly, there is a balance between the 

two tendencies, and this balance is responsible for the 

approximately constant base exchange; that is, a stable 

structure demands that the lack of balance within the 

lattice shall not exceed a certain limit.

As pointed out on pages 40-41 there are three possible octahedral positions within the crystal lattice of 

the layer type of minerals. It has been commonly assumed that exactly two of these positions are occupied, 

as for the heptophyllite micas, represented by muscovite, or exactly three, as for the octophyllite micas 

represented by phlogopite. With three positions available, however, there is no reason to assume that there 

is no variation from the value 2 or 3, although there is 

evidently a tendency to vary only moderately from 

those values.

Nagelschmidt (1938) in discussing montmorillonite 

states, "It seems also that in the micas there is a strong 

tendency for the number of cations in the middle layer 

to be either two or three, or very nearly two or three, 

but not any intermediate number."

Where only Mg+2 proxies A1+ 3 , Li+1 proxies Mg+2, 

or A1+ 3 proxies Si+ 4, conventional mineral formulas can 

be devised, although the small proportion of replaceable 

bases (Na+ 1 or Ca+ 2 ) makes the resulting numbers rather 

large. On the other hand, where two such substitutions 

occur in the same mineral the numbers showing ratios 

become so large that they are cumbersome and fail to 

give a readily comprehensible picture of relations. The 

alternative is to keep the number representing ions in 

the tetrahedral positions definitely fixed—that of a unit 

or half-unit cell—and to use fractional numbers to 

represent proportions of the various ions. Thus, the 

proportions of ions in different positions will be directly 

shown in the formulas and will be strictly comparable

in all members. Another arbitrary convention vail be 

to represent the replaceable ion directly above the ion 

within the crystal lattice that it balances. In tH formulas the exchangeable base has been represented as Na, 

although more commonly it is Ca in natural clays, and 

less commonly other ions. Strictly ,the exchangeable 

bases should be represented as (Na,Ca), but this rela- ~2~

tion is so obvious that the added complexity introduced 

into the equations is not justified. This method also has 

the advantage that assumed end members and all intermediate members may be represented by similar formulas, and the proportions of the various ions may be 

shown as directly for intermediate members as for end 

members.

The same method of expressing the composition of 

minerals has been used by the authors, Hendrichs and 

Ross (1941), in a paper on glauconite and celadonite— 

minerals belonging to the mica group. In a study of 

nontronite, attapulgite, and saponite, Caldwe1 ! and 

Marshall (1942) have used a .method of representation 

that is not essentially different.

Discussion of relationships in the montmorillonite 

series may start with the pyrophyllite and talc formulas, 

which follow, the various formulas used being numbered 

successively:

(1) Pyrophyllite Al 2Si 4Oi0[OH] 2

(2) Talc Mg3Si 4 010[OH] 2

Because the charge is completely balanced within the 

lattice, no replaceable ions are to be expected; and experiment shows that essentially none are present.

If a sodium mineral analogous to muscovite is assumed to have all its Na replaceable, it would be shown 

as follows according to this method of representation:

Na

T

(3) Al 2 [AlSi 3]Oio[OH] 2

A hypothetical mineral in which Mg proxied one-half 

of the Al, ion for ion, could be represented as follows:

Na

T

(4) [MgAl] [Si 4010 [OHj 2

Experiment, however, shows that in montmorTloriite 

only about one-third as much Na or Ca is replaceable 

as represented in formulas (3) and (4), and that substitution of either Mg+ 2 for A1+ 3 , or of A1+ 3 for Si+4 may 

be the factor that induces base exchange.

The observed base exchange means merely that the 

charge on the lattice is unbalanced only to a l:mited 

extent. This can be represented by the followir? formulas, in which fractional numbers are introduced:

Na.33 

T

(5) [AU.67Mg.33] Si 4 010[OH] 2

Na.33 

T

(6) A1 2 .00 [Al. 33Si 3 .67] 010[OH] 2

Na 33

T

(7) [Ah.5oMg.63] [Al.09Si3.9i]0 10[OH] 2

The lack of balance in the charge of the lattice arises 

from the substitution in formula 5 of A1+3 by Mg+2 and 

in formula 6 of Si+4 by A1+3. The more general case is
MINERALS OF THE MONTMORILLONITE GROUP 41

illustrated by formula 7, where both substitutions 

operate and where the lack of balance in the charge of 

the lattice is partly compensated by the increase above 

2.00 of the number of ions with octahedral coordination. 

The negative charge is shown as balanced by Na+1 

placed above the line, the arrow showing the connection 

with the effective negative group. If desirable the Na+1 

may be transferred to follow the [OH] 2 .

METHODS OF CALCULATION OF MINERAL 

FORMULAS

Previous discussions of crystal structure, chemical 

relationships, and base-exchange experiments provide 

the basis for the calculation of mineral formulas of members of the montmorillonite group. These relationships 

as they apply to the development of mineral formulas 

will be briefly restated. The minerals of the montmorillonite group are members of a larger group, all of 

which are characterized by micaceous habit and have 

the same fundamental structure. Of these, two of the 

simplest are as follows:

Pyrophyllite Al 2Si 4010[OH] 2 

Talc Mg3 Si 4 010 [OH] 2

In general the micaceous minerals differ from the 

foregoing minerals in that members of the montmorillonite group contain an additional group of ions, the 

exchangeable bases Ca, Na, etc.; the micas contain K, 

which is not exchangeable; and the brittle micas contain Ca, which also is nonexchangeable. As these 

formulas indicate, the minerals of this type are characterized by a variable number of ions in octahedral positions — A12 or Mg3 in the foregoing formulas. This means 

that there are 3 possible octahedral positions, of which 

2 to about 2.2 are occupied in the montmorillonitebeidellite-nontronite series, and about 3, but not more 

than 3 are occupied in the hectorite-saponite series. In 

tetrahedral coordination there are 4 positions, all of 

which must be occupied. In most members of the group, 

A1+ 3 partly proxies Si+4 in tetrahedral positions. There 

are always 10 oxygen ions and 2 hydroxyl groups.

Titanium is present in the analyses of many of the 

clays, as well as other silicate minerals. In general, 

titanium is an essential part of the crystal structure of 

silicate minerals where it proxies aluminum. However, 

in the clays it is believed to be present as one of the 

titanium oxides and hence is to be considered as an 

impurity. Phosphorous is also believed to be most 

commonly an impurity. For these reasons TiO2 and 

P2O5 are neglected in calculating mineral formulas.

The 10 oxygen ions and 2 hydroxyl groups complete 

the unit used as the basis of calculation, and these ions 

have a negative valency of 22. The most general formula for members of the montmorillonite group is

Octahedral

, —————————————— *^. _____________

+ 3 +3 +2 +2 +3 +3 +2 +1

(1) [Alaa_yy +Feb +Fec +Mgd +Cre +Mnf +Mng +Lih}

Tetrahedral Anions Bases

.y]O10 [OH,F] 2 x. 33

where x represents the exchangeable bases, which on the 

average appear to be near 0.33 and are so assumed 

irrespective of the observed amounts, since these commonly are not determined completely and are of questionable accuracy. A formula can, of course, readily be

calculated in which the observed quantities of excha ngeable bases, rather than assumed values, are used.

The subscripts a_y, b, c, d, e,,, s, h, y, and Si4—y represent 

the proportions of cations in the formula. A shortened 

formula containing only the more typical ions of the 

group will be adequate to illustrate mineral calculations, 

as other ions will be handled by the same methods. 

This is

+ 3 +3 +2 +3

(2) [Ala_y +Feb +Mgd ] [Aly + Si 4 _y]Oio[OH] 2x 3 i

The total number of ions in octahedral positions is 

represented by S; that is

a—y-t-b+a ... =2

Let atomic proportions of Al = A, Fe+s = B, Mg » 

D, Si = Z, Al in tetrahedral positions = Y, and 0 

(oxygen) = R; then the atomic proportions as derived 

from the chemical analysis, and with Al properly allocated between octahedral and tetrahedral positions, 

must bear such a relation to the mineral formulae (2) 

that

(3) [Alt' +s£*l O 10R [OH] 2R XR,

when multiplied by a constant, K, will become

(2) [AUy +Fer +Mfo'] [AC3 +Si4ly]0

It is to be remembered that the subscript to x is assumed 

to be 0.33 in keeping with the general results cf all 

analyses, and does not correspond to the particular 

analysis.

The negative valency of Oi?[OH] 2 = 22. Also the number of ions in tetrahedral positions, that is, Al^'+S'ftiy, 

must always equal 4.

The first step in the calculation is the reduction o* the 

analytical values to atomic proportions. The calculations are based on the distribution of single ions, and 

SiO2 , MgO, and the like, containing single cations, are 

divided by the molecular weights, as usual; but A12 03, 

Fe2O3 , K2Q, and Na2O are divided by one-half the molecular weight. The proportion of exchangeable bases 

depends on the number of ions required to neutralize the 

charge of the remainder of the formula. Thus, one -half 

as much Ca+2 is required as Na+, and for this reason the 

number of equivalents of CaO are to be used, obtained 

by dividing the percentage of CaO by one-half the molecular weight.

To obtain a formula from analytical data it is necessary to determine the number of ions having octahedral 

coordination (2); the factor (K), by which the atomic 

proportions are to be multiplied to give the required 

formula; and the number (Y X K) of aluminum ions in 

tetrahedral coordination. There are thus three unknowns S, K, and Y, which are defined by the three 

equations. From equations (2) and (3) it follows that

(4) ZX4] +.33 = 22

22 - .33

or K =

AX3 + BX3 + DX2 + Zx4

If the determined value of X, the exchangeable base, 

is to be used in the calculation, this equation becomes

K[AX3 + BX3 + Dx2 + Zx4 + X] = 22

22

or ——————————————————————— 

[Ax3 + Bx3 + Dx2 + Zx4 + X] = F
42 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

This equation represents a balance of valence and 

expresses the condition that the sum of the valences of 

the positive ions with the exchangeable base, assumed 

to be 0.33, is 22.00. The factors 3, 3, 2, and 4 are the 

respective valences of Al, Fe, Mg, and Si. This equation 

thus uniquely defines K.

Equations (2) and (3) also indicate that

4

[Z+Y]K = 4 or — - Z = Y

K

This equation expresses the condition that the atoms 

Si + Al, having tetrahedral coordination, equal 4. 

Since Z, the atomic proportion of Si, namely, the percent 

of Si0 2 divided by the molecular weight 'of Si02, is 

known and K is fixed by the first equation, Y X K can 

immediately be calculated. 

Also, it follows that

K[A+B+D-Y] = S

This equation is an expression for the total number 

of ions (S) having octahedral coordination. Since A, 

B, and D are given by the analysis and Y and K by the 

preceding equations, 2 can immediately be calculated.

The method may be illustrated by a calculation of 

the formula of analysis 40, table 1, as follows:

Constituent

Si02 . ........

A1 20, ........

Fe 2O 3 . .......

MgO.........

CaO .........

Na 20 ........

TiO2 .........

H2O..........

Total ......

Percent

49.70

22.10

2.12

2.85

1.08

1.17

.28

21.14

100.44

Factor

60.06

51.00

79.92

40.32

28.00

31.10

Atomic 

proportions 

and 

symbols

0.828=Z

.433=A

.027=B

.071=D

Equivalents 

and symbol

O oo^

39 |.077-X

Substituting numerical values for Z, A, B, D, and X 

in equation (4) and using the assumed value for X 

(.33) rather than the value of x = (.077) X K

22.00 - .33

K = ——————————————————————— = 

.433X3 + .027X3 + .828x4 + .71x2

21.67 ——— = 4.481

4.832

K[Z+Y] = 4.00 or 4.481 [.828+Y] = 4.00

4.00

which gives Y = ——— - .828 = .064 

4.481

K[A+B+D-Y] = 4.481 [.433+ .027+ .071-.064] = 

4.481 X .467 = 2.10 = S

The final formula, therefore, is obtained by multiplying 

each of the above number of ions by their corresponding 

valences and adding X = 0.33, as follows:

a-y = [A-Y]K = [.433-.064]4.481 = 1.66 

b = BK = .027X4.481 = .12 

d = DK = .071X4.481 = .32 

y = YK = .064X4.481 = .29 

4_ y = 4.0-.29 = 3.71

These values give the formula

X.33

T 

[Al,. 66Fe. 12Mg. 32] [Al. 29Si 3 .7i]010[OH] 2

The calculated subscript of x is really 0.077x4.481 = 

0.345, which is in better agreement with the assumed 

value 0.33 than usually is found.

The same method is applicable to other related 

mineral groups, such as the micas. It was usei by 

Hendricks and Ross (1941) in their study of the composition of glauconite and celadonite. Here the analytical value of the bases (essentially K for minerals of 

the mica group) is used in calculations.

In some analyses the amount of Si0 2 was so great as 

to exceed that required for tetrahedral coordination. 

In those specimens Si02 was probably present rs an 

impurity, and it was possibly present also in a minor 

amount in some samples for which formulas could be 

calculated. Similarly, some specimens, particularly 

nontronites, contained more Fe2 03 than could b? accounted for by the formula. In those samples and possibly in some others, iron-bearing impurities such as 

free oxides, hydrous oxides, or more probably mixedlayer minerals of the chlorite type are present.

Thus there are three possible variations from the 

ideal conditions that have been assumed as a bas;<? for 

calculations. The presence of free oxides or hydrorides, 

or of mixed layers of chloritic minerals may make the 

ions that are assumed to occupy octahedral positions 

too large; the presence of SiO2 as quartz, cristpbalite, 

or amorphous silica may make the assumed Si02 too 

large; and the base-exchange value may not be exactly 

0.33. These possible variations, as previously po;nted 

out, probably are so small that they introduce no major 

errors into the calculations. They would involve only 

a minor shift of slightly more or less Al (ions) into 

octahedral positions where it proxies Si and would have 

little effect on the assumed relations.

The results of the calculations as outlined in the preceding sections are given in table 10. The generalized 

formula for minerals of the montmorillonite-beidellite, 

nontronite-saponite group is

+ 3 + 3 +2 +2

[Ala_y, Fe , Fe , Mg ] [AlySi4_y]O10 [OH] 2x.33 

TABLE 10.—Relation of ions to crystal structure

[The analyses that approximate the base-exchange value of one-third of the 

K+1 of a normal muscovite are designated by a dagger (t). Total equiva'ence of 

0.21 or less may be considered low.]

Formulas for montmorillonite-beidellite series

6 

Z

£ "3. 

E «

VI

1

2 

3

4 

5 

6

7 

8

9 

10 

11

12 

13 

14 

15 

16 

17 

18

19 

20 

21 

22

23

Locality

Santa Cruz, 

N. Mex........

Cilly, Styria.. . . .

Amargosa Valley, 

Calif.. ........

Dixon, N. Mex. . . 

Tatatila, Mexico . Maricopa, Calif. . Conejos Quadrangle, Colo. . . . 

San Diego County, 

Calif. .........

Nieder-Bayern,

Near Rideout, 

Utah..........

Ardmore, S. Dak. 

Irish Creek, Va. . . 

Hector, Calif. ....

Polkville, Miss. . . 

Angleur, France. . Montmorillon,

LTpton, Wyo. . ....

Spokane, Wash. . . 

Atzcapozalco, 

Mexico ........ 

Lemon, Miss. ....

Tetrahedral 

coordination

Si

4.00

3.99

3.98 

3.98

3.96 

3.94 

3.94

3.94 

3.93

3.92

3.92

3.91 

3.91 

3.90 

3.90

3.89 

3.88

3.88

3.88 

3.87

3.87

3.87 

3.86

Al

0.01

.02 

.02

.04 

.06 

.06

.06

.07

.08

.08

.09 

.09 

.10 

.10

.11 

.12

.12

.12 

.13

.13

.13 

.14

Octahedral 

coordination

Al+»

1 30

1 43

1.40 

1 00

1.37 

1.63 

1.46

1.38 

1 32

1 44

1 14

1.54 

1.50 

1.49 

1 36

1.36 

1.56

1 64

1.55 

1 43

1.38

1.34 

1.44

Fe+»

0 11

OS

.14

26

.05 '!ii

.18 

06

21

14

.07 

.14 

.13

.24 

.11

05

.19

16

.18

.30 

.16

Fe+*

0.01

.01

...>»...

.09 "".6i"

.05

.02

Mg+s

0.72 

.57

.53 

.95

.72 

.42 

.47

.52 

.79 

.40

.96 

.38 

.42 

.44 

.75 

.49 

.39

.36 

.26 

.52 

.56

.44 

.51

S

2.13

2.08

2.08 

2.22 

2.15 

2.05 

2.07

2.08 

2.17 

2.05

2.24 

2.08 

2.06 

2.07 

2.16 

2.09 

2.06

2.05 

2.02 

2.11 

2.12

2.08 

2.11

Total 

equiva lence of 

external ions 

and predominantion

0.49 Ca 

.48Ca

.61Na 

t .31 Ca 

.15 

.53Ca 

t . 34 Ca

.50Ca 

t .31 Na 

t .31 Ca

t .27Ca 

.70Na 

t .42Ca 

.66 Na 

t .43 Ca, Na 

t .40Ca 

t .34Ca

t .38 Ca 

.50Na 

t .28Ca 

t .40Ca

.12 

t .28 Ca
MINERALS OF THE MONTMORILLONITE GROUP

TABLE 10.—Relation of ions to crystal structure—Continued

Formulas for montmorillonite-beidellite series—'Continued

Sample No.

24 

25 

26

27

28 

29 

30 

31 

32 

33 

34 

35

36 

37

38 

39

40 

41 

42

43

44 

45 

46

47

48 

49

50 

51 

52 

53

54

Locality

Rosedale, Alberta

Santa Monica, 

Calif. .........

West of San 

Antonio, Tex. . . Greenwood, Maine 

Fort Steel, Wyo. . Osage, Wyo. .....

Lemon, Miss.. . . . Otay, Calif. ...... 

Branchville, Conn. 

Leakev, Tex. ..... 

Twin Falls, Idaho

Pala, Calif. ......

Montmorillon, 

France. ....... 

Booneville, Miss. . Maniquipi, 

Mexico ........ 

Clairmont, Calif. . Glen Riddle, Pa. . . 

Pala, Calif. ......

Los Angeles, 

Calif.......... 

Beidell, Colo. .... 

Embudo, N. Mex. 

Fairview, Utah . . . Wagon Wheel 

Gap, Colo.. .... 

South Bosque, 

Tex........... 

Carson District, 

Nev........... 

Pontotoc, Miss.. . Nashville, Ark. .. 

Nashville, Ark.. . . 

Ancon, Canal

rloseland, Va. ....

Tetrahedra 

coordination

Si

3.85

3

QC .85

3 . 85

3.84 

3.83 

3.83 

3.82

3.81 

3.80 

3.78 

3.76 

3.76

3.75

3.72 

3.72

3.71 

3.71 

3.71 

3.69

3.69 

3.64 

3.57 

3.54

3.53 

3.53

3.48 

3.46 

3.35 

3.29

3.26 

3.18

Al

.15

.15

.15

.16 

.17 

.17 

.18

.19 

.20 

.22 

.24 

.24

.25

.28 

.28

.29 

.29 

.29 

• .31

.31 

.36 

.43 

.46

.47 

.47

.52 

.54 

.65

.71

.74 

.82

Octahedral 

coordination

Al+3

1 56

1.50

1.38

1.44 

1.74 

1.46 

1 64

1.41 

1.17 

1.69 

1.72 

1.36

1.62

1.51 

1.40

1.74 

1.66 

1.58 

1.61

1.43 

1.46 

1.53 

1.59

1.52 

1.67

2.00 

1.84 

1.74 

1.73

1.80 

1.84

Fe-H

.21

.33

.16 

.01 

.09

18

.19 

.19

".02 

.37

.01

.07 

.40

.13

.12 'ioi

.22 

.50 

.02 

.31

.37

.07

.05 

.12 

.15 

.23

.29 

.31

Fe+2

/Fe .01 

\Li . 10

.03

.01 

Mn"!6i

Mn .01 

Mn++.01

Mn(.bi) 

.04

"".6i"

'";62" 

.02

Mg+2

.57 }.28

.32

.51 

.30 

.60 

.20 

.52 

.89 

.38 

.35 

.36

.50

.60 

.28

.18 

.32 

.61 

.46

.51 

.08

.72 

.21

.23 

.46

.02 

.15 

.33 

.26

.05 

.02

S

2.13 

2.11

2.06

2.11 

2.05 

2.15 

2.02 

2.13 

2.25 

2.08 

2.09 

2.09

2.14

2.19 

2.08

2.05 

2.10 

2.19 

2.16

2.16

2.04 

2.27 

2.12

2.12 

2.20

.07 

.13

.22 

.22

.16 

.17

Total 

equivalence of 

external ions 

and predominantion

.11

.58 Na

t . 30 Ca

t . 38 Ca 

.48Ca 

t . 35 acid 

.46Na 

.07 

.18 

» .65Ca 

.05 

/No bases 

\reported 

t .25Ca

t .29 Ca 

.21

t -42 Ca 

t .34Na, Ca 

.06 

t .25 Ca

.19 

.40 

t . 32 Ca 

t .42 K, Ca

t .34 Ca 

t .29 Ca

.47 Ca 

. 08 Acid 

.13 

.09

.18 

.19

Formulas for nontronites

55

56

57

58 

59 

60

61 

62 

63

64 

65

66

67

Stary-Krym,

Woody, Calif. .... 

Petrovsk, KrivoiSpokane, Wash . . 

Spokane, Wash. . . 

Santa Rita,

N Mex

Chevy Chase, 

Md............

Szekes-Fejewar,

Stary-Krym,

Tachau, Germany 

Sandv Ridge, 

N. C..........

Spruce Pine,

N. C.. ........

Tachau, Germany

3.77

3.76 

3.74

3.63 

3.51

3.50

3.47

3.45

3.45

3.42 

3.42

3.37

3.36

0.23

.24 

.26

.37

.49

.50 

.53

.55

.55

.58 

.58

.63

.64

0 69

.06 

1 1

.77 

.00

.11

1 02

07

37

.45

39

51

.40

1 26

1.86

8 75

1.10 

1.92

1.91 

1 04

?, 00

1 62

1.61 

1 70

1 58

1.65

.07

.02 

.05

.02

.03

.09

0.03 

.01

.07 

.19 

.10

.04

.12 

.01

.01

1.98 

2.00

2.00 

2.08 

2.07

2.08 

2.06 

2.07

2.11 

2.10

2.09

2.10 

2.14

t .34 Ca 

t .40 Ca

t .36 Ca 

t .28Ca 

t .36Ca

.53Ca 

t .26 Ca

.07

t .33Ca

.07

t .26 acid 

.18

Formulas for samples from soils and chromium-bearing samples

o

"a»

g

68

69

70

71

72

73

74

75

Locality

Maniquipi,

Mexico. . . .

Rothamstead,

England

Putnam

clay, Missouri,U.S.A.

Emerais

mines, Urals

CheghetLakhran

Valley.Urals

CheghetLakhran

Valley,Urals 

Bnlshia-laba

River. ....

Gedmayshkh

Valley,

Russia. . . .

Tetrahedral

coordination

Si

3.24

3.25

3.55

3.56

3.54

3.44

3.29

3.15

Al

0.76

. 75

.45

.44

.46

.56

.71

.85

Octahedral 

coordination

Al+3

1.69

1 35

1.40

1.25

1.10

1.23

1.33

.85

Fe+3

0.36

65

.48

.54

.29

.62

.21

.56

Cr+3

.05

.32

.06

.06

.25

Fe+3

0.03

.05

.04

.03

.02

.56

Mn+=

(Ni.02)

.01

.03

Mg+ 2

0.05

.21

.21

.24

.43

.21

.74

.21

V

2.16

2.21

2.12

2.13

2.21

2.15

2.39

2.43

Total

equivalence 

of external

inns and

predominant 

ion

0 . 30 K, Ca

18

.19

t .36 Ca

t .26 Ca

t .28 Ca

t .29 Ca

.47 Ca

TABLE 10.—Relation of ions to crystal structure—Continued

Formulas for saponite and hectorite

Locality

Hector, Calif. ....

Montreal.. ......

SvardsjO,

Sweden ........

Ahmeek mine,

Mich. .........

Kearsarge mine,

Mich. ..........

San Bernardino

county, Calif. . . .

Glasgow, Scotland

Tetrahedral 

cordination

Si

4.00

3.68

3.46

3.42

3.40

3.33

3.24

Al

.32

.54

.58

.60

.67

.76

Octahedral 

coordination

Al+3

0.01

.21

.27

Fe+3

0.02

.10

.11

.08

.52

Fe+2

0.05

.18

.03

.26

Li+i

0.30

Mg+2

2.67

2.93

2.69

2.72

2.92

2.73

2.33

S

2.98

3.00

3.00

3.01

3.03

3.00

3.01

Total 

equivalence 

of externalio**a

Calculation

N

Anrlysfs

a

I O.M 

Me

.30

C

.23

,?8

a

.!«

Na

.47

C

.!«

a

.£2

Ca

.40 |t .33 

Ca

.24 t .33

When corrected for PzOs content as Caa[PO4]2, sample shows total equivalerce 

of 0.40 f.

TABLE 11.—Calculations of impurities in representative clayp

Samples containing excess silica

Sample 

No.

83

84

85

86

Locality

Quilchina, British Columbia . . . 

Santa Rita, N. Mex. ...........

Tierra Amarilla, Rio Arriba Co., 

N. Mex........ ............

Westcliffe, Colo.. .............

Minimum 

excess 

SiOz

4.4

4.0

3.0

1.1

Minimurr 

value of TJ

2.07 

2.06

2.09

2.10

Samples of nontronites containing an iron-bearing impurity

87

88 

89

QD

91

92

93

Q4

95

Q7

98

99 ,

Magnitnaja Mountains, Urals . . 

Okhansk, Siberia .............

Woody, Calif. ................

Unghwar, Hungary. ..........

Gobitschau, Moravia ..........

St. Andreasberg, Harz ........

St. Andreasberg, Harz .........

Island of Magee, County 

Antrim ....................

Nontron, France ..............

Minimum 

excess 

calculated 

as FezOa

18.7

16.1 

10.1

6.7

5.2

5.0

4.7

9.9

3.6

1.9

1.2

1.0

.7

Maximum 

value of V

2.21

2.07 

2.02

1.96

2.01

2.15

2.14

2.16

2.18

2.20

2.07

2.05

2.04

Samples containing a chlorite or vermiculite impurity (probably as mixed layerr* 

or an iron-bearing impurity

Sample No. and locality

100. Cathkin Hills, Scotland.

101. Webster, N. C..........

102. Blackburn, Scotland....

Ratio

4:3.15 

4:3.47 

4:3.48

The ratios given above are calculated with the assumption of the maximum possible number of ions in 

tetrahedral coordination, that is 4, and the minimum 

possible number of ions in octahedral coordination. As 

previously stated, tetrahedral ions must be 4 and octahedral ions cannot exceed 3. Therefore, the excess abpv* 

the ratio 4:3 illustrates the minimum excess of impurity.
44 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

DERIVATION OF FORMULAS

The foregoing discussion of X-ray structure, the baseexchange experiments, and the large number of chemical 

analyses of clays of extremely varied range in composition provide the basis for the selection of chemical formulas that will present the relationships within the montmorillonite group of minerals. It is believed that the 

coordination of all these factors, including the large 

number of analyses and the extreme range in composition that they represent, presents an unusually adequate 

method for the study of complex mineral groups.

MONTMOEILLONITE-BEIDELLITE SEEIES

Fifty-four of the 103 analyses listed in tables 1 to 7 

correspond to the montmorillonite-beidellite series. 

Silicon in tetrahedral coordination varies continuously 

between 4.00, the montmorillonite end, and 3.18, the 

beidellite end, but it does not go as low as 3.00 in any 

materials so far examined. Apparently, then, montmorillonite and beidellite form a complete series of solid 

solutions. Silicon in tetrahedral coordination exceeds 

3.8 in six-tenths of the samples, and this excess indicates 

the somewhat greater predominance of montmorillonite, 

owing to the large proportion of bentonites included. 

This preponderance probably would not have been 

apparent if an equal number of analyses of soil, vein, 

and shale clays had been available.

The maximum number of ions in octahedral coordination (2) is 2.27; however, 2 exceeds 2.20 only in 6 

samples and 2.16 only in 12. If oxides or hydrous oxides 

of iron or aluminum are present, or if part of the aluminum is external to the lattice, the value of 2 will be 

erroneously high. If excess silica is present 2 will be 

erroneously low. In deriving the formula the presence 

of small amounts of impurities will have slight effect, 

as correction will be made for the impurity by changing 

the amount of silicon in tetrahedral coordination. While 

high and low values of 2 thus are of questionable accuracy, a very large proportion of the values probably 

are accurate to ± .02.

Thus it appears that in the montmorillonite-beidellite 

series the maximum number of ions in octahedral coordination (S) is about 2.20 and the minimum near 

2.05. In the 36 samples in which Si in tetrahedral coordination exceeds 3.72 the average value of 2 is 2.10, 

and in the 19 samples in which tetrahedral Si is less than 

3.72, it is 2.15, which is significantly higher. Thus, apparently, the number of ions having octahedral coordination is slightly greater for the beidellite end of the 

series than for the montmorillonite end. This difference 

is evidence for a tendency toward internal compensation 

of charge as the amount of aluminum in tetrahedral 

coordination is increased. The limited upper value of 2 

is evidence that these minerals do not form complete 

solid solutions with minerals having three ions in octahedral coordination, and the same condition apparently 

is true for pyrophyllite and talc.

There is, further, an inverse relationship between the 

amount of aluminum in tetrahedral coordination and 

the Mg+2 + Fe+2 in octahedral coordination. The 

average value for Mg+2 + Fe+2 in the first 36 samples 

is 0.53 ions in octahedral coordination, and in 18 of these 

samples it is above 0.50. In the next 18 samples the 

average value of Mg+2 + Fe+2 in octahedral coordination is 0.30, and in only 15 percent of them do these ions 

amount to more than 0.50. In other words, the excess 

aluminum in tetrahedral coordination over that required for the base exchange of beidellites is irore commonly neutralized by additional A1+3 and Fe+3 in octahedral coordination than by Mg+2. This is a tendency 

either to prefer the ion of smaller radius or th<* smaller 

total number of ions having octahedral coordination. 

Twenty-six of the 54 samples, or slightly less than

Ca half, have a total equivalence of Na+-«-+K approximately equal to 0.33, as required by the average baseexchange capacity. In 13 it is too high, and in 15 it is 

too low. For those with the higher equivalence the 

average value of 2 is 2.07, whereas for those with the 

lower equivalence it is 2.14. These proportion? strongly 

indicate that the apparent deficiency in external bases 

is partly due to the presence of aluminum or iron external to the lattice. In spine samples, however, it may 

be due to inaccuracy of analysis. The samples in which 

the amount of external base, as obtained analytically, 

is too high probably fall into several classes. The base 

exchange may really be high, or nonexchangeable bases 

may be present. Not infrequently, however, calcium 

carbonate or some sodium salt, such as the carbonate or 

chloride, probably is present. Sodium chloride is known 

to be present in some of the clays from arid regions. In 

any case, it is with just such minor constituents that 

experimental uncertainty and questionable purity of 

materials has the greatest effect. The extent of agreement is perhaps even more striking evidence of the 

general purity of the specimens examined.

The above discussion depends upon the average 

tendency of a large group of samples rather tvan upon 

the pecularities of a particular one. This procedure, 

though seldom followed in mineralogical research, has 

much to recommend it. Results obtained from an impure sample will be sufficiently distinct to (puse suspicion and will be analagous to a curve based on numerous well-spaced points. A few points that do not fall 

correctly do not invalidate the curve. If a sufficient 

number of specimens are analyzed, limits can be placed 

upon the kind and extent of isomorphous replacements.

The foregoing chemical analyses and the cHscussion 

of the chemical and structural relations delimiting their 

interpretation provide the basis for the derivation of 

chemical formulas that will represent relations within 

the montmorillonite group. The analytical data are 

believed to be entirely consistent with the type of 

formulas presented.

A specific chemical formula is necessary to represent 

a named mineral species; it is useful to represent isomorphous mixtures, by means of end members, even though 

these are hypothetical; and it is used in calculations of 

mineral relationships. For these reasons formulas for 

the deduced end members will be presented, although 

as pointed out on page 47 they cannot uniquely represent chemical compositions. For these reasons the endmember formulas presented will probably rot be as 

fully representative as those for the plagiocla^e group, 

for instance; and one should not lose sight of the limitations imposed on the end-member method of representing mineral compositions where an unusually large 

number of variables is involved. For these reasons the 

other type of formulas, 8a, 9a, lOa, etc. (see below), are 

believed to illustrate relationships better. Where formulas representing end members are presented, some 

mineral name is demanded. New names are to be 

avoided wherever possible, and none is proposed in this 

paper, -although this avoidance has necessitated the 

use of chemical modifers for three members.
MINERALS OF THE MONTMORILLONITE GROUP 45

Selection of the most suitable formula for montmorillonite is questionable, within certain rather narrow 

limits. A possible one is

(8) 5Al203 .2Mg0.24Si02 .6H2 0[Na20, CaO] or 

Al&MgSi 12030[OH] 6[Na, ^Ca]

corresponding to

Na.ss 

T 

(8a) [AIi.67Mg.li] Si 4 010 [OH] 2

The only serious flaw in it is that the total number of 

ions in octahedral coordination is 2.00, whereas in all 

actual specimens this number is exceeded.

The mineral name beidellite will be used for the member having a large amount of A1+3 in tetrahedral coordination. A suitable formula is

(9) 13A1 203 . 5A12O3 . 38SiO2 .12H20 . 2Na20or

(9a)

Na ss

T 

Al2 .n [Al. 83Si;

It is possible that the available analyses do not quite 

represent the maximum possible replacement of Si by 

Al and such a greater replacement may be represented 

by the following furmula:

(10) 20A1203 . 9A1 203 

Na2 0] or

54Si02 .18H2 0 . 3[CaO,

(lOa) A1 2 . 22

Na 33

T

010 [OH]

However, the discussion of the value of S on page 

44 indicates that 2.22 is near the maximum value 

and that apparently not more than one-fourth of the 

tetrahedral positions are occupied by aluminum. Na. 33 

is placed above A1. 83 in the foregoing formula (9a) rather 

than above A1. 33, which would be required to balance 

Na.ss directly. This is due to the fact that the positive 

charge resulting from Al in excess of 2 in octahedral 

position is three times as large as the negative charge 

caused by Al in tetrahedral position; hence,

A1. 8 8 - (A1. 17 X 3) = Al., Na .33

The same relation applies to formulas 7 and 10, pages 

40, 48. Notwithstanding the undesirably high 

numbers necessary for the oxide ratios, the beidellite 

formula is an oversimplification, for the usual presence 

of Fe203 and small amounts of MgO have been neglected. 

It is more satisfactory than the montmorillonite formula 

in that it gives about the correct number of ions with 

octahedral coordination. Sample 54, from Roseland, 

Va., closely approximates this formula except that some 

Fe+3 is present, replacing A1+3 with octahedral coordination. The material from Beidell, Colo., is intermediate between beidellite, as written above, and the 

montmorillonite end of the series. It also contains appreciable Fe+3 and Mg+2 in octahedral coordination.

NONTRONITE

Formulas obtained for nontronites in a manner 

analogous to that used for the montmorillonite-beidellite 

series are summarized in table 11. Thirteen of the 

samples examined appeared to be satisfactory nontronites—that is, montmorillonite-type clays containing

large amounts of ferric iron and relatively free of adventitious materials. The amount of aluminum in 

tetrahedral coordination in these, samples ranges-from 

0.23 to 0.63, and a fairly uniform series is represented. 

This range in composition does not justify a distinct 

name for the members richest in Al, and it will be indicated as nontronite (aluminian) (Schaller, 19F1). 

Wider variation probably is limited only by the restricted number of samples. Thus there is evidence for 

a nontronite series in which there is a variation fron a 

limited to a marked substitution of Al for Si, similar to 

that in the montmorillonite-beidellite series.

The maximum number of ions having octahedral 

coordination is 2.11. The average value is 2.06, and 

three of the samples have a lower value of S than vras 

found for any member of the montmorillonite-beidellite 

series, of which four times as many specimens were 

studied. Those nontronites having'the greatest amonnt 

of aluminum in tetrahedral coordination also have the 

higher value of 2. Thus the average value of this summation for seven samples with less than 3.50 Si in 

tetrahedral coordination is 2.09, whereas six samples 

with Si greater than 3.50 have an average of 2.03. 

These averages are strictly in accordance with the behavior of beidellite.

It is striking that the nontronites contain relatively 

small amounts of magnesium. This relationship is 

particularly true for the specimens having chiefly Fe+3 

in octahedral coordination; in fact, of the four samples 

containing less than 0.12 A1+3 in octahedral coordination the highest was 0.04 Mg+2. If anything, the trend 

in content of Mg+2 is opposite to that of the mcntmorillonite-beidellite series, but even at the most it is 

not great. Ca+2

The amount of external Na+ l, K+ 1 , H+ 1, and ~

analytically determined again agrees with the expected 

value, 0.33, in about half the samples. It is too low in 

four samples and too high in only one. Several of the 

samples with low amounts of external base may be acid 

clays. The sample from Sandy Ridge, N. C., is such an 

acid clay; its analysis shows the presence only of Al£03, 

Fe2 03 , Si0 2 , and H2O. Its composition thus is v^ry 

simple and correlates base exchange with the lack of 

balance in the charge of the lattice. The formula is

H.33

[Fe^oAl.to3] [A1.B8S; 3 . 42]0 10[OH] 2

Substitution of Si+ 4 by A1+3 in tetrahedral coordination 

is partly compensated by the external hydrogen ion and 

partly by additional ions in octahedral coordination 

above two, S being 2.09.

Assignment of adequate formulas of the usual type 

for nontronites is even more difficult than for mcmtmorillonite. A compromise is to use an analogous 

formula in which aluminum in octahedral coordination 

is replaced by ferric iron, namely:

(11)

(lla)

5Fe203 . 2MgO . 24Si02 

CaO] or

.

6H2 0 . [Na 20,

This is unsatisfactory in that it shows essential Mg+2 

and suggests that base exchange is principally due to
46 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

substitution in octahedral positions. For these reasons 

it cannot be used to express the composition of some 

samples.

A somewhat better formula is

(12) 6Fe203 .Al 203 .22Si02 .6H2 0[Na20, CaO] or 

Na 33

+ 3 T

(12a) [Fe2 .00] [Al. 33Si 3 . 67]010[OH] 2

It correctly shows the base exchange as arising from 

substitutions of A1+ 3 for Si +4 intetrahedral coordination. 

Nontronite of the aluminian type, is considered to be 

similar to beidellite and its suggested formula is

(13) 13Fe203 . 5A1203 .38Si02 .12H2 0 . 2[Na2 0' 

CaO] or

Na. 33 

+3 T 

(13a) Fe2 .i7 [Al. 83Si 3 . 1 7]0 10 [OH] 2

This formula is more satisfactory than that of beidellite 

in that Mg+2 is not present as an essential constituent. 

It possibly shows too much Fe+3 in octahedral coordination, by about 0.04.

As for beidellite, a formula somewhat higher in Al 

may be possible. It would be similar to No. 10 and 

could be expressed as follows:

20Fe03 . 9A1 203 

Na20] or

Na. 33

T

(14) 54Si02 . 18H2 0 t 3[CaO,

(14a)

Fourteen of the nontronite samples contained an 

iron-bearing impurity, as judged from attempts to 

obtain formulas for them. Many of these samples gave 

no evidence of essential impurities, suggesting the 

presence of mixed-layer material of chlorite type. These 

are listed in table 11. If the iron content is reduced by 

an amount just adequate to give a possible formula, 

this formula will have a maximum value of S for the 

specimen. These maximum values are listed in the 

table, together with the minimum excess of Fe203 . In 

only 4 samples does 2 exceed 2.10, so for the remaining 

10 samples it would appear that the maximum value of 

2 thus calculated is near the true value.

MONTMORILLONITE-BEIDELLITE-NONTRONITE SERIES

These three mineral are members of an apparently 

continuous series of solid solutions; in other words, Fe+3 

and A1+3 can proxy for one another in octahedral coordination in all proportions, and A1+3 can proxy for Si+4 

apparently not to exceed 1 ion in 4. Analyses of the 54 

specimens listed in table 10 and the 13 nontronites in 

table 10 can be represented in terms of end formulas 

for montmorillonite, beidellite, nontronite, nontronite 

(aluminian), and ta1 ^ Thus, the nontronite from Santa 

Rita, N. Mex. (No. 60), the montmorillonite from 

Tatatila, Mexico (No. 6), and the beidellite from Carson 

District, Nev. (No. 49), can be approximately represented as shown in table 12. It will be noted with reference to the sample from the Carson District, Nev., that 

the calculated compositions vary slightly from those 

observed; that is, the composition is not fully represented by end members.

TABLE 12.—Proportions of end members in selected samples

Santa Rita, N. Mex., sample 60

End-member 

component

Nontronite (aluminian). . .

Talc....................

Total ion's. ........

Percent

38 

55 

6 

1

Tetrahedral 

coordination

Si+«

1.20 

2.02 

.24 

.04

3.50

Al+»

0.32 

.18

0.50

Octahedral 

coordination

Al+»

0.10

0.11

Fe+»

0.82

1.10

1.9"

Mg+«, Fe+«

0.02 

.03

0.06

Tatatila, Mexico, sample 6

Beidellite. ...............

Talc. ...................

7

1

4

88

0.22

.03

.16

3.53

3.94

0.06

0.06

0.15

1.48

1.63

o.oi

0.01

0.12

.29

0.42

Carson District, Nev., sample 49

62

2

36

1.96

.08

1.44

3.48

0.52

.01

0.52

1.35

.59

2.00

0.01

0.04

0.09

0.02

Perhaps the most artificial part of such a procedure 

is the use of talc (p. 48) as one end member. This use 

is required by the presence in some of the specimens of 

larger amounts of MgO than correspond to the montmorillonite formula. Though the amount required is 

small,* it further emphasizes the difficulty of assigning 

formulas of the usual type. In order to give a completely satisfactory representation of the various 

analyses by means of end members, about 10 formulas 

for end members would have to be used. These end 

members, moreover, would have unusually large coefficients for their oxide formulas. Notwithstanding the 

somewhat oversimplification, the formulas presented 

probably give a fairly adequate representation of the 

relations and are preferable to the cumbersome presentation that might be devised by the use of numerous 

formulas. However, as previously stated, the endmember method of representing minerals of very complex composition is not fully satisfactory. For this 

reason it is believed that the formulas of these minerals 

making use of decimal fractions, present a simpler and 

more readily understandable picture of the compositions and relations of these minerals. These would be 

as follows:

(60)

Ca. 16

Si 3 . 50]0]0[OH] 2

Ca. 16

T 

(6) [Alj.6sMg.42] [Al.oe Si 3 . 96]010[OH] 2

T

(49) [Al 2 .oo Fe.5o Mg.02] [A1. H Si 3 . 48]010[OH] 2

SAPONITE-HECTORITE SERIES

There is another group of clays in which th-5 number 

of ions having octahedral coordination is 3.00, within 

limits of experimental error. No intermediate members 

between these clays and the beidellites of the mopt-
MINERALS OF THE MONTMORILLONITE GROUP

morillonite-beidellite-nontronite-iron series were found. 

Formulas for this group, given in table 10, indicate that 

the amount of aluminum in tetrahedral coordination 

ranges from zero to 0.76 in the seven samples examined, 

and apparently the beidellite type of substitution is 

operative.

The most interesting of these clays is the hectorite 

from Hector, Calif., sample 76. Reference to the analyses, table 4, shows that it contains very little A1 2 03 or 

Fe203 . About 1 percent Li2 0 is present in a nonexchangeable form — a very substantial proportion, due 

to its low atomic weight. The substitution of Li +1 for 

Mg+2 is the only possible factor accounting for a lack of 

charge in the lattice, and therefore the presence of an 

exchangeable base. A suitable formula for this end 

member is

(15) 16MgO.Li2 0.24Si02 .6[F, H2 0].NasO

which corresponds to

Na T ' 33

(15a) [Mg2 . 67Li.33] Si 4 010 [F, OH] 2

In the calculation of a formula representing analysis 

76, only Si is assumed to occupy tetrahedral positions, 

giving the following formula:

Na 29 

T

Si 4 010 [F, OH]2

This formula indicates that the analysis conforms remarkably well to the assumed end member. The 

analysis shows a small excess of Na2 0 over that represented by the formula, but this is in part due to absorbed salts.

In the actual formula S is 2.98, slightly less than 3.00 

and less than the experimental error.

The rest of the samples listed in table 10 do not show 

Li and owe their base-exchange capacity to the proxying 

of Si+ 4 by A1+ 3 in tetrahedral coordination. The 

saponite from Montreal, Canada, sample 77, is one of 

the simplest of these. Its formula can be written as

(16) 18MgO.Al2 03 .22Si02 .6H2O.Na2 0 or

. T 

(16a) Mg3 [A1.33Si.3.67]0 10 [OH] 2

The actual composition is represented by the following 

formula, which is a close approximation of the preceding formula:

Mg. 15

+ 3 +2

[Fe.02Mg2 . 98] Al.32Si 3 .6S]Oi0 [OH] 2

In the formula of the actual mineral, the exchangeable 

ion is Mg+2 instead of Na+1 as in (16a). The exchangeable base is represented by Na in the general formulas, 

but the actual exchangeable base may be indicated in 

any particular sample. (See pp. 39, 40.)

Substitution of A1+ 3 for Si+ 4 in amounts greater than 

about 0.40 is balanced b*y the proxying of A1+ 3 or Fe+ 3 

for Mg+2 ; thus, in the specimen from Svardsjo, Sweden, 

0.31 (Fe+3 A1+ 3 ) is present in octahedral coordination; 

in the specimen from San Bernardino, Calif., 0.27 A1+ 3 ;

and in the material from Glasgow, Scotland, 0.52 Fe+3. 

In the last two samples the amount of exchangeable 

base as determined analytically is in approximate agre<v 

ment with that required by the formula. This is a ve^y 

exact condition that is not a matter of assumption and 

thus adds further confirmatory evidence to the entire 

scheme. Lack of agreement in several of the oth^r 

samples, the analytical values of which are too low, do^s 

not detract from this evidence. It should really be 

pointed out that agreement as close as 0.47, calculated 

for the base exchange, and 0.16 external base, determined analytically, is really not bad considering all the 

factors involved. All errors are thrown into this one 

place.

Specimen 81 from San Bernardino, Calif., has aoproximately the formula

(17) [16MgO.Al 203] [2Al203 .20Si02] 6H2 0 

[Na2 0, CaO]

.

T 

or (17a) [Mg2 . 67A1.3 3] [Al. 67Si 3 . 3 3]0 10 [OH] 2

For convenience it might be considered to be an aluminian saponite analogous to beidellite. Other substitutions are so varied that a different formula would be 

required for each sample; therefore, an attempt to write 

further end formulas does not seem justifiable. In any 

formula, oxide representations would be quite complex.

SUGGESTED FORMULAS

So far as it has been possible to deduce formulas according to oxide representation, they are as follows:

Formulas of minerals of the montmorillonite group

(8) Montmorillonite

5A1 2O3 . 2MgO . 24SiO2 . 6H2O[Na2O, CaO]

(9) Beidellite

13A1 2O3 . 5A1 2O3 . 38SiO2 . 12H2O[CaO, Na 2O], or

(10) Beidellite

20A1 2O3 . 9A12O3 . 54SiO2 . 18H2O . 3[Na 2O, CaO]

(12) Nontronite

6Fe 2O3 . A1 2O3 . 22SiO2 . 6H2O[Na 2O, CaO]

(13) Nontronite (aluminian)

13Fe 2O3 . 5A1 2O3 . 38SiO2 . 12H2O[Na 2O, CaO]

(14) Nontronite (aluminian)

20A1 2O3 . 9A1 2O3 . 54SiO2 . 18H2O[Na 2 , CaO]

(15) Hectorite

16MgO . Li 2O . 24SiO 2 . 6[F, H2O] [Na 2O]

(16) Saponite

ISMgO . A1 2O3 . 22SiO 2 . 6H2O[Na 2O, CaO]

(17) Saponite (aluminian)

16MgO . A1 2O3 . 2A1 2O3 . 20SiO2 . 6H,O[Na 2O, CaO]

The composition of volchonskoite would demand 

formulas similar to that of nontronite, samples 13 and 14. 

Other elements such as Fe+2, Mn+2, Ni+2, would play 

a role identical with Mg, and separate formulas need not 

be written to represent them as they normally occur in 

very minor proportions.
48 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

Expressed so as to keep the ions occupying tetrahedral 

positions always at 4, these formulas would become:

Structural formulas of minerals of the montmorillonite group

Na.33

(8a) Montmorillonite [Ali. 67Mg. ss]Si 4OLo[OH] 2

Na.ss 

T 

(9a) Beidellite Alu7 [Al. 83Si3.i7]Oio[OH] 2 or

Na.ss

(lOa) Beidellite A1 2 . 22 [AliSi3 ]Oio[OH]2 

Na.ss

(12a) Nontronite Fe 2 . 00 [A1 . ssSis . erJOiofOHJa

Na.ss

(13a) Nontronite (aluminian) Fe 2 . n[Al. ssSis. n]Oio[OH] 2

Na. 33

(14a) Nontronite (aluminian) A1 2 . 2 2 [AhSi 3]Oio[OH] 2

Na.ss 

(15a) Hectorite [Mg2 . 67Li . 3 s]Si4Oi 0 [F, OH] 2

(16a) Saponite Mgs[Al . 3 sSi 3 . 6 7]Oio[OH] 2

(17a) Saponite (aluminian) [Mg2 . erAl . 33] [Al . 6?Si 3 . ss]Oio[OH] 2

In addition to the above formulas, all of which represent clays with base-exchange capacity, other formulas 

must be assumed in order to complete the entire group 

in terms of end members, although these are invariably 

very minor components. Such formulas are as follows:

Formulas of accessory minerals 

Muscovite. ........................... .KAl2AlSi 3O10 [OH,F] 2

Talc. ...................................... MgsSi 4Oio[OH]8

Brucite. ......................................... Mg3 [OH] 6

As has been pointed out, members of the montmorillonite group, especially beidellite, commonly contain 

potassium, which is interpreted as being due to mixed 

layers of a mica mineral. At present it is not known 

whether this is to be considered as bravaisite, as some 

other hydrous mica, or as true muscpvite. Pending a 

complete understanding of bravaisite-hydrous mica 

(illite) minerals, it will be best to represent this interlayered mica mineral as muscovite.

THERMAL STUDIES

Two distinct problems are involved in an investigation of the relations of the water (more precisely of 

water and OH) in clays of the montmorillonite group. 

One is the adsorbed water, particularly the interlayer 

water between the lattice sheets, which is closely related to exchangeable bases; the other is the so-called 

high-temperature water, or the OH" 1 , which is an 

essential part of the crystal lattice. The two problems 

involved different methods of study, as the adsorbed or 

interlayer water can be best investigated by means of 

differential thermal analysis, first proposed by Le Chatelier (1887) and modified by Norton (1939) ; whereas the 

OH" 1, essential water, is best indicated by plotting 

retention of water against change in temperature — that 

is, by dehydration curves. The differential thermal 

method also has been used in a study of clay minerals by 

Hendricks, Nelson, and Alexander (1940) and by Grim 

and Rowland (1942, 1944).

ESSENTIAL HYDROXYL

The dehydration curves of 15 representative clays, 

plotted as retention of water against change in temperature, are given in figures 5, 6, 7. These are based on 

the work of P. G. Nutting, of the Geological Survey 

(Nutting, 1943). The sample is suspended directly from 

the pan of a balance by a fine wire that pasr^ into a 

small electric furnace with intervening insulation to 

protect the balance. This permits weighing of the 

sample without removal from the furnace, anc1 involves 

only a slight correction for temperature. The temperature is determined by a thermocouple and r«ad on a 

calibrated millivoltmeter. The sample is weighed repeatedly at constant temperature until it comes to 

constant weight. This weighing is repeated rt successively higher temperatures until the mineral ceases to 

lose weight. The retention of water is plotted as one 

coordinate and temperature as the other.

There is some question about determination of the 

water present as OH~ from dehydration curves. The 

curves obtained always have an S shape with an inflection point near 300° C., in a region over which the slope 

is changing slowly. This is the expected result when two 

processes having different response to increase in temperature are involved. The inflection marks the point 

where the rate of water loss with increase in temperad ^w ture, that is, the second derivative, -rp-, is zero; in

other words, the decrease in water due to one process is 

just balanced by the increase due to the other process. 

If the correct coordinates are chosen and the functions 

are of the same type, the amount of water lost above the 

inflection point will truly be equal to that involved in 

the second process—that is, hydroxyl water. Since the 

inflection point is difficult to fix in some determinations, 

it has been thought best merely to select 300° C. as the 

zero point. Hydroxyl water is lost below 300° C. but, 

if the analysis is correct, it is compensated for by the 

layer water lost above 300°. If the general practice 

were followed of using 100° C. or 110° C. as the point of 

reference, the curves would be so spread out by the 

variable loss of water at low temperatures as to make 

comparison difficult.

According to the formulas in the section en mineralogy the minerals in the group have the following 

amount of hydroxyl water:

Percent 

Pyrophyllite..........................................5.00

Montmorillonite...................................... 4.93

Nontronite........................................... 4.46

These percentages now will be compared w;th those 

obtained from the dehydration curves.

The dehydration curves of 15 clays are given in 

figures 5, 6, and 7, with that of the Tatatila clay (6) 

reproduced in each one for purposes of comparison. 

Curves for nearly normal montmorillonites, given in 

figure 5, represent clays from Belle Fourche, S. Dak. 

(30); Irish Creek, Va. (13); Otay, Calif. (15); Tatatila, 

Mexico (6); and Polkville, Miss. (16), and another from 

an unknown locality in Mexico (24). Figure 6 gives 

curves of clays from Hamburg, Saxony (10), and Montmorillon, France (37), and a pegmatite clay from Pala, 

San Diego County, Calif. (36). These are clays with 

calcium as the exchangeable J^ase, except that from 

Belle Fourche, which is a sodium clay.

In five of the six montmorillonite dehydration curves 

shown in figure 5, the amount of water lost above 300° 

C. agrees with the expected value within 0.30 percent.
MINERALS OF THE MONTMORILLONITE GROUP 49

3%

2%

1%

2%

3%

6%

V

Belle Fc urche, S.Dak. 

! rish Creek, Vau

Tat^tila, Mexico 

Polkville, Miss.

Mexico

IOO° ZOO0 300° 

FIGURE 5.

400° 500° 600° 700° 800" 

-Dehydration curves of montmorillonite clays.
50 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

6%

5%

4%

3%

1%

3%

4%

5%

6%

7%

8%

10%

\

Tatatila, Mexico 

Mainburg, Bavaria

Montmorillon, France 

San Dieg3, Calif

Wagon Wheel Gap,Colo.

Pontotoc, Miss.

100° 200° 300° 400° 500° 600° 700° 

FIGUKE 6.—Dehydration curves of montmorillonite clays.

8OO°
MINERALS OF THE MONTMORILLON1TE GROUP 51

Spokane, Wash. 

Santa Rrta.N.Mex

100° 200° 300* 400° 500° 600° 100° 800° 

FIGURE: 7.—Dehydration curves of montmorillonite days.

900" 1000°
52 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

The sample from Mexico shows a slightly greater loss, 

5.5 percent, which probably is due to the fact that the 

inflection point is nearer 400° C. Two of the nontronite 

samples, those from Spokane, Wash. (58), and Santa 

Rita, N. Mex. (60), give the expected loss in weight to 

within 0.1 percent. A higher loss, 4.90 percent, was obtained from the specimen from Sandy Ridge, N. C. (65). 

It must be recalled, however, that this is an acid clay 

and thus should contain about 5.0 percent hydroxyl 

water, which is in close agreement with the observed 

amount. These results clearly show that for 10 oxygen 

ions increase in hydroxyl content above 2.00 probably 

does not take place.

Dehydration curves for several specimens in which 

the amount of water lost above 300° C. exceeds 6.0 percent are shown in figure 6. Although the sample from 

Pontotoc, Miss. (50), is partly an acid clay, the additional hydroxyl content is insufficient to account for 

the observed loss of 8.5 percent. The inflection point for 

specimens from Pontotoc, Miss. (50), and Wagon Wheel 

Gap, Colo. (47), are near 300° C., but their dehydration 

curves have an unusually high slope in the region between 400° C. and 500° C. It is possible that some other 

constituent, such as a hydrous oxide of aluminum, is 

present. It also is possible in these clays of the beidellite 

type that additional hydroxyl groups are present. This, 

however, would result in an increase of only about 

1.2 percent in water content, whereas the observed 

values are more than twice that amount. The most 

probable explanation is that the presence of some 

chlorite-type layers modifies the calculated formula. 

Failure to consider this factor or the presence of an 

aluminous impurity gives too high an amount of aluminum in tetrahedral coordination by about 0.10 at the 

most.

The dehydration curves also show the stability range 

of the clays of the montmorillonite group. The clays 

that are approximately normal montmorillonites—that 

is, those from Tatatila (6), Polkville (16), Irish Creek 

(13), Otay (15), Mexico (24), and Belle Fourche (30)— 

begin to break down rapidly at about 500° C. and have 

lost practically all their water at 800° C.

The nontronites (see fig. 7) appear to have a lower 

stability range than montmorillonite, as the clays from 

Sandy Ridge (66) and Santa Rita (61) begin to break 

down at about 400° C. The clay from Spokane (59), 

which contains only about 55 percent of the nontronite 

molecule, begins to break down at a somewhat higher 

temperature; thus its stability is intermediate between 

that of nontronite and montmorillonite, as its chemical 

composition would imply.

The hectorite (76) is unusual in that hydroxyl (OH) 

is largely proxied by fluorine. The curve given in 

figure 7 shows that the curve for hectorite has a sharp 

break at 700° C., about 200° C. higher than is normal 

for the minerals of the group. This difference, no doubt, 

is due to the F, which is evidently more tightly held 

within the lattice structure than OH. The curve in the 

range 300° C. to 650° C. is nearly flat, whereas there is 

a steep slope in other curves of the group—that is, in 

the part just preceding the flexure that indicates mineral 

breakdown. This difference, no doubt, is due in large 

part to the higher temperature at which hectorite breaks 

down and, consequently, to a much more complete 

escape of interlayer water. It results in a sharper flexure, 

for there is a minimum of overlap in the escape of interlayer water and OH+F (so-called high-temperature 

water).

The curve shows that the material was still losing 

weight at 930° C. The total loss indicated is about 6 

percent, though theory demands about 7.32 percent of 

OH+F. Thus the curve does not give a complete quantitative picture of the loss of OH +F. The sharprsss of 

the flexure, the rapid loss of more than 3 percent in 

weight, and the lack of a second flexure in this rar^e indicate that OH and F begin escaping together. It is 

probable that the part still retained at 930° C. is essentially fluorine. The stability of hectorite at higher 

temperatures, as compared with montmorillonite, is in 

contrast to its lower stability under another set cf conditions. Montmorillonite does not break down rapidly 

when subjected to electro-dialysis for removal of bases, 

whereas hectorite under the same conditions readily 

breaks down into gelatinous silica.

INTERLAYER WATER

One of the most striking characteristics of montmorillonite is the reversible swelling produced by water.

Hofmann, Endell, and Bilke (1935) and Hofmann, 

Endell, and Wilm (1938), on the basis of X-ray d'ffraction data have shown that, on swelling, the absorption 

of water takes place between the silicate layers of the 

mineral and causes the spacing of the plane parallel to 

the layers surface to increase, while the other lattice 

dimensions remain unchanged. Variation in clenvage 

spacing with water content for calcium montmorillonite 

is shown by the results of Hofmann and Bilke (1936), 

as follows:

Relation between water content and cleavage spacing in 

montmorillonite

GRAMS H2O PER 100 SPACING OP CLFSAVAGB

GRAMS OP WATER-FREE PLANE (IN

SUBSTANCE ANGSTROM WITS)

10.0 ................. 11.25

13.9 ................. 12.1

19.5 ................. 13.4

24.2 ................. 14.6

29.5 ................. 15.1

36.3 ................. 15.6

41.8 ................. 15.7

59.0 ................. 17.8

Hofmann, Endell, and Wilm, moreover, noted that 

only (hkty and (OOi) reflections appeared on X-ray powder-diffraction photographs. Absence of (hkl) reflections was discussed by Maegdefrau and Hofmann (1937) 

as evidence for absence of regularity in the orientation 

of the parallel layers.

The conclusion that swelling is due to absorption of 

water between silicate layers confirmed the concl'isions 

of Ross and Shannon (1926, p. 85), who stated:

The swelling is primarily due to the micaceous habit end the 

very slight cohesion between crystal plates, so that films p* water 

penetrate between them. Thus, on wetting and slaking, the 

crystalline aggregate that makes up the bentonite subdivic^s into 

micaceous plates that have large lateral extent, but many of them 

probably have a thickness that is best expressed in molecular 

dimensions.

The physical effect of the increase in the thickness of 

the water layer between the lattice sheets is especially 

well illustrated by some of the soda bentonites, which 

may swell to 8 to 10 times their original volume, permitting a great change in volume without destruction of the 

characteristic structure. A sample of bentonite from 

Rosedale, Alberta, was placed in water until it expanded 

to a gel-like mass. Water was removed by repeated 

treatment with alcohol; then bakelite varnish was 

added and allowed to permeate the material and finally 

was hardened by heat. A thin section shows tint the
CLEAVAGE SPACING IN ANGSTROMS
54 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

characteristic volcanic-ash structure was retained without distortions.

Curves showing the amount of water taken up at 

particular relative humidities for montmorillonites, 

with Na+1, H+1, and Ca+2, respectively, as the exchangeable ion, and the corresponding cleavage spacings have been published by Hofmann and Bilke (1936) 

and by Nagelschmidt (1936). Two of the curves published by Hofmann and Bilke (1936, p. 246, curves 1 

and 3) are reproduced as figure 8. These authors found 

that the total amount of water taken up at a given 

relative humidity did not depend particularly upon the 

type of the replaceable ion. The cleavage spacing, however, varied greatly, depending upon the replaceable ion.

Hofmann and Bilke found an essentially monotonous 

change of cleavage spacing with amount of adsorbed 

water, and they suggested that this water is present in 

close packing between the silicate layers. Different 

results were obtained by Bradley, Grim, and Clark 

(1937). They found that the cleavage spacing did not 

increase monotonously but rather by stages indicating 

definite hydrates containing one, two, and three layers 

of water molecules.

A different approach was used by Hendricks, Ne^on, 

and Alexander (1940). They determined by Le Chatlier's differential method the thermal effects accompanying the loss of water as samples with various replaceable 

ions are heated at a constant rate. This method allows 

a somewhat finer distinction of various ways in which 

interlayer water might be held than do static methods.

The essential result of this study is that the first stage 

in the sorption of water by montmorillonite is the hydration of the replaceable ion, if it is magnesium, 

lithium, or calcium. Six molecules of water are required 

for each ion except Li+1, which takes three or less. Other 

ions such as Na+1, K+1, and H+1, apparently do not 

hydrate. The water molecules are in a single layer 

parallel to the silicate layer, and at room temperatures 

they are taken up at a restive humidity of less than 10 

percent. At higher relative humidities an amount of 

water is absorbed adequate to complete the first layer, 

and this is held somewhat less strongly than is the first 

kind. At still higher relative humidities, of 40 percent 

or more, additional layers of water molecules are absorbed. Similar results were obtained by Buswell and 

Dudenbostel (1941).

In most of the samples examined by Hendricks, Nelson, and Alexander the sorption of water in one manner 

was not completed before sorption in another set in, as 

the humidity was increased. In one' sample, however, 

X-ray diffraction results similar to those of Bradley, 

Grim, and Clark were obtained. Total water sorption 

follows closely the type of curves found by Hofmann 

and Bilke (see fig. 9).

OPTICAL PROPERTIES

Optical properties are less definitely deteraiinable for 

the mineral of the montmorillonite group than perhaps 

for any other mineral group. These minerals are all 

definitely crystalline, but no euhedral crystals have 

been observed, and perhaps the nature of the stacking 

of the individual sheets prevents any such development 

in dimensions observable under the oridinary microscope. Areas that show optical unity are in general very 

small but are observable in thin sections of many 

materials even at moderate magnifications. Such areas 

occur in many bentonites, where the replacement of the 

original glass fragments has proceeded inward from the

outer surface, producing parallel groups of micaceous 

plates that meet in a suture line, as shown in plate 3A. 

Even these areas do not permit determination of most 

of the optical properties, and in general only the mean 

index of refraction can be determined and the birefringence approximated. The areas are too small for 

measurement of the optical angle. Indices of refraction 

could be determined on these areas were it possible to 

secure them as units for study in immersion oils, but 

their marked flexibility and distortion on crushing 

destroys their optical unity.

METHODS OF DETERMINATION

There are, however, several methods by which optical 

properties may be approximately determined on these 

clay materials. A few occurrences show a very systematic arrangement of the clay material, and th; « has 

built up a kind of pseudo crystal on which optical properties are determinable. As described later film"' also 

may be used. (See p. 55.)

Under the microscope these pseudo crystals resemble 

optically perfect or nearly perfect grains. A study by 

Hendricks and Ross (1938) shows, however, that these 

grains are made up of alumino-silicate layers, which have 

a rather regular arrangement with respect to one 

another so long as a few orders of reflection (100) can 

be observed; but on a somewhat larger scale th^re is 

only an approximate crystallographic organization of the 

clay material. The grains give very clear-cut optical 

properties, which suggest that the departures from 

crystal uniformity tend to be compensating. Thu^ the 

optical properties determined on these material probably are similar to those determined on films, and closely 

approximate the true ones.

It is well known that dried films of finely dispersed 

clays have fairly definite optical properties. Ros" and 

Shannon (1926) gave optical properties that in some 

specimens had been determined on films, but the method 

seems to have been first described by Grim (1934).

Some clays form granular aggregates that approximate crystal units on drying; for example, the clay concentrated from the altered feldspar from Magnet Cove, 

Ark. (p. 68), consisted of remarkably perfect sphenlites 

made up of clay plates with radial arrangement. In 

general, however, these aggregates are less satisfactory 

for the determination of optical properties than films 

of clay.

Some clays—the natural sodium-bearing clays—disperse rather readily in water, but in general dispersion 

is effected by the following treatment: The clay is 

added to several times its volume of water after vrhich 

a large excess of sodium acetate or sodium chlorde is 

added. This mixture is allowed to stand on a steam bath 

for some hours and, if necessary, is stirred for an hour 

or more with a mechanical stirrer; then the clay is 

thoroughly washed, either by repeatedly allowing it to 

settle and decanting the water or by washing on filter 

paper. This process commonly requires several days 

for completion. By the time the clay is free from excess 

salts it tends to stay in permanent suspension.

Some of this suspension may be allowed to dry on a 

cover glass, using enough material to give a thin film, 

but better results have been obtained by placing the 

clay suspension on a steam bath, where evaporation 

tends to form a coherent film at the surface. This film 

may be removed by lifting off on a glass slide and then 

be allowed to dry slowly at room temperature. The best 

results were obtained with a dry film about 0.03 mm.
MINERALS OF THE MONTMORILLONITE GROUP 55

thick. The great vertical shrinkage of the gel-like film 

tends to bring the individual plates into closely parallel 

orientation, and at the same time there appears to be a 

tendency for the plates to assume an orientation within 

this plane and so build up pseudo crystals, since they 

give a biaxial figure of nearly constant angle. The film 

may be broken up and the fragments examined, but 

better results have been attained by slicing it into narrow ribbons with a razor blade. By working under a 

binocular microscope, 15 to 20 such ribbons to the millimeter may be cut.

A few of the ribbons of clay material will stand on 

edge, aided by the curvature developed on slicing. Two 

indices of refraction may be determined on these and 

one, the maximum, on those that lie flat.

Assuming that the individual crystal plates that make 

up the film have assumed complete parallelism to the 

surface of the film and random orientation perpendicular to that plane, the observed birefringence would be 

as follows, since the optical character is negative:

•— a

The departure of this value from the true birefringence 

will depend on the axial angle, being greatest for an 

angle of 90°. To illustrate: Assume, a = 1.5000; 

7 = 1.5300, the birefringence being 0.0300 and axial 

angle 40°, making £ = 1.5265 by calculation. Then

1.5300 + 1.5265

= 1.5282, and 1.5282 - 1.5000 =

0.0282. That is, the error in determining the birefringence from such a clay film, under ideal conditions, 

would be negligible for small and medium axial angles.

An axial angle of 90° under the same assumptions 

would give a birefringence of 0.0225, where the correct 

value would be 0.0300. These observed values may be 

designated as 7' and a'. A lack of true parallelism of the 

crystal plates would tend also to make the observed 

values converge, and 7' would tend to be low and a' 

high. However, sharp extinction in plates standing on 

edge and complete blackness in those lying flat have 

been observed in some materials, indicating close approximation to true values in the best of such films.

The indices of refraction in tables 13 and 14 were 

determined on films under nearly uniform conditions.

Studies by Kelley, Dore, Woodford, and Brown 

(1939) indicate that, although the change from one base 

to another has little direct effect on the optical properties, a change in the amount of water would probably 

have a material effect. On the other hand, Marshall 

(1930) from studies of anisotropic material in an electric field found that the birefringence varied for different 

bases.

The conversion of the material to a sodium day not 

only permits effective dispersion but a!so makes all clays 

uniform in character. Immersion oils with rather, high 

viscosity penetrate the clay slowsy; therefore the occluded air is expelled by soaking the c'ay in acetone for 

some hours before placing in the immersion oils and then 

allowing it to stand for 6 to 8 hours for the acetone to 

diffuse out and evaporate. Usually 6 to 8 preparations 

of a single sample are made at one time, each treated 

with a different index oil, to cover the probable range of 

indices of refraction. The index oil is then removed with 

absorbent filler paper and fresh oil of the same index of 

refraction is added. The treatment with acetone seems 

to expel only a litt'e of the interlayer water and increases the index of refraction but little; however, it

tends to make the amount of this water more nearly 

uniform in all samples so treated, and to give approximately uniform conditions. In the writers' experiments 

the use of basic organic immersion oils was avoided, and 

all determinations were made in mixtures of a very 

stable petroleum oil and monochlor napthalene.

DETERMINATIONS OF INDICES OF 

REFRACTION

Serdiuchenko (Serdintschenko) (1929) give^ the 

optical properties of the nontronite from Stary-Krym 

(sample 55 of this paper) as follows: Biaxially negative, 

dispersion r > v; 2V about 30°; pleochroism a, bright 

yellow, b, brownish yellow, c, yellow green.

Kurnakov and Cernych (1929) give the indices of 

refraction of nontronite from Baschenowski, Russia 

(see No. 87, table 6), as a = 1.585, $ - 1.593, 7 = 1.608. 

The same authors give the indices of refraction cf nontronite from St. Andreasberg, Harz (No. 95, table 6), as 

a = 1.584, $ = 1.600, 7 = 1.610. Iskyul (1935) gives 

a = 1.585, 7 = 1.605 for material from the same locality 

(No. 95, table 6).

Serdiuchenko and Istikll (1933) give the optical properties of nontronite from Baltatarak (No. 97, table 6) as 

a = 1.606 ± .005, 7 = 1.625 ± .005, 2V medium.

Larsen and Steiger give the optical properties cf nontronite from Woody, Calif. (No. 62), as a = 1.560 ± .01, 

£ = 1.585 ± .01, 7 = 1.585 ± .01, optically negative, 

axial angle small; and for another specimen (No. 59) 

they give a = 1.57, 0 = 1.59, 7 = 1.60; 2V = 6<?°.

Optical properties of the saponite described by 

Palache and Vassar (1925) were determined by Larsen 

as follows: Kearsarge mine (No. 80), optically negative, 

2V small, a = 1.479, £ = 1.510, 7 = 1.511; Al meek 

mine (No. 79), optically negative; 2V moderate, a = 

1.490, £ = 1.525, 7 = 1.527.

The material collected by W. F. Foshag from the 

Princessa mine, Maniquipi, Mexico (sample 68), is 

admixed with 20 to 25 percent of kaolin and a little 

ferric iron but shows very definite optical properties. 

These may be noted for comparison with those of purer 

clays listed in table 13. The following optical properties 

have been described by Ross and Hendricks (1938): 

a = 1.532, $ = 1.544, 7 = 1.557; birefringence = 0.025; 

optically negative; 2V = 60° - 80°. These we~e determined on material that seems not to have been 

thoroughly impregnated with the immersion oil. Determinations on material from which the air and perhaps 

some of the interlayer water were completely expelled, 

are as follows: a = 1.567, 7 = 1.593; birefringence .0026. 

The plates are noticeably pleochroic with the gr?atest 

absorption parallel to the plates.

TABLE 13.—Indices of refraction and birefringence as affected by 

the presence of ferric iron in the montmorillonite-nontronife series

Sample 

No.

6

50

22

35

44

58

65

Locality

Tatatila, Mexico .............

Twin Falls, Idaho ...........

Beidell. Coir;.. ....... ........

Sandy Ridge, N. C. ..........

a'

1 . 523

1.530

1.538

1.544

1 . 548

1 . 568

1.590

Y'

1.548

1.556

1.565

1.570

1.575

1.606

1.630

T-V

0.025

.026

.027

.026

.027

.038

.040

Fer-ic iron 

(percentage)

0.06

2.26

5.38

6.61

8.68

18.54

T7.47

From the data given in table 13 have been plotted the 

curves reproduced in figure 9, which give variation of 

indices of refraction with variation in ferric iron. These 

show a very consistent relation; however, a number of 

indices of low-iron members given in table 14 fail to fit
56 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

1.660

I.64O

Z 

£1.620

DC 

U

* 1.600 

u. 

o

V)

u

§,.580

Z

1.560 

1.540 

1.520

<

x

^

X

X

X

x^

X

x/

X

je^

irj^

V.X

r

f

f^

X

0x

x

x^

X

oX

/

^

x

^* ^^

X

^x

X

rf* x-"^

x

X^

^

"^

^

X

^^-<

x^

X

»^"_x

^"^

^"

D 4- 8 12 16 20 24 28 32 36 4(

PERCENT OF FERRIC IRON

FIGUBB 9.—Curves showing relation of indices of refraction to ferric iron in minerals of the montmorillomte-nontronite series.
MINERALS OF THE MONTMORILLONITE GROUP 57

this curve. This failure probably is due in part to different conditions of determination, but in general it is 

no doubt the result of other factors. In the high and 

moderately high iron members the iron is the dominant 

factor in controlling the indices of refraction. In the 

low-iron members other factors become much more important; f thus, there are variations in the aluminasilica ratio, in the bases, in the water contents, and in 

the effect of these factors on interlayer spacing and 

water content, all of which would effect the indices of

retraction. 

TABLE 14. — Indices of refraction of clays determined on films

Montmoriilonite-beidellite series

Sample

No.

23 

36 

15 

24

45 

84 

38

Locality

Pala, Calif. ...................

Indices of refraction

«'

1.523 

1.513 

1.512 

1.503 

1.503 

1.494 

1.493

Y'

1.555 

1.538 

1.534 

1.536 

1.527 

1.526 

1.526

Birefringence

y'-«'

0.031 

.025 

.022 

.033 

.024 

.032 

.033

Hectorite

76 1.485 1.516 0.031

The indices of refraction in the table 15, determined 

on clay grains described on page 54, were made at room 

temperature and humidity in an air-conditioned laboratory. These grains absorbed oils rather readily; therefore the treatment used for clay films was not necessary.

TABLE 15.—Indices of refraction determined on clay grains

Sample 

No.

46 

22 

47 

44

Locality

Providence mine, Zacatecas, 

Mexico .....................

Pairview, Utah . . . . . . . . . .

Atzcapozalco, Me: 

Wagon Wheel Gai 

Beidell, Colo.

Indices of refraction

a

1.544 

1.544 

1.533 

1.526 

1.502

Y

1.565 

1.565 

1.552 

1.561 

1.533

Birefringence

y-a

0.021 

.022 

.019 

.034 

.031

TABLE 16. — Indices of refraction of minerals related to the montmorillonite group

Talc, Ma 

Pyrophyl 

Vermicul 

Pa. ...

Vermicul 

Webster,

ite, Staley, N. C. . 

te, Brinton quarry,

te (nickeliferous), 

N. C... ..........

Indices of refraction

a

1.540 

1.552

1.562 

1.542

& Y

1.588 1.588 

1.564 1.601

1 . 562 1 . 597 

1.573 1.573

y-a

0.048 

.049

.035 

.031

2V

Small, variable 61°

Nearly uniaxial 

Small

Indices of refraction were also determined on representative clays after interlayer water had been expelled. 

The dehydration curves show that there is a flexure at 

about 200° C., with a marked flattening above that 

temperature. For that reason the indices of refraction 

were determined on material that had been maintained 

at about 210° C. for approximately 36 hours. This 

treatment did not affect the crystallinity of the material. The interlayer water of montmorillonite and 

hectorite introduces the only observable difference between the space lattices of these minerals and those of 

pyrophyllite and talc, minerals without interlayer water. 

Thus, if the interlayer water were expelled from montmorillonite and hectorite, their optical properties should 

approximate those of pyrophyllite and talc. Reference

to table 17 below indicates that low-iron montmorillonite and hectorite after dehydration do have indices 

of refraction comparable to those of pyrophyllite and 

talc, but the birefringences are lower. The lower birefringence observed in water-free hectorite and montmorillonite may have been due to inability to observe 

maximum values and was at least in part due to lack of 

complete parallelism of the component crystal plates.

TABLE 17.—Indices of refraction of clays dehydrated at 210° C.

Sample

No.

44

23

22

76

Locality a

1.559

1.555

1.555

1.555

1.543

1.552

1.539

Y

1.598

1.584

1.585

1.584

1.579

1.600

1.589

&

1.588

1.589

y-«

0.039

.029

.030

.029

.036

.048

.050

'See Larsen and Herman (1934, p. 164).

PLEOCHROISM

In general, the minerals of the montmorillonite group 

are light-colored and show no plepchroism, but ar y that 

have color show distinct pleochroism; thus, films of nontronite from Sandy Ridge, N. C., and Spokane, Wash., 

are bright green parallel to the elongation and yellow 

perpendicular to that direction. The nontronite described by Serdiuchenko (1929) is as follows: a, bright 

yellow; b, brownish yellow; and c, yellow green. The 

vein clays from Beidell, Colo. (44), Princessa mine, 

Mexico (68), and Fairview, Utah (46), show slight 

pleochroism in the red-brown shades.

All the clays of the group are optically negative.

ELECTRON-MICROSCOPE PHOTOGRAPHS

Development of the electron microscope during recent 

years has made possible examination of clay materials, 

including members of the montmorillonite group of 

minerals, under far greater magnification and ra^olving 

power than afforded by optical microscopes. In an 

electron microscope the optical path is similar to that 

of the usual biological or petrographic microscope, but 

there are no polarizing devices. Electrons have not been 

polarized, and there are reasons for doubting thr.t they 

can be polarized. Electrons accelerated under definite 

potential drops, usually greater than 25,000 volts, are 

the radiation used, and lenses are of electrostatic or 

magnetic types. The wave lengths of the e^tron 

radiations, in general, are less than 0.1 A., and the 

numerical aperture of the objective lens is about 0.01. 

The very short wave length makes it possible to obtain 

high resolving power, and the very small numerical 

aperture gives great depth of focus. Resolving power 

is really limited by aberration of the lenses and tl <* construction of the microscope. The resolving pcwer is 

claimed to be adequate in several types of e^tron 

microscopes, which have been constructed to permit 

resolution of features greater than about 20 A. aoart.

Electrons of the velocities used have high coefficients 

of absorption in matter, and so it is necessary to evacuate the microscope and to prepare extremely thin 

mounts of the material to be examined—less than about 

0.1 microns in thickness. For this reason such materials 

are commonly deposited on very thin films of collodion 

or similar substance, mounted on wire nets, whirt.h can 

be inserted in the object position in the microscope.
58 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

Samples of clay minerals thus must necessarily be dispersed in some liquid, and water is generally selected 

for this purpose. A drop of an extremely dilute dispersed mineral suspension is evaporated on the film and 

then examined approximately normal to the film.

Eitel and Radczewski (1940), Ardenne, Endell, and 

Hofmann (1940), Middel, Ruchmann, and Kausche 

(1940), Humbert and Shaw (1941), Endell (1941), Shaw 

and Humbert (1941), Marshall, Humbert, Shaw, and 

Caldwell (1942), Shaw (1942), and Humbert (1942) 

have obtained electron-microscope pictures of mpntmorillonite samples prepared in the manner described 

above. All samples were found to consist of thin plates 

very much smaller in dimensions than could be observed 

by ordinary microscope methods. A photograph (reproduced as plate 7, D) of sample 20, montmorillonite 

from Lemon County, Miss., taken by Mr. H. F. 

McMurdie of the National Bureau of Standards is 

typical. Marshall, Humbert, Shaw, and Caldwell (1942) 

estimated that the mean cross-sectional dimensions of 

the less than 20 m^ fraction, separated by sedimentation, of a clay from Putnam soil was 75 ± 7 ni[A and that 

the thickness did not exceed 5 m[A.

A very thorough study by Shaw (1942) of a bentonite 

from Rock River, Wyo., showed montmorillonite plates 

with a more definite euhedral form than those usually 

observed. Shaw's photograph of the less than 50 m[A 

fraction, separated by sedimentation, is reproduced as 

plate 7, C. This fraction contains particles estimated 

to have an average thickness of 1.9ni[A, which is approximately the structural thickness of two montmorillonite 

unit sheets. These results and those of other observers 

indicate that montmorillonite crystals, after dispersing 

and drying, can contain particles formed of a single 

structural layer. Particles formed by superposition of 

a number of layers in such a way as to maintain some 

euhedral character, such as a definite edge, also are 

present.

Insley and McMurdie (Alexander, L. T., and others, 

1943) and Shaw and his coworkers (Shaw andHumbert, 

1941; Humbert and Shaw, 1941) have shown that hectorite contains lathlike particles and is thus different in 

particle habit from montmorillonite. A typical photograph of the 500 m[j.-200 m[x fraction of hectorite is 

reproduced as plate 8, C by permission of Dr. Shaw. 

The average length of the.particles is 625 m|A and the 

width 63 mjju The difference in particle habit of hectorite and montmorillonite has raised the question of 

possible difference in structure. However, dissimilarity 

of habit does not necessarily imply structural dissimilarity and such an implication is in conflict with the 

X-ray diffraction data. The difference in crystal habit 

is not unlike that of the supposedly isostructural minerals, chrysotile and kaolinite, and pyrophyllite and 

talc. Difference in crystal habit between hectorite and 

montmorillonite can give rise to pronounced differences 

in some properties, such as orientation upon drying, 

swelling in water, and formation of thixottropic gels.

Nontronite from Sandy Ridge, N. C., also was shown 

by Shaw and his coworkers to be lathlike or ribbonlike 

in habit. (See pi. 8,' J5.) The range in size from the 200 

m[A down to the 50 m[j. (equivalent spherical diameter) 

fraction contained particles with an average length of 

567m[j. =±= 57ni[x and width of 108ni[j. =±= 14m;ji.. The third 

dimension was estimated to be about 14nitx ± 3mjju 

Primary particles in nontronite are thus approximately 

the same length as those found in hectorite but twice 

as wide.

The differences in physical form of montmorilonitebeidellite, nontronite, and hectorite, which are shown in 

the electron-microscope photographs, might be assumed 

to indicate a different crystal identity. However, this 

should be considered in connection with the domical 

evidence of isomorphism between the aluminons and 

ferric-iron members and the close relationship, although 

limited isomorphism, between aluminous and magnesian 

members—relationships that are confirmed by X-ray 

diffraction studies. What then is the significance of the 

difference in crystal habit?

Detailed studies under the microscope of thin sections 

indicate that montmorillonite and nontronite show 

equally perfect micaceous cleavage, and selected areas 

show perfect optical continuity throughout the thickness of a thin section. This is shown by photomicrographs of nontronite from Sandy Ridge, N. C., and from 

Dale County, Ala. (pi. 6, C, D), and may be compared 

with one of montmorillonite. (See pi. 3, A.} Microscope 

studies show that hectorite also has a micaceou^ habit 

(see pi. 6, J5), though it is somewhat less well developed 

than that of montmorillonite and nontronite. The 

micaceous habit of nontronite also is indicated by the 

parallel grouping of crystal plates, the "pseudo crystals," that were used in determinations of the index of 

refraction. (See pp. 54, 55.) A thin section o* micaceous plates standing on edge would have a thickness 

nearly a hundred times as great as the width of the nontronite ribbons shown in plate 8, B. The perfect optical 

continuity extending throughout this thickness o* a thin 

section is not compatible with an assumption thr.t these 

areas are made up of superimposed layers of a hundred 

or more of the ribbons shown in plate 8, B.' Thorough 

dispersion, however, does produce ribbons, which appear 

to have been absent in undisturbed material. This difference in habit between nontronite and hectcrite, as 

compared with montmorillonite, seems to be analogous 

to that between chrysotile asbestos and kaolinite, which 

has been investigated by Warren and Hering (1941) and 

discussed by Hendricks (1942). These studies indicate 

that the greater size of the Mg ion results in a strain 

within the crystal lattice, which restricts the width of 

the layer to a few units of structure. Growth along the 

length of the plate, however, is still permitted; in other 

words, the strain is taken up in one directior. The 

fibrous character is due to the extreme extensior of the 

plates in one direction. In the same way, the ribbonlike 

habit of nontronite and hectorite soeni to be due to the 

greater ionic radii and the strain produced by ferric iron 

and magnesium, resulting in a structure giving potential 

cleavability perpendicular to the micaceous plates. Such 

a cleavage would perhaps be analogous to tl at described by Dana (1892, p. 615), who says that, in addition to the eminent basal cleavage, muscovite shows 

a'so planes of secondary cleavage parallel to b, and often 

yields narrow strips or thin fibers parallel to 6.

Asbestoslike brucite has been observed from Hoboken, 

N. J., and from Alaska. Also, slender filaments of 

graphite, another mineral that has a sheet structure 

and that normally has a micalike habit, has b-^en described (Gibson, Riley, and Taylor, 1944).

SYNTHESIS

Information on the composition of the mortmorillonite group of minerals obtained by chemical analysis 

affords a basis for the interpretation of their synthesis 

in the laboratory. Ewell and Insley (1935), Noll
GEOLOGICAL SURVEY PROFESSIONAL PAPER 205 PLATE 1

^wfe^<

r*!MW !*-;.l<*i":?>» \%4

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ALTERATION OF VOLCANIC MATERIALS TO MONTMORILLONITE AND KAOLINITE, MAGNET COVE, ARK. 

A. Tuft fragment showing pumice structure (nearly vertical, wavy-banded structure). Dark gray is montmorillonite; light-gray material at top, with warped structure, is

kaolinite embedded in montmorillonite from which it is derived. Light-gray material below is in part finer-grained kaolinite intergrown with montmorillonite. 

II. I'umice structure shows on right (dark-gray material is montmorillonite; light-gray material is a mixture of fine-grained kaolinite and montmorillonite). An area of warped

kaolinite crystals is on left, embedded in montmorillonite. 

C. Warped and twisted kaolinite crystals embedded in fine-grained montmorillonite. Kaolinite has formed from montmorillonite, as in A and B. The curved and twisted

habit of the kaolinite crystals is characteristic of many occurrences of kaolinite.

D. Angular volcanic-rock fragment embedded in similar but finer-grained material, the whole completely altered to montmorillonite. The while areas were originally feldspar

phenocrysts.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 205 PLATE 2

A, B, AND C, ALTERATION OF VOLCANIC MATERIALS TO MONTMORILLONITE AND KAOLINITE, MAGNET COVE. ARK. 

A. Montmorillonite (gray granular material) enclosing residual portions of a single albite crystal. All the montmorillonite is an alteration product of albite. Black areas are

leucoxene (anatase).

B. Alteration of albite rock to montmorillonite, with several residual areas of albite embedded in montmorillonite. In neither A nor B has any kaolinite formed from albite. 

C. Central area is a phlogopite crystal partly altered, to hydrous mica and then to kaolinite. Dark-gray portion is phlogopite; light-gray portion is largely kaolinite with

hydrous mica on the border. The black areas are leucoxene (anatase).

D. VARVED SEDIMENTARY CLAY FROM FAIRVIEW, UTAH.

The dark zones are due to a concentration of iron oxides. The uniformity of illumination at 45° under crossed iiicols, and almost total extinction at 90°, shows the strong 

tendency for parallel orientation of the individual crystal plates that form the clay. These plates are too small for resolution under the microscope.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 205 PLATE 3

. *> *'.

t ' ;

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(S 

' >' 1'y.J

/^ 

-\ <'^ ; *•'.,:*; 

&4:

.* !««-

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,4. BENTONITE, WAYNE COUNTY, MISS., SHOWING VOLCANIC-ASH STRUCTURE. 

Central area shows the symmetrical arrangement of crystalline clay material derived through the alteration of a single glass fragment. Photographed under crossed .

and turned 45° from extinction. White areas show the perfect parallelism of the vertical crystal plates. Central area is dark because a large number of the platea .. 

in a horizontal plane.

B. VEIN CLAY, TWIN FALLS, UTAH.

The fine-grained clay aggregate is cut by a series of joint cracks, which were later filled by clay material of the same type. The clay material in these veinlets is uniformly

oriented and hence shows its crystalline character under crossed nicols.

C. GRANITE GNEISS, WASHINGTON, D. C.

Lower right shows a single oligoclase grain completely altered to beidellite. Albite-twinning structure is preserved in the different optical orientation of the clay material. Gray

central areas are quartz. Crossed nicols.

D, PLAGIOCLASE PHENOCRYST IN HHYOLITE PARTLY ALTERED TO MONTMOHILLONITE. SANTA RITA, N. MEX. 

White area* represent fresh plagioclaee; gray areas are montmorillonile.
GEOLOGICAL SURVEY PROFESSIONAL PAPEH 20.1 PLATE 4

A, B. BENTONITE, OTAY, SAN DIEGO COUNTY, CALIF., SHOWING VOLCANIC-ASH STRUCTURE

A. Shows several ringlike areas that represent unbroken glass bubbles, some of them—near the top—with the walls between adjacent bubbles still attached. Also near the top 

is a platelike shard that shows perfect orientation of the crystalline clay material. At left center is a shard showing pumice structure. Just below it is a Y-shaped shard, 

the lower arm of which shows a double row of crystal plates with a dark dividing line. Crossed nicols.

B. Shows the platelike structure of one type of bentonite. In several places the double row of plates with a central dividing line is discernible. Crossed nicols.

C, D. BENTONITE, NEAR BEATTY, NEV.

C. Shows the lune-shaped glass fragments that characterize one type of bentonite. Gray areas are montroorillnnite; white areas are zeolite. Crossed nirnU.

D. Large pumice fragments embedded in finer-grained montmorillonile.
GEOLOGICAL SURVEY PROFESSIONAL TAPER 205 PLATE 5

ALTERATION OF ROCKS OF VARIOUS TYPES TO MONTMORILLONITE AND ASSOCIATED MINERALS

A. Replacement of perthitic feldspar by montmorillonite, Santa Rita, N. Mex. 

B. Bentonite, Santa Rita copper mines, Santa Rita, N. Mex. White areas are kaolinite; gray areas are montmorillonite. The kaolinite appears to be the result of partial alteration of montmorillonite, under the influence of sulfate waters derived from oxidizing sulfides.

C. Beidellite, Austin, Nev., resulting from the alteration of plagioclase—shown as light elongated areas—in basalt.

1). Bentonite of Permian age from a depth of about 2,300 feet in oil test well, 20 miles northeast of Carlsbad, N. Mex. Gray areas are montmorillonite, alteration product of 

volcanic glasH; white areas wnre originally fragments of feldspar phenocrysts, now altered to kaolinite; black ureas are pyrite.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 205 PLATE 6

MAGNESIUM AND IRON-RICH CLAYS 

A. White areas are saponite enclosed in celadonite (gray). The saponite and celadonite have formed in a vesicular cavity in basalt, saponite being the ast to form. Reno

Nev. 

B. Hectorite, Hector, Calif., in platelike groups, enclosed in very fine-grained aggregates of the same mineral.

C. Nontronite, Sandy Ridge, S. C., forming sheaflike aggregates of plates. 

D. Nontronite, Dale County. Ala. Areas of perfect plates surround quartz grains.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 205 PLATE 7

'f 38,000

A. BEIDELLITE THAT HAS REPLACED A BIVALVE MOLLUSCAN SHELL, FROM POINTOTOC, MISS.

Original structure of shell is well shown under crossed nicols and is due to the different optical orientation of the clay material of the minute shell lamella. Crossed nicols. 

B. MONTMORILLONITE FROM PEGMATITE NEAR PALA, SAN DIEGO COUNTY, CALIF. Crossed iiicols

C. ELECTRON MICROGRAPH OF MONTMORILLONITE BENTONITE FROM ROCK RIVER, WYO.

Moiitmorillonite bentonite, less than 50/j fraction, showing well-formed particles, which perhaps have crystal edges. The thinnest particles are of the order of 2m/j in thickness, which is about that required by a single structural layer. Other particles are more than 20m/j in thickness indicating an orderly superposition of many structural layers. 

Courtesy of B. T. Shaw and associates, Ohio State University.

D. ELECTRON MICROGRAPH OF MONTMORILLONITE, FROM LEMON, MISS.

Shows featherlike particles without straight boundaries. Many of the particles probably are not more than Sni/j in thickness. Courtesy of Herbert Iiigley and E. B. McMurdic,

National Bureau of Standards.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 205 PLATE 8

1

X 38 00(f

ELECTRON MICROGRAPHS OF HECTORITE AND NONTRONITE

A. Hectorite, from Hector, Calif., showing typical lath-shaped crystals. Most of the laths probably have a thickness greater than lOm/j..

B. Nontronite, from Sandy Ridge, N. C., 200-50m/i fraction. Note the straight edges of many particles. Courtesy of B. T. Shaw and associates, Ohio State University. 

C. Hectorite, from Hector, Calif., 500—200m/i fraction, showing clumps of unseparated needles with radiating acicular particles. Courtesy of B. T. Shaw and associates, Ohio

State University.

D. Hectorite, from Hector, Calif., legs than SOnui fraction, showing very fine acicular character of individual particles. The particle width is about 0.01 n. Courtesy of B. T.

Shaw and associates, Ohio State University.
MINERALS OF THE MONTMORILLONITE GROUP 59

(1936b), Norton (1939), and Hauser and Reynolds 

(1939) have in recent years succeeded in carrying out 

syntheses of montmorillonite-beidellite, and Strese and 

Hofmann (1941) have synthesized hectorite. Their 

experiments do not duplicate the conditions under which 

these minerals have commonly been produced in nature, 

but they nevertheless indicate the importance of some 

factors in paragenesis.

Ewell and Insley (1935), using slightly alkaline coprecipitated gels of A12 03 and Si02 in which the mol 

ratios were lAl203 :2Si02 , succeeded in forming "beidellite" at 350° C. and 390° C. in the presence of 167 and 

260 atmospheres of water vapor, respectively. Their 

material was probably truly a beidellite with the composition near Al2 .i7[A1.83Si 3 .i7]pio[OH] 2X. 33, which would 

approximate the l:2::A! 203 :Si02 ratio, and in which the 

exchangeable base X was probably sodium or A1[OH] 2+,

but part of the sodium may have been present as an 

impurity. Ewell and Insley similarly prepared nontronite using Fe203 and Si02 gels and, as alumina was 

absent, the final composition must have approximated 

[Fei.89lSi 4Oio[OH] 2X.39; but the replaceable base might 

have been less than indicated in the foregoing formula. 

The mixture contained a large excess of Fe203 for this 

type of compound, and it appeared in the final material 

partly as the oxide and partly as an unidentified 

compound.

The most elaborate work on synthesis is that of Noll 

(1936b). He used mixtures of hydrated alumina and 

silicic acid gels in which the mol ratios were Al203 :Si02 :: 

1:4. All experiments were carried out at 300° C. with 

a water-vapor pressure of 87 atmospheres and ir the 

presence of various bases. Results of his experiments 

are summarized in table 18.

TABLE 18.—Conditions of synthesis for hydrous aluminum silicates

(Investigation by W. Noll of the system AbO3 ; SiOj; HjO; R'OH or R" [OHlz. Aluminum hydrate + silica gel + accessory solute; AbO3 :SiO2=l:4. Decree of 

filling, 0.35. Temperature, 300° C.]

Accessory 

solute

N/100 HC1. ...................

HtO...........................

HjO. .........................

NaOH. .......................

NaOH. .......................

NaOH........................

NaOH........................

NaOH........................

NaOH.........................

KOH..........................

KOH.........................

KOH. ........................

CsOH.........................

Ca(OH)j ......................

Ca(OH) 2 ......................

Ca(OH)2 . .....................

Mg(OH) 2 ......................

MtfOK)*.. ....................

Mg(OK)i. .....................

Mg(OH)s .. ....................

Mg(OH)t........ ..............

Be(OH)2.......................

Time in 

hours

18

15

20

16

16

9n

24

on

OA

on

20

on

24

20

on

20

20

on

20

20

20

O4

24

Mg. AhO3

50

50

25

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

50

Mg. SiOs

117.8

117.8

58.9

m o

117.8

235.6

117.8

117.8

117.8

117.8

117.8

117.8

117.8

117.8

117.8

m o

117.8

117.8

117.8

117.8

117.8

117.8

Mg. R'OH or 

R" (OH>2

6.062

fiO

6

0

4.65

6.2

01 n

1.1

11.0

55s

oo ^Q

70

7.3

QC ft

.57

5 79

17.96

28.5

57.15

4.22

42.2

Molecular ratio 

R'jO(R"O):Al 2O3 :SiO2

6. 002: 1:4

.02:1:4

.1:1:4

.15:1:4

2

.1 -A

1:1:4

no.l .4

2:1:4

1:1:4

.2:1:4

.02:1:4

.2:1:4

1:1:4

.02:1:4

.2:1:4

.63:1:4

1:1:4

2:1:4

.2:1:4

2:1:4

X-ray diagram of the 

solid phase

Kaolinite.

Kaolinite.

Kaolinite.

(?)

When alumina and silica were used in absence of bases 

only kaolinite and pyrophyllite were obtained, which 

would be in harmony with their lack of essential base 

exchange. Noll found that an essentially higher concentration of base was required for the appearance of 

montmorillonite than for kaolinite, and his limiting base 

concentration is near the base-exchange capacity of 

montmorillonite. In the experiments in which he used 

NaOH, KOH, CsOH, or Ca[OH] 2 beidellite,

Al2.nCAl.83Si3.nlOloCOHj2X.33,

probably was formed. A montmorillonite type of mineral was formed over a much wider range of concentrations when Mg[OH] 2 was used. The composition probably varied from a montmorillonite-beidellite member 

to montmorillonite of the composition

X.33

T

at the higher Mg[OH] 2 concentrations. The exchangeable base probably was Mg++ or Mg[OH]+. At very 

high Mg[OH] 2 concentrations it is possible that some 

saponite (aluminian) formed as

X.33

T 

[Mg2 . 67Al 33] [Al. 67Si3. 33]0 10[OH] 2

Formation of montmorillonite-type minerals over a very 

broad range of concentrations of Mg[OH] 2 is fur;r in 

harmony with their compositions and with the genetic 

relationships within the group.

The experiments of Noll on synthesis—overemphasize 

the importance of alkaline conditions in the formation 

of minerals of the montmorillonite group; the pres ence 

of bases is of more importance than alkalinity, as ruch.

Noll (1940) emphasizes this in a later paper and spates 

that alkalic rocks will alter to kaolin as a result of either 

acid solution or intensive water leaching. Basaltic or 

gabbroic rocks, on the other hand, will alter to montmorillonite in consequence of the favorable conditions 

resulting from the presence of magnesium.

Strese and Hofmann (1941, abstract 1942) carried out 

synthetic work on magnesian members of the grouo by 

boiling MgCl 2 solutions with silica gels in the presence 

of various bases. Formation of hectorite, showing the 

essential base exchange and other properties, was 

favored by use of NaOH as the base, but also was observed with medium concentrations of KOH. Higher 

concentrations of KOH favored formation of mica-like 

minerals, as might be expected. Antigorite was also 

synthesized by action of MgCl 2 solutions on silica gels.

Norton (1939) studied the hydrothermal alteration of 

albite, nephelite, orthoclase, leucite, petalite, spodumene, lepidolite, anorthite, beryl, pollucite, and kao-
60 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-14

linite in the presence of high pressures of CQ2 and H2 0 

at elevated temperatures. A montmorillonite type of 

mineral was formed only from albite, Na2O.Al203 .6Si02 , 

whereas anorthite (CaO.Al203 .2SiO2 ) gave pyrophyllite. 

Albite probably gave beidellite in these experiments, but 

the conditions are very different from those under which 

it usually forms.

Hauser and Reynolds (1939) studied the devitrification of obsidian and of a glass approximating Wyoming bentonite in composition in the presence of acids 

and bases at 300° C. Both glasses gave montmorillonitetype minerals, but the obsidian was much the more 

stable of the two and altered only in acid solution. 

Basic conditions were not necessary and, in agreement 

with Noll, a wider range of concentrations was effective 

when Mg[OH] 2 was used.

Lovering (1941) has reported a sequence and marked 

zonal relations between sericite, hydrous micas, kaolinite, and minerals of the montmorillonite group. He 

concludes that the sequence of mineralization and the 

control exerted by acid or alkaline solutions corresponds 

with the results of the synthetic experiments of Noll.

ORIGIN AND MODE OF OCCURRENCE

GENERAL FEATURES

Members of the montmorillonite-beidellite-nontronite-saponite group of clay minerals vary greatly in their 

modes of formation. They are stable over a rather wide 

range of temperature; they have formed at temperatures up to at least 200° C. (see pp. 68-69); and they 

have been formed by purely weathering processes, as 

well as by low-temperature hydrothermal processes. 

The important modes of occurrence are (1) in soils, 

either as essential minerals of the clay fraction or in association with other clay minerals; (2) as bentonites; 

(3) in pegmatite veins as an alteration product of other 

minerals; (4) in mineral veins, both as vein minerals and 

as gouge c]ays; and (5) as one of the dominant minerals 

in waterlaid shales.

Some of the factors favoring the formation of the 

montmorillonite type of clay minerals are indicated by 

their chemical composition and by the work on their 

synthesis; others can be deduced from their occurrence 

in various environments. Experiments on synthesis indicate that the formation of montmorillonite is favored 

by alkaline conditions or by the presence of salts of alkalies and alkaline earths, in particular by magnesium, 

which is commonly an essential constituent of these 

minerals. The formation of hectorite necessarily involves the presence of Li, which in this mineral plays the 

role of Mg, and evidently takes place in an alkaline or 

neutral medium, since hectorite is associated with finely 

divided calcite. In some soils the formation of the montmorillonite type of minerals apparently depends upon 

the presence of iron compounds, apparently both ferrous 

and ferric, giving a member of the group containing ferric iron.

As montmorillonite is evidently the typical clay or 

soil mineral with a crystal lattice into which essential 

amounts of iron can enter, a clay-forming system containing iron would favor its formation. Organic material in association with certain bacteria results in 

reducing conditions in the clay-forming system. This 

would favor montmorillonite formation, either by supplying Fe++ directly, where it would play the same role

as Mg++ in the crystal structure, or by means of its 

higher solubility, which would increase the availability 

of iron to the clay-forming system.

Some bentonites are known to contain more Mg++ 

than the glass from which they were derived, and this 

may have been supplied either by marine waters, if alterations occurred in the ocean, or by ground waters, if 

alteration took place after burial. Marine waters are 

slightly alkaline, and ground waters are alkaline or 

neutral.

The presence of potassium in sea water, however, 

favors formation of bravaisite and other mica-like 

minerals as the end products of marine deposition and 

it accounts for the predominance of bravaisite in Ordovician bentonite and marine shales. It seems probable 

that, in general, the formation of the montmorillonite 

type of clay minerals occurs only in the absence of appreciable potassium in the clay-forming system. This 

suggests that in general alteration of glass to montmorillonite was effected in fresh-water bodies or by 

ground water rather than by marine waters. Orthoclase 

and muscovite micas are deficient in the bases Mg, Fe+2, 

and Ca essential for the formation of minerals of the 

montmorillonite group and so are not favorable sources 

of montmorillonite, except in the presence of hydrothermal solutions that add these bases and at the same 

time remove potassium.

A neutral or slightly alkaline medium aids in the removal of silica from weathering feldspars, but it would 

be less effective than acid solutions in removing bases. 

Apparently, there also is a tendency for calcic feldspars 

to give montmorillonite minerals instead of kaolinite 

clay minerals upon weathering.

From the several examples discussed it seems evident 

that weathering under conditions which give ar adequate supply of magnesium or ferric and ferrou^1 iron 

and an alkaline environment, together with necessary 

silica and alumina are the most important factors for 

development of the montmorillonite type of minerals in 

soils. Oxidizing conditions or a parent rock whos<> iron 

is in the ferric state seem to give kaolinite rather than 

montmorillonite. Climate and biotic ecology se?m to 

be of secondary importance. However, extremel^ wet 

and warm climates, by insuring rapid weathering and 

oxidizing conditions, probably favor formation o* kaolinitic and lateritic types of soils from parent materials 

that, under other conditions, might have given montmorillonite-type clays. Dry-soil regimes brought about 

either by lack of rainfall or by prolonged frozen periods 

result in far less drastic weathering and favor formation 

of micaceous minerals of the bravaisite-illite type in 

soils that are dominantly silty.

Minerals of the montmorillonite group have persisted 

for long periods, but they probably are not the normal 

end products of weathering. Under continued leaching 

by acid waters or by long-continued leaching in neutral 

waters under oxidizing conditions, kaolin minerals tend 

to form. The tendency for kaolinite to form may be 

inhibited by the physical relationships; thus, montmorillonite beds and veins may persist indefinitely as a 

result of their imperviousness to water by which they 

protect themselves against further weathering. It 

seems evident that kaolinite seldom if ever reverts to 

montmorillonite under weathering conditions.

Discussion of these factors will be elaborated in consideration of various types of occurrences of clay 

minerals belonging to the montmorillonite group.
MINERALS OF THE MONTMORILLONITE GROUP 61

SOILS AND ROCK WEATHERING

DISCUSSION

The clay minerals' are the essential constituent of all 

soils and shales, and recent mineralogical studies have 

shown that these fall into three distinct groups—that 

is, as kaolinite and halloysite of the kaolin group; members of the montmorillonite group; and members of the 

group of hydrous micas, including bravaisite. Kaolinite 

was long believed to be the typically clay-forming 

mineral, but this is not true for large parts of the earth's 

surface. In areas of deep and long-continued weathering, especially in the tropics and subtropics, and more 

especially where the parent rock is characterized by 

alkalic feldspars, kaolinite with its high alumina does 

form and is commonly accompanied by a high concentration of sesquioxides of alumina and iron. In temperate regions, and in many arid regions, montmorillonite-beidellite or hydrous micas are commonly the 

dominant clay materials.

So far as known, G. P. Merrill (1902, p. 318) was the 

earliest investigator to discredit the assumption that 

kaolinite was the typical clay mineral, but it was rather 

generally held until the X-ray method gave conclusive 

evidence of the real mineral nature of clays. However, 

long before this, microscope studies had shown that 

kaolin minerals were minor constituents of the soils of 

large areas of the earth's surface. The differences in 

indices of refraction, and the still more striking differences in birefringence of the kaolin minerals and of 

montmorillonites or hydrous micas presented evidence 

of the absence of kaolinite in a large proportion of clay 

materials. On the other hand, optical methods do not 

sharply distinguish the montmorillonite group of minerals from bravaisite and some of the micalike minerals. 

However, where the mineral occurs as distinct grains, 

and the iron content is small, montmorillonite has a 

distinctly lower index of refraction than hydrous bravaisite and the micas. A high iron content of montmorillonite or beidellite raises the indices so that they 

cannot be distinguished from bravaisite by optical 

methods alone.

The most widespread occurrence of minerals of the 

montmorillonite group undoubtedly is in the clay fraction of soils and in shales formed generally in fresh water.

Fine detrital materials such as quartz, micas, feldspar, and other igneous or metamorphic rock minerals 

generally are present but can be approximately removed 

by sedimentation. The clay fractions, however, with 

the exception of the clays in some marine shales, which 

are predominately bravaisite or altered micas, are in 

general mixtures of the three principle types of clay 

minerals. Such mixtures of clays have not yet been 

separated into their component minerals and there is 

no great promise of such separations being made. For 

this reason the analyses listed in tables 1 to 5 do not 

contain a representative number of analyses of clay 

minerals from soils and shales. This results in the placing of undue emphasis on montmorillonite-type clay 

minerals from other sources, especially from bentonites.

Beidellite with a high content of ferric iron is probably 

the predominant soil clay mineral of the montmorillonite group. Marshall (1935) has published detailed 

studies, which included chemical analyses of two such 

clays; one, the Putnam clay, our sample 70, a soil type 

occurring in parts of Illinois, Indiana, and Missouri, and 

the other, the Rothamsted clay of England, our sample 

69, table 3. The Putnam clay fractions were examined

by Hofmann, Endell, and Wilm, who found no included 

quartz in the finer fractions. Various fractions gave 

only the diffraction pattern of a member of the montmorillonite group. This agreed with Marshall's optical 

studies, which indicated that the Putnam clay is homogeneous beidellite. Formulas for these two clay;^ are 

given in table 12.

Both the Putnam and Rothamsted clays contain 

appreciable amounts of nonexchangeable K2 0. Since 

the Putnam clay probably is homogeneous, this indicates that it possibly is a mixed-layer mineral with 

layers of the bravaisite or hydrous-mica (illite) type of 

clay material. The analyses, moreover, differ in -many 

minor ways, such as in 1*20 8 content, from most of the 

montmorillonite specimens examined. Marshall assumed that in beidellite the Ti02 proxied for A1203 and 

the P205 for Si02 . In this work, however, they wer-* not 

considered since it seemed more probable that both were 

present as impurities. The Rothamsted clay prol ably 

contains more than 25 percent of kaolinitic clay 

minerals.

Marshall's work fully illustrates the difficulty of obtaining homogeneous clay minerals of the montmcrillonite type from soils. A few clays have formed in very quiet 

waters in such a way that nearly all detrital rock grains 

have settled out before the clay was deposited. Ch« of 

these is the clay from Ancon, Canal Zone (analyses 53, 

table 1) which contains only 0.50 percent K20, and 

which approaches the limiting formula for beidellite in 

composition. Moreover it is characteristically lew in 

MgO. Another clay possibly deposited under sedimentary conditions is the one from Fairview, Utah 

(analysis 46, table 1), which is pictured in plate 2, D. 

It shows closely spaced bedding planes that seem to be 

varves registering annual variations in conditions of 

deposition. The very high K2 0 content (2.25 percent) 

of the Fairview clay, however, indicates that it may 

really be bravaisite or rather than a mixed-layer material made up of both montmorillonite and a mica 

mineral.

As the beidellite end of the montmorillonite-beicfellite 

series is approached there is a decided tendency toward 

the formation of mixed-layer type minerals containing 

potassium. In more fundamental terms the numHr of 

nonexchangeable interlayer ions, essentially of K+, increases with the increase in replacement of silicon by 

aluminum in tetrahedral coordination. This indicates 

that there may be a complete gradation between the 

beidellite-type clay minerals and moderately high 

potassium micalike minerals. A detailed investigation 

of the bravaisite-hydrous mica (illite) type of clay 

minerals is desirable,"and-it is unfortunate that circumstances preclude the inclusion of such a study in 

this paper. Until the mineralogy of these micalike 

minerals can be cleared up, the question of the extent 

of solid solution remains open.

Sufficient observations have been made on the distribution of montmorillonite-type clay minerals in soils 

to permit discussion of factors favoring their formation 

in the environment of a soil. The nature of the parent 

material and the chemical environment during alteration are obviously the fundamental factors. Cheiiical 

environments are determined by physical and biotic 

conditions as well as by parent materials. The physical 

conditions may be continuously arid or humid, or alternately dry and wet; the water table may fluctuate; and 

materials may differ in porosity. Climatic conditions 

in general, as they reflect some of the physical ffe
62 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

and determine the plant cover, will affect the weathering course of the parent materials. However, these are 

but contributory factors, which combine to form the 

physical chemical system that controls soil formation. 

Formation of minerals of the montmorillonite type in 

soils developed under five widely different conditions 

will be discussed to illustrate some of the factors involved. The regions in which the soils formed are 

Australia, Java, eastern and western United States, and 

Russia.

AUSTRALIAN SOILS

Hosking (1940a) has studied the mineral composition 

of Australian soils developed from granitic and basaltic 

rocks under a wide variety of climatic conditions. After 

a study of eight granitic soils Hoskins concludes,

It is apparent that granitic types of parent material will 

weather to kaolinite or halloysite under a wide range of climatic 

(desert to heavy rainfall) conditions. Whereas the clay type 

may remain constant, the variation in mineralogical composition 

of the granite is reflected to a certain extent in the minor constituents of the clay.

From his examination of seven soils developed from 

basaltic rocks Hoskins reached the following conclusions:

In the case of the basaltic soils, the internal moisture conditions 

and developmental trends appear to play an important part in 

determining the mineral clay type formed. Under excessive 

moisture and dominant leaching conditions, where free drainage 

is a feature and there is no approach to drought conditions for 

any period of the year, basaltic parent materials give rise to colloids of the kaolinitic type. Hydrargillite, goethite, and haematite are a further expression of high sesquioxide content and conditions allowing complete oxidation. With a lowering of the 

moisture status, the practical cessation of active leaching, and 

the occurrence of protracted drought periods, montmorillonite 

types become the characteristic mineral type. Where free 

drainage still prevails kaolinite may still be found, but in its 

absence montmorillonite alone occurs. * * * In none of the 

soil samples examined was there any evidence of a mineral of 

the hydrous-mica type.

It seems evident that the dominant factors for montmorillonite formation in the Australian soils studied by 

Hoskins are weathering of a suitable parent material 

under alkaline conditions. Granitic and other highly 

silicic igneous rocks are not suitable source material. 

The basic (basaltic) igneous rocks with calcic feldspars 

and appreciable amounts of ferromagnesium minerals 

weathering under alkaline and possibly reducing conditions will give soils containing clays of the montmorillonite type. These general observations are in 

agreement with studies on soil formation from igneous 

rocks in other regions.

PIEDMONT SOILS

A number of soils characterized by the presence of 

montmorillonite-type clay minerals have formed from 

ferromagnesian igneous rocks in the Piedmont region of 

the eastern United States. In this region residual soils 

have developed from a great variety of crystalline rocks, 

and each rock type gives a distinct soil. Small areas of 

soils derived from ferromagnesian intrusives can readily 

be recognized in large'provinces of granitic and schistose 

rocks. The boundaries between these different soil types 

are commonly very sharp and it is apparent that the 

determining variable in soil formation is the parent 

material. Plant coyer and topography for the various 

soils often differ widely, but they rather reflect than 

determine the course of the soil formation.

Piedmont soils, containing essential minerals of the 

montmorillonite group in their clay fractions, principally belong to the Iredell, Wilkes, and Orange soil 

series. Owing in part to the physical characteristics 

imparted by the montmorillonite type of clays these 

soils seldom are fertile and generally are used ar forest 

or pasture land. The soils are very shallow compared 

with many others in the same region where red and 

yellow soils are most prominent. Their surfaces are not 

uncommonly covered with unweathered parent rock 

and the Iredell soil is known as "nigger-head" land on 

account of the numerous diabase boulders.

A study, detailed in some areas, has been made of 

these soils, but the factors favoring the formation of 

montmorillonite are not yet fully evaluated. It is obvious, however, that the nature of the parent ro°.k is a 

dominantly controlling factor. The diabase int-usiyes 

in the Triassic basins weather to give soils containing 

clays that are predominantly of the montmorllonite 

type from southern North Carolina to Pennsylvania 

(Iredell soils). The parent rocks are dominantly calcic 

plagioclase, with lesser amounts of the ferromagnesian 

minerals, augite and olivine carrying essential amounts 

of ferrous iron. Upon weathering, almost all of the iron 

originally present in the rock becomes a part of the 

crystal structure of the resulting clay mineral, thn being 

an iron-rich member of the montmorillonite group.

Rocks with a composition only slightly differert from 

those weathering to Iredell soils, which are characterized 

by montmorillonite, have been observed to give intensely red and deeply weathered lateritic type of soilr of the 

Davidson and Mecklenberg series in Virginia and North 

Carolina. A thinner soil mantle and lighter red color 

characterizes the Mount Alto type of soils, which are 

derived from rather similar rocks in the Piedmont, region 

of Maryland and southern Pennsylvania. The parent 

rocks from which these soils are derived initially contain 

no greater amounts of ferromagnesian minerals than do 

those giving rise to the Iredell type of soils, but their 

weathering results in the formation of kaolinitic clays 

and in the liberation of the iron as free oxides and 

hydrous oxides (Alexander, Hendricks, and Faust, 

1941). This difference in weathering pattern, however, 

must be due to differences in the parent rocks rather 

than to climatic factors, such as rainfall.

The Orange soil is derived from the Schistose greenstone of the Virgilina district of Virginia and North 

Carolina and from more coarsely crystalline rocks in 

other districts. Its clay fraction contains a member of 

the montmorillonite group, with an appreciate iron 

content, and relatively large amounts of kaolinite. The 

greenstone schist at Virgilina is a very fine grained rock 

and commonly is a recrystallized volcanic tuff. It contains appreciable amounts of ferromagnesian minerals— 

dominantly hornblende and epidote—and fine-gained 

feldspar," and its weathering pattern appears to be 

similar to that of rocks giving the Iredell type cf soils.

Epidote greenstone schists of Albemarle and Orange 

counties, Va., weather deeply to give the intensely red 

Davidson soils, as do the epidote greenstone scHsts in 

central North Carolina. These greenstone schists closely resemble the greenstone schist at Virgilina in ch emical 

composition but contain much less epidote.

The rocks that give the Davidson soil are essentially 

different in mineralogical character from those giving 

the Orange and Iredell soils. The Davidson soil is derived from a rock composed largely of epidote, whereas 

Virgilina greenstone contains abundant hornblende, also 

epidote, and the Triassic diabase contains augite and 

olivine. The iron in the epidote greenstone is in the
MINERALS OF THE MONTMORILLONITE GROUP 63

mineral epidote and hence is essentially all in the ferric 

state. On the other hand, the Virgilina greenstone and 

diabase contain a large proportion of ferrous iron.

The presence of iron in the more soluble ferrous state 

would make it readily available to the clay-forming 

system and so would promote the formation of montmorillonite-beidellite, even though most of it is eventually fixed in the ferric state. The breakdown of epidote 

seems to tend to release iron as oxides and under these 

conditions kaolinite forms rather than montmorillonitebeidellite. Thus, the state of oxidation of the iron in the 

parent rock seems to be one of the factors that influences 

the character of the clay mineral formed. Here then is 

an excellent example where the course of weathering is 

determined by the mineral composition of the rock 

rather than by the over-all chemical composition.

In the weathering of diabasic rocks, the minerals 

decompose almost simultaneously, the augite releasing 

ferrous and ferric iron, magnesium, and silicon to combine with the calcium, aluminum, and silicon of the 

feldspar to form montmorillonite. The same general 

relation seems to hold for certain types of greenstone 

schists characterized by ferromagnesian minerals containing ferrous iron. In these the clay-forming system 

seems to be basic and neutral or reducing, but clearly 

not oxidizing. A different relationship seems to be the 

controlling factor in clay formation in some types of 

rocks. In these the ferromagnesian minerals may 

weather rapidly and their decomposition be far advanced before essential changes take place in the feldspars, which are perhaps less calcic in those rocks. The 

magnesium has been removed by carbonated waters, 

and the iron fixed as ferric oxides before the feldspars 

are decomposed. Under these conditions kaolinite tends 

to form rather than montmorillonite.

Wilkes soils in many localities are derived from amphibolites or other dominantly ferromagnesian rocks 

interbedded with highly silicic layers. These soils occur 

extensively in the region of metamorphic rocks around 

Young's Mountain in Person County, N. C. Amphibolites in neighboring regions are observed to undergo 

lateritic weathering with formation of the red, Davidson 

type of soils. The mineralogic composition of the parent 

rocks and their petrographic characteristics have not yet 

been studied in detail adequate to indicate factors controlling the course of weathering.

Soils of the White Store series are derived from shales 

in the easternmost Triassic basin of North Carolina. 

These soils are impervious, and the clay fraction is predominantly montmorillonite. This clay fraction, however, is inherited from the parent material, a montmorillonite shale, which was apparently deposited in a 

fresh-water lake or a sea of low salinity. Montmorillonite has persisted in the soil, even though it has been 

subject to leaching for a long time in a temperate climate. This shale member is not present in the Triassic 

regions of Virginia, Maryland, and Pennsylvania. Its 

probable source and condition of formation has not been 

studied.

JAVA SOILS

Hardon and Favejee (1939a, 1939b) have examined the 

clay fractions of a number of soils from Java, which were 

derived from five principal types of rocks: lateritic soils 

from ferromagnesian tuffs, "Mergelgronden" soils from 

young marls and calcareous tuffs, limestone soils, yellow 

mountain soils from andesitic tuffs, and white and gray 

earths derived from dacite tuffs. The clay fractions of

the "mergelgronden" soils consist essentially o* the 

montmorillonite type of clay minerals. These soil- are 

formed by the weathering of a soft marl. Some of the 

clays still contain calcium carbonate, and it is probable 

that they were formed under alkaline conditions. These 

clays probably were formed by the weathering of volcanic materials intermixed with the marl.

Some of the clays separated from the marl soils are 

quite pure montmorillonite. One of Hardon's andyses 

(No. 12853) is as follows:

Analysis of montmorillonite from Java

SiO.................. 42.80

A1 2O3 .. .............. 19.03

Fe 2 03

Ti02 . MnO.

CaO.

MgO.

8.68

0.02

0.05

1.73

1.81

K,O................. 0.08

NaiO................ 0.75

Pe 2O5 ..... ........... 0.06

CO2 .......... ........ 0.00

(H2O-). ............ 15.51

(H,O+).............. 8.66

Organic.............. 0.92

100.10

The clay has a base-exchange capacity of 0.86 milliequivalent per gram. The corresponding formula is as 

follows:

X.38

+3 +3 +2 ^-2 t +3 +4 ^_ 

[Alt.34Fe.53Mn.o3Mg.22] [A1.49Si3.5l]0 10 [Ofl]2

Calcium is the predominant exchangeable ion. Since 

these soils are commonly gray or black, it is improbable 

that an appreciable fraction of the ferric iron is present 

as oxides or hydrous oxides. The ferric iron content of 

the montmorillonite is thus probably higher than found 

for the specimens examined in this work.

Some factors essential for development of the montmorillonite type of clay minerals under the tropical conditions of Java appear to be found in the weathering of 

a ferromagnesian volcanic effusive in an alkaline 

medium. These factors are similar to those necessary 

for formation of the montmorillonite type of clay 

minerals under the temperate climates of southern 

Australia and the eastern United States.

GLACIAL SOILS

The grasslands of western United States and Canada 

and the Steppes of central Russia contain large areas of 

soils in which montmorillonite is a prominent constituent of the clay fractions. The ecology of the regions 

is determined to a large extent by the limited rainfall 

and the continental climate. Soils in general are of 

Marbut's "northern prairie" and "northern chermogen" 

types near the boundary between his "pedocals" (soils 

with a zone of calcium-carbonate accumulation?) and 

"pedalfers."

The parent materials of these soils in the United 

States are chiefly calcareous glacial accumulations and 

calcareous loess. Similar types of parent materials and 

similar conditions for soil formation are present in 

central Russia. Clays in the soil are partly inherited 

from the parent material. In the heavy B horizons and 

in clay pans, increased clay concentration is brought 

about by weathering of feldspars in place and there is 

little migration of clay. Feldspar is the source of the 

aluminum and silica for the montmorillonite t;rpe of 

clay mineral that forms under these conditions- Essential magnesium is supplied by the alkaline earth 

carbonate of the parent material which also insures an 

alkaline reaction of the weathering medium.
64 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

In the western part of the Great Plains region of the 

United States the surface material is outwash from the 

Rocky Mountains and the rainfall is scanty. Farther 

east, the glacial accumulation contains much ground 

crystalline rock and reworked shale, which initially contained micaceous minerals of the bravaisite-hydrous 

mica type. Clay fractions of these soils characteristically contain minerals of that type rather than members 

of the montmorillonite group.

Lamar, Grim, and Grogan (1938) have studied gumbotil, a weathering product of glacial material and make 

the following statement:

Gumbotil is derived from glacial till by weathering. * * * 

Gumbotil does not occur on glacial drift as young as the Wisconsin drift but is common on the older drifts—the Illinoin, Kansan, and Nebraskan. It formed under conditions of poor drainage, 

usually just below the soil layer, over broad, flat upland tracts. 

* * * The conversion of till to gumbotil in nature involves 

oxidation, leaching of carbonates and chemical decomposition 

of the silicate materials. Only the most resistant rock and 

mineral fragments remain, and some of these are reduced in size. 

The base of the gumbotil is commonly denoted by a zone containing bands of yellow or brown iron oxide. The original till 

contained large amounts of clay minerals of the illite group, and 

in general the processes of weathering have tended to remove 

alkalies, particularly potassium, and to alter the illite minerals 

to those of the montmorillonite group. These changes took place 

in varying degrees on the same drift sheet because of variations 

in weathering conditions and parent material.

So-called "black-alkali" soils and "slick" spots are 

widely distributed in the western United States. These 

are usually due to the presence of sodium-saturated 

minerals of the montmorillonite type in the c!ay fractions of the soils. The sodium clay is easier to disperse 

than is the calcium clay, and its gels shrink greatly upon 

drying. In the sodium-montmorillonite type "blackalkali" soils the black color results from surface organic 

matter which drops into shrinkage cracks. The tendency 

of the soils to disperse when calcium in montmorillonitetype clays is replaced by sodium must be watched carefully in western irrigation projects, and dispersed clays 

usually must be flocculated before land can be brought 

into successful tillage.

SUMMARY OF SOILS AND ROCK WEATHERING

In general, it may be stated that soils formed by surface weathering from granitic, highly micaceous, or 

other silicic rocks do not appear normally to contain 

montmorillonite as an appreciable part of the clay fraction. Igneous rocks containing essential quantities of 

amphi boles, pyroxenes, or other ferromagnesian minerals tend to give soils characterized by montmorillonite 

clays, but in some physical and chemical environments 

they may undergo lateritic type of weathering. Soils 

derived from sedimentary formations inherit their clays 

in part from the parent formation. Calcareous formations apparently will give various types of clay minerals 

in soils, depending upon the nature of their noncalcareous constituents and the rate of weathering, which 

determines the pH of the weathering system. Climate 

and plant ecology are not the all-prevailing factors for 

soil formation, as now assumed by most pedologists.

The foregoing examples of soil forming processes present a very generalized outline, and many apparent exceptions may be cited. The course of soil formation may 

be closely followed in some types, but others are inadequately understood. Very little work has been 

carried through to a rigorous evaluation of all the factors 

involved in soil formation. It is the difficulty in correlating the various contributory factors that makes many 

soil problems complex. This necessitates many detailed

studies before generalizations on clay paragenesis are 

reliable. Beginnings must be made on areas or occurrences where the relations are not too complex and 

where the maximum number of factors may be evaluated. In some ways soils represent systems more suited 

for study of clay formation than do hydroth^rmal 

deposits, as conditions during soil formation are sometimes constant over long periods and may be determined with some accuracy. On the'other hand, the 

coarser texture, the only partial replacement cf the 

parent mineral, and the associated minerals of veins 

commonly give a superior record of the sequer"!e of 

genetic events.

BENTONITES

Bentonites were first recognized from the western 

United States and for a long time were known only from 

that region; however, since interest has been aroused in 

their commercial value and scientific significance, they 

have been found in nearly all regions of the world. So 

many occurrences have been described that an attempt 

to list localities would be almost hopeless. The:T are 

present even in England (Kerr, 1932), where they were 

once believed to be nonexistent.

Bentonites are in general composed essentially of 

montmorillonite and are among the purest of all clay 

materials. They thus constitute one of the best-known 

sources of essentially pure clay material, and numerous 

clays of this type are represented among the analyses in 

table 1.

Bentonite had long been recognized as a rathe- unusual type of clay material, but D. F. Hewett (1917), of 

the Geological Survey, was the first to recognise its 

mode of origin. He stated that bentonites commonly 

contain minerals characteristic of volcanic rocks and 

that the usual detrital minerals are absent. These facts 

led him to conclude that bentonite was composed of a 

clay mineral that was an alteration product of volcanic 

glass. Later it was found possible to make thin sections 

of clay materials, and Hewett's conclusions as to the 

origin of bentonite were confirmed.

STRUCTURE OF.PARENT GLASS

Thin sections show that-bentonites commonly retain 

the structure of volcanic ash with surprising perfection. 

There are three main types of ash structure, although 

there are complete gradations from one type to anc ther. 

Perhaps the most abundant is the structure of a very 

fine grained pumice, rather large fragments of which are 

represented in plate 4, D. The violent explosive eruptions 

of the parent volcanic vents have drawn out the molten 

glass with its minute included bubbles of volcanic gases 

so that the cooled glass fragments have a seenrngly 

fibrous structure. Another type evidently was formed 

under somewhat similar conditions, but the gas bubbles 

were larger and had the shape of flattened lenses. The 

almost complete fragmentation of the glass by explosive 

action produced glass shards, in large part the platelike 

remnants of the cell walls that surrounded these flat 

lenticular bubbles. Such platelike shards are represented 

in plate 4, B. The third type probably was formed by 

somewhat less violent explosions. This resulted in more 

or less spherical bubbles; and the shards are therefore 

curved plates, lune-shaped fragments, or cusps with 

curved walls, as shown in plate 4, C. A single bubble 

when broken produced shards that form arcs of circles. 

Where two round bubbles were adjoined, remnants of
MINERALS OF THE MONTMORILLONITE GROUP 65

the glass wall between them contain the arcs of two 

circles, and those from three adjoining bubbles form 

triangular segments bounded by curves rather than by 

straight lines. Rarely, unbroken bubbles marked by 

complete circles may be observed in thin sections. (See 

plate 4, A.) These unbroken bubbles are always filled 

with clay material of the same character as the replaced 

bubble walls, showing that some local transportation of 

clay material has occurred.

The size of the glass shards varies within wide limits. 

Some of the finest materials are only a few microns in 

diameter and are perhaps the result of very thorough 

sorting in water. Wind sorting commonly gives larger 

and less well sorted glass fragments with sizes in general 

varying from 0.05 to 0.3 millimeters in greatest dimension. The size is no doubt roughly related to the distance to which the glass fragments were carried from the 

explosive center. Remnants of a very remarkable ash 

shower are scattered from the Panhandle region of Texas 

to east-central Oklahoma, and the more eastern occurrences are marked by a much smaller grain size. In 

general, bentonite beds have a limited lateral extent, 

and variation in grain size cannot be readily traced; but 

there are marked variations in size of grain in the widespread Ordovician bentonites, those in the AlabamaTennessee region being in general coarser grained than 

is ash of the same age in regions to the northeast. In 

some places fragments from 1 to 4 millimeters in diameter have been observed, which had altered to montmorillonite. These probably fell rather close to the volcanic center. Although bentonites are normally derived 

from pyroclastic materials, one occurrence is known 

where a small rhyolitic intrusive had altered to montmorillonite.

PHYSICAL PROPERTIES

Bentonites very greatly in color, although they commonly present some light tint. Cream, light tan, and 

various shades of gray are most commonly observed, 

but pink, lavender, pale blue, pale green, and even pure 

white are not rare. A bentonite from Mississippi is sage 

green. The texture is most often compact but may be 

open or porous. Most bentonites resemble paraffin or 

soap in appearance, especially before thorough drying, 

and in general they show a distinct translucence in thin 

fragments. Only rarely are they highly opaque as are 

nearly all other clay materials.

A few types with sodium as the exchangeable base, as 

illustrated by hectorite and Wyoming bentonites, form 

a gel-like mass on absorption of water; but in general, 

bentonites are characterized by calcium as the replaceable base, and these varieties tend to swell slightly and 

crumble into a granular mass. The gel-forming types 

tend to stay in suspension indefinitely, but the more 

widespread calcium type commonly yields only a small 

percentage of material that remains in permanent suspension. The calcium bentonites may be converted to 

sodium bentonites by the substitution of one exchangeable base for another, and dispersion by mechanical 

means may greatly increase the proportion of material 

that stays in suspension.

DEFINITION

The definition of bentonite by Ross and Shannon 

(1926) which has been generally accepted is, "Bentonite 

is a rock composed essentially of a crystalline claylike 

mineral formed by the devitrification and the accompanying chemical alteration of a glassy igneous material,

usually a tuff or volcanic ash." In describing it they 

state:

It [bentonite] often contains variable proportions of accessory 

crystal prams that were originally phenocrysts in the volcanic 

glass. These are feldspar (commonly orthoclase and oligochse), 

biotite, quartz, pyroxenes, zircon, and various other minerals 

typical of volcanic rocks. The characteristic clay mineral I •»« a 

micaceous habit and facile cleavage, high birefringence and a 

texture inherited from volcanic tuff or ash, and it is usually the 

mineral montmorillonite but less often beidellite.

MODE OF OCCURRENCE

A few bentonites are derived from materials that fell 

on land and were redeposited. These materials contain 

detrital grains, among which quartz is most abundant. 

Where quartz, feldspar, and other detrital minerals become dominant, the material may be called a bentonitic 

arkose. Where bentonitic material is reworked after the 

glass had altered to clay material, or where the si ard 

structure is destroyed before alteration, it is commonly 

impossible to recognize the resulting clay as definitely 

of bentonitic origin. It is probable, therefore, that iruch 

bentonite has become incorporated in sedimentary materials with no way for definitely recognizing its 

presence.

The small amount of quartz in most bentoniter as 

noted by Hewett, has been confirmed by further examination of many hundreds of thin sections. Only a 

few have been noted where the quartz represented 

original igneous phenocrysts, although Bramlette (paper 

in preparation) reports quartz phenocrysts in the San 

Luis Obispo tuff, which is in part altered to a bentonite.

Magmas high in silica tend to produce rocks characterized by quartz phenocrysts, so the absence of quartz 

in most bentonites presents evidence that the glass f>om 

which it was derived was not excessively rich in sPra; 

in other words, the glass that most readily alters to bentonite tends to have the composition of latite, a rock 

the crystalline part of which is composed largely of 

feldspar. Highly silicic glasses evidently have a higher 

stability and so alter less readily. It is not uncommon 

to find geologic sections of volcanic rocks where the 

tuffs of feldspathic composition have altered to bentonite, whereas the rhyolitic rocks in the same section 

are almost unaltered. Seemingly, there is no composition that definitely limits alteration to bentonite, and 

it is evident that conditions of formation—chemical 

environment, depth of burial, and time—also are important factors in controlling its formation.

In the region near Petaca, N. Mex., the base of the 

section of volcanic rocks is marked by a light-cole red, 

friable, highly permeable rhyolite tuff that has resisted 

alteration, while higher and less silicic beds in the s^.me 

geologic section have altered to bentonite.

The comparative sparsity of ferromagnesian minerals 

other than biotite in bentonites is evidence that the 

rocks from which they were derived were in general not 

basaltic and probably in general not andesitic in 

character. This restriction is no doubt due, at leart in 

part, to the fact that the types of volcanic rock characterized by calcic plagioclase and large proportion* of 

ferromagnesian minerals do not form glassy volcanic 

ash as readily as do the alkalic types. Where basaltic 

glass does occur, however, as in Hawaii, volcanic glasses 

are present and alter very readily to an amorphous 

material, called palagonite, which is high in iron and 

very high in water and quite unlike the crystalline clay 

material that characterizes bentonite. It is poss'ble, 

however, that given suitable time and conditions,
66 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

material would undergo further change and crystallize 

into a clay mineral. An exception is the beidellitenontronite, sample 59, table 2, occurring between two 

basalt flows in Spokane, Wash., where it is evidently an 

alteration product of basaltic material.

Ross, Miser, and Stevenson (1929) have described 

the bentonite represented by analysis 51, table 1, from 

near Nashville, Ark., where it is associated with fresh 

fragments of igneous rock. Because the rock fragments 

are dense and finely crystalline, they have escaped the 

alteration that changed the unstable glass to clay. The 

crystalline fragments and the glass shards, which subsequently altered to clay, are so intimately associated 

that they are believed to be two phases of the same 

parent magma. The chemical ana^sis and the norm of 

these unaltered crystalline fragments are given below.

Chemical composition and norm of volcanic rock from Mine Creek 

near Nashville, Ark.

CHEMICAL COMPOSITION

SiO2 ................. 62.97

A1 2O3 . .............. 19.00

1.13

.26

.54

2.86

Fe 2O3

FeO

MgO

CaO.

Na2O

K2O................. 10.80

H2O................ .72

H2O + ... ............ 1.26

TiO.................. .26

PjO.................. .10

99.90

NORM

Quartz............... 3.60

Corundum............ 1.80

Orthoclase............ 63.94

Albite........ ........ 24.10

Anorthite............. 1.95

Salic................. 95.39

Hypersthene.......... 1.13

Ilmenite.............. .46

Apatite............... .34

Femic................ 1.93

Water........ ........ 1.98

99.30

The figures given above show that about 92 percent 

of the Arkansas rock was normative feldspar. The 

glasses from which bentonites have most commonly 

formed probably do not have such an extreme composition as that given above, but they seem in general to 

be highly feldspathic.

An analysis of the clay derived from the volcanic 

glass is given below.

Analysis of Mine Creek bentonite

SiOj................. 45.12

AM),................ 28.24

FetO3 ................ 4.12

MgO......... ........ 2.32

CaO................. .88

H,O................. 18.72

99.40

Aluminum is the element least likely to be removed 

during alteration of glass to bentonite, and for purposes of calculation it may be assumed that A12O3 was 

neither added or abstracted. This assumption implies 

that nearly all of the alkalies and an appreciable proportion of silica were removed and that approximately 

one and one-half parts of trachyte glass were required 

to produce one part of bentonite. If part of the A12O3 

also were removed, the proportion of removed material 

would be greater.

The clay mineral derived from this rock contained 

2.32 percent of MgO, whereas the parent rock contained 

only 0.26 percent. The additional MgO must have been 

derived from sea water or from ground water in the 

presence of which the alteration from glass to clay 

mineral occurred. This evidence coincides with that 

presented by Noll (1936) that the optimum conditions 

for the formation of montmorillonite are in the presence 

of salts of magnesium.

M. N. Bramlette has had analyzed a vitric volcanic 

ash that is undergoing alteration to montmorillcnite on 

the north side of Santa Monica Mountains, Los Angeles 

County, Calif. Analyses are as follows:

TABLE 19.—Analyses of bentonite and related rocks, Santa Monica 

1 Mountains, Calif.

SiOj...................... ........

AhOi.............................

Fe»Oj.............................

FeO.. ............................

MgO ..............................

CaO..............................

NazO. ............................

KjO. .............................

H»O— .............................

H*0 + ............................

TiOj. .............................

PtOs..............................

MnO..............................

BaO ..............................

Total ........................

I

70.30

12.81

.42

1.80

.27

1.18

2.10

2.48

.68

7.12

.32

.07

.17

99.72

2

65.66

12.71

1.94

1.60

.71

1.44

1.10

1.73

4.34

8.03

.40

.06

.00

.17

99.89

3

50.03

16.75

5.83

.47

2.78

1.20

.26

.60

13.53

7.52

.65

.24

.01

.16

100.03

1. Washed concentrate of larger and least-altered shards of vitric volcanic ash. 

R. E. Stevens, analyst.

2. Untreated vitric ash, including finer ash and thin films of bentoritic alteration. Charles Milton, analyst.

3. Bentonite from a detailed section 1.18 miles from locality of samples 1 and 2 

above. Charles Milton, analyst.

The formula of the montmorillonite represented by 

analysis 3 is

Na.ji

+ 3 +2 |

[AU.3eFe.34Fe.o3Mg.33] [Si 3 . 85Al. 15]O10[OH] 2

If the A1 2O3 is assumed to be constant, about one and 

one-half parts of glass are required to produce one part 

of bentonite. As before, there must have been a manyfold increase in magnesium and iron while the alkali 

content was decreasing. Perhaps the most interesting 

feature of the analyses is that the amount of water required by the hydroxyl content of montmorillonite was 

picked up by the glass before devitrification set in. The 

analysis of the Santa Monica tuff (sample 1) shows that 

it is a rhyolite and indicates that under suitable conditions .glassy ash of rhyolitic character may form bentonites. Bramlette finds a relation between depth of 

burial, intfuding the time during which leachir<* continued, and the degree of alteration to bentonite. This 

implies that alteration was brought about by ground 

water.

The conditions under which alteration to bentonite 

takes place are not fully known and probably differ for 

different deposits. Some bentonites contain marine 

fossils, notably those in southwestern Arkansas and 

those in Mississippi that contain beautifully preserved 

echinoids, and were obviously deposited in a marine 

embayment. Alteration, however, may have followed 

rather than accompanied deposition. Some of tl ^ bentonites in the southwestern states appear to have been 

deposited in local saline lakes. In other region" there 

is no evidence as to whether the ash fell in wate~ or on 

land, or of the character of the water into which it fell. 

There is no basis for postulating saline waters as a 

necessary factor in the formation of bentonites; some 

bentonites probably have formed in fresh-wate" lakes 

or by the percolation of nearly fresh ground water 

through the porous ash beds. Bentonites have formed 

in northernmost Alaska and in the tropics; therefore, 

climate is probably not a completely determinative 

factor in their formation and persistence.

Very commonly beds of bentonite are marked by a 

concentration of chertlike material at their bases where
MINERALS OF THE MONTMORILLONITE GROUP 67

they are in contact with less pervious beds. This concentration shows that alteration occurred under conditions that allowed the waters to move downward and 

redeposit leached silica. In such bentonites alteration 

did not take place in the standing water of lake or sea. 

In other occurrences there is no evidence as to where the 

silica was deposited, but its removal points to the circulation of water during alteration. Some of the bentonites of the Black Hills and Wyoming regions contain 

a large proportion of silica in finely granular or colloidal 

form, and there the silica seems not to have been removed during alteration.

Bentonite thus seems to form most readily from rock 

types normally rich in potassium and sodium, but with 

only moderate amounts of calcium and magnesium. 

Most bentonites, however, are low in potassium and 

sodium whereas at least some of them contain more 

essential calcium and magnesium than did the parent 

glass. The essential character of magnesium in highsilica montmorillonite—the type characterizing bentonites—has been pointed out in a preceding section. 

(See pp. 39, 40.) There is reason, therefore, to believe that such montmorillonite forms only where Mg 

is available in the rock or where it can be abstracted 

from ocean, lake, or ground waters. The fixation of 

calcium in preference to sodium in most bentonites is 

obviously due to the relation discussed under base exchange (see pp. 37, 38); that is, bivalent calcium is 

much higher than univalent sodium in the replacement 

series. The reason for the very thorough removal of 

potassium, in view of the possible formation of minerals 

of the bravaisite type of micaceous clays, is not so clear. 

The formation of montmorillonite necessitates the removal of very essential amounts of silica, and the presence of alkalies markedly increases its solubility. Thus 

there may be a marked tendency for escaping colloidal 

silica to flush alkalies out of the system, possibly before 

crystallization to montmorillonite. We know little, 

however, about the relative stability of montmorillonite 

and the bravaisite type of mica, and can only conclude 

that montmorillonite is, in general, the more stable 

mineral of the two under the conditions of formation of 

bentonite. The Ordovician bentonites mentioned on 

page 65 are composed of bravaisite, but they seem to 

represent an unusual type of alteration, and it is not 

clear whether their mica minerals formed directly or 

resulted from the recrystallization of montmorillonite 

in the presence of solutions that supplied K2O.

ORDOVICIAN BENTONITE

The Ordovician bentonites are made up of a clay 

material belonging to the bravaisite type which contain 

5 to 7 percent of K2O. These are composed of a mineral 

that does not belong to the montmorillonite group, but 

may be briefly mentioned. The term metabentonite 

has been applied to the potash-bearing Ordovician 

bentonites as a group (Kay, 1931). This term was 

suggested by Ross (1928) but only for those occurrences 

that gave definite evidence of metamorphism. As the 

Ordovician bentonites, with a few exceptions, are entirely unmetamorphosed, a term implying severe 

metamorphism is unsuitable.

The Ordovician bentonites have a very wide distribution and have been described by Nelson (1922) from 

Alabama, Tennessee, Kentucky, and Virginia; they 

have been reported also from Pennsylvania, New York, 

the Georgian Bay region of Ontario, and Minnesota;

and Ross (1928) has discussed the Virginia mate4al and 

its relations. Over very wide areas these bentorftes are 

characterized by abundant euhedral to subhedral 

crystals of biotite, which represent the only crystalline 

part of the ash fall, the igneous-rock phemcrysts. 

More rarely, they contain biotite, together with phenocrysts of orthoclase, and in a few places, as in some of 

the bentonite beds at Chickamauga Dam nea~ Chattanooga, Tenn., orthoclase and albite are tH only 

phenocrysts. Quartz phenocrysts are totally absent, 

and soda feldspars may or may not be present,- Thus, 

the mineral composition indicates that the parent rock 

of the Ordovician bentonites was a potash-rid latite.

VOLCANIC MATERIALS AT MAGNET 

COVE, ARK.

An unusually complete record of the paragenesis of 

a group of clay minerals is presented in the Magnet 

Cove area in Arkansas. About 13/£ miles northwest of 

Magnet post office, altered pyroclastic rocks have been 

exposed in the course of futile-mining operation.

Magnet Cove has long been a noted locality for interesting minerals and intrusive igneous roc^s, and 

recently Ross (1941) has described volcanic materials 

from that locality. Volcanic activity, widespread in 

Cretaceous time throughout the Gulf Coastal region, 

has been discussed by Ross, Miser, and Stephenson 

(1929).

Steam-shovel mining has exposed an agglomerate 

containing highly altered rock fragments of various 

types, the largest of which are 10 feet or more in diameter. Between these fragments is a fine-grained, nearly 

white bentonite. The included blocks represent volcanic materials, together, with intrusive and sedimentary rocks derived from underlying rock masses. The 

rock types represented, their variable composition and 

texture, the fragmental character, and the interstitial, 

altered volcanic ash, are evidence of violent explosive 

activity and indicate that the area was the throat of a 

volcanic vent.

The fine-grained interstitial materials were originally 

glass, but the larger rock fragments were all crystalline. 

The freshest of the igneous material is composed wholly 

of feldspar, but nepheline rocks characterize tin region 

and probably were also present, as was some type of 

mica-bearing rock.

The feldspar rock is composed entirely of pis gioclase 

whose composition is near the albite end of tin series. 

As nearly pure albite does not crystallize directly from 

a magma, the rock evidently has undergone albitization. 

The mica-bearing rock is so altered that its original 

character is not evident. The glassy material- are of 

two types, one originally a very fine grained ash, the 

other made up of pumice fragments, some of which 

were as much as 5 millimeters in diameter. (S<>e pi. 1, 

A,B.)

These materials have all been very thoroughly altered, seemingly by moderately hot solution^ or by 

volcanic vapors, which rose through the porous mass. 

Alteration products extend far below the zone of oxidation, as fresh pyrite is abundantly present; alteration, 

therefore, was not a weathering process. Permeation 

of the vent-filling materials by altering solution", which 

was facilitated by their porous character, promoted 

very thorough alteration. The secondary minerals 

formed were montmorillonite, kaolinite, hydrous micas, 

unusually abundant leucoxene (anatase), apatite, pyrite,
68 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

and rutile. The fresh mica remaining is phlogopite, 

part of which, at least, is evidently the result of early 

hydrothermal reactions. The fine-grained ash and the 

pumice fragments were first altered to montmorillonite 

and later underwent partial further alteration to 

kaolinite.

The different alteration of feldspar rock and glassy 

materials is probably due to the greater permeability of 

the glassy fragments! materials. The nearly pure feldspar rock was partly to completely replaced by montmorillonite, but it did not undergo further alteration 

to kaolinite. A representative specimen is illustrated 

in plate 1, A, B. The only other alteration of feldspar 

rock was the local development of micas and hydrous 

micas, which no doubt preceded the development of 

montmorillonite. The abundant leucoxene, which 

seems to have formed at the same time as the clay 

minerals, shows as black areas. (See pi. 2, A, B, C.) 

An X-ray examination of this material indicated that 

it was anatase. Most of the kaolinite is the result of 

further alteration of montmorillonite, as illustrated in 

plate 1, A, B, C, but a little has been derived from 

phlogopite, which was first altered to hydrous micas, 

as shown in plate 2, C. In none of the numerous specimens examined was there any direct alteration of feldspars to kaolinite.

The alteration of glass and feldspar rocks to montmorillonite was evidently brought about by primary 

solutions, or more probably by juvenile vapors as they 

escaped through the porous mass, and was similar to 

the alteration process at Yellowstone Park, which is 

described on pages 68-69.

The presence of abundant pyrite associated with 

rutile in the interstitial clay material shows that the 

clay was formed under reducing conditions, and the 

similarity of these relations to those described by Fenner at Yellowstone indicates that these solutions were 

neutral or alkaline. (See pp. 68-69.) Under these 

conditions montmorillonite was formed through the 

alteration of both glass and alkalic feldspars by juvenile 

vapors. Those vapors that came into contact with air 

would become acid through oxidation of H2S to H2S04. 

As volcanic activity waned the vapors would tend to 

condense and penetrate back into the porous aggregate 

of pyroclastic materials. In zones where acid conditions 

prevailed there would be further reactions that would 

change the earlier clay materials to kaolinite. The acid 

character would be confined to the upper part of the 

volcanic neck, and in depth only montmorillonite would 

be" expected.

The chemical analysis given below represents montmorillonite that was an alteration product of albite. 

A specimen was selected with only a small proportion 

of residual albite—one similar to but containing somewhat less albite than is shown in plate 2, B. This was 

dispersed in water with a mechanical stirrer, and sodium 

acetate was added to convert it to a sodium clay. After 

repeated washings, the clay portion remained in permanent suspension, and repeated overnight settling 

eliminated even the finest of the residual feldspar. 

After the separation the suspended material was treated 

with calcium acetate, to change it back to a calcium 

clay, and washed as before. Examination under the 

microscope showed that the resulting material was an 

unusually pure clay of the montmorillonite type. 

Washing, however, had evidently not removed all the 

calcium acetate, and probably other organic materials 

formed from the acetic acid by catalytic action of the

clay, and as a result 4.07 percent of organic material rnd 

an excess of CaO remained in the clay. Aside from tHs, 

however, the material was a typical bentonite.

Analysis of montmorillonite from Magnet Cove, Ark.

[K. J. Murata, analyst]

SiO2 ................. 49.33

AljO................. 17.40

FetOi................ 3.05

MgO................. 2.37

CaO..............

TiO2 .. 

H2O4.91

.04

11 04

H,O + ............... 7.74

F.................... .31

Organic impurity...... 4.07

Less O=F2 .

100.26 

...... .13

100.13

The following formula represents the chemical composition, assuming a normal amount of exchangeable 

Ca:

Ca.iT

T

[Alt'sBFetsMg.td [Si,+,4,Al.M]Oio[F,OH],

This formula would require about 1.98 percent of exchangeable CaO, indicating an excess of about 2.93 percent of CaO, which was combined, no doubt, with the 

portion reported as organic material. The foregoing 

chemical analysis of material from Magnet Cove is tl at 

of an almost typical montmorillonite and provider a 

clear example of a montmorillonite derived from 

feldspar.

The derivation of the clay minerals at Magnet Cc^e 

and their relation to one another is shown in plater 1 

and 2. Plate 2, A, represents a single feldspar crystal 

(white) partly replaced by montmorillonite (gray). Tie 

black areas are aggregates of leucoxene.

Plates-2, A and B represent a rock that was originally 

pure feldspar, but which has been so completely replaced 

by montmorillonite that only a few residual areas of 

feldspar remain.

Plate 1, B shows excellent preservation of the fibre ns 

structure of the pumice. The medium-gray areas r^e 

montmorillonite, and the lighter areas that accenturte 

the fibrous structure are kaolinite. On the left is an 

area where typical curved crystals of kaolinite have 

formed within the groundmass of montmorillonite 

(gray). The black is leucoxene.

Plate 1, A shows a sharper separation between are**s 

of montmorillonite (dark gray), and those of mixed 

montmorillonite and kaolinite 0ight gray). An area of 

curved kaolinite crystals occurs at the top of the figu-e. 

As in plate 1, B, the fibrous structure shows best in the 

areas partly altered from montmorillonite to kaolinite, 

but also is discernible in the areas of pure montmorillonite.

Plate 1, C shows typical curved and twisted crystals 

of kaolinite lying in a groundmass of montmorillonite.

Plate 2, C shows a mass that was originally phlogop;te 

partly altered to kaolinite. White areas are kaolinite 

and gray ones residual areas of phlogopite. Black are*>s 

are leucoxene.

CLAYS OF THE GEYSER REGION IN 

YELLOWSTONE PARK

The formation of clays at Magnet Cove, Ark., de^p 

in a volcanic neck and in the presence of escaping ma somatic emanations, bears a close resemblance to that in 

the geyser region of Yellowstone Park.
MINERALS OF THE MONTMORILLONITE GROUP 69

The geyser and hot-spring region was intensively 

studied between 1925 and 1935 by Alien and Day (1935) 

of the Carnegie Geophysical Laboratory. As a part of 

this study two areas were explored by drilling and the 

materials were studied by Fenner (1936). One of these, 

in the Norris Basin, was selected as representing an 

area of acid-sjilfate waters. In the well, pressures up to 

297Yi pounds per square inch and temperatures up to 

205° C. were recorded.

Alien's work had led him to believe that the acidity 

in such areas was a superficial condition, caused by the 

oxidation of H2S and that it gave way to the alkaline 

condition at a moderate depth. Fenner (1936), after 

detailed studies of the drill cores came to the following 

conclusions:

There is considerable evidence in descriptions given in the 

literature that kaolinite is formed in acid solutions. The beidellite-montmorillonite-nontronite group seems to be indicative of 

alkaline conditions, but the evidence is less positive. * * *

Four samples of material from the Norris hole were examined 

for the writer by Posnjak. In thoroughly leached surface rock

* * * Posnjak found kaolinite, together with much cristobalite, 

quartz, and tridymite. * * * From depth of 95 feet, white clay 

replacing phenocrysts of feldspar * * * was found by X-rays 

to be mostly kaolinite, with a little residual feldspar, but it may 

contain some beidellite also. * * * From 243 feet, creamcolored clay, from feldspar casts, was the beidellite type of clay.

* * * These results, considered in connection with the character and occurrence of the materials examined, show that in the 

surface rock feldspars have been wholly kaolinized. In the core 

from 95 feet only a few feldspar phenocrysts and * * * a little 

of the groundmass have been altered, and the clay is probably a 

mixture of kaolinite and beidellite. At greater depth the alteration has produced beidellite only. * * * The effect of acidity 

is thus apparent in the formation of kaolinite as far down as 95 

feet, and probably extends somewhat beyond that, but only a 

small amount of feldspar has been altered at this depth, and 

therefore the action is not vigorous. This does not imply, of 

course, that meteoric waters do not descend below the shallow 

zone of acid leaching. They doubtless descend much deeper, but 

at such a slow rate that the acidity that they tend to impart is 

overcome by the alkalinity of the bicarbonates. * * * In these 

rocks the beidellite clays in the casts are usually white, but some 

are light brown or light ?reen. From appearance and optical 

measurements they probably vary from nearly pure hydrated 

aluminum silicate to those members in which considerable ferric 

oxide and magnesia are present. * * * Destruction of feldspars 

and formation of the beidellite type of clay is apparently now the 

active process in the deeper levels at Norris. It is going on slowly 

and has not yet brought about sufficient changes to obscure previous processes, but the clay penetrates almost everywhere

* * * An interesting feature is the close association of beidellite with pyrite. In innumerable instances the casts of feldspars 

replaced by clay contain bright little cubes of pyrite, and portions 

of proundmass that contain an unusual amount of clay are likely 

to have much pyrite. This does not always hold, but the association is so common that one feels that the same alkaline solutions that formed the clay introduced the pyrite. * * * In the 

Norris cores it appears that the alteration of feldspars to beidellite is now in operation. It has followed rather than preceded 

base exchange in the feldspars.

Thus, Fenner presents evidence that kaolinite was 

formed at higher levels where alteration took place 

under influence of acid waters, and that at deeper 

levels where the waters were alkaline, beidellite-montmorillonite formed.

The relations between montmorillonite and kaolinite 

at Magnet Cove and Yellowstone Park illustrates the 

essential factor controlling their formation. Kaolinite 

is most commonly the clay mineral derived from alkalic 

feldspars, but this is the result of formation in the absence of bases or under conditions that favor their 

removal. Genetic processes taking place under hydrothermal or vapor-phase conditions are commonly 

characterized by the introduction of bases including 

magnesium and iron. Where these bases are introduced,

minerals of the montmorillonite group tend to form; 

where they are removed, kaolinite forms.

The relationships at Magnet Cove and in the Yellowstone hot-spring area show that feldspar alters to a clay 

mineral of the montmorillonite type under the influence 

of vapors or solutions, which add bases to the system 

and which are alkaline or alkalic in character. On the 

other hand, the discussion of soil formation indicated 

that feldspathic rocks tend to alter to kaolinitic material 

where leaching is the dominant process. It is evident 

that the controlling factor in the formation of clays of 

the montmorillonite type is the availability of bases, of 

which magnesium is the most essential. Granitic rocks 

are low in ferromagnesian minerals, and ever where 

these are present in the parent rock, the iron and magnesium tend to be removed under leaching corditions. 

In the absence of these bases, kaolinite forms. In the 

presence of mineralizing solutions these bases are commonly contributed to the system, and feldspar that was 

originally without essential amounts of associated 

minerals, containing either iron or magnesiuir, alters 

to montmorillonite.

VEIN AND GOUGE CLAYS

Montmorillonite-beidellite is the mineral that forms 

gouge clays in some mineral veins. In many of these 

it is evidently a hydrothermal mineral formed during 

the deposition of the vein, but in others it seems to be 

an alteration product of aluminous minerals which 

were reduced to extreme fineness during fault movements and thus rendered easily alterable. OtNr vein 

clays seem to have been deposited from colloid1 al solution during a very late stage of mineralization, or even 

from cold waters. In some veins the gouge clay is later 

than mineralization and seems to have been deposited 

in open spaces by percolating waters, which entered 

through such minute capillary spaces that all but the 

finest fractions representing colloidal clay material were 

filtered out. The beidellite (Nos. 44,47, tables 1 and 10) 

from the type locality, Beidell, and from Wagon Wheel 

Gap, Colo., and the varved clay from Fairviev7, Utah 

(No. 46; see pi. 2, D), .seem to be clays of this type. 

They form large platy masses whose optical properties 

closely simulate those of single crystals. This simulation 

is due to the uniform optical orientation of the minute 

crystal plates that make up the larger platy masses. 

The most thorough mixing with a spatula does not 

destroy the anisotropic character of the moist clay. It 

merely drags the submicroseopic plates out into oriented 

smears, which continue to show the characteristic high 

birefringence of the original aggregates. Many samples 

of clays of this group show the same property in lesser 

degree.

Other vein-forming clays are No. 35, Twin. Falls, 

Idaho (see pi. 3, B); No. 49, Carson District, Idaho; 

Nos. 39 and 68, Maniquipi, Mexico; No. 54, Roseland, 

Va.; No. 13, Irish Creek, Va.; and No. 41, Glen Riddle, 

Pa. The clay from Irish Creek, Va., is the result of very 

thorough alteration along a shear zone several feet wide 

within granodiorite. This zone lies only a few feet from 

cassiterite-bearing veins, but the absence of ary other 

products of mineralization associated with the montmorillonite shows that its deposition was later tl an that 

of the cassiterite and may have been an unrelated 

episode.

The color of the vein clays when free frorr colorimparting impurities, is commonly bluish gray o~ slightly greenish. The most important impurities are hydrous
70 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

ferric iron (limonite), which gives yellowish colors, and 

ferric iron (hematite), which gives red colors. It is commonly believed that greenish colors are due to ferrous 

iron, but ferric iron in combination, as in some silicates, 

may give tints that appear to be greenish rather than 

red. In discussing colors, however, it must be remembered that very dark yellows appear to the eye to have 

greenish shades, when in fact no green is present; thus, 

olive green (23 yellow M of the Ridgway Color Standard) contains no green whatever. Nontronite, however, 

is chalcedony yellow (25TG-Y,b of the same color 

standard) and does contain a green component. The 

Ordovician bentonites are commonly blue-green, a color 

no doubt due to the combined effect of iron and potassium.

The pegmatite clays represented by samples 36 and 

42 from Pala, Calif, (see pi. 7, B), are of slightly different 

origin. The Pala clay occurs in the gem-bearing pegmatites of San Diego County, Calif., where it is at least 

in part an alteration product of feldspars, spodumene, 

and tourmaline. W. T. Schaller (personal communication) believes that this alteration was probably a late 

hydrothermal process, though large masses of the clay 

give no clear evidence of its mode of origin. Similar 

clays have been described by Brush and Dana (1880), 

who evidently believed that they were formed by 

mineralizing solutions. An analysis of this clay is given 

in table 1, No. 33, and one of a similar clay from Greenwood, Maine, in No. 28.

Nontronite appears to be widely but sparsely distributed. Much of it is associated with mineral veins, 

and a number of samples have come from small veins 

of unknown origin in the schists of the southern Piedmont region of this country. The beidellite-nontronite 

from Spokane, Wash., is an alteration product of basaltic volcanic glass. It would be interesting to know if it 

first altered to the palagonite mentioned on page 65 and 

later assumed the crystalline form.

The clay called faratshite from Madagascar (Lacroix, 

1914) has been shown by Hendricks (1939) to be a 

mixture of kaolinite and nontronite.

Saponite also seems to be most commonly associated 

with mineral veins, but the related hectorite from 

Hector, Calif., described by Foshag and Woodford 

(1936), is an exception, being derived from bentonitic 

material. Saponite is reported also by Meddle (1879) 

from amygdaloidal cavities in basalt, especially from 

Scotland where it occurs associated with celadonite. 

The same association has been noted in basalt from 

Reno, Nev., where it appears to have developed at a 

late stage in the cooling of the extrusive rock, and thus 

seems to be a deuteric mineral.

PYROPHYLLITE AND TALC

Pyrophyllite and talc are not commonly grouped with 

clay minerals, but the relationship of pyrophyllite to 

montmorillomte, and of talc to hectorite, makes a comparison advisable. Pyrophyllite occurs as foliated, often 

radiating groups or as the fine-grained massive material 

that was named by Hermann (1829). Talc has long 

been known, having been referred to as "magnetis" by 

Theoprastus about 315 B. C. in his work, "On stones." 

The name talc also is old, being derived from the Arabic. 

Talc occurs in talc schists, as massive material, and in 

beautiful foliated masses usually of a pale apple-green 

color and commonly associated with hydrothermally 

altered serpentines and dunites.

Analyses of several pyrophyllite and talc specimens

are listed in table 20. Only a few recent analyse."" of 

talc and pyrophyllite are available, but the older ones 

given by Dana (1892) no doubt are good, and all asree 

so consistently that those quoted in this paper (table 28) 

are believed to be representative. They have l«en 

selected from the published analyses because of their 

low alkali and alkaline-earth content. All specimens 

represented by these analyses consist of well-crystall: zed 

material; they include the radiated pyrophyllite f~om 

the Hemp mine, Moore County, N. C., and the large 

talc folia from Harford County, Md.

Formulas, derived from the analyses are given beT ow. 

In these particular talcs, A1+ 3 is substituted to a 

slight extent for Si+4 in tetrahedral coordination. 

The valency is balanced by substitution of Fe+3 or A1+3 

for Mg+2 in octahedral coordination. The numbe~ of 

ions having octahedral coordination does not differ 

significantly from 3.00. -The slightly higher value, 3,01, 

for the Harford County, Md., specimen is within the 

analytical error but is probably due to the presence of 

a slight amount of ferric iron, which was included as 

ferrous in the analysis. A summation greater than 3.00, 

of course, is not possible according to the present concept of the talc structure.

In the pyrophyllite samples, also, A1+3 proxies to a 

slight extent for Si+4 in tetrahedral coordination. The 

resulting decrease in the positive charge is balanced by 

an increase above 2.00 in the ions haying octahedral 

coordination. Though this increase is slight and within 

experimental error it probably is real.

TABLE 20.—Analyses of pyrophyllite and talc

SiOj. .........

AhOs.........

FeuOa. ........

FeO..........

MgO.........

Na 2O ..........

H20. .........

Total.. ....

Pyrophyllite

1

66.04 

28.25 } •"

.12

.06 

5.02

100.13

2

64.53 

29.40 

/ .00 

\ .67

.28 

5.45

100.33

3

66.25 

27.91 

1.08

5.25

100.49

Talc

4

58.68 

3.75

5.52 

26.80

5.33

100.08

5

61.37 

1.96

1.47 

30.23

5.37

100.40

6

62.63 

.55 

1.07

31.36

4.84

100.45

1. Moore County, N. C. Ellen Rist, analyst (unpublished analysis, made in the 

laboratory of the Bureau of Plant Industry).

2. Moore County, N. C. Charles Baskerville, analyst (Pratt, 1900, p. 26!\

3. Carbonton, Moore County, N. C. Oscar D. Alien, analyst (Brush, 18S2, 

p. 219).

4. Harford County, Md. J. G. Fairchild, analyst (Wells, 1937, p. 112).

5. Fichtelgebirge, Bavaria. J. G. Fairchild, analyst (Wells, 1937, p. 112).

6. Deep River, N. C. H. C. McNeil, analyst (Clarke, 1910, p. 295).

Chemical formulas 

PYROPHYLLITE

1. Moore Co., N. C.

+3 + 2 + 3 

[Ali.gyMg.oiFe.os] j

2. Moore Co., N. C.

+3 + 2 +4

Ions having

octahedral

coordination

2 . 01

[AlooFe. d [Si 3 9oAl.o]Oio[OH] 2Na.04. .............. 2 03

3. Deep River, Moore Co., N. C.

................... 2.01

4. Harford Co., Md.

5. Fichtelgebirge, Bavaria

TALC

3.01

[Si3t94iAl.yOio[OH] 2 ............... 3 . 00

6. Deep River, N. C.

[Sis!,4 Al.8+4]O 1 o[OH] 2 ................... 3 . 00
MINERALS OF THE MONTMORILLONITE GROUP 71

One of the pyrophyllite specimens from Moore 

County, N. C., contains a small amount of sodium. 

This is included in the formula as it would be in a 

paragonite mica. Although the amount of sodium in 

this sample is small, both alkali and alkaline-earth ions 

are present to an appreciable extent in many pyrophyllite and talc analyses. A specimen of beautifully 

bladed pyrophyllite from the Hemp mine, Moore 

County, N. C., analyzed by R. C. Wells, is typical. The 

sample was clean platy pyrophyllite containing no 

observable mica, and it seems probable that the potassium in its analysis, as given below, is present in mixed 

layers and not as sericite.

Analysis of pyrophyllite from Moore County, N. C.

[R. C. Wells, analyst]

SiO2 ................. 64.32

AliOi................ 29.05

Fe 2Q*................ .20

MgO................. .05

CaO................. None

K2O................. 1.20

Na 2O................ None

TiO2 ..... ............ .30

H2O................. 5.27

100.39

The chemical formula calculated from this analysis 

is as follows:

In this sample about 10 percent of the layers would 

be micalike. It is probable that these micalike layers 

would only statistically affect the X-ray diffraction 

pattern, which would not be expected to be appreciably 

different from that of pure pyrophyllite. Thus C 0 would 

te expected to change from 8.90 A., as characteristic of 

pyrophyllite, to 9.00 A., while a 0, b 0, and @ would not 

necessarily be changed.

The finer-grained talc and pyrophyllite samples often 

might contain adventitious materials. Massive talc, 

especially, contains fine-grained chlorites as impurities, 

but these impurities are not likely to be found in the 

foliated varieties. An analysis of a foliated talc from 

Huntersville, Fairfax County, Va., is given below 

(Clarke, 1915).

Analysis of talc from Huntersville, Va.

[E. A. Schneider, analyst (Clarke, 1910, p. 295)]

MgO........... ...... 30.95

MnO................. Tr.

H,O-... ............ .07

H2O+... ............ 4.84

SiOi................. 62.27

A1 2O3 ................ .15

Fe 2O3 ................ .95

FeO.................. .85

100.08

The formula percentages calculated from the foregoing analysis are as follows:

+3

0.985[Fe.o+42Fe.o4Mg2. 9 i]+ [Si 3 .9 9Al.o 1]01 o[OH] 2 

.015[Mg[OH]2]

The calculation giving an excess of magnesium merely 

shows that a small fraction of chlorite layers is present 

in the specimen.

Pyrophyllite differs in composition from the analogous montmorillonite in several important ways. Very 

little water is lost at low temperatures; in other words, 

interlayer water is absent. It contains no Ca+2, and 

little or no Na+2 or other replaceable bases, those 

present being essentially nonreplaceable; furthermore, 

it shows no appreciable admixture with-a hypothetical 

ferric end member. The Mg+2 content also is very 

small. Montmorillonite, on the other hand, invariably 

contains appreciable amounts of magnesium and iron 

and offers evidence of complete solid solutions between 

the Fe+3 and Al+3 end members.

Talc similarly differs from the analogous sappnite. 

In talc, as in pyrophyllite, replaceable bases and interlayer water are absent. Although small but appreciable 

amounts of A1+ 3 and Fe+3 or Fe+2 are present in some 

specimens, there is no indication of a continue us series 

of solid solutions with an aluminum end member like 

pyrophyllite, or with an analogous beidollite-like 

material. Thus, it seems obvious that the essential difference between montmorillonite and saponite and between pyrophyllite and talc is in the substitution of ions 

that characterize members of the montmorillonite 

group. The most essential of these substitutions are 

Al for Si in tetrahedral positions and of Mg for Al in 

octahedral positions. Both induce valence deficiencies, 

which are balanced by the exchangeable ions between 

the sheets. Associated with these bases is interlayer 

water. The X-ray defraction patterns of montmorillonite and pyrophyllite and of saponite and talc are 

similar except for the (OOZ) reflections, and tin substitutions described involve no great chemical differences; 

but they have fundamentally altered the physical properties of the two groups of minerals.

A logical explanation for these differences between 

pyrophyllite and montmorillonite on the one band and 

talc, hectorite and saponite on the other, is probably 

afforded by the lattice limitation of montmorillonite, 

which, in turn, is due to, the presence of the exchangeable 

ions. Thus there need not be exact matching in dimensions between successive layers in montmorillonite. The 

well-crystallized talcs and pyrophyllites, on the other 

hand, require lattice continuity to macroscopic dimensions. For this reason, too, they probably are far more 

limited in the extent and kind of mixed-layer lattices 

that they might form. Pyrophyllite would be expected 

to form mixed layers with muscovite-type micas but not 

with clorite or brucite, whereas talc would form mixed 

layers with phlogopite-type micas, chlorite, and brucite 

but not with pyrophyllite or muscovite mica.

Talc and pyrophyllite are the results of mets morphic 

or hydrothermal processes. Talc is commorly associated with graphite, serpentine, or chlorite; and pyrophyllite is commonly associated with sericite. Both 

probably are formed at moderately high temperature or, 

commonly, as a result of dynamic metamorphism. 

These conditions would tend to inhibit the interlayer 

water characteristic of montmorillonite and saponite. 

Talc and pyrophyllite have yet to be found a^ constituents of soils that characteristically are formed by 

low-temperature weathering.' Montmorillonite, on the 

other hand, is predominantly a product of weathering 

formed in the presence of liquid water. Where there is 

evidence that minerals of the montmorillonite group 

were formed by thermal processes, there is every reason 

to believe that they were deposited late in the paragenetic sequence, when the temperature was low— 

that is, as much as 200° C. (pp. 68-69) but in general 

below that temperature. Under these conditions interlayer water and exchangeable cations are present. The 

mode of origin of montmorillonite and saponite, therefore, is dependent on conditions that permit th<> formation of materials characterized by interlaye" water, 

solid solution between an unusually large grouo of end 

members, and the exchange of bases; in contrast, the 

origin of pyrophyllite and talc is dependent on conditions of high temperature and high pressure, which 

inhibited interlayer water, permitted very limited 

isomorphism, and so precluded the presence of exchangeable bases.
72 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

BIBLIOGRAPHY

AARNIO, B.

1938, Uber die Tone Finnlands und ihre Eigenschaften: 

[Finland] Voltion Maatukimuslaitos Agrogeologiska 

Julkaisu, vol. 46, 27 pp. 

ALDRICH, D. G., HELLMAN, N. N., and JACKSON, M. L.

1943, Hydration control of montmorillonite as required for its

identification and estimation by X-ray diffraction

methods: Soil Sci. America Proc., vol. 57, pp.

215-231.

ALEXANDER, L. T., FAUST, G. T., HENDRICKS, S. B., INSLEY,

HERBERT, and McMuRDiE, H. F. 

1943, Relationship of the clay minerals halloysite and endellite:

Am. Mineralogist, vol. 28, pp. 1-18: 

ALEXANDER, L. T., HENDRICKS, S. B., and FAUST, G. T.

1941, Occurrence of gibbsite in some soil-forming materials:

Soil Sci. Soc. America Proc., vol. 6, pp. 52-62. 

ALEXANDER, L. T., HENDRICKS, S. B., and NELSON, R. A. 

1939, I, Minerals present in soil colloids; II, Estimation in 

some representative soils: Soil Sci. Soc. America 

Proc., vol. 48, pp. 273-279. 

ALLEN, C. T., and DAY, A. L.

1935, Hot Springs of Yellowstone National Park: Carnegie

Inst. Washington Pub., pp. 1-525. 

ANGEL, F.

1913, Uber Wolkonskoit: Zeitschr. Kristallographie, Band 52,

pp. 568-579. 

ANTIPOV-KARATAEV, J. N., and BRUNOVSKY, B. K.

1936, Chemische und rontgenographische Untersuchungen 

kolloider Fraktionen einiger Bodenarten: Kolloid 

Zeitschr., Band 75, pp. 325-337.

ARDENNE, M. V., ENDELL, KURD, and HOFMANN, ULRICH. 

1940, Deutsche Keram, Gesell. Ber., Band 21, pp. 209-227, 

Investigation of the finest fraction bentonite and 

clay soil, with the universal electron microscope: 

Ceramic Abs., vol. 24, p. 154, 1941. 

BAYLEY, W. S.

1920, Kaolin in North Carolina, with a brief note on hydromica: Econ. Geology, vol. 15, pp. 236-246. 

BERGMANN, C.

1857, Gramenit: Neues Jahrb., Jahrg. 1857, pp. 395-397. 

BERNHARDI, J. J., and BRANDES, RUDOLPH.

1822, Mineralpgische-chemische Untersuchung zwei neuer 

ung^arishes Mineraolien des muschligen und des 

erdigen Chloropals: Jour. Chemie und Physik (Beitr. 

Chemie und Physik), Band 5, pp. 29-44. 

BERTHIER, PIERRE.

1926, Analyse de L'halloysite: Annales de chimie et de

physique, tome 32, pp. 332-335.

1827, Nontronite noveau mineral: Annalin de chimie et de 

physique, tome 36, pp. 22-27. Also a translation, 

Description of nontronite, a new mineral discovered 

in the Department of Dordogne: Edinburgh Jour. 

Sci., vol. 10, pp. 150-153, 1829. 

BRADLEY, W. F., GRIM, R. E., and CLARK, W. L.

1937. A study of the behavior of montmorillonite upon wetting: Zeitschr. Kristallographie, Band 97, pp. 

216-22. 

BRAGG, W. H.

1938. Clay: Royal Inst. Great Britain Proc., vol. 30, pp.

39-67. 

BRAGG, W. L.

1932, The structure of silicates, 2d ed., Akademische Verlagsgesellschaft, Leipzig. 

1937, Atomic structures of minerals, Cornell Univ. Press,

pp. 37-40. 

BRAMMALL, ALFRED, and LEECH, J. G. C.

1937, Some aspects of clay chemistry: Royal Coll. Sci.

London, Sci. Jour., vol. 7, pp. 69-78. 

1940, in collaboration with Dunstall, W. J., Montmorillonite

in fuller's earth, Nutfield, Surrey: Geol. Mag., vol.

77, pp. 102-112. 

BRAY, R. H.

1929, Base-exchange capacity studies of certain soil types in

Illinois: Am. Soil Survey Assoc. Bull. 11, pp.

161-168. 

1937, Chemical and physical changes in soil colloids with

advancing development in Illinois soils: Soil Sci.,

vol. 43, pp. 1-14. 

BRAY, R. H., GRIM, R. E., and KERB, P. F.

1935, Application of clay mineral technique to Illinois clay

and shale: Geol. Soc. America Bull., vol. 46, pp.

1909-26.

BREITHAUPT, AUGUST.

1829, Pinguit: Jour. Chemie und Physik, (Jahrb. Cto^mie

und Physik), vol. 55, pp. 303-304. 

1832, Vollstandige Charakeristik des Mineral-System, 2d ed.,

p. 99, Dresden. 

1841, Vollstandiges Handbuch der Mineralogie, Band 2, p.

544, Dresden and Leipzig. 

BRUSH, G. J.

1862, Tenth Supplement to Dana's Mineralogy: Am. Jour.

Sci., vol. 34, 2nd ser., p. 219. 

BRUSH, G. J., and DANA, E. S.

1880, On the mineral locality at Branchville, Conn., Spodumene and its alteration products: Am. Jour. Sci., 

vol. 20, 3d ser., p. 284. 

BUSWELL, A. M., and DUDENBOSTEL, B. F.

1941, Spectroscopic studies of base-exchange materials: Am.

Chem. Soc. Jour., vol. 65, pp. 2554-2558. 

CAILERE, SIMONNE.

1935, Sur les caracteres specifiques de la bowlingite: Acad. 

sci. Paris Comptes rendus, tome 200, pp. 1483-1485. 

CALDWELL, 0. G., and MARSHALL, C. E.

1942, A study of some chemical and physical properties of the 

clay minerals nontronite, attapulgite, and saponite: 

Univ. of Missouri, Coll. Agr., Agr. Exper. Sta. 

Research Bull. 354, pp. 1-51. 

CAYEUX, LUCIEN.

1916, Introduction a 1'etude petrographique des riches 

sedimentaire: Paris, Imprimerie nationale, pp. 

230-232. 

CLARK, G. L., RIEKEN^ F. F., and REYNOLDS, D. H.

1937, X-ray diffraction studies of two micron fractions of 

some genetic soil profiles: Zeitschr. Kristallographie, Band 96, pp. 273, 286. 

CLARKE, F. W.

1910, Analyses of rocks and minerals: U. S. Geol. Survey

Bull. 419.

1915, Analyses of rocks and minerals: U. S. Geol. Srrvey 

Bull. 591, p. 339.

COLLINS, J. H.

1877, Remarks on gramenite from Smallcombe, anc1 the 

chloropal group of minerals: Mineralog. Mag., vol. 

1, pp. 67-72. 

CORNET, I.

1943, Sorption of NHS on Montmorillonite: Jour, of C^em.

Physics, vol. 11, pp. 217-226. 

CORRENS, C. W.

1933, Uber die Bestandteile der Tone: Deutsche geol.

Gesell. Zeitschr., Band 85, pp. 706-12. - 

1936, Die Petrographie der Tone: Naturwissenschaften,

Jahrg. 24, pp. 117-24.

1938a, Die Tone: Geol. Rundschau, Band 29, pp. 20T-219. 

1938b, Zur Frage der Neubildung von Glimmer in jrngen 

Sedimenten: Geol. Rundschau, Band 29, pp. 

220-222. 

CORRENS, C. W., and MEHMEL, MARTIN.

1936, Uber den optischen und rontgenographischen Nachweis 

von Kaolinit, Halloysit und Montmorillonit: Zeitschr. Kristallographie, Band 94, pp. 337-348. 

CORRENS, C. W., and SCHLUNZ, F. K.

1936, Mineralogische Untersuchungen dreier Mecklenburg'scher 

Boden: Zeitschrift fur Pflanzenerahrung, Dimming 

und Bodenkunde. Band 44, pp. 316-26. 

CORRENS, C. W., and SCHOTT, WOLFGANG.

1933, Uber den Einfluss des Trocknens auf die Korngrossenverteilung von Tonen: Kolloid Zeitschr., Band 65, 

pp. 196-203. 

Cox, S. H.

1882, On a new mineral belonging to the serpentine group:

New Zealand Inst. Trans. vol. 15, p. 409. 

CRONSTEDT, AXEL.

1758, Mineralogie, Stockholm, English translation by Magellan, 

John Hyacinth de, An essay toward a system of 

mineralogy, p. 98, London, 1788. 

DAMOUR, A. A., and SALVETAT——.

1847, Et analysis sur un hydrosilicate d'alumine trouve a 

Montmorillon (Vienne): Annales de chemie et de 

physique, 3d ser., torn 21, pp. 376-383. 

DANA, E. S.

1892, System of mineralogy, 6th ed., New York, John Wiley

& Sons. 

DARTON, N. H., and SIEBENTHAL, C. E.

1909, Geology and mineral resources of Laramie Basin, Wyo.: 

U. S. Geol. Survey Bull., 364, pp. 34, 58-61.
MINERALS OF THE MONTMORILLONITE GROUP 73

DENISON, I. A., FRY, W. H., and GILE, P. L.

1929, Alteration of muscovite and biotite in the soil: U. S. 

Dept. Agr., Tech. Bull. 128, 32 pp.

DUFRENOY, A.

1856, Traite de minfralogie, 2 ed., tome 3, p. 385, Paris. 

DUNN, L. R. L.

1938, Note on some bentonitic clays from Hawkes Bay, New 

Zealand: New Zealand Jour. Sci. Tech., vol. 20 

(33), pp. 162-165. 

EAKLE, A. S., and ROGERS, A. F.

1914, Wilkeite, a new mineral of the apatite group, and 

okenite, its alteration product, from southern 

California: Am. Jour. Sci., 4th ser., vol. 37, pp. 

262-267. 

EDELMAN, C. H.

1935. Relations between the crystal structure of minerals and 

their base-exchange capacity: Third Internat. Cong. 

Soil Sci. Trans., vol. 3, pp. 97-99, London.

1936. Problemes mineralogiques se rapportant a 1'etude du 

sol: Congr. Internat. Mines, 7, 1935, pp. 577-595.

1938. Ergebnisse der sedimentpetrologischen Forschung in den 

Niederlanden und den angrenzenden Gebieten, 

1932-1937: Geol. Rundschau, Band 29, pp. 223-271. 

EDELMAN, C. H., BAREN, F. A., VAN, and FAVEJEE, J. CH. L.

1939. I, General discussion of the mineralogical composition 

of clays and qualitative X-ray analysis of some Dutch 

soils: Mededeelingen Landbouwhoogeschool Wageningen, Deel 43, pp. 1-33, Wageningen, H. Veenman 

und Zoene. 

EITEL, WILHELM, and RADCZEWSKI, O. E.

1940. On recognition of montmorillonite clay minerals in 

supermicroscope pictures: Naturwissenchaften, vol. 

28, pp. 397-398; Chem. Abstracts, vol. 35, p. 3195, 

1941. 

EKBLAW, G. E., and GRIM, RALPH E.

1936, Some geological relations between the constitution of 

soil materials and highway construction: Illinois 

Geol. Survey Rept. Investigations No. 42, pp. 5^-16. 

ENDELL, JOACHIM.

1941, The study of the fine structure of clays by means of the 

super-microscope: Keram. Rundschau, vol. 49, pp. 

23-26; Chem. Abstracts, p. 4558, 1941.

ENDELL, KURD, HOFMANN, ULRICH, and MAEGDEFRAU, EDMUND. 

1935, Uber die Natur des Tonantiels in Rohstoflfen der deutchen Zement-Industrie: Zement, Band 24, pp. 

625-632. 

ENDELL, KURD, HOFMANN, ULRICH, and WILM, DIEDERICH.

1933. Uber die Natur der keramischen Ton: Deutsche 

Keram. Gesell, Ber., Band 14, pp. 407-438.

1934. Uber Quartz und glimmerhaltige Tone: Sprechsaal.

Band 67 [20] 24 pp. 

ENGELHARDT, WOLF VON.

1937a, Mineralogische Beschreibung eines mecklenburgischen

Bodenprofils: Chemie der Erde, Band 11, pp. 17-37.

1937b, Uber silikatische Tonminerale: Fortschr. Mineralogie,

Band 21, pp. 276-337. 

ERDELYI/JANOS.

1927, Unknown hydrosilicate gel from the Rac quarry near 

Szekes-Fejevar, Magyar Chemiai Folydirat, Budapest, 

vol. 33, pp. 133-135. 

EWELL, R. H., and INSLEY, HERBERT.

1935. Hydrothermal synthesis of kaolinite, dickite, beidellite, 

and nontronite: U. S. Nat. Bur. Standards, Jour. 

Research, vol. 15, pp. 173-185. 

FAVEJEE, J. CH. L.

1939a, [General discussion of the mineralogical composition 

of clays], II, Quantitative X-ray analysis of some 

Dutch soils: Mededeelingen van de Landbouwhoogeschool, Wegeningen, Deel 43, pp. 43-51, 

Wegeningen, H. Veenman & Zonen.

1939b, Zur Methods der rontgenographischen Bodenforschung: 

Zeitschr. Kristallographie, Band 100, pp. 425-436.

1939c, Quantitative rSntgenographische Bodenuntersuchung:

Zeitschr. Kristallographie, Band 101, pp. 259-270. 

FENNER, C. N.

1936. Bore-hole investigations in Yellowstone Park: Jour.

Geology, vol. 44, no. 2, pt. 2, pp. 225-315. 

FISHER, C. A.

1905. The bentonite deposits of Wyoming: U. S. Geol. 

Survey Bull., 260, pp. 559-563.

1906. Geology of Bighorn Basin, Wyo.: U. S. Geol. Survey

Prof. Paper 53, pp. 29, 56-58. 

FLEISCHER, MICHAEL.

1943, Illite (= Bravaisite): Am. Mineralogist, vol. 28, p.

214; also, vol. 28, p. 470.

FOSHAG, W. F., and WOODFORD, A. O.

1936, Bentonite magnesium clay mineral from California:

Am. Mineralogist, vol. 21, pp. 238-44. 

GALPIN, S. L. 

1912, Studies of flint clays and their associates: Anr, Ceramic

Soc. Trans., vol. 14, pp. 301-346. 

GEDROIZ, K. K.

1929, Der asorbierende Bodencomplex und die abs-orbierten, 

Bodkenkationen als grundlage der ge7»etischent 

Bodenklassifikation: Kolloidchem. Benefte, pp. 

149-260. 

GEILMANN——, and ROSE, HERMAN.

1927, In Rosenbusch, Harry, Mikroskopische PhyMographie 

der Mineralien und Gesteine, Band 1, 2.d Halfte, 

spezieller Teil, p. 594. 

GEISEKING, J. E.

1939, Mechanism of cation exchange in the montm oriHonitebeidellite-nontronite type of clay minerals: Soil 

Sci., vol. 47, pp. 1-12. 

GIBSON, J., RILBY, H. L., and TAYLOR, J.

1944, Filamentous carbon: Nature, vol. 154, p. 544. 

GLOCKER, E. F.

1839, Handbuch der Mineralogie, 2d ed., p. 537, Nurnberg. 

GRAHAM, R. P. D.

1918, Saponite from the Canadian Northern Railway tunnel, 

Montreal: Royal Soc. Canada Trans., 3rd ser., vol. 

12, pp. 199-201. 

GRIM, R. E.

1933. Petrography of fuller's earth deposits, Olnrsted, 111., 

with a brief study of some non-Illuurs .earths: 

Econ. Geology, vol. 28, pp. 344-63.

1934. Petrographic study of clay minerals—a laboratory note: 

Jour. Sedimentary Petrology, vol. 4, pp. 45-47.

1935. Petrology of the Pennsylvania shales and noncalcareous 

underclays of Illinois: Am. Ceramic Soc. Bull., vol. 

14, pp. 113-19, 129-34, 170-76.

1939, Relation of the constitution to the properties of clays: 

Am. Ceramic Soc. Jour., pp. 141-151.

1941. The clay minerals in soils and their significance: Illinois 

State Geol. Survey, Circ. 65, pp. 9-14.

1942. Modern concepts of clay minerals: Jour. Geo1 ., vol. 50,

pp. 225-275. 

GRIM, R. E., and ALLEN, V. T.

1938, Petrology of the Pennsylvanian underclays of Illinois: 

Illinois State Geol. Survey Rept. of Investigations, 

no. 52, pp. 1485-1513. 

GRIM, R. E., and BRAY, R. H.

1936, Mineral constitution of various ceramic clrys: Am.

Ceramic Soc. Jour., vol. 19, pp. 307-15. 

GRIM, R. E., BRAY, R. H., and BRADLEY, W. F.

1936. The constitution of bond clays and its influence on 

bonding properties: Am. Foundrymen's Assoc. 

Trans., vol. 7, pp. 211-28.

1937. Mica in argillaceous sediments: Am. Mireralogist,

vol. 22, pp. 813-29. 

GRIM, R. E., LAMAR, J. E., and BRADLEY, W. F.

1937, The clay minerals in Illinois limestones and dolomites:

Jour. Geology, vol. 45, pp. 829-43. 

GRIM, R. E., and ROWLAND, R. A.

1942, Differential thermal analyses of clay minerals and other 

hydrousm inerals: Am. Mineralogist, vol. 127, pt. 1, 

pp. 746-761; pt. 2, 801-818. 

GRIM, R. E., and ROWLAND, R. A.

1944, Differential thermal analysis of clays and shales, a control and prospecting method: Am. Cerrmic Soc. 

Jour., vol. 27, pp. 65-76. 

GRUNER, JOHN W.

1934a, The structure of vermiculites and their collapse by 

dehydration: Am. Mineralogist, vol. 19, pp. 557- 

575. 

1934b, The crystal structure of talc and pyrophyll'te: Am.

Mineralogist, vol. 19, pp. 537-74. 

1935, Structural relationships of nontronites and nrontmorilIpnites: Am. Mineralogist, vol. 20, pp. 4T5-83. 

1940a, Cristobalite in bentonite: Am. Mineralogist, vol. 25,

pp. 587-590.

1940b, Abundance and significance of cristobalite in bentonite 

and fuller's earths: Econ. Geologist, vol. 35, pp. 867-875. 

HAGNER, A. F.

1939, Adsorptive clays of the Texas Gulf Coast: Am. Mineralogist, vol. 24, pp. 67-108.

HANNEY, J. B.

1877, On bowlingite, a new Scottish mineral: Mineralog. 

Mag. vol. 1, pp. 154-157.
74 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

HARDON, H. J.

1939, Investigation on the composition of the clay fraction of 

the most important soil types of the Dutch East 

Indies: Mededeelingen van het algemeen Proefstation voor den Landbouwhoogeschool, Deel 37, 

Bintenzord.

HARDON, H. J., and FAVEJEE, J. CH. L.

1939, Mineralogic investigations of clays and clay minerals, 

III. Qualitative X-ray analysis of the clay fraction 

of the principal soil types of Java: Mededeelingen 

van de Landbouwhoogeschool te Wageningen, Deel. 

43, Wageningen, H. Veenman & Zonen. 

HARDY, F., and FOLLETT-SMITH, R. R.

1931, I. Studies in Tropical Soils; II. Some characteristic 

igneous rock soil profiles in British Guiana, South 

America (includes citations of the studies of Harrison, 

J. B.): Jour. Agri. Sci., vol. 21, pt. 4, p. 750. 

HARRISON, J. B.

1931, Field and laboratory notes on the katamorphism of 

igneous rocks under tropical and temperate conditions 

(unpublished manuscript quoted by Hardy, F., and 

Follett-Smith, R. R., 1931). 

HAUSER, E. A., and REYNOLDS, H. H.

1939, Alteration of glasses to montmorillonite: Am. Mineralogist, vol. 24, pp. 590-598. 

HAUY, R. J.

1801, Traite" de Mineralogie, vol. 4, p. 444, Paris. 

HEDDLE, M. F.

1879, The mineralogy of Scotland, Saponite: Royal Soc.

Edinburgh Trans., vol. 29, pp. 91-102. 

HEIDE, F.

1928, Beitrage zur Mineralogie und Petrographie der Rhon. 

I. Montmorillonit von Unter-Rupsroth bei der 

Milseburg: Chemie der Erde, Band 3, pp. 91-97. 

HELLMAN, N. N., ALDRICH, D. G., and JACKSON, M. L.

1942, Further note on an X-ray diffraction procedure for the 

positive differentiation of montmorillonite from 

hydrous mica: Soil Sci. Soc. Am. Proc., vol. 7, pp. 

194-200. 

HELLMERS, J. H., and KOHLER, R.

1935. Die Bestimmung von Tonerde und KieselsSuregel in 

Boden auf optischem Wege: Preuss. geol. Landesanstalt Laborat Mitt, Heft 21, pp. 22-54. 

HENDRICKS, S. B.

1936. Crystal structure of kaolinite, Al 2O3 .2Si02.2H2O, and 

composition of anauxite: Zeitschr. Kristallographie, 

Band 95, pp. 247-52.

1938. On the crystal structure of talc and pyrophyllite: 

Zeitschr. Kristallographie, Band 99, pp. 264-275.

1939. Random structures of layer minerals as illustrated by 

cronstedtite (2FeO.Fe 2O3.SiO22H2O). Possible iron 

content of kaolin: Am. Mineralogist, vol. 24, pp. 

529-539.

1941. Base exchange of the clay mineral montmorillonite for 

organic cations and its dependence upon absorption 

due to van der Waals forces: Jour. Physical Chemistry, vol. 45, pp. 65-81.

1942. Lattice structure of clay minerals and some properties

of clays, Jour. Geology, vol. 50, pp. 276-290. 

HENDRICKS, S. B., and ALEXANDER, L. T.

1939, Minerals present in soil colloids: Soil Sci., vol. 48, pp.

257-267.

1940a, A quantitative color test for the montmorillonite type 

of clay minerals: Jour. Am. Soc. Agronomy, vol. 

36, pp. 455-458.

1940b, Semiquantitative estimation of montmorillonite in 

clays: Soil Sci. Soc. Am. Proc., vol. 5, pp. 95-99. 

HENDRICKS, S. B., and FRY, W. H.

1930, The results of X-ray and microscopical examinations of

soil colloids: Soil Sci., vol. 29, pp. 457-78. 

HENDRICKS, S. B., and JEFFERSON, M. E.

1938. Crystal structure of vermiculites and mixed vermiculite-chlorites: Am. Mineralogist, vol. 23, pp. 

851-862.

1939. Polymorphism of the micas (Hendricks) with optical 

constants (Jefferson): Am. Mineralogist, vol. 24, 

pp. 729-771. 

HENDRICKS, S. B., NELSON, R. A., and ALEXANDER, L. T.

1940. Hydration mechanism of the clay mineral montmorillonite, saturated with various cations: Amer. Chem. 

Soc. Jour., vol. 62, pp. 1457-1464.

HENDRICKS, S. B., and Ross, C. S.

1938, Lattice limitation of montmorillonite: Zeitschr. Kristallographie, Band 100, pp. 251-264.

1941, Chemical composition and genesis of glauconite and' 

celadonite: Am. Mineralogist, vol. 26, pp. 683-708. 

HERMAN, RUDOLPH.

1829, Zerlegung des Pyrophyllites: Annalen der Bhysik und

Chemie (Poggendorff), Band 15, pp. 592, 593. 

HEWETT, D. F.

1917, The origin of bentonite: Washington Acad. Sci. Jour.,

vol. 7, pp. 196-198. 

HICKLING, GEORGE.

1908, China clay, its nature and uses: Inst. Min. Eng. Trans., 

vol. 36, pp. 10-33, England.

HlLGARDE, E. W.

1916, A peculiar clay from near the city of Mexico: Nat.

Acad. Sci. Proc., vol. 2, pp. 8-12. 

HOFMANN, ULRICH.

1942, Neues aus der Chemie des Tons: Die Chemie (Andrew.

Chemie, neue Folge) Jahrg. 55, pp. 283-289. 

HOFMANN, ULRICH, and BILKE, WILHELM.

1936, Uber die innerkrustalline Quellung und das Brsenaustauschvermogen des Montmorillonits: KoFoidZeitschr., Band 77, pp. 239-251.

HOFMANN, ULRICH, ENDELL, KURD, and BILKE, WILHELM. 

1939, Die Quellung von Bentonit und sein technische An wendung: Zeitschr. Electrochemie, Band 41, p. 469-471. 

HOFMANN, ULRICH, ENDELL, KURD, and WILM, DIEDERICH.

1933. Kristallstruktur und Quellung von Montmorillonit: 

Zeitschr. Kristallographie, Band 86 (A), pp. 340-347.

1934. Rontgenographische und kolloidscnemische Untersuchungen iiber Ton: Zeitschr. Angew. Chemie, 

Band 47, pp. 539-547. 

HOFMANN, ULRICH, and GIESE, K.

1939, Uber den kationenaustausch an Tonmineralien: Kclloid

Zeitschr. Band 87, pp. 21-36. 

HOLZNER, JULIUS.

1934. Neue Daten iiber den Stilpnochloren von Gobits^hau 

(Mahren) zugleich Bericht iiber eine ungewohnliehe 

untauschreaktion: Centralbl. Mineralogie, 1934, 

Abt. A, pp. 250-256, 399-405.

1935. Vorlaufiges tiber den chemischen Aufbau der Tonmineralien und entsprechender Silikate dreiwertiger 

und zweirwertiger Metalle: Chemie der Erde, Band 

9, pp. 464-485. 

HOSKING, J. S.

1940a, The soil clay mineralogy of some Australian soils 

developed on granitic and basaltic parent material: 

Australian Council Sci. and Indus. Res., vol. IS, pp. 

206-216, Melbourne.

1940b, The constitution and classification of the clay minerals: 

The Australian Chem. Inst. Jour, and Proc., V)l. 7, 

pp. 234-250, Melbourne. 

HOSKING, J. S., and BURVILL, G. H.

1938. A soil survey of part of the Denmark Estate, western 

Australia: Australian Council, Sci. and. Ind. Research, Bull. 15. 

HOUSER, E. A., and RENOLDS, H. H.

1939. Alteration of glass to montmorillonite: Am. Mir^ralogist, vol. 24, pp. 590-598. 

HUMBERT, R. P.

1942. Particle shape and the behavior of clay as revealed by 

the electron microscope: Am. Ceramic Soc. Bull., 

vol. 21, pp. 260-263. 

HUMBERT, R. P., and MARSHALL, C. E.

1943. Mineralogical and chemical studies of soil formation 

from acid and basic igneous rocks in Missouri: 

Missouri Univ. Agri. Exper. Sta. Research Bull. 359, 

pp. 5-60. 

HUMBERT, R. P., and SHAW, BYRON.

1941, Studies of clay particles with the electron micros-rope: 

I. Shapes of clay particles: Soil Sci., vol. 52, pp. 

481-483.

ISKYUL, E. O.

1935, On the nontronite from St. Andreasberg, Harz.: T-udy

Lomonossov. Inst., Akad. Nauk, S. S. S. R., ro. 5,

pp. 149-156. 

JACKSON, M. L., and HELLMAN, N. N.

1941, X-ray diffraction procedure for positive differentiation

of montmorillonite from hydrous mica. Soil Sci.

Soc. Am. Proc., vol. 6, pp. 133-145. 

JACKSON, W. W., and WEST, J.

1933, The crystal structure of muscovite, KAl 2 (AlSi 2 )O10 (OH)*

Zeitschr. Kristallographie, Band 76, pp. 211-227,

1931; also Band 85, pp. 160-164.
MINERALS OF THE MONTMORILLONITE GROUP 75

JACOB, ARTUR, HOFMANN, ULRICH, and LOOPMANN, H.

1935, Die Identification der Tonsubstanz des Bodens durch 

Rontgenuntersuchungen: 3d Internat. Cong. Soil Sci. 

Trans., vol. I, pp. 85-88.

JACOB, ARTUR, HOPMANN, ULRICH, LOOPMANN, H., 

MAEGDEPRAU, EDMUND, and WILM, DIEDRICH.

1935, Chemische und rontgenographische Untersuchungen iiber die mineralische Sorptionssubstanz 

im Boden: Vereins deutscher Chemiker, Beihefte 

21; Review in Angew. Chemie, Band 48, pp. 585-586. 

JUNG, HERMANN.

1937, Zur Kenntnis des Montmorillonits: Chemie der Erde,

Band 11, pp. 287-293. 

KAMMERER, A. A.

1831, Auszug aus Briefen: Neues Jahrb., Band 2, p. 420. 

KAWAMURA, KAZYMI, and FUNABIKI, SHINGO.

1936, The soil colloids of middle and western Japan, iy, The 

rontgen analysis of the colloids: Jour. Sci. Soil 

Manure, vol. 10, pp. 281-294. 

KAY, G. M.

1931, Stratigraphy of the Hounsfield metabentonite: Jour.

Geology, vol. 39, pp. 361-376. 

KBLLEY, W. P.

1935, The evidence as to the crystalinity of soil colloids: 3rd

Internat. Cong. Soil Sci. Tran., vol. 3, pp. 88-91. 

1939, Base exchange in relation to sediments in recent marine 

sediments, a symposium: Am. Assoc. Petroleum 

Geologists, pp. 454-465. 

1942, Modern clay researches in relation to agriculture: Jour.

Geology, vol. 50, pp. 307-319. 

KELLEY, W. P., BROWN, S. M., and LIEBIG, G. F., Jr.

1940, Chemical effects of saline irrigation water on soils: Soil

Sci., vol. 49, pp. 95-107. 

KELLEY, W. P., and DORE, W. H.

1937, The clay minerals of California soils: Soil Sci. Soc.

America Proc., vol. 2, pp. 115-120. 

KELLEY, W. P., DORE, W. H., and BROWN, S. M.

1931, Nature of base-exchange material of bentonite, soils, 

and zeolites as revealed by chemical investigation 

and X-ray analysis: Soil Sci., vol. 31, pp. 25-55. 

KELLEY, W. P., DORE, W. H., WOODPORD, A. O., and BROWN, 

S. M. 

1939, The colloidal constituents of California soils: Soil Sci.,

vol. 48, pp. 201-255. 

KELLEY, W. P., and JENNY, H.

1936, The relation of crystal structure to base exchange and 

its bearing on base exchange in soils: Soil Sci., vol. 

41, pp. 367-382. 

KELLEY, W. P., JENNY, H., and BROWN, S. M.

1936, Hydration of minerals and soil colloids in relation to 

crystal structure: Soil Sci., vol. 41, pp. 259-274. 

KELLEY, W. P., WOODFORD, A. O., DORE, W. H., and BROWN, 

S. M.

1939, Comparative study of the colloids of a Cecil and a 

Susquehanna soil profile: Soil Sci., vol. 47, pp. 

175-193. 

KENNGOTT, G. A.

1853, Ubersicht der Resultate mineralogischer Forschungen 

im Jahre 1852: K. k. geol. Reichanstalt (Bundesanstalt), Jahrbuch 1853, Beilage Band 4, p. 633, 

Wien, 1854. 

KERR, P. F.

1931. Bentonite from Ventura, Calif.: Econ. Geology, vol. 

26, pp. 153-168.

1932. Montmorillonite or smectite as constituents of fuller's 

earth and bentonite: Am. Mineralogist, vol. 17, 

pp. 192-98.

1937. Attapulgus clay: Am. Mineralogist, vol. 22, pp. 534-50.

1938. A decade of research on the nature of clay: Am.

Ceramic Soc. Jour., vol. 21, pp. 267-286. 

KERR, P. F., and CAMERON, E. N.

1936, Fuller's earth of bentonitic origin from Tehachapi,

Calif.: Am. Mineralogist, vol. 21, pp. 230-237. 

KERSTEN, C.

1839, Chemische Untersuchung des Wolchonskoits: Annalen 

der Physik und Chemie (Poggendorff), Band 47, 

pp. 489-493. 

KIRWAN, RICHARD.

1794, Elements of mineralogy, 2d ed., vol. 1, p. 184, London. 

KNIGHT, W. C.

1897, Mineral soap: Eng. Min. Jour., vol. 63, pp. 600-601. 

KOKSCHAROW, NIKOLAI.

1853, Materialien zur Mineralogie Russlands, vol. 1, p. 145, 

St. Petersburg.

KRANTZ, AUGUST.

1857, Sitzungsberichte der Niederrheinischen Gesell. fur 

Natur- und Heilkunde zu Bonn, in Verhandlun^en 

des Naturhistorischen Vereins der preussisc ^r 

Rheinlande und Westphalens, Jahg. 14, pp. XLIVXLV. 

KURNAKOV, N. S., and CERNYCH, V. V.

1928, Physico-chemical investigation of some hydrous ferric 

• iron and aluminum silicates, nontronite, hisingerite, 

gramenite, melinite, smectite, and allophane: Soc. 

Russe Mine'ralogie, Mem. ser. 2, vol. 57, pt. l,pp. 

61-74. 

LACROIX, ALFRED.

1910, Mineralogie de la France et de ses colonies, tome 4, p.

749, Paris.

1914, Sur 1'opale et sur une nouvelle espece mine'rale (faratsihite) de Faratsiho (Madagascar): Soc. frpng. 

mine'ralogie Bull., tome 37, pp. 231-236. 

LAMAR, J. E., GRIM, R. E., and GROGAN, R. M.

1938, Gumbotil as a potential source of rotary drilling nrnd, 

bonding clay, and bleaching clay: Illinois Geol. 

Survey Information Circular 39, pp. 1-23. 

LAPPARENT, JACQUES DE

1934, Constitution et origine de la leverrierite: Acad. sci.

Paris, Comptes rendus, vol. 198, p. 669-671. 

1938, Formules structurales et classification des argues:

Zeitschr. Kristallographie, Band 98, pp. 233-58. 

LARSEN, E. S., and BERMAN, HARRY.

1934, The microscopic determination of the nonopaque

minerals: U. S. Geol. Survey Bull. 848, p. 164. 

LARSEN, E. S., and STEIGER, GEORGE.

1928, Dehydration and optical studies of alunogen, nontrorite, 

and griffithite: Am. Jour. Sci., 5th ser., vol. 15, pp* 

1-19. 

LARSEN, E. S., and WHERRY, E. T.

1917, Leverrierite from Colorado: Wash. Acad. Sci. Jour.,

vol. 7, pp. 208-217. 

1925, Beidellite, a new mineral name: Washington Acad.

Sci. Jour., vol. 15, pp. 465-466. 

LE CHATELIER, H.

1887, De 1'action de la chaleur sur les argiles: Soc. fr^nc..

Mine'ralogie Bull., tome 10, pp. 204-211. 

LINDGREN, W., and HILLEBRAND, W. L.

1904, Minerals from the Clifton-Morenci district, Ariz., 

Morencite: Am. Jour. Sci., 4th ser., vol. 18, pp. 

456-457. 

LOUDERMILK, J. D., and WOODPORD, A. O.

1934, Secondary montmorillonite in a California pegmatite:

Am. Mineralogist, vol. 19, pp. 260-267. 

LOVERING, T. S.

1941, The origin of the tungsten ores of Boulder County, 

Colo.: Econ. Geology, vol. 36, pp. 229-279.

McCONNELL, DUNCAN.

1937, The substitution of SiO4 and SO4 groups in the apatite

structure; ellestadite, the end number: Am. Minri*alogist, vol. 22, pp. 977-986. 

MAEGDEFRAU, EDMUND, and HOFMANN, ULRICH.

1937a, Glimmerartige Mineralien als Tonsubstanzen: Zeitschr.

Kristalographie, Band 98, pp. 31-59. 

1937b, Der Kristallstruktur des Montmorillonits: Zeitschr.

Kristallographie, Band 98, pp. 299-323. 

1940, Quantitative determination of minerals in clays by the

X-ray method. Deutsche Keram. Gesell. F?r.,

Band 21, pp. 383-412; Ceramic Abstracts, vol. 24,

p. 180, 1941. 

MALLARD, F. E.

1878, Sur le bravaisite, Substance minerale nouvelle: Soc.

franc, mine'ralogie Bull., tome 1, pp. 5-8. 

MARSHALL, C. E.

1934, The role of aluminum in the reactions of the cbys:

Soc. Chem. Industry Jour., vol. 53, pp. 1-11. 

1935a, Mineralogical methods for the study of silts and clays:

Zeitschr. Kristallographie, Band 90, pp. 8-34. 

1935b, Layer lattices and base-exchange clays: Zeitschr.

Kristallographie, Band 91 (A), pp. 433-49. 

1935c, The importance of the lattice structure of the cHys

for the study of soils: Soc. Chem. Industry Jonr.,

vol. 54, pp. 1-5. 

1935d, The chemical composition and crystal structure of

clay minerals: 3d Internat. Cong. Soil Sci. Trrns.,

vol. 3, pp. 95-97. 

1935e, Properties of clay fractions: 3d Internat. Cong. Soil

Sci. Trans., vol. 1, pp. 88-90.
76 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1943-44

MARSHALL, C. E.—Continued

1936a, The chemical constitution as related to physical

properties of clays: Ceramic Soc. (Eng.) Trans.,

vol. 35, pp. 401-11. 

1935b, The constitution of the clay minerals: Sci. Prog., no.

119, pp. 422-433. 

1937a, Soil science mineralogy: Soil Sci. Soc. America Proc.

vol. 1, pp. 23-31. 

I937b, Colloidal properties of clays as related to their crystal

structure: Jour. Phys. Chem., vol. 41, pp. 935-42. 

MARSHALL, C. E., and GUPTA, R. S.

1933, Base exchange equilibria in clays: Jour. Soc. Chem.

Industry, vol. 52, pp. 433T-443T. 

MARSHALL, C. E., HUMBERT, R. P., SHAW, B. T., and 

CALDWELL, 0. G. 

1942, Studies of clay particles with the electron microscope:

II, The Fractionation of beidellite, nontronite,

magnesium, bentonite, and attapulgite: Soil Sci.,

vol. 54, pp. 149-158. 

MAUGUIN, CH.

1927, Etude du mica muscovite au moyen des rayons X:

Acad. sci. Paris, Comptes rendus, tome 185, pp.

288-291. 

1928a, Etudes des micas (non flores) au moyen des rayons X:

Acad. sci. Paris, Comptes rendus, tome 186, pp.

879-881. 

1928b, Etude des chlorites au moyen des rayons X: Acad. sci.

Paris, Comptes rendus, tome 186, pp. 1852-1855. 

MEHMEL, MARTIN.

1935, Uber die Struktur von Halloysit and Metahalloysit:

Zeitschr. Kristallographie, Band 90, pp. 35-43.

1937. Beitrag zur Frage des Wassergehalts der Mineralien 

Kaolinit, Halloysit, und Montmorillonit: Chem. der 

Erde, Band 11, pp. 1-16.

1938. Anwendung optischer und rontgenographischer Methoden bei der Untersuchung der Tone: Deutsch. 

Keram. Geselsch, Ber., Band 19, pp. 295-328.

1939. Datensammlung zum Mineralbestimmen mit Rontgenstrahlen, Teil 1: Fortschr. der Mineralogie, Kristallographie, und Petrographie, Band 23, pp. 91-118. 

MERRILL, G. P.

1902, What constitutes a clay: Am. Geologist, vol. 30, p. 318.

MEULEN, J. B., VAN DER.

1935, The relation between the phenomenon of cation exchange with silica-alumina complexes and - their 

crystal structure: Rec. travaux chim., tome 54, pp. 

107-110. 

MIDDEL, V., REICHMANN, R., and KAUSCHE, G. A.

1940, Supermicroscopic investigation of the structure of 

bentonites: Wiss. veroffentlishe Siemans-Werken, 

Werkstoff-Sonderheft, pp. 334-341; Chem. Abstracts, 

vol. 35, p. 4167, 1941. 

MISER, H. D., and STEVENS, R. E.

1938, Taeniolite from Magnet Cove, Ark.: Am. Mineralogist, 

vol. 23, pp. 104-110.

NAGELSCHMIDT, GUNTHER.

1904, Rontgenographische untersuchungen an Tonen: Zeitschr. Kristallographie, Band 87-A, pp. 120-145.

1936, On the lattice shrinkage and structure of montmorillonite: 

Zeitschr. Kristallographie, Band 93, pp. 481-487.

1938. On the atomic arrangement and variability of the 

members of the montmorillonite group: Mineralog. 

Mag., vol. 25, pp. 140-155, London.

1939. The identification of minerals in soil colloids: Jour.

Agr. Sci., vol. 29, pt. 4, pp. 478-501. 

1943, The mica of certain coal-measure shales in South Wales:

Mineralogy Mag., pp. 297-303, London. 

NELSON, W. A.

1922, Volcanic ash bed in the Ordovician of Tennessee,

Kentucky, and Alabama: Geol. Sco. Am. Bull.,

vol. 33, pp. 605-616.

NELSON, R. A., and HENDRICKS, S. B.

1943, Specific surface of some clay minerals, soils, and soil 

colloids: Soil Sci., vol. 56, pp. 285-296.

NlCKLES, M. J.

1859, Sur la saponite nouvel hydrosilicate d'alumine: Annales 

de chimie et de phisique, tome 56, p. 46.

NOLL, W.

1930, Zur Kenntniss des Nontronite: Chemie der EHe, Band

5, pp. 373-384. 

1936a, Mineralbildung im system Al 203-Si02-H20: Neues

Jahrb., Beilage Band 70, Abt. A, pp. 65-115. 

1936b, Synthese von Montmorilloniten, Ein Beitrag zur 

Kenntniss der Bildungsbedingungen und des Chemismus von Montmorillonit: Chemie der F^ie, vol. 

10, pp. 129, 154.

1937. Uber das Vorkommen von Montmorillonit ir einigen 

Zersetzungsprodukten von Basal ten des westlichen 

Vogelberges: Chemie der Erde, Band 2, pp. 294-306.

1938. Fortschritte in der Erkenntnis der Tonrnnerale: 

Deutsch. keram. Gesell. Ber., Band 19, pp. 176-205.

1940, Arbeiten iiber die Synthesis und naturliche BiHung der 

Tonminerale: Hannoverische Hochshulgemeinschaft, 

Heft 17, pp. 1-2.

NORDENSKIULD, GUSTAP.

1890, Om. Mineral fran drushal vid Taberg i VarmHnd (On 

a mineral from a drusy cavities near Taberg in Varmland, Sweden): Geol. Fdreningens Forh., Pand 12, 

pp. 348-358, Stockholm. 

NORTON, F. H.

1933, Notes on nature of clay, II: Am. Ceram. Sec. Jour.,

vol. 16, pp. 86-92. 

1939a, Hydrothermal formation of clay minerals in the

laboratory: Am. Mineralogist, vol. 24, pp. 1-18. 

1939b, Critical study of the differential thermal method for 

the identification of the clay minerals: Am. Ceramic 

Soc. Jour., vol. 22, pp. 54-63. 

NORTON, F. H., and HODGDON, F. B.

1932, Notes on nature of clay: Am. Ceram. Soc. Jcur., vol.

15, pp. 191-205. 

NUTTING, P. G.

1931, Adsorption and base exchange: Wash. Acad. Srti. Jour.,

vol. 21, pp. 33-36. 

1943, Some standard thermal dehydration curves: U. S.

Geol. Survey Prof. Paper 197-E. 

ORCEL, J.

1935, Differential thermal analysis for determinatior of constituents of clays, laterites, and bauxities: Cong. 

Internat. Mines, Met., et Geol. AppL, 7 Sessicn, vol. 1, 

pp. 359-73, Paris. 

OWEN, D. D.

1852, Description of two new minerals and a new earth: Acad. 

Nat. Sci. Philadelphia Jour., vol. 2, pt. 2, pp. 179-183. 

PALACHE, C., and VASSAR, H. E.

1925, Some minerals of the Keweenawan copper deposits, 3,

saponite: Am. Mineralogist, vol. 10, pp. 417-418. 

PAULING, LINUS.

1930, The structure of micas and related minerals. Nat. Acad.

Sci. Proc., vol. 16, pp. 123-129. 

PAVER, H., and MARSHALL, C. E.

1934, The role of aluminum in the reactions of clays: Soc. 

Chem. Industry Jour., vol. 53, pp. 750-760, London. 

PRALOW, W.

1938, Mikroskopische, rontgenographische, und cl ?mische 

Untersuchung einiger Proben des estlSndischen 

Blauen Tons: Chem. Erde, Band 11, pp. 48^497. 

PRATT, JOSEPH HYDE.

1900, Talc and pyrophyllite deposits in North Carolinr: North

Carolina Economic Papers, No. 3, pp. 7-29. 

REED, C. E.

1938, Recent advances in knowledge of the collodial properties 

of clay suspensions and gels: Am. Inst. Min. Met. 

Eng. Tech. Pub. 871, 23 pp. 

RlES, H.

1927, Clays, their occurrence, properties and uses: 3d ed.

John Wiley & Sons, New York. 

ROBORGH, R. H. J., and KOLKMEIJER, N. H.

1936, Uber die Struktur des Adsorption Komplexes der Ton:

Zeitschr. Kristallographie, Band 94, pp. 74-79. 

ROSENBUSCH, HARRY.

1927. Microskopische Physiographie der Mineral^n und 

Gesteine, Band 1, 2d Halfte, spezieller Teil, pp. 

591-596. 

Ross, C. S.

1928. Altered Paleozoic volcanic materials and their recognition: Am. Assoc. Petroleum Geologists Bull., vol. 12, 

pp. 143-164.

1941, Titanium deposits of Nelson and Amherst Counties, Va., 

compared with those of Magnet Cove: U. S. Geol. 

Survey Prof. Paper 198, pp. 1-38, pi. 20.

1943, Clays and soils in relation to geologic processes: Wash. 

Acad. Sci. Jour., vol. 33, pp. 225-235.
MINERALS OF THE MONTMORILLONITE GROUP 77

Ross, C. S., and HENDRICKS, S. B.

1941, Clay minerals of the montmorillpnite group; their mineral and chemical relationships and the factors of 

base exchange: Soil Sci. Soc. America Proc., vol. 6, 

pp. 57-62. 

Ross, C. S., and KERR, P. F.

1931a, The clay minerals and their identity: Sedimentary

Petrology Jour., vol. 1, pp. 55-65. 

1931b, The kaolin minerals: U. S. Geol. Survey Prof. Paper

165, pp. 151-180. 

1934, Halloysite and allophane: U. S. Geol. Survey Prof.

Paper 185-G, pp. 135-148. 

Ross, C. S., MISER, H. D., and STEPHENSON, L. W.

1929, Water-laid volcanic rocks of early Upper Cretaceous age 

in southwestern Arkansas, Southeastern Oklahoma, 

and northeastern Texas: U. S. Geol. Survey Prof. 

Paper 154, pp. 175-202. 

Ross, C. S., and SHANNON, E. V.

1925. Chemical composition and optical properties of beidellite: 

Washington Acad. Sci. Jour., vol. 15, pp. 467-468.

1926. Minerals of bentonite and related clays and their physical 

properties: Am* Ceram. Soc. Jour., vol. 9, pp. 77-96. 

Ross, C. S., and STEPHENSON, L. W.

1939. Calcareous shells replaced by beidellite: Am. Mineralogist, vol. 24, p. 393-397. 

RUSSEL, M. B., and HADDOCK, J. L.

1940. The identification of the clay minerals in five Iowa soils 

by the thermal method: Soil Sci. Soc. America Proc., 

vol. 5, pp. 90-94.

SCHACHTSCHABEL, P.

1940, Research on the sorption of clay minerals and organic 

soil colloids and the estimation of the fraction of these 

colloids in the sorption in soils: Kolloid Beihefte, 

Band 51, pp. 199-276. 

SCHALLER, W. T.

1930, Adjectival ending of chemical elements used as modifiers 

to mineral names: Am. Mineralogist, vol. 15, pp. 

566-579. 

SEDLBTSKY, I. D.

1938. The genesis of the minerals of soil colloids in connection 

with types of erosion and soil formation: Priroda, 

vol. 27, no. 1, pp. 34-44, Akad. Nauk, U. S. S. R.

1939. Genesis of montmorillonite and kaolinite and conditions 

of their joint occurrence in the colloids of soils and 

clays: Acad. sci., U. R. S. S., Comptes rendus 

(Dokl.), vol. 22, pp. 510-514. 

SERDINTSCHENKO, D. P. See Serdiuchenko. 

SERDIUCHENKO, D. P. (Incorrectly given as Serdintschenko.) 

1929, 0ber die Nontronite aus zwei siidrussischen Graphitlagerstaten: Centralbl. Mineralogie, Abt. A, pp. 

47-55.

1933, Chrom-nontronites and their genetical relations with the 

serpentines of the northern Caucasus: Soc. russe 

mine'ralogie Mem., 2d ser., vol. 62, pp. 376-390, 

English summary, pp. 376-391. 

SERDIUCHENKO, D. P., and ISKULL, E. V.

1933, Nontronites from the Baltatarak and Samenhet deposits 

in Kazakstan: Soc. russe mine'ralogie, Mem., 2d ser., 

vol. 62, pp. 103-110, English summary, p. 110. 

SHAW, B. T. *

1942, The nature of colloidal clay as revealed by the electron 

microscope: Jour. Phys. Chem., vol. 46, pp. 1032- 

1043. 

SHAW, B. T., and HUMBERT, R. P.

1941, Electron micrographs of clay minerals: Soil Sci. Soc.

America Proc., vol. 6, pp. 146-149. 

SHEARER, J., and COLE, W. T.

1940, X-ray analysis of some Tasmanian soil colloids: Jour. 

Royal Soc. of Western Australia, vol. 26, pp. 133-137.

SlEBENTHAL, C. E.

1905, Bentonite of the Laramie Basin, Wyo.: U. S. Geol.

Survey Bull. 285, pp. 445-7. 

SIEFERT, H.

1934, Geochemische Tarnung in Anomalen Mischkristallen: 

Mineralog. und Petrog. Mitteil., Band 45, pp. 

191-208. 

SPENCE, Hugh S.

1924, Bentonite: Canada Dept. Mines, Mines Branch, no. 626,

pp. 1-36. 

STEVENS, R. E.

1938, New analyses of lepidolites and their interpretation: 

Am. Mineralogist, vol. 23, pp. 607-628.

STREME, H.

1917, in Doelter's Handbuch der Mineralchemie, Bani 2,

Abteilung 2, p. 38. 

STRESE, H., and HOFMANN, ULRICH.

1941, Synthesis of magnesium silicate gels with two-dinensional structure: Zeitschr. anorg. allg. Chemie, vol. 

247, pp. 65-95. Abstract in Chem. Abstracts, vol. 36, 

p. 5074, 1942. 

STUCKEY, J. L.

1935, The pyrophyllite deposits of the Deep River region of 

North Carolina: Econ. Geology, vol. 20, pp. 442-463. 

SVANBERG, L. F.

1840, Saponit: Stockholm. Akad. Handl., p. 153.

1842, Saponite: Annalen der Physik und Chemie (Pog^endorff), Band 57, pp. 165-170.

1841, 0ber einige neue schwedische mineralien, Piotin: 

Annalen der Physik und Chemie (Poggendorff), F*nd 

54, pp. 267-268. 

TERMIER, PIERRE.

1889. Sur une phylite nouvelle, la leverrierite: Acad. sci. P^ris, 

Comptes rendus, tome 108, p. 1071.

1890. Etude sur la leverrierite: Annales des mines, tome 17,

pp. 372-398. 

THOMPSON, H. S.

1850, On the absorbent power of soils: Jour. Royal Agric. Soc.

vol. 11, pp. 68-74, London. 

TOMKEIF, S. I.

1934, Differentiation in basalt lava, Island of Maeree, County 

Antrim: Geol. Mag., vol. 71, pp. 501-512, London. 

TOMLINSON, W. H., and MEIER, A. E.

1937. On the origin of montmorillonite: Am. Mineralogist,

vol. 22, pp. 1124-26. 

TRUOG, EMIL, and JONES, R. J.

1938. Fate of soluble potash applied to soils: Indust. and Eng.

Chemistry, vol. 30, pp. 882-885. 

TYLER, S. A., and MARSDEN, R. W.

1938, The nature of leucoxene: Jour. Sedimentary Petrob^ry,

vol. 8, pp. 55-57. 

URBAIN, PIERRE.

1937-, Introduction & l'6tude pe"trographique et ge'ochimique des

roches argileuses, Hermann & Cie, pp. 1-61, ParX 

1938, Texture microscopique des roches argilleuses: Soc. Ge"ol.

France Bull., 5th ser., tome 7, pp. 341-346. 

VOLK, N. J.

1933, Formation of muscovite in soils, and refinement" in 

specific gravity separations: Am. Jour. Sci., 5th ser., 

vol. 26, pp. 114-129. 

VOLK, G. W.

1938, The nature of potash fixation in soils: Soil Sci., vol. 45,

pp. 263-276. 

WARREN, B. E., and HERING, K. W.

1941, The random structure of chrysotile asbestos: Phys. F.ev.f

vol. 59, p. 925. 

WAY, J. T.

1850, On the power of soils to absorb manure: Roy. Agric. Soc.

Jour., vol. 11, pp. 313-379. 

1852, On the influence of lime on the absorptive properties of

soils: Roy. Agric. Soc. Jour., vol. 13, pp. 123-143. 

1855, On the power of soils to absorb manure: Roy. Agric. Soc.

Jour., vol. 15, p. 491. 

WEINSCHENK, E.

1897, Beitra?e zur Mineralogie Bayern: Zeitschr. Krystallographie, Band 28, p. 153. 

WELLS, R. C.

1937, Analyses of rocks and minerals: U. S. Geol. Survey Pull.

878, p. 108. 

WHERRY, E. T.

1917a, Clay derived from volcanic dust in the Pierre of South 

Dakota: Washington Acad. Sci. Jour., vol. 7, pp. 

576-583. 

1917b, Bentonite in Wyoming: U. S. Geol. Survey Bull. 624,

p. 342.

1925, Bentonite as a one-dimensional colloid: Am. Mineralogist, vol. 10, pp. 120-123. 

WHERRY, E. T., Ross, C. S., and KERR, P. F.

1930. Progress in the study of the clay minerals: Conoid 

Symposium Annual, pp. 191-193.

WIEGNER, G.

1931. Some physico-chemical properties of clays, I, Base exchange or ionic exchange, II, Hydrogen clays: Soc. 

Chem. Industry Jour., vol. 50, 103 T. 

WINCHELL, A. N.

1933, Elements of optical mineralogy, 3d ed., p. 433, New 

York, John Wiley & Sons.

INDEX

Page 

Abstract........................................................... 23

Acknowledgments................................................... 25

Analyses, tables of.................................................. 34-35

See also particular minerals. 

Atomic planes in crystal structure..................................... 29-30

Base exchange, determination of...................................... 38-39

physical chemistry of............................................ 37-38

Beidellite, nomenclature of........................................... 26

photomicrographs showing................................. pis. 5, C, 7, A

Bentonite, analyses of............................................... 66

definition of.................................................... 65

micrographs showing............................. pis. 3, A, 4, 5, B, D, 7, C

mode of occurrence of............................................ 65-67

Ordovician..................................................... 67

parent glass of, chemical analysis of............................... 66

structure of...................................... .64-65, pis. 3, A, 4

physical properties of............................................ 65

Bibiography........................................................ 72-77

Birefringence, relation of, to presence 9f ferric iron...................... 55

Chlorite, analyses of samples containing................................ 35

Chlorite impurity, calculation of...................................... 43

Chromium-bearing samples, analyses of................................ 35

calculation of formulas for........................................ 43

_„-.description of................................................... 32

Clay minerals, nomenclature of....................................... 25-29

synthesis of..................................................... 58-60

Clays, analyzed, impurities in........................................ 36-37

Analyzed, location and description of.............................. 30-33

minor constituents in........................................ 37

calculation of impurities in....................................... 43

gouge.......................................................... 69-70

of geyser region................................................. 68

varved......................................................... pi. 2, D

vein-forming.............................................. 69-70, pi. 3

See also Montmorillonite. 

Crystal structure, atomic planes in..................................... 29-30

relation of ions to............................................... 41-43

Dehydration curves................................................. 48-52

Electron-microscope photographs............................... 57-58, pis. 7, 8

Fenner, C. N., quoted............................................... 69

Formulas, mineral, calculation of...................................... 41-43

mineral, derivation of............................................ 44-48

Geyser region in Yellowstone Park, clays of............................ 68

Gruner, J. W., quoted................................................ 30

Hectorite, analysis of..............................................27, 35, 38

calculation of formulas for........................................ 43

micrographs showing...................................... pis. 6, B, 8, A

nomenclature of................................................. 27

Hendricks, S. B., and Alexander, L. T., quoted......................... 29

Hosking, J. S., quoted............................................... 62

Hydrous micas, nomenclature of...................................... 28-29

Hydroxyl, essential.................................................. 48-52

Illite. See Hydrous micas.

Indices of refraction, determination of................................. 55, 57

Iron-bearing samples, analyses of....................................... 35

calculation of impurity in........................................ 43

description of..........................................'......... 32-33

Kaolin minerals, nomenclature of...................................... 28

Kaolinite, volcanic materials altered to............................... pis. 1, 2

Kerr, C. P., quoted................................................... 26

Lamar, J. E., Grim, R. E., and Grogan, R. M., quoted.................. 64

Magnet Cove, Ark........................................... 67-68, pis. 1,2

Marshall, C. E., quoted.............................................. 30

Methods of study................................................... 24

Mixed-layer minerals, calculation of...............'.................... 43

structural relations of............................................ 29

Montmorillonite, analyses of.......................................25, 63, 68

essential hydroxyl in............................................. 48

micrographs showing............................. pis. 1, 2, 3, D, 4, C, 5, 7

nomenclature of................................................. f 5-26

relation between water content and cleavage spacing in.............. 52-53

Montmorillonite-beidellite series, analyses of samples of.................. 34

calculation of formulas for........................................ 42

derivation of formulas for........................................ 44-45

description of samples of......................................... 30-32

Montmorillonite-beidellite-nontronite series,

derivation of formulas for........................................ 46

proportions of end members in selected samples of................... 46

Montmorillonite clays, base-exchange capacities of...................... 39

dehydration curves of............................................ 48-52

mineraiogic relations of.......................................... 39-41

origin and mode of occurrence of.................................. 60-70

synthesis of..................................................... 58-69

Noll, W., experiments in synthesis by.................................. 59

Nontronite, analyses of...........................................27, 2", 35

calculation of formulas for........................................ 43

derivation of formulas for........................................ 44-46

description of samples of......................................... 32

essential hydroxyl in............................................. 48

micrographs showing...................................pis. 6, C, D, 8, B

nomenclature of................................................. 27-28

Optical properties, methods of determination of......................... 54-55

Pleochroism........................................................ 57

Previous work...................................................... 24-25

Pyrophyllite, analyses of.............................................. 70, 71

essential hydroxyl in............................................. 48

formulas for.................................................... 70

Refraction, determinations of indices of................................ 55-57

Rocks, weathering of................................................ 61-64

Ross, C. S., and Shannon, E. V., quoted................................ 52, 65

Saponite, analysis of................................................. 26

calculation of formulas for........................................ 48

nomenclature of................................................. 26-27

Saponite-hectorite series, analyses of samples of......................... 35

derivation of formulas for........................................ 46-47

description of samples of......................................... 32

Silica, excess, analysis of............................................. 32-33

excess, calculation of............................................. 43

samples containing, description of................................. 32-33

Soils, analysis of samples of........................................... 35

Australian...................................................... 62

calculation of formulas for samples from........................... 43

description of samples from....................................... 32

glacial......................................................... 63-64

Java........................................................... 63

Piedmont...................................................... 62

rock weathering in relation to..................................... 61--64

Talc, analyses of.................................................... 70, 71

Thermal studies..................................................... 48-54

Titanium, role of, in clays............................................ 36, 37

Vermiculite, analyses of samples containing............................. 35

structural relations of............................................ 29

Vermiculite impurity, calculation of................................... 43

Volcanic Ash, structure of..................................64-65, pis. 3, A, 4

Volcanic materials, alteration of.................................. pis. 1, 2, 3, A

Volcanic rock from Arkansas, analyses of............................... 66-68

Volchonskoite, nomenclature of....................................... 28

Water, hydroxyl.................................................... 48-52

interlayer...................................................... 52-54

79

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