Mixed-Layer Glay in the Pierre Shale 

and Equivalent Rocks, 

Northern Great Plains Region

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1064-A
Mixed-Layer Clay in the Pierre Shale

and Equivalent Rocks, 

Northern Great Plains Region

By LEONARD G. SCHULTZ

GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS 

OF LATE CRETACEOUS AGE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1064-A

X-ray diffraction techniques for interpretation 

of mixed-layer illite-smectite clay, 

chemical composition of the clay, 

and developmental history of the clay

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1978
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Schultz, Leonard Gene, 1926-

Mixed-layer clay in the Pierre Shale and equivalent rocks, northern Great Plains region.

(Geochemistry of the Pierre Shale and equivalent rocks, northern Great Plains) (Geological Survey Professional

Paper 1064-A) 

Bibliography: p. 27 

1. Clay minerals Great Plains. 2. Geology, Stratigraphic Cretaceous. I. Title. II. Series. III. Series:

United States Geological Survey Professional Paper 1064-A. 

QE389.625.S3 552'.5 77-26718

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

Washington, D.C. 20402 

Stock Number 024-001-03115-9
CONTENTS

Page

Abstract ........................................................................................ 1

Introduction .................................................................................. 1

General geology and mineralogy .................................................. 1

Definition of mineral and mixed-layer terms ................................ 3

Interpretation of X-ray diffraction traces of mixed-layer clay ...... 5

Illite-smectite ......................................................................... 5

Illite-beidellite-montmorillonite ............................................. 7

Peak positions of IS-ordered clay .......................................... 9

Common proportions of illite:beidellite:montmorillonite ............. 10

Chemistry of mixed-layer clays ..................................................... n

Structural formulas calculated from whole-rock samples ...... H

Shale ............................................................................... 11

Bentonite ...................................................... ..... ........ 12

Page

Chemistry of mixed-layer clays Continued

Structural formulas calculated from purified mixed-layer clay 12 

Formulas inferred for individual illite-, beidellite-, and

montmorillQnite-type layers ........................................ 15

Structural OH" ............................................................... 16

Illitic mixed-layer clay in bentonite .............................................. 18

Depth diagenesis .......................................................................... 19

Main Pierre Shale study ........................................................ 20

Complementary studies .......................................................... 22

Cuttings from deep wells ................................................ 22

Montana disturbed belt .................................................. 23

Summary and interpretation ......................................................... 24

Selected references ........................................................................ 27

ILLUSTRATIONS

————————————— Page

FIGURE 1. Diagrammatic section of the Pierre Shale and some of the equivalent rocks ............................................................................... 2

2. Index map showing sample localities and pertinent physiographic features ................................................................................. 3

3. Schematic structure of an aluminum smectite or illite layer ......................................................................................................... 4

4. X-ray diffraction traces of four clay samples that illustrate the range of composition of mixed-layer illite-smectite in rocks of Pierre

age and the new and old methods of expressing proportions of layers ................................................................................. 6

5. Graph showing relationship between the proportion of smectite and mixed-layer clay interpreted from peak heights and

the percentage of illite-type layers interpreted from peak migration ..................................................................................... 7

6. Diagram used to interpret proportions of layers in mixed-layer clay ........................................................................................... 9

7. Graph showing relationship between the proportion of beidellite- and illite-type layers in mixed-layer clay determined from migration curves and the change in peak height after glycerol treatment of Li*-saturated and heated clay .................................. 10

8. Graph showing illite content of 11 IS-ordered mixed-clay samples determined from Sooa/Iw related to ^-spacing of the first

basal reflection of the clay treated with ethylene glycol ........................................................................................................ H

9. Diagram showing proportions of layers in mixed-layer illitite-smectite in the Pierre Shale and equivalent rocks ....................... 12

10. Graphs showing cation abundance in structural formulas ............................................................................................................ 13

11. Diagrams showing tetrahedral Ali3 , total layer charge, and interlayer Kv+ ions of structural formulas related to proportions

of illite:beidellite:montmorillonite .......................................................................................................................................... 15

12. Differential thermal analysis curves typical of shale and bentonite of Pierre age ......................................................................... 17

13. Graph showing change in bulk density and porosity of shale as depth of burial increases .......................................................... 20

14. Map showing average porosity of samples from generalized localities of the pierre Shale and Equivalent strata ........................ 21

15. Maps showing average proportion of illite-type layers in mixed-layer illite-smectite from generalized localities of the Pierre Shale

and equivalent strata ............................................................................................................................................................. 21

16. Graph showing common proportions of illite-type layers in mixed-layer clay originally deposited and that developed by depth

diagenesis in the Pierre Shale and equivalent strata ............................................................................................................. 26
IV CONTENTS

TABLES

Page

TABLE 1. Typical mineralogical composition of the Pierre Shale and equivalent rocks ............................................................................... 4

2. Average structural formulas of mixed-layer illite-smectite clay in the Pierre Shale and equivalent rocks .................................... 13

3. Chemical analyses and structural formulas for 15 purified samples of mixed-layer illite-smectite from the Pierre Shale and equivalent rocks ............................................................................................................................................................................... 14

4. Average structural formulas of layers in mixed-layer illite-smectite from the Pierre Shale and equivalent rocks ........................ 16

5. Dehydroxylation peak temperature and size and amount of deviation from ideal structural water in 15 purified mixed-layer

illite-smectites from the Pierre Shale and equivalent rocks ................................................................................................... 17

6. Mineralogy and dehydroxylation peak temperatures of samples of bentonite, adjacent shale, and an argillized sill of volcanic rock 

from the Dearborn River locality within the Montana disturbed belt and of three bentonite samples from equivalents of the 

Pierre Shale outside the belt that contain mixed-layer illite-smectite .................................................................................... 19

7. Mineralogy and dehydroxylation peak temperatures of shale and bentonite cuttings of Pierre age at depths of 10,000 feet or more 

in the Powder River basin and of samples from stratigraphically equivalent rocks from the nearest outcrops at the Salt Creek 

locality ..................................................................................................................................................................^ 22

8. Mineralogy and dehydroxylation peak temperatures of shale and bentonite samples from the Montana disturbed belt ............. 24
GEOCHEMISTRY OF THE PIERRE SHALE AND 

EQUIVALENT ROCKS OF LATE CRETACEOUS AGE

MIXED-LAYER CLAY IN THE PIERRE SHALE AND 

EQUIVALENT ROCKS, NORTHERN GREAT PLAINS REGION

By LEONARD G. SCHULTZ

ABSTRACT

Mixed-layer illite-smectite is the most abundant constituent in much of 

the Pierre Shale and equivalent rocks. X-ray diffraction data on both peak 

positions and intensities have been used to distinguish illite-, beidellite-, and 

montmorillonite-type layers and to evaluate their proportions. The 

proportion of beidellite is fairly constant at about one-third of the layers. In 

most samples, illite makes up from 20 to 60 percent of the layers. 

Montmorillonite makes up from 5 to 60 percent of the layers and is 

generally complementary to illite. Smectite in bentonite commonly is 

composed of 60-90 percent montmorillonite-type layers, the remainder 

being beidellite-type. Interlayering is random.

Average structural formulas for the mixed-layer illite-smectite inferred 

from 191 whole samples of shale and 22 samples of bentonite and from 15 

purified and fractionated samples of shale are as follows (X denotes 

exchangeable cations, ~ denotes approximate):

X+ (Alt' +i Mg+2 ) A1+ 1 Si+<) O-2 (OH)

Illite-smectite in shale ..... 

Individual layers in shale:

Illite .........................

Beidellite ..................

Montmorillonite ......

Smectite in bentonite ......

0.7 0.6 2.8 0.5 O.I 0.6 0.6 7.4

1.2 .2 2.9

.2 1.0 2.9

.1 .7 2.8

.1 .6 3.0

1.0 

.6 

.1

.2

7.0 

7.4 

7.9 

7.8

20 ~ 4

•20 ~4

/20 ~M

20 4

The mixed-layer clay differs little in marine and nonmarine rocks. 

Thermogravimetric measurements of clay purified from shale generally 

indicate that OH" deviates slightly but significantly from the ideal, and the 

nonideal structural OH- apparently causes dehydroxylation at 

temperatures consistently lower than those typical of montmorillonite in 

bentonite that has an ideal 4(OH). The relatively high dehydroxylation 

temperatures of bentonite persist even where its normal montmorillonitic 

composition has been altered to highly illitic mixed-layer clay.

In the Montana disturbed belt in northwestern Montana something akin 

to depth diagenesis, as it is commonly called in the Gulf Coast, has altered 

much of the clay to regularly interlayered IS-ordered mixed layer clay 

containing 60-85 percent illite-type layers. Such clay occurs in shale or 

claystone, in bentonite, and even locally in argillized intrusive igneous rock. 

Almost all of the randomly mixed-layer clay outside the disturbed belt 

apparently is essentially as it was deposited and has not been altered by 

depth diagenesis. In the few places outside the disturbed belt where clay has 

been altered diagenetically, such as in cuttings from deep wells in the 

Powder River basin, it is the montmorillonite in bentonite beds rather than 

the mixed-layer clay in the shale that is altered first.

INTRODUCTION

The Pierre Shale Project has been a long-term study by H. 

A. Tourtelot, the late J. R. Gill, and L. G. Schultz of the 

geochemistry, mineralogy, and physical properties of a large 

mass of sedimentary rock, mainly shale, such as has 

contributed significantly through geologic time to the 

makeup of the Earth's crust. Several preliminary and related 

reports have been published as U.S. Geological Survey 

Professional Papers 390, 391-A-C, and 392-A and B. The 

final report of the Pierre Shale Project, informally referred 

to herein as the main Pierre Shale report, will be 

Professional Paper 1064-B. The purpose of the present 

report is to give details of the interpretation of X-ray 

diffraction patterns and of the chemical composition of the 

mixed-layer clays in the Pierre Shale. The range of composition of these mixed-layer clays is considered in terms of their 

origin, particularly with regard to depth diagenesis.

The bulk of the material in this report came directly from 

the Pierre Shale Project and is representative of the rock unit 

and its equivalents in the area of study across the northern 

Great Plains. Two smaller complementary investigations are 

also described that are directed specifically to the problem of 

depth diagenesis. One is of well cuttings in the Powder River 

basin, and the other is of mixed-layer clay in the Montana 

disturbed belt.

GENERAL GEOLOGY AND 

MINERALOGY

In the northern Great Plains the Pierre Shale and 

equivalent rocks form a wedge (fig. 1) composed in the west 

of 2,000-5,000 feet of predominantly nonmarine claystone, 

sandstone, and conglomerate, and in the east along the 

Missouri River of less than 500 to 1,000 feet of dark-gray 

marine shale containing some marl. In the west, volcanic
GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

Central 

Montana

South Dakota

Missouri River

area

s* Mary Rive, 

Formation

Elkhorn 

Mountains 

Volcanics

200 MILES

APPROXIMATE SCALES

FIGURE 1. Diagrammatic cross section of the Pierre Shale and some of the equivalent marine and nonmarine rocks. Nonmarine rocks are shaded.

debris derived locally from the Elkhorn Mountains volcanic 

field is abundant, and thin bentonite beds occur throughout. 

Sediments of the Pierre Shale and equivalent rocks came 

mostly from the western side of the basin. In the area studied 

the name Pierre Shale applies strictly only to rocks along the 

Missouri River and around the Black Hills, other formation 

names being used to the west (fig. 1). However, in this report 

Pierre Shale and equivalent rocks refers to the whole body of 

rock investigated, including the equivalent marine and 

nonmarine rocks. For the main study of the Pierre Shale, 

this rock body was sampled at stratigraphic sections 

indicated by the numbered localities in figure 2. Details of 

the composition of these rocks will be given in the main 

Pierre Shale report. Only a brief summary is given here as a 

framework for the occurrence of the mixed-layer smectitic 

clays.

The common mineralogical composition of the Pierre 

Shale and its equivalents is fairly uniform (table 1). 

Variation in proportions of minerals is considerably greater 

in the western predominantly nonmarine claystone than in

the eastern marine shale, but the average composition differs 

little. The most consistent differences are the slightly higher 

quartz and considerably higher plagioclase content of the 

western samples. In both, kaolinite and chlorite generally are 

minor and make up less than 10 percent of the clay minerals, 

illite commonly makes up 15-20 percent of the clay minerals, 

and mixed-layer clays make up half or more of the 

clay-mineral content and 30-60 percent of the total rock. 

These mixed-layer clays are the principal concern in this 

report.

Composition of bentonite in the Pierre Shale and its 

equivalents varies widely, as shown in the main Pierre Shale 

report, but the composition shown in table 1 is representative of most of it. Completely expanding smectite is 

predominant and may be the only constituent found in 

bentonite. A few percent of kaolinite is found in about 

one-tenth of the bentonite samples, whereas chlorite or illite 

are rare or absent. Quartz rarely makes up more than a few 

percent of most bentonite, and plagioclase and biotite may 

be abundant, particularly at the base of the bed.
116° 114°

MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 

112° 110° 108° 106° 104°_____102°_____100° 96°

> Banff I

) Calgary

50° % ALBERTA

100 200 MILES

100 KILOMETERS

-A...

48°

I

—————-I_____CANADA

4 UNITED STATES" "| 

46°

D,

w)

Elkhorn 

Mountains 

Volcanics

MONTANA 14

15

44°

13

iT 

£<a

IDAHO

Powder 

^. River 

o ^ Basin

WYOMING

42°

161______________________X2

FIGURE 2. Index map of the northern Great Plains region, showing sample localities and pertinent physiographic features. Numbers 

indicate the general area of localities from the main study of the Pierre Shale and equivalent rocks, which are described by Schultz, 

Tourtelot, and Gill (1978). Letters indicate localities in the Montana disturbed belt (table 8); well-cutting locality (table 7) is starred (*).

DEFINITION OF MINERAL AND 

MIXED-LAYER TERMS

Most minerals are essentially homogeneous crystals 

composed of orderly repetitions of unit cells of similar 

composition and geometry. Mixed-layer clay minerals differ 

in that individual layers of the crystal have different 

compositions and properties.

Commonly, layers in mixed-layer clays of Pierre age are 

illite-type and smectite-type. Two kinds of smectite-type 

layers also are differentiated, those that are 

montmorillonite-type and those that are beidellite-type. 

Structures of all of these clay-mineral types (fig. 3) have a 

middle sheet of alumina octahedra and two outer sheets of 

silica tetrahedra. In aluminous varieties like those in the 

Pierre Shale, only two of every three octahedral cation 

positions are filled (fig. 3). Cations substituted for Al+3 and 

Si44 cause a net negative layer charge. The negative layer 

charge of montmorillonite is due mainly to substitution of

divalent Mg+2 ions for trivalent Al +3 ions in the octahedral 

sheet (fig. 3). The negative layer charge of beidellite is due 

mainly to substitution of trivalent A1+3 for quadrivalent Si+4 

ions in the tetrahedral sheets. The negative layer charge of 

smectites is balanced by positively charged interlayer 

cations, most commonly Na+ and Ca+2 , which are exchangeable and commonly hydrated. The layers are said to 

be expandable because water or other substances readily 

penetrate between the layers. Two organic liquids commonly 

used to expand smectites to known thicknesses are ethylene 

glycol, which produces a 17-angstrom (A) layer thickness, 

and glycerol, which produces an 18-A layer thickenss. The 

characteristic feature of illite layers is that they are nonexpanding and have a fixed layer thickness of 10 A. This is 

because illite layers have a net negative layer charge that is 

balanced predominantly by interlayer K+ cations. The K+ 

cations are no hydrated, and they fit snugly into the silica 

rings of adjacent layers. Bonding of K+ to the rings of oxygen 

anions prevents penetration of water between the illite
GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

TABLE 1. Typical mineralogical composition, in percent, of the Pierre 

Shale and equivalent rocks

SHALE

West

Clay minerals .......................................................................... 50-65

Clay composition:

Mixed-layer ......................................................... 50-100

lllite .................................................................... 0-20

Chlorite ............................................................... 0-5

Kaolinite ............................................................. 0-20

Quartz ..................................................................................... 20-30

Plagioclase .............................................................................. 5-10

K-feldspar and other minerals ..................................................... (')

East

Clay minerals ............................................................................... 75

Clay composition:

Mixed-layer ................................................................ 75

lllite ........................................................................... 15

Chlorite ...................................................................... 5

Kaolinite .................................................................... 5

Quartz ..................................................................................... 15-20

Plagioclase, K-feldspar and other minerals .................................. (')

BENTONITE

Clay minerals .......................................................................... 80-100

Clay composition:

Smectite ................................................................... 100

Quartz ....................................................................................... 0-5

Plagioclase ................................................................................ 0-10

Biotite ....................................................................................... 0-10

'A minor amount.

layers. The layer charge of illite, like that of beidellite, is 

predominantly in the tetrahedral layers, and the charge is 

somewhat larger than in the smectites.

Throughout this paper, layers of mixed-layer clay are 

referred to as illite-type or montmorillonite-type. The 

hyphenated "type" terminology is used to alert the reader to 

two things. First, the layers are parts of a mixed-layer clay. 

Second, strictly speaking, what is being referred to is not a 

clay layer as normally thought of, but rather the interlayer 

space and the nearest halves of the two adjacent layers. The 

identity of the layer is determined by the behavior of the 

interlayer area, which in turn is determined primarily by the 

makeup of the adjacent layers. In mixed-layer clay, a 

classical layer must commonly be asymmetrical. For example, in figure 3, the upper half of the layer cross section 

together with the nonexpanding interlayer space having K+ 

ions and part of the adjacent layer above (not shown) would 

behave like and could be said to be an illite-type layer, 

whereas the lower half of the layer cross section and 

corresponding adjacent parts below would behave like and 

could be said to be either montmorillonite- or smectite-type.

Interlayering of different types of clay minerals in 

mixed-layer clay may be random or regular, and partly 

regular types also are possible.

Random interlayering implies no detectable influence of 

one layer on adjacent neighbors; in other words, the 

probability of one type of layer being adjacent to another is 

the same as its abundance in the mixed-layer crystal. Basal 

X-ray diffraction peaks of randomly interlayered clay 

normally lie (migrate) between d-spacing positions of the 

types of layers involved, depending upon the proportion of 

the layers. However the 17-A and 10-A d-spacings of 

glycolated smectite and of illite do not migrate, although 

higher basal-order reflections do. X-ray peaks of mixed-layer clay are commonly designated by the basal order of 

the mineral layers between which they are migrating; for 

example, the peak resulting from the third basal order of 

smectite-type layers and the second basal order of illite-type 

layers is herein designated as an S0o3/ Ioo2 reflection. Randomly interlayered illite-smectite is the most common type 

of clay in the Pierre Shale.

Regular or ordered interlayering implies a periodicity in 

the sequence of layers; in other words, one type of layer tends 

to follow a different type, and tends not to be adjacent to 

layers like itself. Any degree of regularity theoretically may 

occur, from none (random) to the maximum permitted by 

the proportion of layers. However, regularity is easily

EXPLANATION

O O-2 Si+4

OH- O Al+3

^ ) Interlayer cation

- - - Boundary of unit 

cell

> SiO4 tetrahedra 

A1O4(OH)2 octahedra

• SiO4 tetrahedra

( ) Na+ and Ca+2 in smectite

FIGURE 3. Schematic structure of an aluminum smectite or illite layer, 

adapted from Bragg(1937, p. 206). Above is a plan view of a silica sheet 

showing one complete ring of linked silica tetrahedra and parts of 

others, with a hydroxyl at the center of the rings. Below is a layer cross 

section showing two outside silica sheets sharing apical oxygen ions 

with a middle sheet of alumina octahedra. Certain O"2 ions that 

otherwise would be superimposed have been slightly displaced. Al +3 

substitutes for Si*4 in beidellite and illite tetrahedra; Mg" substitutes 

for A I*" 3 in montmorillonite octahedra.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION

recognized only when the degree of regularity is high. 

Complete regularity commonly occurs only when the two 

types of layers involved are equally abundant. Such regular 

interlayering produces a superorder reflection at a d-spacing 

that is the sum of the thicknesses of the two layers, and this 

superorder is accompanied by a rational sequence of basal 

reflections. For example, 10-Aand 17-A ordered interlayers 

produce a basal sequence of 27-A, 13.5-A, 9-A, 6.75-A, ... 

peaks. Completely regular mixed layering is rare in the 

Pierre Shale and usually involves chlorite-type layers which 

are not considered in this report.

Still another kind of partly regular interlayering has been 

described by Reynolds (1967) and Reynolds and Hower 

(1970). Probably the most common such interlayering 

involves "an illite-montmorillonite superlattice (IM), randomly interstratified with additional illite" (Reynolds and 

Hower, 1970, p. 26); in other words, no two smectite layers 

(or expanding interlayer areas) are adjacent to each other. 

The characteristic feature of such clay is that, unlike 

randomly interlayered types, it produces an X-ray peak that, 

as the proportions of layers change, migrates in the area 

between 10 A and 17 A commonly near 12-13 A. Such clay 

is abundant in some equivalents of the Pierre Shale, and in 

this report such clay is said to have IS-ordered interlayering. 

The IS in this paper, rather than the IM of Reynolds and 

Hower, reflects the use in this paper of smectite rather than 

montmorillonite as the general term for expandable clay.

Readers needing a more detailed explanation of mixed 

layering in clay minerals are referred to MacEwan, Amil, 

and Brown (1961) and to the papers of Reynolds and Hower 

mentioned above.

INTERPRETATION OF X-RAY 

DIFFRACTION TRACES OF 

MIXED-LAYER CLAY

X-ray diffraction procedures used for quantitative evaluation of the mineralogy of the Pierre Shale (Schultz, 1964) are 

fairly standard for most minerals. Procedures used for 

mixed-layer expandable clays are in part unconventional 

and have been changed from the earlier work. The new 

method for evaluation of mixed-layer clays is described in 

detail below and is compared with the old method. The 

methods are illustrated by sets of X-ray diffraction traces for 

four samples in figure 4. The new method involves a two-step 

breakdown of the mixed-layer clay. The first step, which 

distinguishes proportions of illite-type and smectite-type 

layers, is illustrated in figure 4 A-D, and is described in the 

next section of the report. The second step, which divides 

smectite into beidellite- and montmorillonite-type layers is 

illustrated in figure 4 A'-D' and is described in the 

second-following section. Both steps involve a conventional 

peak-migration method for evaluation of proportions of 

layers as well as an alternative unconventional peak-height

method that can be applied to many samples for which peak 

migration cannot be used.

ILLITE-SMECTITE

Almost all expandable clay from shale in the Pierre Shale 

shows evidence of interlaying with nonexpandable illite-type 

layers. Completely expandable smectite (not shown in fig. 4) 

is common only in bentonite. One of the more completely 

expanding mixed-layer clays from a shale is illustrated in 

figure 4A. Treated with ethylene glycol, the first basal 

reflection at 17 A differs little from that of a completely 

expanding smectite from a bentonite bed, but the higher 

order reflections are fairly broad and slightly shifted from 

the ideal d-spacings (S.oo2'So03<fig. 4). As the proportion of 

nonexpanding layers increases (fig. 4#), the higher order 

basal reflections shift or migrate farther toward the 

d-spacing positions of the nonexpanding illite-type layers 

(Iooi> Ioo2). but the first basal reflection remains near 17 A, 

merely becoming broader and less intense, until it almost 

completely disappears (fig. 4Q.

Clays like those in figure 4A-C are randomly interlayered. 

Clays having appreciably more illite-type layers than the one 

shown in figure 4Cinvariably have IS-ordered interlayering. 

As previously defined. IS-ordered illite-smectite diffracts 

X-rays in the approximately 11.5-14-A range that is not 

possible for randomly interlayered illite-smectite. Most 

IS-ordered clay from the Pierre Shale shows no super order 

reflection that is characteristic of completely ordered mixed-layer clay. However, a few do give a small reflection from 

glycolated material that is almost invariably near 31 A. The 

31-A d-spacing, however, is nowhere near a rational multiple 

of the much stronger basal reflection between 11.5 A and 

14A.

All four clay samples in figure 4 A-D produce well-defined reflections near 10 A and 5 A when heated to 300° C. 

Therefore, all are mixed-layer combinations of only two 

kinds of layers. One kind is nonexpanding illite, having an 

unvarying layer thickness of 10 A. The other kind is 

expanding smectite, which contracts to a thickness of 

approximately 9.6 A when heated. The 10-A- and 

9.6-A-layer thicknesses are so nearly similar that they 

produce a fairly sharp X-ray reflection not noticeably 

displaced from the 10-A position (I00) , fig. 4), but the second 

order is noticeably displaced from 5 A (Ioo2).

The most useful migration curve for determining proportions of illite- and smectite-type layers is that involving the 

8003 and Ioo2 d-spacings for glycolated material. As will be 

explained later in connection with figure 6, the Soo3/I()02 

reflection indicates approximately 20, 45, 60 and 75 percent 

nonexpanding illite-type layers, respectively, for the samples 

of figure 4 A-D ("new" method). However, owing to the 

weak intensity of the reflection and some interference from 

other minerals, only about one-tenth of the total number of 

samples could be analyzed by peak migration such as was
GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

S=

S=

New Value 

1=76

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M=0

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DEGREES 2 9 20

FIGURE 4. X-ray diffraction traces of four clay samples that illustrate the range of composition of mixed-layer 

illite-smectite in rocks of Pierre age and the new and old methods of expressing proportions of layers. Curves 

A'-D' are of the same four clays as A-D, but are Li+ treated. Other treatments are: solid line, ethylene glycol; 

long-dashed line, glycerol; and short-dashed line, heated. Short arrows show ideal 001 peak positions of 

smectite (S), illite (1), beidellite (B), and montmorillonite (M); X is mixed-layer clay (see text); long arrows 

show 20 positions or angstrom (A) values of observed peaks. CuK« radiation.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION

done with the exceptionally pure and well-oriented clays 

illustrated in figure 4.

During the early part of the Pierre Shale study, owing to 

the limited usefulness of peak migration, a peak-height 

method previously described (Schultz, 1964) was used to 

indicate the amount of expandable clay. This "old" 

peak-height method (fig. 4) divided what is now called 

mixed-layer clay into two components. One component, 

which would give a 17-A reflection of the observed relative 

height, was called montmorillonite (in the group sense S in 

fig. 4); the other component, which was assumed not to 

diffract at the 17-A position, was called mixed-layer clay (X 

in fig. 4). The basis for the separation was the observation 

that, for 40-millimeter long oriented aggregate mounts and a 

1° beam slit, completely expanding aluminum smectite gave 

a 17-A reflection about 4'/£ times the height of the 10-A 

reflection after heating. The absolute height of the reflections depended greatly upon the degree of preferred orientation, but the ratio of the 17-A peak height to the 10-A peak 

height from the same sample mount was fairly reproducible. 

In figure 4A, heights of the 17-A and 10-A reflections in 

arbitrary units are 105 and 30, respectively. A clay having a 

30-unit peak height when heated, if it were entirely expanding smectite, should give a 135-unit (30 X 4'/£) peak height 

at 17 A when solvated with ethylene glycol. The observed 

105-unit peak height is 78 percent of the expected height, so 

78 percent of the expanding clay was attributed to smectite 

(S) and the remaining 22 percent to mixed-layer clay (X). 

Values for smectite (S) and mixed-layer clay (X) are similarly 

calculated by the "old" method for figure 4 B-D.

A reasonably consistent relationship developed, as the 

Pierre study progressed, between the old peak-height 

method and the proportion of illite-and smectite-type layers 

that could be interpreted from peak migration. As shown in 

figure 5, the 17:10 A ratio decreases from about 4.5 for 

completely expanding smectite to zero where illite-type 

layers make up 60-65 percent of the layers. Thus, in samples 

where illite and smectite are the only two components of the 

mixed-layer clay, as is the case with most rocks of Pierre age, 

the 17:10-A ratio can be used in conjunction with figure 5 to 

estimate the approximate proportion of the two different 

layers. The estimate from the new peak-height method is 

more realistic than that from the old method, because rarely 

does any unmixed smectite occur in shale of the Pierre. Most 

samples do contain essentially unmixed illite or mica, which 

gives a 10-A reflection after all sample treatments. An 

adjustment for such illite must be made in the 10-A peak 

intensity (Schultz, 1964, fig. 3, table 2) before the peak height 

ratios (fig. 5) are used.

ILLITE-BEIDELLITE-MONTMORILLONITE

Beidellite, as pointed out in connection with figure 3,

differs from montmorillonite in that the layer charge of

beidellite is mainly in the tetrahedral layers owing to

20 40 60 

ILLITE-TYPE LAYERS. IN PERCENT

80

FIGURE 5. Relationship between the proportion of smectite (S) and 

mixed-layer clay (X) interpreted from peak heights (Schultz, 1964) and 

the percentage of illite-type layers interpreted from peak migration 

(S00_,/I(K,2 , fig. 4). Plotted points are individual samples. Solid line is 

curve used to interpret the percentage of illite-type layers. A, 

angstrom.

Al +3-for-Si+4 substitution, whereas the layer charge of 

montmorillonite is mainly in the octahedral layers owing 

mostly to Mg+2-for-Al+3 substitution. The Greene-Kelly 

(1955) lithium test uses this charge-site difference to differentiate between beidellite and montmorillonite by making only 

the latter nonexpanding. The process is herein called a 

Li+-200° C-glycerol treatment. Exchangeable cations are 

replaced with Li + by treatment with 3N LiCl. Overnight 

heating at 200 ° C volatilizes interlayer water, and the layer 

thickness decreases to about 10 A. If the layer is 

montmorillonite-type, the small Li+ ions enter vacant cation 

sites in the octahedral layer (fig. 3) and neutralize the 

negative layer charge caused by Mg+2 -for-Al +3 substitution. 

Thus, the uncharged layer becomes nonexpanding and 

retains a 10-A thickness when treated with glycerol. 

Beidellite, similarly treated, remains expanding because its 

layer charge is in the tetrahedral layer which contains no 

vacant cation sites for the Li+ ions to enter.

Proportions of beidellite- and montmorillonite-type 

layers can be qualitatively evaluated by visually comparing 

X-ray traces before and after the Li+-200° C treated clay is
GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

reexpanded by glycerol (fig. 4 A '-D') with traces of the same 

clay not treated . with Li+. If no reexpansion of the 

Untreated clay occurred, the 10-A reflection (fig. 4 A'-D') 

before and after glycerol treatment would be unchanged, 

and all smectite layers would be interpreted as montmorillonite. If the trace of the glycerol-treated clay were 

essentially the same as that of the natural clay expanded with 

ethylene glycol (fig. 4 A '-£>'), then smectite layers are 

interpreted as beidellite. In the latter case (fig. 4D and 4D') 

the traces shown are not actually identical, because clays 

expanded by glycerol and ethylene glycol produce slightly 

different spacing about 18 A and 17 A, respectively. (In 

practice, the Li+-treated clays were compared with 

Mg+2-glycerol-treated samples routinely run for identification of vermiculite. These are not shown in figure 4.) 

Commonly, results are somewhere in between(fig. 4 A'-C), 

with an appreciable drop in the intensity of the 10-A 

reflection after glycerol treatment and a rise in the baseline 

but no development of an 18-A reflection comparable to the 

17-A reflection of the ethylene glycol-treated clay. Apparently both types of layers are present.

The proportion of beidellite-type layers is conventionally 

evaluated from the position of the 001 reflections of the 

Li-2000 C-glycerol-treated clay. Greene-Kelly (1955) used 

the second beidellite reflection (fig. 4Df, 6002) and its shift 

toward the approximately 9.6-A position of collapsed 

montmorillonite. For the Pierre Shale, use of this refelction 

was generally not possible because the angular shift was 

small, and free illite at 10 A interfered; a shift of the fourth 

beidellite reflection (fig. 4D', 8004) toward higher angles was 

more useful. Also, Greene-Kelly used the lithium test for 

two-component interlayering of beidellite- and 

montmorillonite-type layers. For the Pierre Shale, a third 

component, the illite-type layers, must be taken into 

account.

Proportions of illite-, beidellite-, and 

montmorillonite-type layers are interpreted from the 

triangular diagram in figure 6. The numbered heavy lines 

within the triangle refer to the angular position of the peak in 

the 18°20 region for Li+-200°C-glycerol-treated material, 

for example, at 18.65° for figure 4A'. The corners of the 

triangle correspond to the biedellite (8004) spacing of 4.5 A at 

19.73° 28, the second basal spacing of collapsed montmorillonite of 4.8 A at 18.5° 20, and the second basal spacing 

of illite (looi) of 5.0 A at 17.74°20. The scale to the right of the 

triangle from 15.6° to 17.7°20 is the migration curve between 

the 5.7 A (S003) of smectite solvated with ethylene glycol at 

15.6° 20 and the 5.0 A (I002) of illite at 17.74° 20 that is used to 

determine the proportion of illite layers in figures 4A-D. For 

example, in figure 4A, the S0o3/Ioo2 reflection at 15.85°2-0 

corresponds in figure 6 to 20 percent illite this is where the 

20 percent illite in the new interpretation in figure 4A comes 

from. Then, the 18.65°20 position from the

Li-200°C-glycerol treated sample (fig. 4A') intersects the 

20-percent-illite line in figure 6 at the letter A. From A the 

composition lines project to 25 percent beidellite and 55 

percent montmorillonite, as recorded for the new composition in figure 4A. All proportions have been rounded to the 

nearest 5 percent. The Soo3/Ioq2 migrati°n curve on the right 

side of figure 6 is adapted from one calculated for randomly 

interlayered clay (MacEwan, Amil, and Brown, 1961, p. 

417), and, as pointed out by Reynolds and Hower(1970, p. 

33), the Soo3/Ioo2 migration curve is almost identical for 

IS-ordered and randomly interlayered illite-smectite.

The problem with the beidellite-montmorillonite interpretation, as with the illite-smectite interpretation, is that 

most samples give X-ray traces not nearly so good as those in 

figure 4A '-D', and the relevant reflections cannot be 

measured. Here again, as data accumulated from the Pierre 

Shale study, a rough relationship was noticed between the 

proportion of beidellite-type layers that remained expandable as interpreted from figure 6 and the change in intensity 

of the 10-A reflection before and after glycerol treatment. 

This change is labeled as a 30-percent drop in figure 4 A 'and 

50-, 55- and 60-percent drops in figure 4R-D'. This 

relationship is shown in figure 7. The decrease in intensity 

appears to depend not only on the proportion of reexpandable beidellite-type layers but also on the proportion of 

illite-type layers. For mixed-layer clays having few illite-type 

layers, the percent decrease in height of the 10-A reflection is 

approximately equal to the inferred percent of 

beidellite-type layers (for other smectites see Schultz, 1969 

fig. 2). If illite-type layers are dominant, however, the percent 

decrease in the 10-A peak height is equal to nearly double the 

percent beidellite-type layers. The lines relating the three 

factors in figure 7 are very approximate even more so than 

the peak-height relationship in figure 5 but the height 

relations have made possible the estimation of proportions 

of illite-, montmorillonite-, and beidellite-type layers in 

mixed layer clays from most samples of Pierre Shale.

The following example illustrates the peak-height method 

for figure 4B and 4R. The 17:10-A height ratio of 55:30=1.83 

would indicate, in figure 5, 44 percent illite-type layers, 

rounded to 45 percent. This illite value together with the 

50-percent peak drop from the Li+ test (fig. 4R) gives slightly 

more than 30 percent beidellite-type layers (fig. 7) which, 

when rounded to 30 percent, leaves 25 percent 

montmorillonite-type layers. The good agreement with 

proportions derived from migration curves (fig. 4B, "new 

value") is by no means universal, but values usually agree 

within 5 or 10 percent. The critical requisites for application 

of relative peak heights to the interpretation of mixed-layer 

clays are that the oriented aggregate must not be broken up 

during solvation or heating and that essentially all the layers 

must be either illite- or smectite-type having a basal 

^/-spacing near 10 A when heated.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION

PEAK POSITIONS OF IS-ORDERED CLAY

Unlike the 17-A reflection for randomly mixed-layer 

illite-smectite (fig. 4A-C), the first basal reflection for 

glycol-treated IS-ordered clay does vary over a considerable 

range, from about 11.5 A to very broad reflections at about 

14 A. Figure 8 shows the relationship between proportions 

of illite-type layers from 11 samples for which the Soo3/ Ioo2 

curve on the right side of figure 6 could be used and the 

^/-spacing of the first reflection. The range of d-spacings

observed is considerably greater than that shown by 

calculated diffraction profiles of maximum IS-ordered 

material described by Reynolds and Hower (1970, fig. 5, 

table 3). The d-spacings of about 13 A observed for 70 

percent illite-type layers are close to that calculated by 

Reynolds and Hower. However, several observed d-spacings 

of about 14A probably indicate that IS-ordered clay having 

less than 70 percent illite-type layers in the Pierre Shale tends 

to have less than maximum ordering. Conversely, many

60 70 80 90 Percent 100 

MONTMORILLONITE=4.8A

FIGURE 6. Diagram used to interpret proportions of illite: beidellite: montmorillonite layers in mixed-layer clay. The heavy lines within the 

triangle are the 20 positions of the B"oo4/Ioo2 reflections (fig. 4A '-£>'),and the points at which these lines intersect the legs of the triangle 

are from migration curves (MacEwan, Amil, and Brown, 1961, p. 417) adapted for an 18-A primary ^-spacing appropriate for glycerol. 

The scale next to the right-hand leg of the triangle is for the S003 /l oa2 reflection (fig. 4A-D) adapted for the 17-A primary J-spacing 

appropriate for ethylene glycol. Letters A-D within the triangle indicate composition of samples on figure 4. CuK^radiation; °, degrees 

20; A, angstrom.
10 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

Percent illite-type layers 

in mixed-layer clay

0=0-5

1=10-15

2=20-25

3=30-35

4=40-45

5=50-55

6=60-65

7=70-75

8=80-85

BEIDELLITE-TYPE LAYERS, IN PERCENT

FIGURE 7. Relationship between the proportion of beidellite- and illite-type layers in mixed-layer clay determined from migration curves 

and the change in peak height after glycerol treatment of Li+-saturated and heated clay. Illite-type layers rounded to nearest 5 percent.

observed d-spacings of about 12 A or less indicate that when 

there is appreciably more than 70 percent illite-type layers, 

superlattices larger than IS may be present, though no 

patterns like that for ideal ISII ordering (IMII ordering of 

Reynolds and Hower, 1970, fig. 7) were observed. Figure 8 is 

the ^-spacing curve used to evaluate the proportion of 

illite-type layers in IS-ordered clay in the Pierre Shale where 

only the first basal reflection could be measured.

COMMON PROPORTIONS OF 

ILLITE: BEIDELLITE: 

MONTMORILLONITE

Proportions of the three common types of layers in 

samples of mixed-layer clay from individual samples of the 

main Pieere Shale study are shown in figure 9. Beidellite
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 11

content is relatively uniform, averaging about 35 percent and 

deviating little more than 10 or 15 percent from the average. 

In most shales the proportions of illite-type layers vary more 

widely, from 10 or 15 percent to 60 percent; more than 60 

percent illite-type layers occurs mostly in samples having 

IS-ordered mixed-layering from the Dearborn River locality 

(fig. 2, loc. 2). Montmorillonite is generally complementary 

to the proportion of illite-type layers. Almost all smectite 

having no illite-type layers is from bentonite, and conversely, 

few of the approximately 200 bentonite samples examined 

contained detectable illite.

Beidellite layers are slightly more abundant in nonmarine 

than in marine rock, but the difference is slight. Excluding 

shale from Dearborn River, which has been altered, 

mixed-layer clays in marine rock are more illitic, but again 

the difference is slight.

CHEMISTRY OF MIXED-LAYER CLAYS

One objective of the Pierre Shale study was to determine 

whether sediments deposited in different environments, such 

as marine and nonmarine, could be distinguished by their 

chemistry or mineralogy. The mixed-layer illite-smectite was 

of particular interest in this regard because it is the most 

abundant component in rocks of Pierre age. Also, because of 

its fine grain size and large surface area, the mixed-layer clay 

might be expected to be particularly sensitive to changes in 

chemical environment. Two methods were used to determine 

the chemical makeup of the mixed-layer clay: (1) chemical 

analysis of the whole rock and then correcting for other 

mineral constituents (Schultz, 1964), and (2) chemical 

analysis of the purified mixed-layer clay component. The 

first method was used on 223 samples, for which chemical 

analyses are the basic data for much of the main Pierre Shale 

report. However, as minerals other than mixed-layer clay 

amounted to half or more of most samples, the structural 

formulas calculated for the clay may involve considerable 

error. The formulas calculated are neverthelass believed to 

be useful because any systematic errors that may be involved 

apply equally to marine and nonmarine samples; thus, a 

valid comparison of these two groups can still be made. The 

second method was used on only 15 purified samples, but the 

calculated structural formulas are much more accurate.

STRUCTURAL FORMULAS CALCULATED 

FROM WHOLE-ROCK SAMPLES

The transformation of a chemical analysis of a mineral to 

a structural formulas (table 2) involves showing where each 

chemical element is thought to occur in the structure of the 

mineral (fig. 3). Of the 223 chemical analyses in the main 

Pierre Shale report, structural formulas for mixed-layer clay 

that seemed fairly reasonable could be calculated from 219 

analyses. These include 191 from shale, 22 from bentonite, 

and 6 from marl. Formulas for shale and bentonite are 

considered separately below, but formulas for marl are not

14

13

12

1160 65 70 75 80 

ILLITE-TYPE LAYERS, IN PERCENT

85

FIGURE 8. Illite content of 11 IS-ordered mixed-layer clay samples 

determined from Soo3/l'oo2 (fig- 6) related to ^/-spacing of the first basal 

reflection of the clay treated with ethylene glycol. The solid line was 

used to interpret the percentage of illite-like layers when the S003 / Ioo2 

reflection was indistinct. CuK« radiation.

considered here because of the small number of samples and 

the small amount of clay that they contain.

SHALE

The range of composition of mixed-layer clay in marine 

and nonmarine shale is shown in figure 10, and averages are 

given in the upper part of table 2. The general similarity 

between mixed-layer clay in marine and nonmarine rocks is 

as expected from their mineralogical similarity (fig. 9). One 

of the bigger differences is the presence of more K+ ions in 

the nonmarine rocks. This difference is not in agreement 

with similar proportions of illite-type layers in marine and 

nonmarine rocks indicated in figure 9. Average K+ content 

for nonmarine rocks, exclusive of the nine chemically 

analysed samples from the Dearborn River locality (loc. 2), 

is 0.65. This average exceeds the 0.59 K+ average for marine 

rocks even though the average content of illite-type layers, 

exclusive of the Dearborn River locality samples (fig.9), is 

about 5 percent less than for marine rocks (fig. 9). These 

differences are small, and the lack of agreement between the 

K^ ion and content of illite-type layers probably indicates 

that the differences are not significant. Certainly, in view of 

the wide variation in proportions of illite layers shown in 

figure 9 and R+ shown in figure 10 within both marine and 

nonmarine samples, neither the amount of illite-type layers 

nor the amount of K+ in the mixed-layer clay would seem to 

be a reliable indicator of environment of deposition.

Tetrahedral Al +3 is generally higher in nonmarine rocks 

than in marine rocks. This difference is not due exclusively to 

abundant illite-type layers in samples from the Dearborn 

River locality without these, average tetrahedral Al+3 is 

still 0.52. The higher tetrahedral A1 +3 for nonmarine rocks is 

in accord with their somewhat higher beidellite content 

(fig.9). The difference appears to be real but much too small 

to serve as a reliable environmental indicator (fig. 10).
12 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

Two aspects of the structural formulas calculated from 

whole-rock analyses seemed unusual. First, the sum of the 

octahedral cations is less than the ideal 4.00 per unit cell for 

more than 80 percent of the samples (fig. 10), and second, the 

average of 0.44 tetrahedral A1+3 (table 2) seems too low for 

clays in most of which illite- and beidellite-type layers 

predominate. Both of these features could be caused by 

under-correction for quartz, a possibility that will be 

considered later.

BENTONITE

The structual formulas for smectite in bentonite differs in 

generally expected ways from those for mixed-layer clay in 

shale of Pierre age (table 2). As previously mentioned, 

smectite in nearly all bentonite samples in the Pierre Shale 

and in all of the 22 bentonite samples in table 2 are not 

interlayered with illite-type layers, and mortmorillonite 

layers predominate over beidellite. Therefore interlayer K+ 

ions are few, and tetrahedral Al +3 , which is low in 

montmorillonite, is low in the bentonite. The total interlayer

charge, averaging 0.94 for the bentonite samples, is only 

about two-thirds that for the shale samples. The amount of 

iron in the octahedral layer is about half that in shale, and the 

amount of magnesium is nearly the same. As the amount of 

minerals other than smectite is small in bentonite, averaging 

about 10 percent, the correction for other minerals is small, 

and the average structural formula for bentonite is a more 

reliable one than for shale. Note, for example, that the sum 

of the octahedral cations is very close to the ideal 4.00 per 

unit cell.

STRUCTURAL FORMULAS CALCULATED 

FROM PURIFIED MIXED-LAYER CLAY

Chemical analyses of mixed-layer illite-smectite concentrated from 15 selected samples of the Pierre Shale and 

equivalent rocks are given in table 3 together with calculated 

structural formulas. The samples were selected to give a 

representative range of proportions of illite-, beidellite-, and 

montmorillonite-type layers in marine and nonmarine

EXPLANATION

o Marine 

n Nonmarine 

A Bentonite

Solid symbols represent 

ssmples from Dearborn 

River locality

100

10 20 30 40 90 100 

MONTMORILLONITE

FIGURE 9. Proportions of layers in mixed-layer illite-smectite in the Pierre Shale and equivalent rocks. Letters M, N, and n are, 

respectively, the means for marine, nonmarine, and nonmarine exclusive of rocks from the Dearborn River locality (loc. 2). Values, in

percent, are at the 5 percent intersections about which the symbols cluster.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 13

TABLE 2. Average structural formulas of mixed-layer illite-smectite clay in the Pierre Shale and equivalent rocks, in cations per On(OH)4

K + Na+

Interlayer

Ca+2 Mg+ -'

Octahedral

Al H +

Total 

charge Al + ' He- F... M... Sum

Tetrahedral

Al*' Si 4'

Calculated from chemical analyses of whole-rock samples

Shale samples: 

191 total .........

134 marine ...... 

57 nonmarine .. 

Bentonite samples: 

22 total ...........

0.63 

.59 

.69

.09

0.42 

.42 

.41

.40

0.09 

.09 

.08

.12

0.01 

.01 

.01

.07

0.01 0.01 

.01 .01

.00 .02 

.02 .01

1.29 2.62 

1.25 2.64 

1.30 2.57

.94 3.00

0.68 0. 

.68 

.71

.35

11 0.51 

11 .51 

13 .54

04 .59

3.92 

3.94 

3.95

3.99

0.44 

.41

.55

.20

7.56 

7.59 

7.45

7.80

Calculated before fractionation of 15 shale samples below

15 shale samples .... 0.66 0.38 0.13 0.00 0.00 0.00 1.30 2.60 0.69 0. 10 0.48 3.87 0.47 7.53

Calculated from chemical analyses of purified < 0.25/1 samples (table 3)

Shale samples: 

15 total ...........

7 nonmarine ... 

Nonmarine less 2 

Dearborn River 

samples .......

0.58 

.52 

.63

.47

0.58 

.58 

.58

.68

0.02 

.02 

.02

.02

0.00 

.00 

.00

.00

0.00 0.00 

.00 .00 

.00 .00

.00 .00

1.20 2.80 

1.14 2.81 

1.25 2.79

1.19 2.78

0.53 0 

.50 

.56

.62

.15 0.58 

.15 .62 

.16 .54

.16 .49

4.06 

4.08 

4.05

4.05

0.64 

.60 

.68

.69

7.36 

7.40 

7.32

7.31

rocks, and are arranged in table 3 from left to right generally 

in order of increasing percent of illite layers and of 

decreasing montmorillonite layers. Average formulas for all 

15 samples and for the 8 marine and 7 nonmarine samples 

are given in the lower part of table 2. Total estimated 

amounts of impurities are 0-18 percent and average 7 

percent, in contrast with an average of 49 percent for other 

minerals estimated for the original 15 whole-rock samples.

The mixed-layer clay in the purified samples is not 

identical with that in the whole-rock samples because they 

represent only those clay particles smaller than 0.25 

micrometer (p.). According to X-ray analyses, the < 0.25M 

purified mixed-layer clays tend to be slightly less illitic and 

more montmorillonitic than mixed-layer clay in the samples 

as a whole, with average illite:beidellite:montmorillonite 

proportions of 43:32:25 and 47:37:16, respectively.

100 I I I

0.5 1.0 0 0.5 0 0.5 0 0.5 0 0.5 0.5 1.0 1.5 

Na+ Ca+2 Mg+2 Al+3 H+ Total charge

INTERLAYER CATIONS PER UNIT CELL

i i i i i i i i i wr

0.5 1.0 0

Fe+3 Fe+2

0.5 0 0.5 1.0 3.5 4.0 0

Mg+2 Sum

0.5 1.0

OCTAHEDRAL CATIONS PER UNIT CELL AI+3 TETRAHEDRAL IONS 

PER UNIT CELL

FIGURE 10. Cation abundance in structural formulas calculated per full unit cell, ideally O2o(OH}4 . Cumulative curves are for 134 marine

(solid line) and 57 nonmarine (dashed line) shales and siltstones.
14 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

TABLE 3. Chemical analyses and structural formulas for 15 purified samples of mixed-layer illite-smectite from the Pierre Shale and

equivalent rocks

[Samples were Na-saturated and purified of free iron oxides by Na-dithionate-citrate treatment (Jackson, 1958, p. 168) 

and concentrated by size-fractionation below 0.25/1* Samples are arranged generally from most montmorillonitic on 

the left to most illitic on the right; illite:beidellite:montmorillonite ratios are in table 5. M, marine; N, 

nonmarine;<less than. Analyses by L. Artis, S. Bolts, G. Chloe, J. Glenn, N. Smith, and D. Taylor]

Locality No. 

(fig. 2) .................

Depositional 

environment ........

.

...... 259550

23

M

16

M

7

N

1 CQR I *)

9

N

159763

7

M

259543

M

1 SQ707 1 ^OSm

13 5

M N

1 ^OHOS 1 ^Q"77fl

I 6

N M

1 SQ7R 1

N

8

1 Q

M M

159834

2

159835

2

Chemical analyses, in percent

SiO2 ....................

AkO3 ..................

FelOa ..................

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

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

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

NazO ...................

K2 0 .....................

H2 0 ....................

H2O+ ...................

TiOl ....................

C02, .....................

60.2 

16.1

5.2 

.72 

2.4 

.08

1.9 

2.2 

5.6 

4.9 

.50 

.08

55.2 

19.0 

4.6 

1.1 

2.8 

.08

2.3 

2.0 

7.1 

5.4 

.48 

<.05

54.9 

18.9 

4.4 

1.6 

2.3 

.13

2.1 

2.6 

7.0 

5.6 

.66 

<.05

52.3 

20.2 

5.1 

1.1 

2.5 

.00

2.3 

2.3 

7.6 

5.6 

.52 

.09

50.7 53.3 

22.4 18.7 

4.3 6.5 

1.7 1.0 

3.1 2.6 

.14 .13

2.1 2.0 

3.0 3.0 

6.2 6.3 

5.8 5.7 

.66 .56 

<-.05 .11

51.7 49.8 

21.7 20.5 

4.5 6.4 

1.0 1-3 

2.6 2.4 

.11 -12

2.0 2.5 

2.6 2.3 

6.7 7.9 

6.3 5.2 

.64 1.2 

<.05 .05

49.1 

20.6

7.5 

1.2 

1.9

.22

2.6 

2.4 

8.0 

5.5 

.78 

<.05

52.5 

21.0 

3.8 

1.3 

3.2 

.26

1.7 

3.8 

5.9 

6.0 

.54 

.12

50.0 

22.8 

4.7 

1.4 

2.1 

.12

2.2 

3.4 

6.7 

5.6 

1.0 

.08

50.4 

23.0 

4.1 

1.1 

2.6 

.23

1.7 

3.2 

6.0 

6.7 

.70 

.10

50.9 

24.6 

2.7 

1.4 

2.6 

.04

1.4 

4.4 

4.5 

6.6 

.44 

<.05

52.2 

23.3 

3.3 

1.3 

2.6 

.00

1.5 

5.3 

5.0 

5.0 

.13 

.05

52.3 

19.1 

4.5 

1.9 

3.6 

.16

1.0 

6.2 

4.4 

5.9 

.47 

.08

Impurities corrected for, in percent

Illite .....................

1 

10 

0

2

1 

0 

1

3

4 

0 

2 

8

1 

0 

0

2

1 1 

0 0 

4 1 

9 3

1 0 

0 0 

4 0 

8 0

0 

0 

0 

0

1 

0

2 

5

0 

0 

3 

0

0 

0 

3 

10

0 

0

5 

13

0 

0 

1 

0

0 

0 

0 

0

Interlayer cations per C>2o(OH)4

Kt ........................

Nai. ......................

Ca+2 .....................

Total charge

0.42 

.59 

.01 

1.03

0.34 

.66 

.01 

1.02

0.43 

.68 

.02 

1.15

0.41 

.66 

.00 

1.07

0.50 0.53 

.68 .58 

.02 .02 

1.22 1.15

0.43 0.43 

.64 .71 

.02 .02 

1.11 1.18

0.45 

.73 

.03 

1.24

0.68 

.51 

.04

1.27

0.64 

.63 

.02 

1.31

0.53 

.55 

.04 

1.16

0.77 

.46 

.01 

1.25

0.95 

.40 

.00 

1.35

1.13 

.27 

.03 

1.46

Octahedral cations per Oa(OH) 4

Al« ......................

Fe-1- 3 .....................

Fe+2

Mgt2 ....................

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

2.69 

.62 

.10

.57 

3.98

2.77 

.50 

.13 

.60 

4.00

2.74 

.51 

.21 

.53 

3.99

2.83 

.56 

.13 

.54 

4.06

2.76 2.60 

.49 .72 

.22 .12 

.72 .57 

4.19 4.01

2.84 2.72 

.51 .70 

.13 .16 

.60 .52 

4.08 4.10

2.67 

.82 

.15 

.41 

4.05

2.79 

.42 

.16 

.71 

4.08

2.91 

.52 

.17 

.46 

4.06

2.94 

.46 

.14 

.60 

4.14

3.05 

.28 

.19 

.60 

4.12

3.01 

.34 

.15 

.54 

4.04

2.62 

.48 

.14 

.76 

4.00

Tetrahedral cations per 0 2o(OH)1

A1+3 ......................

Si+« ......................

0.26

7.74

0.32 

7.68

0.38 

7.62

0.59

7.41

0.84 0.48 

7.16 7.52

0.64 0.78 

7.36 7.22

0.86 

7.14

0.62

7.38

0.84 

7.16

0.78

7.22

0.82

7.18

0.77 

7.23

0.57 

7.43

Therefore, the chemical differences shown on table 2 

between the 15 samples before and after fractionation do not 

necessarily represent error. The higher K+ content for the 15 

whole-rock samples clearly reflects more illite-type layers. 

The lower total interlayer change for the fractionated sample 

is in accord with thier lower illite and higher montmorillonite content. The high Nat- and low Ca+2-ion 

content of the purified samples results from the 

Na+-citrate-dithionate treatment intended to reduce and 

remove free ferric iron oxides, because the treatment also 

replaces Ca^ 2 cations with Na+ . The higher Fe+2 and lower 

Fe^ 3 contents of the purified samples probably indicates that 

some Fe+3 in the mixed-layer structure was reduced to Fe+2

by the dithionate treatment, but the lower total Fe of the 

<0.25/i fraction of 0.68 as compared with 0.79 before 

fractionation may indicate that free iron oxide on the order 

of 0.5 percent is present in most samples and has been 

calculated incorrectly into structural formulas of mixed-layer clays in whole-rock samples. These iron differences 

apparently do not indicate that the more montmorillonitic 

fractionated clays are iron poor, because no such trend is 

evident in table 3.

Two differences between the structural formulas from the 

15 <0.25j« fractions and from the 191 whole-rock samples 

indicate errors in correction for impurities in the latter (table 

2). These differences for the 0.25ju clays are the higher
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 15

content of tetrahedral Alf 3 , averaging 0.64, and the higher 

sum of octahedral cations, averaging 4.06. As previously 

pointed out, what seem to be unrealistically low values 

calculated from whole samples of shale could result from 

systematic underestimation of quartz. The fact that all of the 

15 formulas show total octahedral cations very close to or 

above the ideal 4.00 per unit cell (table 3) indicates that the 

quartz corrections applied to whole-rock samples were, in 

fact, too low. An average of 24 percent quartz was corrected 

for when the 191 structural formulas for marine and 

nonmarine shale were calculated (table 2). If, instead, 26 

percent quartz were corrected for, then the tetrahedral part 

becomes (A1062 Si 736), and the octahedral part of the 

structural formula becomes (A1 262 Fe^Fe ;;2l2 Mg 054 ), 

having a sum of 4.00 cations. The main difference between 

this formula and that for the 0.25ju mixed-layer clay is the 

higher iron: in formulas computed from the whole-rock 

sample, which apparently reflects removal of free iron oxide 

from the purified < 0.25M samples.

In several other respects, however, the formulas for the 

fractionated samples support conclusions derived from the 

structural formulas calculated from whole-rock samples 

(table 2). Differences in total layer charge, iron content, and 

tetrahedral A1+ 3 between the mixed-layer clays and smectites in bentonite are confirmed. Small differences in average 

K+ of marine and nonmarine clays are also shown by the 

<0.25M clays, but are due to the two exceptionally illitic 

clays from Dearborn River. Finally, the seven fractionated 

nonmarine clays, like the unfractionated clays, tend to have 

more tetrahedral Al+3 and less octahedral Al+3 than the 

marine clays, but proportions of Mg+2 are reversed. All of 

the differences are much too small for use as indicators of 

environment of deposition. The most remarkable feature of 

the mixed-layer clay in the marine and nonmarine rocks 

appears to be its overall compositional similarity.

FORMULAS INFERRED FOR INDIVIDUAL ILLITE-, 

BEIDELLITE-, AND MONTMORILLONITE-TYPE LAYERS

Three features of the structural formulas of the 15 

mixed-layer clays (table 3) should be related to proportions 

of illite:beidellite:montmorillonite: (1) interlayer K+ ions 

should be concentrated in illite; (2) total layer charge 

balanced by interlayer cations should be higher in illite than 

in montmorillonite; and (3) the amount of tetrahedral A1+3

FIGURE 11 . Tetrahedral A 1*3, total layer charge, and interlayer K+ ions of 

structural formulas related to proportions of illite:beidellite:montmorillonite. A and flare averages, respectively, of 15 purified samples 

(table 3) and of 22 bentonite samples (table 2). Individual purified 

samples (table 3) are plotted as points except Dearborn River locality 

samples plotted as +. Solid lines are drawn through averages for shales 

so as to come as close as possible to plotted positions of individual 

samples, especially the 13 mixed-layer samples that are randomly 

interstratified. Dashed lines indicate appreciable differences between 

Dearborn River (loc. 2) samples and the other samples.

should be low in montmorillonite and high in beidellite and 

illite. In figure 11 these three measures are plotted for the 15 

purified samples (table 3), together with the average values

20 40 60 80 100 

MONTMORILLONITE-TYPE LAYERS, IN PERCENT

20 40 60 80 

ILLITE-TYPE LAYERS, IN PERCENT

100
16 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

for all 15 shale samples and for the 22 bentonite samples in 

table 2. The bentonite samples have an average proportion 

of 18 percent mortmorillonite layers.

Average structural formulas for individual layers of the 

mixed-layer clay given in table 4 are in part based on three 

generally accepted premises and a fourth one taken from 

table 3: (1) the total oftetrahedral Al f3 and Si+4 is 8.0; (2) the 

total of octahedral cations is about 4.0, but may vary slightly 

from this perhaps by 0.1 or 0.2 at the most; (3) the layer 

charge of montmorillonite is mostly in the octahedral layer, 

whereas the layer charge of beidellite and illite is mostly in 

the tetrahedral layer (fig. 3); and (4) the population of the 

octahedral layer of Fe+3 , Fe+2 , and Mg+2 is essentially the 

same for all three layers, as indicated by lack of consistent 

variation of values from the mixed-layer clays having a high 

proportion of montmorillonite layers to those having a high 

proportion of illite layers (table 3). Concerning this fourth 

point, montmorillonite-rich bentonites are low in iron (table 

2), but this low iron content seems not to apply to 

montmorillonite layers in shales as shown by 

montmorillonite-rich clays in samples 259550 and 259578 

(table 3).

Variable features of the structural formulas in table 4 are 

interpreted from figure 11 as follows: Intersections of the 

average lines with 0 and 100 percent on the left and right 

sides of the graphs, respectively, indicate the probable 

composition of the end members. The zero-percent intersection includes two types of layers, the probable proportions of 

which are indicated just inside the zero line. Total layer 

charge and tetrahedral Al"1" 3 projected to zero montmorillonite represent the average for illite and beidellite. 

Assuming about two to three times as many illitic as 

beidellitic layers (fig. 9), the 1.3 average total charge is 

allotted 1.4 to illite and 1.0 to beidellite; the 0.8-0.9 average 

tetrahedral A1+ 3 is allotted 1.0 to illite and 0.6 to beidellite. 

For 100 percent montmorillonite the average tetrahedral 

Al+3 must be near zero, probably no higher than 0.1; the 

average total layer charge must be 0.8 or 0.9; values of 0.1 

and 0.8 are assigned (table 4) so as to balance the layer 

charge. The average line for K+ ions clearly does not project 

to zero for zero illite, indicating that 0.1 or 0.2 K+ ions must 

be present before a detectable number of nonexpanding 

illite-type layers develop. The average K+ for 100 percent 

illite-type layers projects to about 1.1 or 1.2; the latter 

number is used in table 4 so as to reach a total interlayer 

charge of 1.4 without assuming too many cations that are 

normally exchangeable. Also, for the illite in table 4 extra 

octahedral Al+3 is assumed, thereby increasing the total of 

octahedral cations to 4.1 and reducing octahedral layer 

charge to -0.4; otherwise, the total charge of -1.7 (-0.7 

octahedral and -1.0 tetrahedral) could be balanced only by | 

an unreasonably large K + ion level that is not reached even in ' 

the IS-ordered clays from the Dearborn River locality. The , K+ ion and total interlayer charge in illite-type layers from ' 

Dearborn appear to be about 0.2 greater than in a unit cell of |

TABLE 4. Average structural formulas of layers in mixed-layer illite-smectitefrom the Pierre Shale and equivalent rocks inferredfrom tables 2 and 

3, and figure 11

[X, exchangeable cations]

Total cations . 1.4 

Total charge .. +1.4

4.1 

-.4

Beidellite 0.2 2.9 0.5 0.6 ;

Total cations . 1.0 4.1

Total charge .. +1.0 -.4

Montmorillonite (*I', * I',) (Al +3 Fet3 - ' O.I 0.7'

Total cations . .8 

Total charge .. -.8

4.0

-.7

8.0

-1.0

1 0.6 Sl 7.4^°20

8.0

-.6

8.0 

- 1

illite-type layers in normal, randomly mixed-layered clay. A 

total of 4.1 octahedral cations is similarly assumed for 

beidellite so as to decrease the octahedral-layer charge from 

-0.7 to -0.4 units per unit cell and keep it below the 

tetrahedral-layer charge of -0.6 as is required by definition.

STRUCTURAL OH

Structural water, OH", makes up four of the anions in an 

ideal unit cell of smectite or illite (table 2) and occurs in the 

octahedral layers ideally at the center of the silica rings at 

sites not shared with silica tetrahedrons (fig. 3). Structural 

OH- has been investigated by two techniques: (1) Differential Thermal Analysis (DTA), which measures heat that is 

either absorbed or given off as chemical reactions or 

structural changes occur on heating; and (2) Thermal 

Gravimetric Analysis (TGA), which measures changes in 

weight of the sample on heating.

Differential thermal curves typical of most shale and 

bentonite of Pierre age are illustrated by the two traces in 

figure 12. The endothermic reactions around 150°- 200° C 

are due to volatilization of adsorbed and interlayer water. 

Reations between 550° and 750° C are due to loss of 

structural OH- from the clay-mineral layers, where two OHmolecules combine to form a volatile H:O molecule and a 

residual O~2 ion.Reactions between 850°to 1000° C are due to 

final breakdown of the layer structures of the clay and 

formation of new minerals. The loss of structural OH- is the 

reaction of interest here.

Bentonite (fig. 12) typically shows a dehydroxylation 

endotherm having its major peak between 650° and 750° C. 

Such relatively high temperatures are generally considered 

to be characteristic of montmorillonite, which is, of course, 

the major constituent of bentonite. Shale of Pierre age, 

except for a few samples from the Dearborn River locality 

(table 5, samples 159834 and 159835), invariably has its 

major dehydroxylation endotherm between 550° and 600° C 

with or without a small shoulder between 650° and 700° C.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 17

SHALE

Area of low-temperature

dehydroxylation

endothernrv

Area of high-temperature

dehydroxylation

endotherm

BENTONITE

580'

Peak 

Temperatures

100 200 300 400 500 600

TEMPERATURE, IN DEGREES C

700 800 900 1000

FIGURE 12. Differential thermal analysis curves typical of shale and bentonite of Pierre age. Patterns between curves and dashed lines

indicate how peak areas in tables 5-8 were interpreted.

Only those samples in which montmorillonite-type layers are 

dominant, like 259550 and 259578 in table 5, gave an 

appreciable but still subordinate high-temperature endotherm. Initially the lower dehydroxylation temperature 

for the shale was thought to result from abundant 

Al+3 -for-Si+4 substitution such as is characteristic of illite 

and beidellite. However, this explanation seemed inadequate for some of the more montmorillonitic samples, and 

initial calculations of structural formulas for smectitic clay 

in many Pierre Shale samples (fig. 10) did not indicate very 

high tetrahedral Al"1" 3 .

The just-mentioned conflict led to a study of 

Greene-Kelly's (1955) lithium test as a means for differentiating between beidellite and montmorillonite in the Pierre 

Shale and in aluminous smectites generally (Schultz, 1969). 

From this study it was concluded that the lithium test is 

reliable. A second conclusion was that tetrahedral Al" 3 , 

though generally related to dehydroxylation temperature, is 

not the primary cause for major variations. Instead, the 

primary cause of variation indicated by thermal gravimetric 

analyses of 21 smectites was the amount of structural OH~. If 

the amount of structural OH~ is ideal, and the OH- is 

presumably all in ideal positions in the octahedral layer at 

the center of silica rings (fig. 3), then the smectite 

dehydroxylates at high temperatures and gives major DTA 

peaks between 650° and 750° C, regardless of the tetrahedral 

or octahedral location of the layer charge. Conversely,

| nonideal amounts or possibly nonideal distribution of 

; structural OH" cause appreciably lower temperatures of

dehydroxylation, and major DTA peaks occur between 550°

and 600° C.

j TABLE 5. Dehydroxylation peak temperature and size and amount of 

deviation from ideal structural water in 15 purified mixed-layer 

illite-smectites from the Pierre Shale and equivalent rocks

Sample

No.

259550 .......

259578 .......

159766 .......

159812 .......

159763 .......

259543 .......

159727 .......

159803 .......

159825. .......

159778 .......

159781 .......

159787 .......

259574 .......

159834.......

159835 .......

Locality 

no. 

(fig. 2)

23

16

7

9

7

22

13

5

1

6

6

8

18

2

2

'Differential Thermal Analysis 

^Thermal Gravimetric Analysis 

'IS-ordered mixed layering.

Illite: 

beidellite: 

montmorillonite 

ratios

15:15:70 

20:20:60 

25:45:30 

30:35:35 

35:35:30

40:25:35 

40:30:30 

40:45:15 

40:50:10 

50:35:15

55:35:10 

55:35:10 

65:30:5 

'60:30:10 

'80:10:10

(fig. 12). 

. by ¥. O. Simon

TGAJ 

DTA1 structural water 

peak temperature in as deviation 

°C and area (in=) from idea!4.0(OH) 

low high in percent

585(1.6) 

600(1.7) 

600(2.3) 

590(2.3) 

585(2.4)

590(2.4) 

560(2.4) 

565(2.2) 

600(2.3) 

590(2.3)

570(2.2) 

575(2.3) 

565(2.5) 

610(3.1) 

630(3.2)

690(1.2) 

685(1.0) 

685(0.1) 

690(0.1) 

675(0.1)

675(0.1) 

665(0.3) 

660(0.2) 

690(0.1) 

700(0.1)

600(0.2)

None 

None 

None 

None

-0.4 

-.55 

-.4 

+.2 

-.1

-.1

+.4 

+.5 

-.3 

+.3

-.4

-.2 

-.3 

-.2 

-.0
18 Cil()Cm: \1ISlRY 01 IHF IMT-RRl- SHALI AM) 1 QlIVAl F\ I ROCKS

The only mixed-layer clays investigated from the Pierre 

Shale and equivalents that are of purity comparable with the 

21 smectites (Schult/. 1969) are the 15 samples in table 3. 

Thermal gravimetric analyses (TGA) of these 15 clays were 

made on the same apparatus by the same operator at the 

same time as for the 21 smectites and were interpreted using 

the same methods (Schultz, 1969. fig. 5). The measured 

amount of high-temperature structural water was corrected 

for the amounts of kaolinite and quartz or cristobalite shown 

in table 3. Any illite impurities were assumed to have the 

same OH" content as the mixed-layer clays, and no 

correction was made. For example, in the first sample in 

table 5, 259550, the measured structural OH" was 4.08 

percent of the ignited weight of the sample. This OH" occurs 

in 89 percent of the sample, as 11 percent is quartz and 

cristobalite (table 3). Therefore the amount of OH" in pure 

clay would be 4.08-^0.89=4.57 percent. If a clay with the 

structural formula calculated for sample 259550 contained 

the ideal 4(OH)- per unit cell, the structural OH" should be 

4.95 percent of the ignited weight. The sample therefore 

contains 4.95-4.51=03$ percent less than the ideal structural 

water, rounded to -0.4 percent in table 5.

The estimated maximum error in measuring the TGA 

curves is 0.1 percent of the weight of the sample at both the | 

low and high ends of the temperature range (about 300° and 

1000° C, respectively) for loss of OH~, or a total of 0.2 

percent if the errors are in opposite directions. For9 of the 15 

illite-smectites (table 5) measured OH" deviates more than 

0.2 percent from ideal. The other six samples could still have 

an ideal amount but nonideal distribution of OH", or, in 

view of the possible error of measurement, the actual OH" 

content of all but the last sample could deviate more than 0.2 

percent from ideal but still be measured as within 0.1 to 0.2 

percent of ideal if errors of measurement at about 300° and 

at 1000° C were both nearly maximum and in opposite 

directions.

One unusual feature of the data in table 5 is the 

dehydroxylation peak temperatures of the last two 

samples 610° and 630° C which are in the 600°-650° C 

range between the normal low- and high-temperature 

dehydroxylation endotherms. Both are IS-ordered mixed-layered clays from Dearborn River (loc. 2). Possible 

significance of these intermediate temperatures is considered 

in the next section of this report.

ILLITIC MIXED-LAYER CLAY

IN BENTONITE

Although clay in most bentonite of Pierre age is highly 

montmorillonitic and contains no detectable illite layers, six 

exceptions were found in the main study of the Pierre Shale 

and its equaivalents (fig.9). These six are compared with 

nearby shales in table 6. Three of the bentonites are from the 

Dearborn River (loc. 2) section, and for the exceptionally 

thick 3-foot bentonite bed from the Telegraph Creek

Formation, two analyses are given one for the middle and 

the other (Ba, not shown in fig. 9) from the basal 6 inches. 

The latter is of particular interest because it contains the 

most illitic mixed-layer clay found in the Pierre Shale. All 

five of the mixed-layer clays from both bentonites and shales 

from the Telegraph Creek and Eagle Formations are highly 

illitic and have IS-ordered mixed layering. Three of the five 

samples, like the two from claystones at the Dearborn River 

locality in table 5 (loc. 2), have their main dehydroxylation 

endotherm peak in the normally vacant 600°-650° C range, 

and the shale from the Telegraph Creek Formation with its 

peak at 640° C is so close to the 650°-750° C range for 

smectites having ideal structural OH" as to suggest that 

development of IS-ordered mixed-layering may also involve 

ordering of O H". Such an interpretation is in accord with the 

nearly ideal OH" content measured for the two Dearborn 

River samples in table 5. However, the dehydroxylation 

temperatures of the bentonite sample (Ba) containing 85 

percent illite and of the Eagle bentonite (table 6) are not 

especially high, so that any tendency toward ordering of OH" 

in IS-ordered clay is not consistent. Still another noteworthy 

feature shown by the rocks at the Dearborn River locality 

(table 6) is the composition of an argillized sill in the lower 

part of the Two Medicine Formation. It is composed 

predominantly of a mixed-layer clay very similar to much of 

that in most of the shale and bentonite. Several other 

altered sills or welded tuffs in the lower part of the section 

also contain similar clays, though in smaller amounts. The 

occurrence of IS-ordered clay in materials so different as 

shale, bentonite, and an altered igneous sill indicates that the 

clay is a locally developed phenomenon.

Not all of the clay from the Dearborn River locality has 

IS-ordered mixed layering, especially in the upper part of the 

section. A good example is the bentonite sample from the St. 

Mary River Formation (table 6) that has barely detectable 

10-percent illite-type layers. The nearest shale is much more 

illitic but is not IS-ordered. These samples occur only about 

500 feet stratigraphically above the other samples at the 

Dearborn River locality (table 6) and show that development of ordering in mixed-layer clay can vary greatly over 

short distances.

The three bentonite samples in table 6 from the Columbus 

(loc. 6) and Shawmut (loc. 5) localities all contain mixed-layer clays that are considerably less illitic than those in 

nearby shales, and all are randomly interlayered. Yet even 

though the range of proportions of illite-type layers in the 

three bentonite samples, 15-35 percent, is within the range of 

illite-type layers in clays from shale (table 5), the bentonite 

samples give high-temperature dehydroxylation endotherms 

from 675° to 700° C that are bigger, mostly much bigger, 

than the low-temperature endotherms. Shale associated with 

the three bentonite samples shows almost no 

high-temperature dehydroxylation. This tendency for illitic 

mixed-layer clays in bentonite to retain the 

high-temperature endotherm characteristic of the usual
MIXI D-LAYER CLAY. NORTHERN GREAT PLAINS REGION 19

montmorillonite in bentonite is further shown by the fairly 

large endotherms at 670° and 705° C for the Eagle and 

Telegraph Creek bentonite samples at the Dearborn River 

locality. Only the most highly illitic part of the Telegraph 

Creek bentonite shows no appreciable high-temperature 

dehydroxylation endotherm.

A noteworthy feature of the two illitic bentonite samples 

from the Columbus locality (loc. 6, table 6) is that the one 

from the Bearpaw Shale is more illitic than the one from the 

Claggett Shale, which is 300 feet stratigraphically lower. 

Other bentonite beds at both the Columbus and Shawmut 

(loc. 5) localities contain normal clays composed only of 

smectite.

DEPTH DIAGENESIS

Studies by Powers (1959), Burst (1959), and Weaver 

(1960), among others, and more recently by Perry and 

Hower(1970), mostly in connection with Tertiary sediments 

of the Gulf Coast, have convincingly demonstrated the 

concept of depth diagenesis of clay minerals. In summary, 

mostly from the last-mentioned work, depth diagenesis

involves conversion of expanding clay (smectite in this 

report) that is found at shallow depths to less expanding 

illite-smectite mixed-layer clay at greater depths of burial. 

Development of nonexpanding layers is first noted at depths 

on the order of 5,000 feet, above which 80 percent or more of 

the layers commonly are expanding, and it continues to 

about 10,000 or 15,000 feet, where only 15-20 percent of the 

layers are expanding. No further decrease in expanding 

layers is found at greater depths. At the greater depths 

mixed-layer clay having from 60 or 65 to 85 percent 

illite-type layers is IS-ordered; less illitic mixed-layer clay at 

shallower depths is randomly interstratified. Discrete illite 

(mica), which is abundant at shallow depth, apparently 

breaks down at greater depth, thereby supplying K+ ions 

that are adsorbed on smectite-type layers converting them to 

illite-type layers. Diagenesis of the clays actually seems to be 

dependent more on temperature than depth, the IS-ordered 

mixed layering first developing at about 100° C. With 

varying thermal gradient in different areas, the depths at 

which the changes in mixed layering occur vary over several 

thousand feet.

TABLE 6. Mineralogy and dehydroxylation peak temperatures of samples of bentonite, adjacent shale, and 

an argillized sill of volcanic rock from the Dearborn River locality (loc. 2) within the Montana disturbed 

belt and of three bentonite samples from equivalents of the Pierre Shale outside the belt (fig. 2) that 

contain mixed-layer illite-smectite

[Ba, sample from base ot bentonite: B. bentonite; S. shale]

Locality name and 

number (fig.2) ........

Dearborn River

2

Columbus 

6

Shawmut

5

Formation 

Rock type

Telegraph Creek 

Formation

Ba B S

Eagle 

Sandstone

St. Mary River Claggett 

Formation Shale

Bearpaw 

Shale

Claggett 

Shale

Proportions of layers in mixed-layer clay, in percent

Illite ..................... i85 '75 '75 '70 '60 '75 10 60 15 60 35 50 20 55

Beidellite .............. 15 25 20 25 30 20 40 30 35 35 30 40 40 35

Montmorillonite ..0 0 5 5 10 5 50 10 50 5 35 10 40 0

Proportions of clay minerals in total clay, in percent

Mixed-layer .....

Illite .................

Chlorite ............

Kaolinite ..........

.... 95

.... 0

.... 0

.... 5

100

0

0

0

60

30

6

4

100

0

0

0

7?

70

3

5

93

0

0

7

100

0

0

0

100

0

0

0

9S

0

0

5

46

37

6

II

99

0

0

1

6?

79

5

4

98

0

0

7

69

20

4

7

Proportions of minerals in total sample, in percent

Quartz. ............

K-feldspar ......

Plagioclase .....

Calcite ............

Dolomite ........

...... 100

...... 0

...... 0

...... 0

...... 0

...... 0

80

15

0

4

1

0

55

?9

0

5

5

7

70

15

0

6

8

0

65

77

0

5

0

0

70

17

0

17

0

0

85

7

0

1

10

0

65

16

7

70

1

0

70

3

0

10

15

0

45

37

?

10

0

17

80

7

0

P

0

0

65

75

0

7

1

0

100

1

0

0

0

0

60

27

0

4

0

4

Dehydroxylation peak temperature, in °C, and area, in square inches

Low 

High

590 615 640 580 620 620 585 600 550 580 560 590 550 570

?.6 1.5 1.6 1.5 1.8 1.3 0.9 1.3 0.2 1.2 0.7 1.1 0.2 1.2

690 705 ( 2 ) 670 ( 2 ) 740 695 ( 2 ) 690 ( 2 ) 675 750 700 (=)

0.1 0.5 None 0.7 None 0.5 1.2 None 1.4 None 0.9 0.2 1.8 None

'IS-ordered mixed layering. 

2 No high-temperature peak.
20 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

No direct information is available on past temperatures in 

the Pierre Shale. Temperature can only be inferred from 

average thermal gradients in sediments, commonly 8°-10° C 

per 1,000 feet, and from inferred amounts of overburden. In 

the discussion that follows, therefore, depth of burial rather 

than temperature is the primary factor considered.

MAIN PIERRE SHALE STUDY

Variation in illite-smectite proportions in the Pierre Shale 

and its equivalents is almost identical with that in sediments 

of the Gulf Coast, both in proportions of layers and in 

development of IS-ordered layering in clay having more 

than about 60 percent illite-type layers. The temptation is 

great, therefore, to attribute variations in illite-smectite 

proportions in the Pierre Shale to depth diagenesis. Hower 

and Hall (1970) came to just such a conclusion in central 

Montana. In the Two Medicine Formation in the general 

area around the Dearborn River locality (fig. 2, loc. 2) they 

found the same IS-ordered illite-smectite as was found in this 

study; a little farther to the east on the Sweetgrass arch, in the 

general area of locality 3 (fig. 2), they found only randomly 

mixed-layer illite-smectite. Hower and Hall attributed the 

more illitic clays near the Dearborn River locality to greater 

depth-of-burial than on the Sweetgrass arch. More recently, 

Hoffman (1976) and Hoffman, Hower, and Aronson (1976) 

have attributed diagenesis of clays around the Dearborn 

River locality and elsewhere in the Montana disturbed belt 

(fig. 2) to burial under thrust sheets of the belt, and they have 

used the clays to interpret the temperatures involved 

(100°-200° C) and the time of thrusting (Late Cretaceous to 

Paleocene).

Clearly, the clays at the Dearborn River locality have 

responded authigenically to some sort of elevated 

pressure-temperature equilibrium akin to depth diagenesis. 

However, as explained below, there are three general 

difficulties with depth diagenesis as the sole explanation of 

variation in the illite-smectite ratio in the Pierre Shale and 

equivalents as a whole. First, no general relation exists 

between the illite-smectite ratio and stratigraphic position, 

geographic location, or inferred depth of burial that is 

consistent with pervasive depth diagenesis. Second, except 

at Dearborn River, the clay in bentonite is so different from 

that in nearby shale as to indicate little or no diagenesis in 

either. Third, the abundance discrete illite and the proportion of illite-type layers in mixed-layer clay of individual 

samples are not compatible with the former altering into the 

latter.

Past depth of burial of the sampled parts of the Pierre 

Shale can only be estimated, as the overburden has been 

removed from outcropping sediments. Generally, the 

thickness of oveirdng beds is thought to decrease from west 

to east, at least from the center of the basin to the Missouri 

River, much as the thickness of the Pierre Shale itself 

decreases through this interval (fig. 1). The maximum 

overburden that can be inferred with confidence is about

12,000 feet in the middle of the Pierre basin of deposition, in 

the southern part of the Powder River basin, reconstructed 

as follows:

Age Formation Thickness, in feet

Miocene .............. Arikaree ..............

Oligocene ............ White River ........

Eocene ................ Wasatch ..............

Paleocene ............ Fort Union .........

Late Cretaceous .. Lance ..................

Total

1,000

1,000

2,400

4,500

3,200

12,100

Overburden along the Missouri River probably was never 

more than a few thousand feet. Changes in thickness of 

overburden westward from the Powder River basin are less 

certain, but the great thickness of lower Tertiary sedimentary rocks listed above are not known to extend as far west as 

the Dearborn River area.

Variation in porosity of Pierre Shale agrees only partly 

with the burial depths inferred above. As overburden 

amounts increased, sediments generally and shales in 

particular were compacted by the squeezing out of water, 

and porosity decreased. The general change in expected 

porosity as depth increases is shown in figure 13, and 

porosities of shale samples calculated from bulk and grain 

densities from the main report of the Pierre Shale and 

equivalent strata are averaged for localities in figure 14. As 

expected, porosity is greatest in the east. An average porosity 

of about 25 percent in the Missouri River area corresponds 

to an overburden of only one or two thousand feet. 

Porosities of 15-20 percent in the middle of the basin

30

POROSITY, IN PERCENT 

25 20 15 10

BULK DENSITY

FIGURE 13. Change in bulk density and porosity of shale as depth of burial 

increases. Solid line is an average from Athy (1930), Hedberg (1936), 

Johnson (1950), Dickinson (1953), and B. K. Proshlyakov as cited in 

Maxwell (1964, p. 698). Dashed lines indicate range of values.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 21

NUMBER OF SAMPLES POROSITY, IN PERCENT

FIGURE 14. Average porosity of samples from generalized localities of the main study of the Pierre Shale and equivalent strata (fig. 2). 

Localities 19, 20, and 21 are combined and localities II, 12, and 24-26 are omitted because only two or fewer values were determined.

correspond to an overburden of only a few thousand 

feet much less than the overburden inferred above for the 

Powder River basin. Porosities of 5-10 percent, corresponding to overburdens on the order of 10,000 feet that are 

necessary for depth diagenesis to IS-ordered clay, occur only 

in the westernmost localities. The average porosity at the 

Dearborn River locality (loc. 2) of 7 percent is the lowest 

measured but is not much different from the average 

porosity at other western localities such as Livingston or 

Augusta-Choteau (fig. 2, Iocs. 1, 3), where mixed-layer clay 

is randomly interlayered.

Average illite content of mixed-layer clay in the upper 

three and the lower three fossil zones of the Pierre Shale and 

equivalent rocks as used in the main Pierre Shale report is 

shown for individual localities in figure 15. Except for the

highly illitic mixed-layer clay at the Dearborn River locality 

(loc. 2), averaging 65 and 67 percent illite-type layers in the 

upper and lower parts of the Pierre, there are no conspicuous 

regional trends. Nor is mixed-layer clay in the southern part 

of the Powder River basin, like the 31 and 47 percent illite in 

the upper and lower parts at the Salt Creek locality (loc. 10), 

much different from average. The most consistenly illitic 

mixed-layer clay, averaging 50 percent or more of the layers, 

comes from the lower part of the Pierre Shale in the Missouri 

River area, but here the overburden, estimated both from 

reconstruction and from porosity, was minimal. Although 

mixed-layer clay in the lower part of the Pierre Shale and 

equivalents is generally more illitic than in the upper part 

(fig. 15), within individual sections adjacent samples may 

differ greatly. In the Missouri River area the highly illitic

7

19

76 M 21

38 9

—20 —— 17 —————

5 

5

44 30

28 ^

23 35

72

« A8 

8 44 30

Upper

part,

Pierre

Shale

and

equivalents

Lower

part, 

Pierre 

Shale

and 

equivalents

33

35 31

67 40

45 46 42

52 54 33

1 —— 44 —— 36 —————

"— 'N 45 

34

47 47

————— c

57

43

57

50 57 51 

_ ————— ——— • — *-^~x

50

NUMBER OF SAMPLES ILLITE-TYPE LAYERS, IN PERCENT

FIGURE 15. Average proportion of illite-type layers in mixed-layer illite-smectite from generalized localities of the main study of the Pierre

Shale and equivalent strata (fig. 2).
22 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

character of the mixed-layer clay in the lower part of the 

Pierre is so consistent that this characteristic has been used 

as one compositional criterion for correlation of beds 

(Schultz, 1965); yet the change to less illitic clays in the upper 

part of the Pierre is abrupt, taking place over a few feet, 

rather than a gradual change over hundreds of feet as would 

be expected from depth diagenesis.

Finally, there is the question of whether discrete illite 

originally deposited in the Pierre Shale and equivalents has 

dissolved and furnished K+ ions to diagenetically alter 

expandable smectite-type layers to nonexpanding illite-type 

layers of mixed-layer clay. If so, discrete illite should have a 

negative correlation with the proportion of illite-type layers 

in mixed-layer clay. Instead, the opposite relation is 

observed. In marine rocks discrete illite has a correlation 

coefficient of+0.47 with the proportion of illite-type layers. 

In nonmarine rocks this correlation, +0.07, is also positive, 

but the magnitude of the coefficient is not significant. The 

much lower coefficient in nonmarine rocks is due in 

considerable part to samples from the Dearborn River 

locality (loc. 2), which contain exceptionally illitic mixed-layer clay but very little discrete illite. Discrete illite in the 

Pierre Shale is thought to be almost entirely of nonvolcanic 

origin. Whether the scarcity of discrete illite at the Dearborn 

River locality is due to its solution during diagenesis or to the 

scarcity of nonvolcanic detritus is not certain. The locality is 

very near the volcanic source (figs. 1,2), so a high proportion 

of volcanic detritus is expectable. In any case, the general 

tendency in most samples, other than those from the 

Dearborn River locality, for discrete illite to occur with the 

more illitic mixed-layer clay is incompatible with pervasive 

depth diagensis in most of the Pierre Shale and its 

equivalents.

Thus, with the exception of the Dearborn River locality, 

the variations in the illite:smectite ratio that might be 

expected from depth diagenesis are not observed. The 

strongest argument for in-place development of the highly 

illitic IS-ordered mixed-layer clay at the Dearborn River 

locality is that such clay occurs not only in claystone but also 

in bentonite, which presumably once had the usual montmorillonitic composition characteristic of most bentonite. 

Conversely, in most of the Pierre Shale and its equivalents, 

the great difference between the highly montmorillonitic 

clay of bentonite and the illitic mixed-layer clay in shale 

seems an equally strong argument against depth diagenesis.

COMPLEMENTARY STUDIES

The bentonite-shale relationship at the Dearborn River 

locality (loc. 2) is illustrated by only a few bentonite samples 

from the main study of the Pierre Shale (table 6). Also 

evidence of past burial depths is uncertain and partly 

contradictory, particularly as to relative amounts of overburden in the central and western parts of the basin. 

Complementary studies made to help clarify the problems of 

overburden thickness and significance of bentonite are 

described in the following sections of this report.

CITTINGS FROM DEEP WELLS

The well cuttings used in this study came from depths that 

are known within a few tens of feet. These depths give a 

minimum unequivocal depth of burial. Cuttings from three 

wells near the axis of the Powder River basin (* in fig. 2) were 

obtained from American Stratigraphic Co., Casper, Wyo. 

The cuttings all came from the lower part of the Pierre Shale 

from depths of 10,000-12,350 feet. All three wells started in 

the Wasatch Formation, so that actual past overburden 

thicknesses must have been greater than the well depths (see 

unnumbered table in the section "Main Pierre Shale study") 

and generally in the range affected by depth diagenesis.

The average mineralogy of the 12 samples of shale cuttings 

analyzed is given in table 7. Also given for comparison is the 

average mineralogy of stratigraphically equivalent samples 

at the nearest outcrop sampled, about 20 miles to the west on 

the Salt Creek anticline (fig. 2, loc. 10). The average 

compositions of the shale are nearly identical. This similarity 

indicates either that the outcrop samples were once buried at 

depths similar to the cuttings and have been similarly altered 

by depth diagenesis or that neither the outcrop samples nor 

the well cuttings were so affected.

TABLE 7. Mineralogy and dehydroxylation peak temperatures of shale 

and bentonite cuttings of Pierre age at depths of 10,000 feet or more in the 

Powder River basin and of samples from stratigraphically equivalent 

rocks from the nearest outcrops at the Salt Creek locality (loc. 10, fig. 2)

[<, less than]

Well cuttings Salt Creek

Bentonite Shale Bentonite Shale

Number of samples ...... 10 12

Proportions of layers in mixed-layer clay, in percent

Illite ...........................

Beidellite ....................

Montmorillonite ........

35

35

30

50

35

15

0

10

90

45

35

20

Proportions of clay minerals in total clay, in percent

Illite ..............

.............. 100

.............. 0

.............. 0

60

25 

7 

8

100 

0 

0 

0

60 

25 

5 

10

Proportions of minerals in total sample, in percent

Dolomite .......

.............. 95

.............. 2

.............. 0

.............. 4

.............. 1

.............. 0

Dehydroxylation peak temperature,

Low ..............

High..............

(')

.............. (')

None 

.............. 685

55 

30 

1

7 

1

7

in °C, and

( 2 )

590 

1.1 

680 

0.3

95 

0 

0

1 

2 

0

55 

30 

2 

5 

3 

8

area, in square inches

M59721

None 

715 

3.0

3 159720 

575 

1.2 

665 

0.4

'Composite sample from 10,000-10,030 ft. depth.

-10,000-10,01 Oft.

'Individual samples from main Pierre Shale report.

4 No low-temperature peak.

''Small dehydroxylation endotherm due to small sample.
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 23

The most significant features of the well cuttings were 

particles of bentonite and the composition of these particles. 

The light-colored bentonite particles contrast with the much 

more abundant dark shale particles from which the bentonite samples were separated. The maximum quartz 

content of any of the 10 bentonite samples was 7 percent and 

in most it was only 1-2 percent, amounts that are typical of 

most bentonite but much lower than those of shale analyzed 

from the Pierre Shale and its equivalents (table 1). None of 

the bentonites contained chlorite or free illite and two 

contained small amounts of kaolinite. These also are 

features typical of bentonite but not of shale. The atypical 

feature of all 10 samples of bentonite cuttings was the 

presence of mixed-layer illite-smectite. The proportion of 

illite-type layers ranged between 20 and 45 percent and was 

consistently lower than in the accompanying shale particles, 

though only slightly so in some cases. The mixed-layer 

composition of the bentonite particles proves that they 

cannot be drilling mud and must therefore be from bentonite 

interbedded with the Pierre Shale and its equivalents. Their 

mixed-layer composition must be caused by depth 

diagenesis. In contrast, the normal composition of the three 

bentonite samples from outcrops at the Salt Creek locality 

(loc. 10) indicates no alteration of the bentonite, and 

therefore, presumably, no alteration of the shale. The close 

similarity between mixed-layer clay in the surface shale and 

in the well cuttings pointed out in the preceding paragraph 

indicates that neither groups of shale have been appreciably 

altered by depth diagenesis.

The explanation for the consistently altered condition of 

bentonite from the Powder River basin well cuttings in 

contrast to the unaltered bentonite only 20 miles away on the 

Salt Creek anticline lies in the post-Cretaceous history of the 

Powder River basin. Conglomerates of Eocene age at the 

western edge of the basin near Buffalo, Wyo. (Sharp, 1948), 

contain boulders of metamorphic and igneous rock derived 

from the core of the Bighorn Mountains to the west (fig. 2). 

The presence of these boulders shows that the Bighorn 

Mountains were uplifted and the Powder River basi,n began 

to form well before the Eocene Epoch. Therefore, the lower 

Tertiary sedimentary rocks that make up most of the thick 

overburden of the Pierre Shale in the center of the basin 

likely were t.'inner on the flanks of the basin. As previously 

pointed out in connection with figures 13 and 14, no more 

than a few thousand feet of overburden is indicated by 

porosity of shale at the Salt Creek locality (loc. 10). Such 

overburden on the edge of the basin was too thin to produce 

any change in the composition of the bentonite, let alone to 

produce the abundant illite-type layers of the mixed-layer 

clay in the shale.

The amount of bentonite cuttings in the well samples was 

much too small for differential thermal analysis. However, 

one small sample was composited from three adjacent 

10-foot samples from one well and was sandwiched next to 

the thermocouple between layers of alundum in the sample 

well of the DTA apparatus. As shown in the lower part of

table 7, the sample gave a dehydroxylation endotherm 

having a single peak at 685° C. Thus, like the few other 

mixed-layer clays from bentonite in the main study of the 

Pierre Shale and its equivalents (table 6), it tended to retain 

the high temperature of dehydroxylation in the 650°-750° C 

range that is typical of normal unaltered bentonite.

MONTANA DISTURBED BELT

The Montana disturbed belt is a narrow zone of closely 

spaced thrust faults east of the Rocky Mountain front 

extending from central Montana well into Canada (Mudge, 

1970). Thrusting was from west to east. The only 

stratigraphic section in this belt that was sampled for the 

main Pierre Shale study was that at the Dearborn River 

locality (fig. 2, loc. 2); this is also the only section from which 

IS-ordered mixed-layer clay was identified, though others 

(Hower and Hall, 1970; Hoffman, 1976; Hoffman and 

others, 1976) have identified such clay at many other 

localities in the disturbed belt. Maiklem and Campbell 

(1965) described IS-ordered mixed-layered clay from one 

bentonite and randomly mixed-layered clay from several 

other bentonite beds from the Pierre-equivalent strata west 

of Calgary in the Foothill belt of Alberta, Canada, a 

northern extension of the Montana disturbed belt (fig. 2). To 

determine just how closely and consistently IS-ordered 

mixed layering is related to the disturbed belt, other 

localities from the south end of the disturbed belt near Wolf 

Creek to Marias Pass about 100 miles to the north were 

sampled in 1972 on a field trip arranged by John Hower and 

John Hayes. The localities are given letter designations in 

figure 2 to distinguish them from the numbered localities of 

the main study of the Pierre Shale and equivalents.

Mineralogy of the additional samples from the disturbed 

belt is shown in table 8. At four of the seven localities 

bentonite beds, as well as shale or claystone, were sampled. 

Two of these bentonite localities, Marias Pass and Sun 

River, actually are in the Colorado Shale, below Pierreequivalent rocks (fig. 1), but their mineralogy still illustrates 

the alteration of clay in the disturbed belt. At five of the 

seven localities the mixed-layering is IS-ordered. At Wolf 

Creek, where the clay in the bentonite is not IS-ordered, it is 

still highly illitic and is very similar to that in the adjacent 

claystone. As at the Dearborn River locality (table 6), the 

mixed-layer clay in the bentonite is nearly as illitic or more so 

than in the shale. Thus, in the disturbed belt there seems to be 

no appreciable tendency for bentonite to lag behind shale 

during alteration.

Only one of the IS-ordered clays listed in table 8, the 

bentonite from the Sun River locality (loc. S), gives a 

dehydroxylation peak temperature within the intermediate 

temperature range of 600°-650° C that seemed to 

characterize several of the clays at the Dearborn River 

locality (table 6). Therefore, this feature and any intermediate degree of OH" ordering that it may indicate for 

IS-ordered clay in the shale or claystone is erratically 

developed. All four bentonite samples in table 8, however,
24 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

TABLE 8. Mineralogy and dehydroxylation peak temperatures of shale 

and bentonite samples from the Montana disturbed belt (fig. 2)

[B, bentonite; S. shale]

Locality

Rock type .............. B

Proportions of layers in mixed-layer clay, in percent

Illite ............... '70 '60 55 '60 '80 '70 "70 '60 '60 45 50

Beidellite ........ 25 25 25 35 20 20 25 35 35 35 30

Montmorillonite 5 15 20 5 0 5 5 5 5 20 20

Proportions of clay minerals in total clay, percent

Mixed-layer ... 99 67 64 78 99 100 90 70 89 93 85

Illite ............... 0 28 28 14 0 0 0 15 0 0 0

Chlorite .......... 0055000150015

Kaolinite ........ 1533101001170

Proportions of minerals in total sample, in percent

Total clay ....... 75 55 45 55 95 75 75 50 70 85 65

Quartz ............ 27 30 18 30 1 18 6 23 20 6 17

K-feldspar ...... 00000001000

Plagioclase ..... 2 1 6544653 10 3

Calcite ............ 015 9000 12 9000

Dolomite ........ 00 15 5000 12 000

Dehydroxylation peak temperature, in °C, and area in square inches

Low 

High

595 570 580 585 605 590 590 580 590 560 560

0.8 1.3 0.9 1.4 1.4 2.0 1.2 1.2 2.2 1.2 1.5

710 680 (2 ) (2 ) 695 690 705 (2) 670 700 660

1.4 0.1 NoneNone 1.4 0.2 1.0 None 0.1 1.7 0.1

'IS-ordered mixed layering. 

2 No high-temperature peak.

Description of localities

M, Colorado Shale, 2.5 miles east of Marias Pass, in railroad cut north of U.S. Route 2, unsurveyed. 

T, St. Mary River Formation, in gully 0.25 miles south of road along Two Medicine River and 4.

miles southwest of U.S. Route 89, NW'/4 NE'/i sec. 26, T. 31 N., R. 10 W. 

C, Two Medicine Formation, 18 miles west of Choteau along Bellview Road, NEW sec. 24, T. 24 N.,

R. 8 OW. W

S, Colorado Shale, in gully just south of bridge over Sun River, NE'/SSW'/i sec. 36, T. 22 N., R. 9 W. 

B, Two Medicine Formation, 100 yards east of Bowmans Corner, NE!4NW',4 sec. 30, T. 18 N., R.

W. 

D, Eagle Sandstone, 1 mile south of Dearborn River bridge on U.S. Route 287, SW'/J sec. 34, T. 17

N., R. 4 W. 

W, Two medicine Formation, in town of Wolf Creek south of U.S. Route 91 across from Union Oil

Co. gas station, NE'/i sec. 2, T. 14 N., R. 4 W.

have a large component of their dehydroxylation endotherms peaking between 650° and 750° C, so that this 

characteristic and the ordering of OH" that it implies is 

retained from the original montmorillonitic bentonite.

The data in tables 6 and 8 clearly show that alteration of 

expanding clays and IS-ordered mixed layering is related in 

some way to the Montana disturbed belt. The exact 

relationship, however, is not clear. Stacking of thrust sheets 

to achieve the required overburden would seem an obvious 

explanation in a zone of thrust faults. Such an explanation is 

particularly appealing at the Marias Pass locality (loc. M), 

which is well into the disturbed belt, where one can see in the 

mountain face just to the north a thick section of Precambrian metasedimentary rocks thrust eastward over the 

Cretaceous rocks. However, as at the Dearborn River 

locality (loc. 2), most of the sample localities are on the 

eastern side of the disturbed belt where only Cretaceous 

strata are involved, deformation is mostly open folding, and

the thrusts tend to be small (Mudge, 1970). Furthermore, 

porosity measurements (fig. 14) indicate no great difference 

in overburden between the Dearborn River locality within 

the distrubed belt and the Augusta-Choteau locality (loc. 3, 

fig. 2) about 15 miles to the north and just outside the 

disturbed belt. Yet, mixed-layer clays from the 

Augusta-Choteau locality, having avarages of 28 and 35 

percent illite-type layers, are among the least illitic found 

(fig. 15). Initially, alteration of the IS-ordered clays at the 

Dearborn River locality was thought possibly to be related 

to heat from small intrusive sills in the section or to heat from 

the younger Upper Cretaceous Adel Mountain Volcanics of 

Lyons (1944) that occur less than 10 miles to the east and that 

may have once covered the Dearborn River locality. 

However, such volcanic rocks are not present near the 

northern localities sampled within the disturbed belt.

SUMMARY AND INTERPRETATION

Mixed-layer illite-smectite is the most abundant constituent of the Pierre Shale and its equivalents. The lithium 

test is used to differentiate smectite layers into montmorillonite and beidellite. X-ray methods have been 

developed for use of relative peak heights where conventional peak-migration methods cannot be used, so that the 

threefold breakdown of mixed-layer clay into illite-, 

beidellite-, and montmorillonite-type layers can be made for 

most samples. Within the mixed-layer clay, beidellite makes 

up about a third of the layers, commonly ranging from 25 to 

45 percent. Proportions of illite-type layers range much 

more widely between samples, commonly from 20 to 80 

percent, but rarely few or no such layers are present. 

Montmorillonite is generally complementary to the amount 

of illite, commonly from 0 to 60 percent of the layers. 

Mixed-layer clay containing as much as about 60 percent 

illite-type layers is randomly interlayered; when it contains 

more illite-type layers it is IS-ordered. Such ordered 

interlayering occurs only in the Montana disturbed belt. The 

ranges and average proportions of layers differ little between 

marine and nonmarine rock.

Smectitic clay in bentonite differs from that in shale in that 

it rarely is interlayered with illite-type layers. Montmorillonite usually makes up 60-90 percent of the layers, the 

remainder being beidellite. The only consistent exceptions to 

the usual composition of bentonite in the Pierre equivalents 

are in the Montana disturbed belt, where many bentonite 

beds contain IS-ordered clay, and in deep wells from the 

Powder River basin, where bentonite contains less illitic, 

randomly interlayered illite-smectites. Only three other illitic 

bentonite samples were noted out of about 200 analyzed.

The mixed-layer clay in the Pierre Shale and its 

equivalents has an average structural formula of 

K^Na^Ca^Al^Fe^Fe;2 Mg^Al^Si^K):2, (OH) 4 , 

which differs little between marine and nonmarine rocks. 

From chemical analyses of purified mixed-layer clays, 

structural formulas have been inferred for illite, beidellite,
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 25

and montmorillonite layers (table 4) that show expectable 

differences in K+, total layer charge and tetrahedral A1+3 . 

Illite-type layers in IS-ordered clay seem to have a slightly 

higher K+ content and total layer charge than in other 

mixed-layer clays. The average structural formula for 

smectite in bentonites of K,+, jX^Al^Fe^Fe^Mg^) 

(Al^Si^JOjofOH); (X represents exchangeable cations) 

differs consistently from that of mixed-layer clay in shale in 

that the structural formula has much lower K+, lower total 

layer charge originating mostly in the octahedral layer, less 

iron, and little tetrahedral A1+-1 . Montmorillonite layers in 

mixed-layer clays in shales are similar to those in bentonites 

except that they do not seem to have the low iron content.

Smectite clay in bentonite also differs from that in shale in 

that it contains an ideal 4(OH)~ per unit cell as measured 

thermogravimetrically, whereas the OH~ in shale is not of 

ideal amount or is not ideally located within the layer. The 

ideal 4(OH)~ of bentonitic montmorillonite is volatilized at 

relatively high temperatures giving dehydroxylation peaks 

between 650° and 750° C, whereas the nonideal OH" of shale 

gives dehydroxylation peaks between 550° and 600° C. The 

lower temperature is dominant even in mixed-layer clays 

from shale having 60-70 percent montmorillonite-type 

layers. On the other hand, the uncommon illitic mixed-layer 

clay found in some bentonite beds tends to retain the high 

temperature of dehydroxylation characteristic of bentonite, 

even in highly illitic IS-ordered clay. IS-ordered clay in shale 

has either a low temperature of dehydroxylation or, in some 

clay, an intermediate temperature between 600° and 650° C, 

which may indicate a tendency toward OH "ordering. This 

intermediate dehydroxylation is, however, very inconsistently developed.

Depth diagenesis or some type of diagenetic alteration can 

be shown to have consistently affected only the IS-ordered 

mixed-layer clay from the Montana disturbed belt, where 

such regularly interlayered clay occurs in most shale or 

claystone, bentonite, and even argillized igneous instrusive 

rock. In contrast, the less illitic randomly interlayered clay in 

shale outside the disturbed belt generally cannot be shown to 

have developed by depth diagenesis; instead, most such 

interlayered clay must have developed before rather than 

after deposition.

The most highly illitic mixed-layer clay in rocks of Pierre 

age outside the disturbed belt occurs most consistently on 

the east side of the basin of deposition along the Missouri 

River. Past overburden amounts here, both as reconstructed 

from thicknesses of younger rocks at their nearest outcrops 

and as inferred from porosities of the shale, were minimal 

and were much less than necessary to alter the mixed-layer 

clay in the shale. Bentonite in the Missouri River area 

contains smectite with no illite-type interlayers, and it also 

obviously has not been altered by depth diagenesis. Farther 

to the west, but still east of the Montana disturbed belt, a few

bentonite samples do contain somewhat illitic mixed-layer 

clay that apparently results from depth diagenesis. Mixed-layer clay in shale closely associated with these somewhat 

illitic bentonite beds is consistently much more illitic than 

that in the bentonite. The possibility was considered that the 

onset of alteration in the bentonite lagged appreciably 

behind alteration in the shale, because montmorillonite in 

bentonite, which has a mostly octahedral layer charge, might 

logically be expected to alter with more difficulty to illite, 

which has a mostly tetrahedral layer charge, than would the 

more beidellitic and illitic mixed-layer clay in the shale. Such 

a lag, however, seems unimportant. First, mixed-layer clay 

in shale along the Missouri River is as illitic or more so than 

that in shale associated with the somewhat illitic bentonite 

beds. Therefore, as the clays in shale and bentonite on the 

Missouri River are unaltered, there is no logical reason to 

infer that similarly illitic mixed-layer clay in shale farther 

west is altered either, just because nearby bentonite is 

somewhat altered. In addition, a comparison of mixed-layer 

clay from the Wolf Creek locality (loc. W, table 8) with that 

in the three pairs of samples from the Columbus and 

Shawmut localities (Iocs. 6 and 5, table 6) shows that 

illitization of the bentonite at the Wolf Creek locality closely 

approaches the proportion of illite-type layers in the shale, 

and yet any illitization of the shale has not advanced beyond 

that in the shale at the Columbus or Shawmut localities. 

Therefore, the depth-temperature conditions that produced 

the lower degree of illitization of bentonite at the Columbus 

and Shawmut localities cannot logically be inferred to have 

increased the proportion of illite-type layers there in the 

shale. Apparently bentonite does not lag appreciably behind 

shale in depth diagenesis. Therefore the montmorillonitic 

composition of most bentonite in rocks of Pierre age 

indicates not only a lack of depth diagenesis of the bentonite 

but also no possible alteration by depth diagenesis of the 

associated shale.

The extent to which depth diagenesis is believed to have 

modified the mineralogy of the mixed-layer clays in the 

Pierre Shale and equivalent strata is summarized in figure 

16. Mixed-layer clays originally deposited commonly contained 20-60 percent illite-type layers, a composition that is 

retained in most of the Pierre Shale and equivalents (fig. 

16/4); accompanying bentonite contains no illite-type layers. 

At a few isolated outcrops, overburden thicknesses on the 

order of 10,000 feet and resulting high temperatures have 

caused development of some illite-type layers in smectites of 

bentonite (fig. 165), but only minor additional development 

of illite-type layers has occurred in the shale in the originally 

least illitic mixed-layer clay. Examples are the bentonite at 

the Columbus and Shawmut localities (Iocs. 6 and 5, table 6), 

some of the least illitic bentonite from the Powder River 

basin wells (table 7), and the upper part of the Dearborn 

River locality (table 8, St. Mary River Formation). Still 

more rarely, diagenetic development might have advanced 

to form nearly half illite-type layers in the bentonite (fig.
26 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

75

50

25

Most of

the Pierre

Shale and

Equivalent

rocks

(table 1)

Deep well cuttings 

(table 7)

Rare 

outcrops 

(loc. 5-6, 

table 6}

Wolf

Creek

(loc. W,

table 8)

Most of

the Montana

disturbed

belt 

(table 8)

A D

FIGIRE 16. Common proportions of illite-type layers in mixed-layer clay 

originally deposited (stippled pattern) and that developed by depth 

diagenesis (lined pattern) in the Pierre Shale and equivalent strata. 

Interlayering is random except in the Montana disturbed belt where it 

is mostly IS-ordered.

16Q and increased the proportion of illite-type layers in all 

mixed-layer clays in the shale that originally had appreciably 

fewer illite-type layers. Examples are the bentonite and shale 

from the Wolf Creek locality (loc. W, table 8) and possibly 

some of the more illitic bentonite from the Powder River 

basin wells that are almost as illitic as mixed-layer clays in 

accompanying shale. Even here, however, newly developed 

illite-type layers in the shale probably are far subordinate to 

the original illite-type layers. Only in the Montana disturbed 

belt (fig. 16D) do authigenically developed illite-type layers 

unequivocably and appreciably augment the original illite-type layers.

If illite in the mixed-layer clays and its wide range of 

proportions throughout much of the rocks of Pierre age is 

not due to depth diagenesis, what has caused it? If both the 

mixed-layer illite-smectite of the shale and the smectite of the 

bentonite are altered volcanic debris, as seems to be the case, 

why are they so different?

The closest relationship between any one factor and the 

"illite: smectite ration of the mixed-layer clays in the Pierre 

Shale and its equivalents seems to be the rapidity of burial or 

the length of contact with weathering conditions at the land 

surface or with sea water before burial. Clay deposited slowly 

so that it was in contact with surface conditions for long 

periods of time tends to contain highly illitic mixed-layer 

clay. For example, the shale in the lower part of the Pierre 

Shale along the Missouri River is relatively thin and must 

have been deposited much more slowly than the much 

thicker shale in the upper part or in stratigraphically 

equivalent shale farther west (fig. 1). As previously mentioned in connection with figure 15, this lower shale from the 

Missouri River area contains some of the most consistently 

illitic mixed-layer clay in the Pierre Shale. The high 

proportion of illite-type layers cannot have been caused by 

depth diagenesis, because past overburden was minimal. 

Therefore, long contact of mixed-layer clay with seawater 

before burial may have resulted in fixation of K+ on some of 

the more highly charged beidellite-type layers, converting 

them to illite-type layers. The amount of rock thus altered, 

however, was too small to increase the average proportion of 

illite-type layers in marine rock as a whole, as the average is 

not appreciably different from that in nonmarine rock (fig. 

9). Conversely, mixed-layer clays having few illite-type 

layers consistently occur in altered tuffaceous rocks in which 

the clay occurs as rounded sand-size aggregates that 

originally were grains of volcanic glass. These aggregates 

disintegrate readily in water and could not have been 

transported and deposited in their present condition. Instead, they must have been deposited as grains of volcanic 

glass that were altered to clay after deposition. Thus, the 

highly montmorillonitic clays in such tuffs probably had 

little contact with surface conditions. The ultimate example 

would be bentonite, which is composed almost entirely of 

wind-transported volcanic glass that altered to montmorillonite after burial.

Dacitic volcanic ash, such as that from which bentonite 

and most smectitic clay in the Pierre Shale and its 

equivalents formed, commonly contains about 60-65 percent 

SiO2 and about 16 percent A1 2O,, whereas the clays contain 

only about 50 percent SiO2 and 18-20 percent A1 2 O. The 

alteration thus releases a considerable excess of SiO2. In 

bentonite beds, buried under a layer of mud when alteration 

occurs, the excess SiCh cannot be readily flushed away. 

Because of the excess SiCh, Al+3 could not compete 

effectively with Si+4 for cation positions in tetrahedral sheets 

of the forming montmorillonite layers, and the layer charge 

was limited almost entirely to the octahedral sheet, owing to 

Mg+2-for-Al +3 substitution. Apparently the homogeneous 

uniform composition of the silica sheets limited OH-almost 

entirely to ideal cation positions at the center of the silica 

rings (fig. 3).
MIXED-LAYER CLAY, NORTHERN GREAT PLAINS REGION 27

Most of the volcanic debris, particularly the larger 

fragments, probably did not travel nearly so far as the fine 

glass particles that formed the bentonites, but fell instead in 

land areas near the volcanoes. Long periods, perhaps 

thousands of years, separated the volcanic eruptions; during 

this time the terrestrial debris was subjected to alteration 

considerably different from that of the bentonite beds to 

weathering agents like rain, sun, and organic acids from 

plants. Circulation of water probably was fairly open so that 

much of the excess silica could be washed away. Furthermore, the coarse volcanic debris probably cooled slower 

than the bentonite ash, so that crystallization advanced to a 

sodic plagioclase-quartz stage, compared with a calcic 

plagioclase-biotite stage indicated by the phenocrysts in 

bentonite (Schultz, and others 1978).

For either or both of the above reasons (1) leaching of 

silica and (2) crystallization of quartz the altering volcanic 

debris must not have had nearly as great a silica excess as the 

altering bentonite. Consequently, Al +3 could compete with 

Si+4 for cation positions in the tetrahedral sheets, and the 

newly forming smectitic clay tended to be more beidellitic 

than in the bentonites. Because of this inhomogeneous 

composition of the silica sheets, OH~ distribution also 

tended to be erratic and nonideal. Furthermore, the net layer 

charge on the octahedral layer due to Mg+2_for-Al+3 

substitution tended to be as large as that of the clay in 

bentonite; this octahedral charge, combined with the additonal tetrahedral charge, apparently was large enough to 

fix many of the K+ ions re leased during alteration of the tuff, 

and illite-type layers developed.

Weathering of most tuffaceous debris probably started 

where it initially fell. Much of this weathered debris was 

eroded and redeposited downstream where it was again 

weathered. Several such cycles of erosion, redeposition, and 

weathering probably occurred before the altered volcanic 

sediment mostly mixed-layer clay reached the sea. In the 

marine environment, some additional development of 

illite-type layers may have occurred locally owing to fixation 

of K+ during periods of exceptionally slow deposition. 

However, judged from the similarity of mixed-layer clays in 

most of the Pierre Shale and equivalent nonmarine rocks, 

most of the mixed-layer clay was little altered in the marine 

environment. Only locally within most of the area studied 

has overburden been sufficient to alter the clays originally 

deposited in Cretaceous time. Diagenetic alteration has been 

pervasive only in the Montana disturbed belt, in most of 

which the piling up of thrust sheets and elevated 

temperatures owing to the augmented overburden seem to 

account for the alteration. However, highly illitic IS-ordered 

clay in some areas on the edge of the distrubed belt, where 

thrusting is minor, may indicate that factors other than 

overburden may also have been important; perhaps 

something like shearing stress or elevated thermal gradients 

due to nearby igneous activity had an effect.

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28 GEOCHEMISTRY OF THE PIERRE SHALE AND EQUIVALENT ROCKS

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