Clay Mineralogy of

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Pleistocene Lake Tecopa 

Inyo v County, y * California

$ WE VET Ft Of E S S I 0 K A L ]PAfER

Clay Mineralogy of 

Pleistocene Lake Tecopa, 

Inyo County, California

By HARRY C. STARKEY and PAUL D. BLACKMON

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1061

In a saline lake with pH above 8.5 

authigenic sepiolite and detrital 

lithium-bearing saponite were 

deposited in mudstones which 

were interlayered with 

volcanic tuffs

UNITED STATES GOVERNMENT PRINTING OFFICE, W ASHINGTON : 1 9 7 9
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Starkey, Harry C.

Clay mineralogy of Pleistocene Lake Tecopa, Inyo County, California.

(Geological Survey Professional Paper 1061)

Bibliography: p. 33

Supt.ofDocs.no.: 119.16:1061

I. Clay minerals—California—Inyo Co. 2. Meerschaum—California—Inyo Co. 3. Saponite—California—Inyo Co.

4. Saline waters—California—Inyo Co.

L. Blackmon, Paul D., joint author. II. Title. III. Series: United States Geological Survey Professional Paper 1061.

QE389.625.S73 549'.6 77-608318

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

Washington, D.C. 20402 

Stock Number 024-001-03185-0
CONTENTS

Page

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

Introduction ......................

Location and geographic setting., 

Acknowledgments .............

...................... I

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

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

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

Scope of investigation .................................... 3

Previous work ........................................... 3

Geology of the basin .......................................... 3

Description of the mudstones and siltstones .................... 4

Laboratory methods ......................................... 5

Nonclay minerals ............................................ 9

Quartz and plagioclase feldspar ........................... 9

Potassium feldspar...................................... 16

Carbonates............................................. 16

Magadiite .............................................. 17

Zeolites and amorphous material......................... 18

Saline minerals ......................................... 18

Page

Nonclay minerals—Continued

Other minerals.......................................... 18

Clay minerals............................................... 20

Chlorite and chlorite-mica ................................ 20

Mica and illite .......................................... 20

Lithian saponite ........................................ 23

Montmorillonite ........................................ 25

Sepiolite ............................................... 25

Tecopa basin occurrence ............................. 25

Previous work on sepiolite ........................... 26

Modes of sepiolite formation ......................... 26

Sources of magnesium and silica ...................... 27

Sepiolite formation in Lake Tecopa ................... 30

Discussion ................................................. 31

Sequence of events ...................................... 31

Conclusions ................................................ 31

References cited ............................................ 33

ILLUSTRATIONS

FIGURE 1. Index and sample locality maps, Pleistocene Lake Tecopa drainage basin, Calif.........................

2. Photograph from sample locality 54 looking northeast toward Shoshone, Calif..........................

3. Generalized stratigraphic section of the Pleistocene Lake Tecopa deposits .............................

4. Photograph of sample locality 44 showing gravel mixed with clay .....................................

5. Flow sheet for laboratory procedures used for mineral identification of Pleistocene Lake Tecopa deposits 

6-11. Transmission electron micrographs:

6. Suspended sepiolite ..........................................................................

7. Sepiolite fibers more than 2|*m long ..........................................................

8. Surface replica of sepiolite ....................................................................

9. Suspended saponite..........................................................................

10. Suspended crystals of magadiite from Lake Tecopa basin .......................................

11. Suspended crystals of magadiite from Trinity County, Calif......................................

12. Vertical distribution of clay-mineral content in stratigraphic section .................................

13. X-ray diffraction curves of typical gradation from mica to mixed-layer mica-saponite ..................

14. Photograph of mud cracks in saponite at sample locality 57..........................................

15. Map of samples taken within 1 m of tuff A and their clay mineral content.............................

16. Map of samples taken within 1 m of tuff B and their clay mineral content.............................

Page

2

4

4

5

9

14

14

15

15

19

19

21

22

24

28

29
IV CONTENTS

TABLES

Page

TABLE 1. Localities and particle size distribution of samples .................................................................... 6

2. Mineral content of samples ........................................................................................ 10

3. X-ray diffraction data for magadiite ................................................................................ 18

4. Comparison of X-ray diffraction spacings of sepiolite and saponite from Pleistocene Lake Tecopa with those of

	previously reported occurrences ................................................................................ 20

5. Chemical comparison of Pleistocene Lake Tecopa saponite with other smectites ........................................ 23

6. Weight percent of Mg, Li, and F in samples from Pleistocene Lake Tecopa mudstones and siltstones ..................... 24

7. Li and F in samples outside of lake beds in local drainage ............................................................. 26
CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA, 

INYO COUNTY, CALIFORNIA

By HARRY C. STARKEY and PAUL D. BLACKMON

ABSTRACT

Pleistocene Lake Tecopa in southeastern Inyo County, Calif., was 

formed when the Amargosa River was blocked at the southern end 

of its valley. The lake acted as a settling basin for detrital material 

being transported by the river. This detritus consisted of clays, 

quartz, feldspars, and micas which became mudstones and 

siltstones. These mudstones and siltstones, much eroded and 

dissected after the draining of the lake, extend over the entire basin 

and are interbedded with tuffs formed by the intermittent deposition of volcanic ashfalls in the former lake waters. These lightcolored mudstones and siltstones are tough and well indurated and 

break with a conchoidal fracture.

The predominant clay mineral in these detrital beds is a lithiumbearing saponite, which is found not only in the lake beds but also in 

the area beyond the boundaries of the lake, especially in fluvial 

deposits in the drainage basin of the Amargosa River to the north. 

This saponite does not contain enough lithium to be classified as a 

hectorite, and we have observed no indications that this clay consists of a mixture of two phases, such as hectorite and a diluent.

Some authigenic dioctahedral montmorillonite, found only in 

small quantities close to the tuffs, was formed by alteration of the 

volcanic glass of the tuffs and was then admixed with the overlying 

or underlying detrital clays.

The only authigenic clay-type mineral found in any significant 

quantity is sepiolite, found near the edges of the lake basin and 

stratigraphically located mainly within a meter of the two uppermost tuffs. This sepiolite probably was precipitated when silica 

became available to the magnesium-bearing lake water through 

dissolution of the volcanic ash. Precipitation of sepiolite probably 

did not occur within the tuffs owing to the presence of alumina in 

solution. Zeolites were produced there and sepiolite formed outside 

the margins of the tuffs.

Also formed by the high-pH lake waters were water-soluble 

minerals, which were found widely dispersed in crusts or streaks on 

the clays. Much of the calcite was likely precipitated from the lake 

waters, especially near the north end of the lake where calciumbearing fresh water came into contact with the CO2-rich lake waters.

Magadiite, a sodium silicate mineral reported only twice previously in the United States, was found in small quantities in the 

southern end of the basin. This mineral is indicative of a minimum 

pH of 8.5.

The authigenic minerals formed in the lake reflect the presence of 

silica-rich tuffs and the high-pH, alkaline character of the lake 

waters.

INTRODUCTION 

LOCATION AND GEOGRAPHIC SETTING

The basin of former Pleistocene Lake Tecopa, in 

southeastern Inyo County, Calif., is located in the Mojave Desert about 32 km east of Death Valley National 

Monument within Tps. 20, 21, and 22 N., and Rs. 6 and 

7 E. The area is shown in figure 1. The nearest large city is Las Vegas, Nev., which is located about 97 km to 

the east.

The town of Shoshone is located in the northern part 

of the basin, and the town of Tecopa is located in the 

southern part; Shoshone is on State Route 127, which 

runs generally north-south through the basin.

The lake basin is bounded on the west by Dublin 

Hills, Ibex Hills, and the southern part of the Greenwater Range and on the east by the southern parts of 

the Nopah Range and Resting Spring Range. To the 

south are the Sperry Hills through which the 

Amargosa River flows, draining the study area.

The lake beds cover an area about 18 by 23 

kilometers. The elevation of lowest exposures of the 

beds is at about 396 m near the south end of the lake, 

and the highest beds are at about 549 m at the margins 

of the lake. The hills and mountain ranges to the east 

and west of the basin have elevations of about 

914-1,219 m. Sperry Hills to the south are about 

610-762 m above sea level.

ACKNOWLEDGMENTS

We gratefully acknowledge the help of those 

members of the U.S. Geological Survey who furnished 

technical assistance during this study. Elaine L. 

Brandt, (Mrs.) Johnnie Gardner, John C. Hamilton, 

Violet Merritt, Wayne Mountjoy, and Daniel R. Norton supplied the various chemical analyses. Toribio G. 

Manzanares did most of the fractionations and ran
CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

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Edge of Pleistocene Lake Tecopa

EXPLANATION 

Sample locality and number 

Line of measured section

Spillway of 

Pleistocene Lake

Island in Pleistocene 

Lake Tecopa

FIGURE 1.—Index and sample locality maps, Pleistocene Lake Tecopa drainage basin, California.
GEOLOGY OF THE BASIN

most of the X-ray patterns. Donald C. Noble, now at 

Michigan Tech University, provided data on unaltered 

glasses from Nevada.

SCOPE OF INVESTIGATION

This report is concerned primarily with the identification, distribution, and origin of the clay minerals 

found in the Lake Tecopa basin. It also discusses the 

types and amounts of other minerals present. The 

Lake Tecopa basin is suited for this type of study 

because: (1) it covers a relatively small area of about 18 

by 23 km, (2) the clay beds are separated by tuffs 

which can be followed easily and, (3) the clay beds are 

dissected by gullies so that a few tens of meters of 

clays can be sampled without drilling. A total of 158 

samples from 94 sample sites were examined, the locations of which are shown in figure 1. Samples that were 

collected from the area outside the basin are shown in 

the area covered by the map in the inset. The word 

"basin" is used in this report to mean only the area actually covered by the waters of Pleistocene Lake 

Tecopa.

PREVIOUS WORK

Numerous geologic investigations have been carried 

out in the Lake Tecopa area. Melhase (1926) described 

the geology and the mining operations of 

"amargosite," which he claimed to have named in 

1920. This clay, which is actually montmorillonite, at 

the time was known also as "soap rock" or "natural 

soap." Noble, Mansfield, and others (1922) described 

the mudstones and saline deposits in their investigations of the nitrate beds. Noble (1926) gave a brief 

description of the geology of part of the Amargosa 

valley in his investigation of a colemanite deposit near 

Shoshone. Later (1934) he gave a more complete 

description of the rock formations in the area. Dietrich 

(1928, p. 88) described the clay beds as trending north 

and south, as being 1.8-2.4 m thick, and as being 

overlain by 1.2-1.8 m of volcanic ash. Blackwelder 

(1936, 1954) gave brief outlines of the general geology 

and history of the Lake Tecopa area, pointing out that 

the deposits on the lake floors had been deeply entrenched and eroded after the lake drained. Wright and 

Troxel (1954) described the lake beds as mostly 

siltstone with subordinate layers of volcanic ash and 

bentonite clay. Although in the past the clays in the 

beds of Lake Tecopa have been described by several 

authors, the scope of the work often was limited by the 

available analytical methods. Identifications and conclusions were based on physical and chemical properties and upon the commercial uses to which the clays 

could be put.

Sheppard and Gude (1968), in their study of the 

authigenic silicate minerals in the tuffs of Lake 

Tecopa, described those tuffs in detail and we will use 

them as stratigraphic markers for the present study. 

They reported the occurrence of sepiolite in two of 

their samples, but they noted that neither Droste 

(1961) nor Kerr and Langer (1965) had reported 

sepiolite in their studies of other playa clays of the Mojave Desert. Sepiolite, however, is known to occur in 

desert lakes of high salinity (Bradley, 1930; Schroter 

and Campbell, 1942; Hardie, 1968; Parry and Reeves, 

1968). The nearest probable occurrence of sepiolite to 

Lake Tecopa, known to the present authors, is that 

reported by Foshag and Woodford (1936). They state, 

"In the bentonitic clay of Ash Meadows near 

Shoshone, Calif., there are numerous lumps of mixed 

lime, magnesium carbonates and a magnesium silicate 

close to sepiolite," but they do not give an exact sample site location. Ash Meadows is about 40 km north of 

Shoshone (fig. 1).

GEOLOGY OF THE BASIN

Most of the geologic investigations that have been 

made of Lake Tecopa in the past have been primarily 

concerned with the saline deposits and especially with 

the possiblity of finding nitrate deposits. The reports 

of these investigations have also described the general 

geology of the area surrounding Lake Tecopa.

Sheppard and Gude (1968) gave a detailed report on 

the mineralogy of the tuffs associated with the 

mudstones, and an excellent review of the previous 

geologic work in the area. Therefore, only a brief 

description will be given here to acquaint the reader 

with the mudstones in the basin. For a more complete 

understanding of the geology, the works of Bailey 

(1902), Campbell (1902), Thompson (1929), Miller 

(1946), Mason (1948), and Blanc and Cleveland 

(1961a,b) should also be consulted.

The Amargosa River, the headwaters of which arise 

in Pahute Mesa, about 45 km north of Beatty, Nev., is 

the principal surface drainage of the Death Valley 

region. Its channel extends southward across the 

California-Nevada State line and through the area 

under study before it turns northwest into Death 

Valley. The river is mostly dry except for short 

distances where water is supplied by the springs 

located at Shoshone and Tecopa, Calif., and by infrequent heavy rains. The Greenwater Valley enters the 

Lake Tecopa basin from the west, and the Chicago 

Valley enters from the east. Both, however, have small 

drainage areas.

The Amargosa River has eroded the mudstones 

which were deposited in Lake Tecopa when, during
CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

Pleistocene time, the river was blocked south of the 

site of the present town of Tecopa. These mudstones 

appear to be almost horizontal but have a definite 

slight dip toward the center of the basin. This dip was 

probably produced by the original slope of the basin 

floor and by postdepositional compaction. Toward the 

center of the basin, near the river, the clay beds have 

been eroded to a low, hummocky surface. Away from 

the center of the basin, the hummocks become progressively higher and then become buttes capped with 

gravel which, near the margins of the basin, coalesce to 

form gravel-capped pediments dissected by steep-sided 

washes with gravelly alluvium covering the bottoms 

(fig. 2). The washes, which are as much as 9 m deep 

make traverse difficult but provide excellent exposures 

for the study of the mudstones. The gravel covering 

the buttes, mesas, and pediments has developed a thin 

covering of desert varnish.

' 1s '^

FIGURE 2.—View from sample locality 54, looking northeast toward 

Shoshone. Washes in foreground and middle distance are eroding 

gravel-capped pediments covered with desert varnish. Light area 

in the middle distance is central portion of the Pleistocene Lake 

Tecopa basin where surface is eroded away exposing clays 

beneath.

DESCRIPTION OF THE MUDSTONES AND 

SILTSTONES

A generalized section of the stratigraphy is shown in 

Figure 3. The uppermost sample collected in the lake 

beds for this study was located about 12m above tuff 

A. The lowest lake beds are not exposed. The beds have 

a more or less uniform gentle dip toward the center of 

the basin. The distances between the bottom of tuff A 

and the top of tuff B ranged from 6 to 8.5 m and the 

distance between the bottom of tuff B and the top of 

tuff C was approximately 23 m. The lowermost sample 

was collected from 7.6 m below tuff C.The tuffs range 

in thickness from several centimeters to about 4 m.

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EXPLANATION

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LABORATORY METHODS

Near the center of the basin, erosion has been more 

complete leaving gently rounded hills covered with 

clay, some of which have a "popcorn" surface. Sheppard and Gude (1968) attribute this "popcorn" appearance to the weathering of montmorillonite-rich 

mudstones.

The mudstones become more silty and sandy toward 

the margin where they are mixed with alluvial material 

washed out from the higher land surrounding the basin 

(fig. 4). Toward the center of the basin where the 

mudstones contain less silt and sand, they are indurated and are very tough. These clays break with a 

conchoidal or subconchoidal fracture, but some are 

almost impossible to sample unless a sledge and moil 

are used.

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FIGURE 4.— View of sample locality 44 showing gravel mixed with

clay.

The colors of the clays are pastel and vary from 

shades of white to buff, yellow, green, brown, or red. 

Some of the samples contain white veinlets or specks 

which have been identified as calcite or amorphous 

silica, and some contain small black specks of biotjte or 

manganese oxide.

LABORATORY METHODS

A flow sheet (fig. 5) is shown which outlines the basic 

procedures used to prepare and analyze the samples. 

The samples were soaked in water for as much as a 

week, during which time the more indurated samples

were crushed lightly with a rubber pestle. They were 

then disaggregated by agitation in a mechanical stirrer 

for 10 minutes and the resulting slurry was put 

through a 230-mesh sieve to remove the sand. The clay 

and silt were then separated by centrifugation according to Stokes' Law. The sand and silt fractions and an 

aliquot of the clay slurry were dried and weighed to 

determine percent composition. A portion of the clay 

slurry was sedimented onto unglazed porcelain tiles by 

vacuum to obtain oriented aggregates for use in the 

X-ray analyses of the clays. The salt solutions and the 

glycerine used to characterize the clay minerals were 

applied to the surface of the clay-coated tiles by pipet 

and were sucked through the sample by vacuum.

The X-ray diffraction patterns of the randomly 

oriented total samples and of the various fractions 

were compared with standard patterns for identification and estimation of the nonclay minerals present. 

To identify the clays the oriented aggregates were 

treated with glycol to expand the smectite layers, were 

heated to 400° C to collapse the smectite layers, and 

then were heated to 550° C to destroy any kaolinite 

and to increase the intensity of the 001 chlorite reflection. Glycerine treatments of KC1- and MgCl 2- 

saturated oriented aggregates were discontinued when 

they did not indicate the presence of any vermiculite.

The sample numbers, localities, and the results of 

the fractionation procedure, plus the percentages of 

water-soluble material present, are listed in table 1. It 

will be noted that not all the sums of the various fractions and the water-soluble materials total 100 percent. Part of this error is due to timing of determinations: the determinations of the water adsorbed on the 

clays were not always made at the same time that the 

total samples were weighed for fractionation. The adsorbed water varied with the relative humidity, 

especially in those samples containing considerable 

amounts of zeolites.

The amounts of water-soluble material present were 

determined by the specific conductance of leachates or 

by gravimetric measurements of evaporated leachates. 

Almost all samples gave some indication, however 

slight, of the presence of salts although most of them 

indicated no more than 1 percent. The amounts of 

water-soluble material are also reported, as parts in 

ten, with the other minerals determined by X-ray diffraction (table 2).

Small variations in the amounts of mineral present 

are not significant. The wide range of composition, the 

presence of many minerals in one sample, the veiling of 

small amounts of clay minerals in the presence of 

others, the varying degrees of orientation, and the
TABLE 1.—Stratigraphic locations, descriptions, and particle size distribution in percent, of samples 

taken from Pleistocene Lake Tecopa basin and drainage

[All samples carrying a letter prefix were supplied by Arthur J. Gude, 3d and Rirhard A. Sheppard]

Sample 

No.

2 ———— 3 ———— A ———— 5A ———— D

6 ———— 7A ____ (2 _ gA _ (^ _ D ———— V ——— W ——— X ———— 

Y ———— Z ———— 

9 ———— 10 ———— 11 ———— D ———— E ———— 13 ———— D ———— 16A ———— 

B ———— D ———— ISA ———— B ———— 19 ———— ?nA

Stratigraphic- locations and descriptions

Dark-yellowish-brown zone about 61 cm thick, 3-6 m above tuff A ——— ————————— Hard mudstone 2.9 m below tuff A-- — ———— - — - — - —— — - — - —— —— —— — —— - —— — - Friable mudstone 1.5 m below tuff A-- — ————— — — —————— ——— —— —— - — —— — --- Porous mudstone 1.7 m above tuff B- — — ——— ——— ————— — ——— — ———— - — ————— Friable mudstone 3.5 m below tuff B —— - ———— ——————— —— ———— ——— — ——— — —— - Crumbly mudstone 3.2 m below tuff C; 46 cm above 8 cm-thick tuff below tuff C — Sand >62pm 8 51 43 20 6 27 81 62 18 1248 9 8 25 11 3 6 6 1 Trace3 43 29 31 Trace 4 Trace 1 7 418 1 Trace 1 2 16 4 6 2 34 2 1 9 

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CLAY MINERALOGY OFPLEISTOCENE LAK TECOPA
LABORATORY METHODS

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TABLE 1.—Stratigraphic locations, descriptions, and particle size distribution in percent, 

taken from Pleistocene Lake Tecopa basin and drainage--Continued

Sample 

No.

Stratigraphic locations and descriptions Sand 

>62y

55

A

56 

57A

58—— 

59____ 

60—— 

61A—— 

B——

Black limestone from Dublin Hills————————————————— 

Gray limestone from Dublin Hills———————————————

Silty bed 9 m below surface and 9-12 m above tuff A——- 

Mud-cracked surface material from Amargosa River bed—— 

Silty sediment 30 cm above river bed—————————————

Silt, 4.6 m above river bed———————————————————

Silty sediment in bank of stream bed————————————

Silty sediment in bed of stream———————————————

Mud-cracked surface sediment in stream bed-————————

Silty material about 61 cm above river surface in bank62———— Silty material along bed of intermittent stream——————————————————————

ST1-5—— Limestone immediately above tuff B———————————————————————————————

2-3—— Mudstone just below tuff A————————————————————————————————————

3-8B— Claystone about 6 m above tuff C——————————————————————————————————————-

3_10— 2.7 m mudstone just above tuff C——————————————————————————————————

4-2—— 61-cm-thick mudstone immediately below tuff A—————————————————————

T4-6C—— Tuff A, 61 cm above base——————————————————————————————————

7A—— Mudstone below tuff A——————————————————————————————————————

8A—— Mudstone below tuff B—————————————————————————————————————

B—— Tuff B, 36 cm above base————————————————————————————————————————

C—— Tuff B, 1 m above base——————————————————————————————————————————————

D—— Top of tuff B————————————————————————————————————————

12——— Near middle of tuff B———————————————————————————————————————

24E—— Tuff B, 1 m above base———————————————————————————————————

26C—— Tuff A, 1 m above base—————————————————————————————————————————————

31A—— Base of tuff B————————————————————————————————————————————

39C—— Top of tuff A———————————————————————————————————————————

D—— Silicified mudstone above tuff A———————————————————————————————————————

49C—— Upper part of tuff A—————————————————————————————————————

58C—— Tuff A, 9.8 m above base————————————————————————————————————

64A—— Middle of tuff B———————————————————————————————————————————

66B—— Upper part of tuff A————————————————————————————————————————

E—— Near the middle of tuff B———————————————————————————————————

70C—— Mudstone 61 cm below tuff A—————————————————————————————————————

78——— Upper part of tuff A———————————————————————————————————

79——— 1.4 m above base of tuff B————————————————————————————————————

8QB—— Middle of tuff A——————————————————————————————————————————

83——— At upper part of tuff B———————————————————————————————————

93B—— Upper part of magadiite unit on "island"——————————————————————————

TB-109B- Top of tuff C—————————————————————————————————————————————

118— 1.8 m above tuff C—————————————- ———————————————————————————————

119B- 2.1 m below tuff C—————————————————————————————————————————

D- 7.6 m below tuff C—————————————————————————————————————————

122C- Top of tuff C————————————————————————————————————————————

123C- Mudstone 1.2 m above tuff C—————————————————————————————————————

127B- 1.5-m-thick silty mudstone immediately below tuff B——————————————————

149A- Near the middle of tuff B—————————————————————————————————————

151B- Between tuffs B and C————————————————————————————————————————

90

86

45

42

73

79

75

75

11

53

81

7

5

4

3

6

7

1

28

38

35

40

49

60

28

37

23

83

37

36

11

11

28

2

24

37

52

49

19

36

16

29

18

27

7

47

52

65
NONCLAY MINERALS 

Sample

Disaggregation, sieving, centrifugation

___________1___________

Sand (>62//) Silt (2-62//) Clay (<2/t)

Leaching of water-soluble material

I 1

Specific conductance Gravimetric measurements 

of leachates of evaporated leachates

I 1

X-ray diffraction, X-ray diffraction, X-ray diffraction, Oriented X-ray of 

random orientation random orientation random orientation aggregate total sample,

| random 

Air drying orientation

__________1______

T T

X-ray diffraction X-ray diffraction X-ray diffraction

1 I I

Glycol treatment Heat to 400° C

I I

X-ray diffraction X-ray diffraction

I

Heat to 550° C

I

KCI-saturated 

fraction

I

X-ray diffraction

1

X-ray diffraction

I

MgCb-saturated 

fraction

1

X-ray diffraction

I I

Glycerine treatment Glycerine treatment

I I

X-ray diffraction X-ray diffraction X-ray diffraction 

FIGURE 5.—Flow sheet illustrating procedures used for mineral identification.

sampling irregularities make comparisons of small 

amounts subject to error.

Chemical determinations were made as needed when 

X-ray determinations were inconclusive. For instance, 

when single small peaks at 30.4° 20 on the diffraction 

patterns indicated possible manganocalcite, the carbonates were removed by dilute hydrochloric acid. 

Qualitative manganese determinations were made on 

the resulting solutions.

The amount of amorphous material in the clay-size 

fraction was difficult to estimate either optically or 

from X-ray patterns. Therefore, samples appearing to 

contain more than two parts in ten of amorphous 

material were boiled for 2Yz minutes in 0.5 N sodium 

hydroxide, according to Mallory's (1965) method, and 

the weight loss sustained was reported as amorphous 

material, after the percentages of previously determined water-soluble material were subtracted.

Transmission electron micrographs (figs. 6-9) were 

made to illustrate the morphology and to confirm the 

presence of the different types of clay minerals. X-ray 

diffractograms and chemical analyses were made of 

concentrations of individual minerals to better determine their characteristics.

NONCLAY MINERALS

Although this investigation is primarily concerned 

with clay mineralogy, identification of the nonclay 

minerals is needed, because they may be either a 

source of materials necessary for the formation of 

authigenic clays or indicators of detrital origins.

QUARTZ AND PLAGIOCLASE FELDSPAR

Quartz, which is mainly detrital, is found in most 

samples but only in small quantities, the largest 

amount being the 3 parts in 10 found in sample 47. 

This sample was taken in the area where sediments 

from Chicago Valley were brought into the lake basin. 

In general, the amounts of quartz are highest near the 

edges of the basin, at the northern end where the 

Amargosa River enters the basin, and near the junctions of the basin with Greenwater Valley and with 

Chicago Valley.

Stratigraphically, the greatest amounts of quartz 

are found in the youngest sediments above tuff A. Between tuff A and tuff B (fig. 3) there appears to be a 

general decrease in quantity of quartz, but in the 7.6 

meters between tuff B and the intermediate tuff still
TABLE 2.—Mineralogic composition (in parts in ten) of samples from Pleistocene Lak

estimated from X-ray diffraction patterns of randomly oriented mounts of 

fractions, and of oriented clay mounts

[Soluble materials were determined by specific-conductance measurements or gravimet

<1, between 5 and 10 percent, Tr., less than percent; +, indicates a value betwe

preceding the symbol and the next higher number; ?, amount too small for positiv

Leaders (——), not found]

Sample

1A

52 — ——

2

3

4—

SA

6-

7A

p

8A

D —— —

V —— —

w ——

y _____

9 ______

10 ______

11 ______

1 ?A

A A

D — ——

F

N

4-1

l-i

o-

<

1+

<

Tr.

1+ 

1+

1+

1+ 

Tr.

1

1+

<

Tr. 

Tr.

Tr.

1

1+

<

Tr.

Tr.

Tr.

Tr.

i

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<

^7

Tr. 

Tr.

1+

1+

1 +

1+

1+

Tr.

Tr.

Tr.

Tr.

1+

2

1+

Tr. 

Tr.

Tr.

Tr.

Tr.

TV

K-feldspar

Tr.

Tr. 

Tr.

Tr.

Tr.

Tr.

Tr.

TV

Calcite

2

Tr.

6

1+

1

Tr. 

Tr.

5+

Tr.

Tr. 

1+

1+

1+

Tr.

1+ 

1

2

1+

4+

1+

Tr. 

1+

2+

1+

2+

14-

Dolomite

Tr. 

Tr.

Tr. 

<

< ?

Tr.

Other COs minerals

Tr.

Tr.

Tr.

Sepiolite

1+

"^

3+

4

Tr.

2+

1+

2+

1+

2+

1+

<-i

Chlorite

Tr.

Tr.

"^

Tr.

Tr.

Tr.

Tr.

Tr.

Chlorite-mica

Tr. 

Tr.

Tr.

Tr.

Tr. 

Tr.

ra 

o•H

2

1+

1+

2+

1+ 

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1+

1+

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3 

2+

2 

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1+ 

3

2+

1+

2+

2+

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1

1+

2+ 

1+

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1+

3

2+

94.

Saponite

"^

Tr.

Tr.

Tr.

Tr.

Tr. 

Tr.

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1

<

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1

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1+

14-

Mica-saponite Montmorillonite

Tr . —— 

1

—— Tr.

Tr . ——

Tr . ——

Tr . ——

Tr. ——

Tr . ——

Tr . ——

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1+ ——

1+ ——

1+ ——

1 ——

<-i

Mica-montmorillonite 

Clinoptilolite

1+

—— Tr.

Tr . Tr .

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2

1+

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V

•H 

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<

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Oze

T

T
NONCLAY MINERALS 11

H H V V V V

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1 1

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+ 1 • • 1 + -

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1 H H 1

+ . ^ . . 1

H H H 1 H '

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v v v H H

1 • 1

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1 H 1

1 + • 1

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.

H

III II 

III II 

III II

• • • • 1

H H H HI

III II 

III II 

III II

III II 

III II 

III II

III II 

III II 

III II

+ • -1

v H H 1

+ +

v V

N CN CN m CM

1 

1 

1

H

1

1

1 1 1

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f

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H H

III II

III II III II

III II III II III II III II III II

0 < « U < •& {**•

HVHHHH H VHHHHE1 1 1 1 1 1 1 1 1 1 1 1

!>• 1 1 1 1 1 1 1 1 1 1 1 1 

1 1 1 1 1 1 1 1 1 1 1 1

1 1 + 1 1 1

IrHlrHlt-ICNJrH rH 1 !>• 1 1 

1 V | | H V VI II

. . . . | . . IIH H H H 1 H H 1 I H

1 1 1 1 1 • 1 1 1 1 1 1 1 1 1 1 1 1 }-t 1 1 1 1 1 1 1 1 1 1 1 1 H 1 1 1 1 1 1 1

HHHHHv IH HHV IH

1 1 1 1

1 1 1 1 rH C^- rH rH rH (>• O- )-l V 

1 1 1 1 1 V V V H E1 1 1 1 1 1 1 1 1 1 1 

1 1 1 1 1 1 1 rH rH 1 1 1 1 

1 1 1 1 1 1 1 V V 1 1 1 1

III 11+ • 1 1 1 1 

1 1 1 1 1 1 V H 1 1 1 1

II • II 111 +

| | C-. H ,H rH 1 1 1 1 1 rH 

II H V V 1 1 III

+ • 'It + + + 4

V H V H 1 1 V

CNjrr">rr">csjrocN)-irr"> ,— ),— I,H,_|,HC. 

H v

• 1 1 1 II • 1 • 1 H 1 1 1 o- 1 1 o- )-i I )-i | 

Hill II H 1 H 1

1 1 1 1 1 1

(J 0- 0- H !>• 1 1 1 1 0-

H HI III

III 1 1 +1 

III II 1 ir> fl fl 1 1 1 1 1 1 1 1

1 1 1 • 1 1 III 

1 1 1 H 1 1 o- 1 1 1 o1 1 1 H 1 1 III

• • 1 • II III 1

rH rl rl 1 VI !>• 1 1 1 1 1 1 1 

VHHIH II Illll

VVHHHV IH HV

+ + + + II 1 + 4

rHCNCNflCNrHI 1 |O- rHrHrHrII 1 V V

H* £ ' ' + A o

H H H H V H

j j j ^ ' __, ^ + +, n' n' n' '

1 1 1 H 1 H H H H H E1 1 1 1 III 

till III 

1 1 1 1 III 

1 1 1 1 III

' ' ' - ' .i ' «il

CO O>> O rH CM

H H H H H

1 1 1 1 1 1 1 1 1 1 1 1

1 + ^ 

1

1 1 1 1

1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1

III* 

1 1 1 H

J ^H )-l )-l CN 

H H H

1 + +

1 rH rH rH 

1 V

| ...

1 H H H

1 H H H

1 1 1

1 1 1 1 1

1 1

1 rH 1 1

1 1

J CN rH rH rH

1 1 1 1 

1 1 1 1 

1 1 1 1

1 1 1 1 

1 1 1 1 

1 1 1 1

+ 111 

1 1 1

1 1 1 

!>• 1 1 1

1 1 1

1 1 1 1 

1 1 1 1 

1 1 1 1

V V H H

1 1 1 1 H 1 1 1 1 1 1 1 1

H H

J )-l rH )-l rH 

H H V H

1 1 

1 1 

1 1

1 O

3 <3 <J <J 

ro -*

HHHHV VHHVH
TABLE 2.—Mineralogic composition (in parts in ten) of samples from Pleistocene Lak

estimated from X-ray diffraction patterns of randomly oriented mounts of 

fractions, and of oriented clay mounts—Continued

Sample

35A _ _ _

B -——

B -——

r -

D ————

37

38A -

QQ

40A

41 - -

42A

___

B —— —

r -

44 ~

45- -

46 - —

47 - —

48 —

49 ————

50 —

51 --

52 ——

54

55

56 —— ——

R ————

Quartz

1+

Tr. 

1+

Tr.

Tr.

Tr.

1+

Tr.

1

*

1+

1

1+

3

1+

1+

1+ 

1+

1+

1+ 

Tr.

Tr.

1+ 

2+

14-

Plagioclase

Tr.

Tr.

2

Tr.

Tr.

Tr.

Tr. 

Tr.

1+ 

1

Tr.

Tr.

1

Tr.

1 

1

2

1

1+

1+

1+ 

1+

2+

1+

Tr.

1+

2+

q

K-feldspar

2+

1+

2+

1+

1+

Tr.

2

1+

2+

Tr.

Tr.

Tr.

Tr..

1 

5

Calcite

Tr.

1+

1+

2

1+

7+

4+

1+

3 

1+

14-

Tr.

2

64-

24-

Tr.

Tr.

Tr.

< r

14- 

7

Q 1

9+

< r

^i

Dolomite

7

Tr.

Tr.

1

Tr.

Tr.

Tr.

Tr.

Tr.

Tr.

Tr. 

2+

1

2

TTOther C03 minerals 

Sepiolite Chlorite 

Chlorite-mica

Tr —

? 2+ —— ——

7 1+ ——— ———

——— 1 ——— ———

? 1+ ——— ———

<1 —— Tr. ——

<:L

7

Tr. —— —— —— 

Tr. 2 —— ——

—— —— 1+ Tr.

7'

—— —— 1 1+

—— —— <1 Tr .

—— —— Tr. Tr.

- TT- TTcfl 

0 

•H

1+

1+

2

2+

2+

4+

2+

1+

1+

2 

2+

3+ 

2+

^

1+

2+

1 

2

Tr.

2+

1+

2+

2 

1+

1+ 

Tr.

Tr.

1+

2+

9

Saponite

1+

Tr.

1+

1+

2+

Tr.

1+

1+

1

Tr. 

4+

1+

3+

Tr.

1+ 

Tr.

Tr.

Tr

Mica-saponite Montmorillonite

Tr. ——

Tr. ——

1+ ——

Tr. ——

1

1+ ——

1+ ——

Tr. ——

Tr. ——

1+ ——

<

1 ——

Tr. ——

Tr

Mica-montmorillonite 

Clinoptilolite

—— 1+

—— Tr.

—— 1

—— Tr.

—— Tr.

—— Tr.

—— Tr.

—— Tr.

—— Tr . 

—— Tr .

—— <i

Phillipsite

?

Tr.

Tr.

Tr.

Tr.

Tr.

Tr.

7

Tr.

Tr.

Tr.

Tr.

Tr.

Other zeolites 

—— 

—— 

—— 

—— 

Tr

—— 

—— 

—— 

—— 

—— 

—— 

—— 

_ 
NONCLAY MINERALS 13

. 1 . . . 1 1 . .

VJlV-lV-lrJ 1 1 }-l J-l rH rH J-l J

HIHHH 1 1 H H V V H E1 1 1 1 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

• 1 1 1 1 1 1 1 1 1 1 +

VJ 1 1 1 1 1 1 1 1 1 1 CNI

H 1 1 1 1 1 1 1 1 1 1

. | . . | | .

VHvH HIHHIIH

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1

Illll 1 1 1 1 rH rH J-l

1 1 1 1 1 1 1 1 V V H

... 1 1 1 1

H H H H V 1 1 1 1 H

Illll 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

Illll 1 1 1 1 1 1 1

11 +

J-l J-l C^- CN J-l C^-| | rH rH J-l rH

H H H II V H

1

1 + + 1 + •

H V H """ 1 V H \

1 • IIIV 1 H v V V | | | H

. . . . | . |

J-l J-l J-l J-l rH ,_) | }4 ,_) | iH 0

HHHHV V IHV IV

Illll II 11+ H

Illll 1 1 CNI | | rH C

Illll II II

(•III Illll

1 J-i 1 1 1 1 1 1 1 1 c1 H 1 1 1 Illll

+ . . . . 1 . , 1

,-H ,-H J-l J-l J-l J-l | C-. ^|| )

V H H H HI Hll E-

• + + + • •

VHVH H • H H

11111 iii i

Illll IIICNCNJrHI

Illll III VI

+ + + +!••+!

rH rH CO rH ,— 1 CO | J-l J-l rH | rH !

V 1 H H 1 V E-

+ + + +1 II

V 1 v | I v E1 1 1 1 1 1 1

1 1 1 1 1 1 1

i i i m o i u <

1 in co co i — l CN V»D r*

1 1 1 1 1 1 1 <!PQ i rH CNI ro co ^r <r

00 C^ O rH CNI H H

in in VD VD VD co

•H H H H H

II II II II II II

II 1

II 1 J-l

II 1 H

1

1 H H

II II

II II

1 1 II

1 + + +

1 <T rH rH rH

1

+ 1

rH J-l rH rH |

V H VI

II III

II III

II III

II III

II III

II III

+ +

rH J-l rH rH CO

V H

+ ' • ' +

/ H H H

v 

1 III

lo- III

1 III

II 1 1

II II

-II III

-J 1 1 III

II III

II III

II III

II III

• 1 1 III

-ill III

Hll III

+ •

HHV

II III

II III

II III

• • + +

-1 J-l rH rH rH J-l

H H V H

+ +

H V V

II III II III C < pa o Q i

-CO CNI

rH

HHHHHHI HH

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 + 1 + + 1 1 +1

| CO | rH in | | rH |

II II 1

1 • 1 1 1 1 II

1 J-l C- 1 1 1 1 II 

1 H 1 1 1 1 II

1 1 1 1 1 1 1 '

1 1 1 1 1 1 0- 1 J-l

1 1 1 1 1 1 1 H

+ 1 + + • + 1 + r~ I rH in Vj J^- vc 1 rH

1 H 1

• +

C'- rH CO rH CNI rH rH J-l VD 

V V H

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 1 1 1 1 1 1 II

1 1 • 1 II

J-l rH | 1 J-l | C^- 1 1 r

H 1 1 H 1 1 1 \

• + 1 j • 1 • • 1 H

H 1 1 H 1 H HI

V V V V V

1 • • 1 1

| J-l J-l | r-\ C~ rH |

1 H H 1 V V|

1 1 1 1 1 1

1 1 1 1 1 1 rH C--

1 1 1 1 1 1 v

1 1 • 1 1 1 II

1 1 J-l 1 1 1 II

1 1 H 1 1 1 II

1 1 1 1 1 1 II

1 1 1 1 1 1 II

1 1 1 1 1 1 II

1 1 1 1 1 1 II

1 1 1 1 1 1 1 1 r1 1 1 1 1 1 1 1 V

Illll 1

1 1 1 1 1 rH CNI | J 

1 1 1 1 1 V IE1 1 1 + • II

1 rH 1 1 rH J-l II

1 II H II

1 1 1 1

rH | | rH 1 | rH J-l rV | | V | | V H V

+

vv VVHH HE11 1 1 1 1 1 II

1 1 1 1 1 1 1 II

-£3O-<3OQC_?O -<i3PQP

<3° v»D i — 1 CT* 0s* CO ^f V*O

CNICNICOCO ^±m vcvc

H E1

1

1

11

1

•

H

1

1

1

I H

l

H \

1

1

1

1

1

1

i H

H 1 r

/ 1

\

CN]

1

1

1

1

H 1

1

• +

-1 CN]

H

1

1

1

H rH J

' E-

. .

H H E1

L\ O

CO O

r^- r*

1 H H

1 1 1

1 1 1

1 1 1 <

1 1 1

1 1 1

1 1 1

• 1

c- Vi 1

H 1

1 1 1

1 1 1

1 1 1

h it i rH

V

• • 1

H VJ J-i 1

/ H H 1

1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

h 1

H C-- | rH

1

1 + +

/ 1

V 

11 c--

1

1

1 c1 1

1 1

1 1

1 1

1 1

1 1

1 1

1 1 rH

1 1 v

1 1 +

1 1 rH 

1 1

.

J-l J-l rH

HHV

• 1 1

H 1 1 V

•

•H V V

1 1 1

i PQ i

} CT* CO CO ** r~*- oo co

H H H E-

+ + 1

r-- CNI i

"> -f \

1 1 1 1

1 1 1 1 1

1

1 «-• J-l J

1 H E1 1 1

1 1 1

1 1 1

l l H

)-l 1 1 CH 1 1

1

H 1 E1 1

c- 1 1

1 1

1 1 1

1 1 1

1 1 1

1 1

1 1 rH r1 1 V \

1

1 V

J-l rH J.

H V E1

1 J-i

1 H

1 1

1 1

1 1

1 1

1 1

.

J-l C^-

H

y^ \-t rHHV

+ + m m rI i

rH 1 1

V 1 |

1 1

rH 1 | rV 1 |

1 ' '

1 PQ | ft

FQ ^ CO O

cr> O iH i3^ i — 1 i — 1 i— i

L/"1)

H

H H E1

1

1

1

1

1

• 1 -i 1

H |

1

1

1

- + H

>l CNI vj

H

1

1

1

1

1

1

+

H rH r-

/ \

-1 C^«

-1

C^-

}-4

H

V

-( rH

•

J-l

H

-1 rH J

v E1

3 Q C. •\ rH 0

H iH V H H E1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

• • 1 J-i J-i 1

H H 1

1 1 1

1 1 1

1 1 1

hi • + 4

o l J-i <r r1 H

1 +

• 1 C-- rH 

1

1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

1 1 1

• 1

H J-i 1 J-i J.

/ H 1 H EH Ev

J-l J-l J-l C^

H H H

J-l rH C^- J.

H V E1 1

1 c- 1

1 1 1

1 1

1 1

1 1

1

rH C-- |

V

+

v v E-

• l

rH J-l | C^

H 1

• + • 1

J-l rH J-l |

H H 1

• 1 +

-1 1 rH rH J-

-i 1 E1 1 1 1 P O CQ ^ P^

i co r^- ^ i—

0 CN OJ <t VH i — 1 i — 1 * — 1 * —

01

rt

W

c

•H 4J "O rH 

rH W Cfl J-l

03 -ri toO CO

CX J-i cfl Ol

O U S CO

ri CM ro j-
14 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

FIGURE 6.—Transmission electron micrograph of sample 21 showing suspended sepiolite. Note the presence of rhombohedral calcite

crystals.

FIGURE 7.—Transmission electron micrograph of sample 1A showing suspended sepiolite fibers more than 2 micrometers long.
NONCLAY MINERALS 15

/ * TX*

• j '^a^^ ^V "\_ . w.v - ,-./•.

FIGURE 8.—Transmission electron micrograph of sample 12B, surface replica of sepiolite, showing some of the fibrous bundles protruding

from the surface.

-J I

FIGURE 9.—Transmission electron micrograph of suspended saponite. Note the very short fibers of saponite.
16 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

farther down in the column higher amounts are again 

found.

Sediments sampled near the center of the basin and 

stratigraphically below the intermediate tuff contain 

little or no quartz. Some samples taken at the same 

levels, but near the Chicago Valley junction at the edge 

of the basin, contain some quartz but the amounts are 

small.

Some cyclical variation in quartz content 

throughout the stratigraphic column is suggested, but 

sampling was insufficient to prove this. Changes in 

source and (or) volume of the incoming sediments could 

easily account for stratigraphic variations in the quantity of quartz and changes in lake-water chemistry, 

perhaps due to rainfall fluctuations, may also have had 

some influence. During periods of fresh-water incursions the pH of the lake waters would have been 

lowered producing conditions in which the quartz 

would have been more stable. Conversely, during dry 

periods evaporation of the lake waters would have 

caused an increase in salinity and pH.

Frondel (1962) stated that quartz, especially 

microcrystalline varieties, is slowly attacked by 

alkaline solutions at room temperatures. This solubility of quartz may account for the lack of it in the older 

and central part of the lake sediments where it would 

have been subjected to attack by the alkaline water? 

for longer periods of time than was the quartz of the 

more recently deposited sediments or the quartz that 

was located around the basin edge where the alkalinity 

of the lake waters would have been diluted by runoff.

The feldspars observed in this suite of samples consisted of potassium feldspar of probable authigenic 

origin and detrital plagioclase. The plagioclase was 

found as a minor constituent in most samples of the 

mudstones. It was found in very small quantities, or 

not at all, in samples of the tuff. Samples collected 

from along the streambeds north of the lake (fig. 2), 

especially from the bed of the Amargosa River, contained more plagioclase feldspar than did any of the 

other samples. A fairly high plagioclase content was 

also found in the Chicago and Greenwater Valley 

drainages and near their points of entry into the 

Tecopa basin.

POTASSIUM FELDSPAR

The potassium feldspar, determined to be authigenic 

by Sheppard and Gude (1968), was found mostly in the 

central part of the basin, especially in or near the tuffs, 

and was almost completely absent from the samples 

taken outside the basin except at locality 45. At locality 45, a rock sample with a gray and red laminated appearance proved to be half potassium feldspar, almost

half cristobalite, and a trace of mica. The greatest concentration of the potassium feldspar was found in the 

west-central part of the lake basin below tuff 

B—within the area designated by Sheppard and Gude 

as the potassium feldspar zone.

CARBONATES

Calcite is found throughout the lake beds, and outside the former lake boundaries. However, some 

samples taken just within the boundaries of the lake 

showed little or no calcite. Most of these samples were 

taken near the entrance of present intermittent 

streams or close to the mountains where runoff from 

storms would most likely have been heavy. The area 

where Greenwater Valley enters the Lake Tecopa 

basin, and where runoff is likely to be heavy at times, 

also contains little calcite. The largest amounts of 

calcite were found at the northern end of the valley 

where the Amargosa River flowed into the lake. In that 

area the fresh waters of the river, bearing calcium 

which had been taken into solution from the larger 

drainage area upstream, mixed with the carbonate-rich 

lake brines of high pH and precipitated calcite.

Dolomite is found most extensively as detritus at 

the edges of the basin but small amounts occur in the 

mudstones within the basin. The amounts of dolomite 

are less than 10 percent and usually only a trace. In 

general, dolomite is not found in a sample with 

sepiolite although there are some minor exceptions. 

Sheppard and Gude (1968) have recognized several 

thin beds of finely crystalline dolomite in the Lake 

Tecopa basin deposits.

Jones (1961), on the basis of the pattern of distribution of the dolomite in the Deep Spring Lake playa in 

California, attributed the formation of the dolomite to 

direct precipitation from solutions. Peterson, Bien, and 

Berner (1963) concluded from radiocarbon dating 

studies that the dolomite in Deep Spring Lake was 

precipitated directly from solution with no intermediate calcite phase. They stated that very slow 

crystal growth (on the order of hundreds of Angstroms 

per thousand years) seems necessary to form dolomite. 

Clayton, Jones, and Berner (1968), basing their results 

on X-ray diffraction, solubility, and isotopic 

measurements, concluded that the fine-grained 

dolomite in Deep Spring Lake was formed by direct 

precipitation from solution, although they did not rule 

out the possibility that detrital or authigenic calcite in 

the lake could have been transformed into colomite.

The amount of dolomite in the Lake Tecopa area is 

not sufficient to draw conclusions from the distribution as to its origin as did Jones (1961), and the scope 

of the present paper precludes the use of more
NONCLAY MINERALS 17

sophisticated techniques necessary to determine the 

origin of the dolomite. If the dolomite in Lake Tecopa 

was precipitated at the same rate as that described by 

Peterson, Bien, and Berner (1963) in Deep Spring 

Lake, the size of the largest dolomite crystals based on 

the age of the lake should be approximately 0.1 

micrometers. At least some of the dolomite was found 

in the sand- and silt-sized fractions, which indicates 

that perhaps some of the dolomite in Lake Tecopa is 

detrital, or was formed by the magnesium replacement 

of calcium in some of the calcite. Part of the finegrained dolomite in the clay-sized fraction could have 

been precipitated directly from the alkaline lake 

waters. Probably the thin dolomite beds reported by 

Sheppard and Gude (1968) were authigenic in origin. 

Minor amounts of other carbonate minerals were 

also found. A probable identification was made of a 

trace of siderite in a couple of samples, and aragonite 

was identified in the sample from site 54. 

Manganocalcite and rhodochrosite were found in less 

than 20 percent of the samples, usually in trace 

amounts. They were found not only in the mudstones 

of the basin but in the detritus along the Amargosa 

River upstream from the basin and in Chicago Valley 

to the east. These occurrences would indicate that 

these minerals are detrital but it is possible that some 

of them that were found in the mudstones were 

precipitated from solution.

MAGADIITE

Magadiite was first described by Eugster (1967), 

who reported its occurrence at Lake Magadi, Kenya, in 

the High Magadi beds of Late Pleistocene to Holocene 

age. He, and later Jones, Rettig, and Eugster (1967), 

stated that the magadiite was precipitated from 

alkaline, silica-rich brines of high pH in waters of a 

more dilute precursor of present-day Lake Magadi. According to Eugster (1969), if such brines come in contact with fresh water, a stratification of the lake may 

occur producing magadiite saturation at the interface. 

Solubility curves for magadiite (Eugster, 1969) suggest a minimum pH value of between 8.5 and 9.0, 

which may be the lower limit of pH for precipitation of 

magadiite. Bricker (1969) stated that above pH 9, 

magadiite would be soluble because of the increased 

silica solubility, and below pH 9, its solubility would 

be increased owing to decreasing [Na+]/[H+] ratio.

The mineral has since been reported from two 

localities in the United States: Trinity County, Calif. 

(McAtee and others, 1968), and Alkali Lake, Oreg. 

(Rooney and others, 1969). Magadiite is reported in 

Lake Tecopa as the third occurrence in the United 

States. The sample, T4-93, containing the magadiite

was one of those furnished to the authors by Sheppard 

and Gude, who stated (Oral Commun., 1975) that the 

deposit is small, probably not exceeding a couple of 

cubic yards, including the impurities associated with it 

which make up about one-fourth of the sample. The 

sample locality was on the main "island" in the 

southern part of the basin just north of Tecopa; the 

deposit was not connected to the other clay beds. The 

elevation of the magadiite was estimated to be about 

488 m, stratigraphically about the level of tuff A.

The X-ray powder diffraction data from the Lake 

Tecopa sample are compared with those of the Trinity 

County, Alkali Lake, and Lake Magadi samples in 

table 3 and show very good agreement. Comparison of 

electron micrographs (figs. 10, 11) of the Lake Tecopa 

magadiite and the Trinity County magadiite show the 

Trinity County material to be made up of crystals 

about 2-2 x/2 times the size of the Lake Tecopa crystals. 

Whereas the Trinity County crystals are square or rectangular, the Lake Tecopa crystals appear to be broken 

and irregular and mixed with other detrital minerals 

commonly found in the mudstones of the basin.

The precipitation of magadiite in Lake Tecopa is 

somewhat analogous to that of the sepiolite, as 

described in the section on sepiolite. Apparently the 

tuff supplied the silica to the water which was 

disseminated into the surrounding detrital beds where 

it came into contact with their high pH (about 9.0 or 

more) high-sodium pore waters. Bricker (1969) has 

stated that under these conditions, but in the absence 

of reactive aluminous phases, magadiite could 

precipitate when an incursion of fresh water dropped 

the pH to about 9.0 and the [Na+]/[H+] ratio was at 

least 5. Any reactive alumina made available in the 

dissolution of the tuff would have been used to form 

the zeolites found in the tuffs or at the interface of the 

tuffs with the detrital beds.

If the Tecopa magadiite was precipitated in the manner described by Eugster (1967) for the waters of Lake 

Magadi, then the mineral may have covered a larger 

area in the Lake Tecopa basin than is apparent from 

the present-day deposit. Subsequent erosion that has 

scoured the clay beds and tuffs would have removed 

most of the magadiite, or the sodium could have been 

leached out leaving behind silica which converted to 

microcrystalline quartz (Eugster, 1969) or opaline 

silica.

However, it is probable that one or more sodium-rich 

springs existed on or near the "island" where the 

magadiite is found. Chemical analyses of the spring 

waters in various localities throughout the basin show 

that the sodium content of the springs of the nearby 

town of Tecopa is higher than that of any of the others.
18 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

TABLE 3.—X-ray diffraction data for magadiite 

[Leaders (——), not found or not reported]

Lake Tecopa, Lake Magadi, Trinity, Alkali Lake, 

Calif. Kenya 1 Calif. 2 Oreg. 3

I/I I/I15.49

7.82

5.57

5.18

5.01

4.46

4.00

3.90

3

r o

. 0 J

3.55

3.43

3.31

3.19

3.14

2.99

100

10

6

16

18

25

19

8

15

25

85

70

55

65

8

15.41

7.775

5.612

5.181

5.007

4.464

4.008

3.909

3.543

3.435

3.296

3.200

3.146

2.994

100

9

4

19

16

18

9

4

12

80

35

10

50

3

15.77

7.79

7.19

6

QO

. OO

5.62

5.18

5.01

4.69

4.46

4.00

3.93

3

f O .bz 

3.54

3.43

3.30

3.20

3.14

2.99

15.80

7.82

7.25

5.68

5.21

5.03

4.48

4.00

———

3

f O .bz 

3.53

3.44

3.30

_____

3.14

_____

100

26

10

10

26

21

24

17

——

O£zb 

25

74

53

___

57

——

!Eugster (1967) .

2McAtee and others (1968) .

3 Rooney and others (1969) .

This higher sodium content could have been sufficient 

to form magadiite at this location, all other factors being present. In other areas possibly the [Na*]/[H*] 

ratio was not high enough to precipitate magadiite 

because of the prevalence of magnesium and other cations.

ZEOLITES AND AMORPHOUS MATERIAL

, The principal zeolites found in the clay formations 

are clinoptilolite and phillipsite, with the phillipsite being found most often in the samples containing significant amounts of potassium feldspar. Although zeolites 

in small amounts may be found in almost any part of 

the study area, the greatest concentrations are found 

in or near the tuffs where they were formed. Sheppard 

and Gude (1968) made a thorough study of the formation of zeolites in this area and the reader is referred to 

their work for a discussion of their origin. Other 

zeolites found in small quantities are erionite and 

analcime.

No attempt was made to identify the exact composition of the amorphous material although it probably is 

volcanic ash in various stages of devitrification. Opal,

opaline silica, tridymite, and cristobalite were identified, which demonstrates that uncombined silica is 

abundant in the area. When the sum of the various 

mineral constituents of a sample was less than 100 percent the presence of amorphous material was 

suspected, whether or not it was indicated by the 

typical broad flourescent hump seen between 15° and 

30° 26 on X-ray diffractometer patterns. Optical examination revealing glass shards was accepted as 

proof of the presence of amorphous material. Only 

when the amount of amorphous material was 

estimated to be 2 parts in 10 or more was the sodium 

hydroxide leaching technique employed to confirm the 

amount present.

SALINE MINERALS

No specific effort was made to sample for saline 

minerals except at locality 29 in the present-day playa. 

However, a salty taste was noted in some samples; a 

salt crust and occasional specks and streaks of gypsum 

were observed also in some samples. Saline minerals 

were found to some extent in all but a dozen samples. 

The amounts varied from a trace to almost 50 percent. 

The largest quantities were found on the western side 

of the lake at localities 8, 14, 15, and 16, and at the 

playa locality, site 29.

Most of the saline minerals were soluble enough to 

be leached out of the samples during the fractionation 

procedures. Some of the gypsum, however, was not entirely removed during these procedures and was converted to bassanite when the fractions were oven-dried 

at 110°C.

Halite is the predominant evaporite; next in abundance is gypsum. Traces of thenardite and gaylussite 

were found infrequently. Only one sample contained a 

trace of soda nitre, even though, at one time, there 

were several nitrate claims in the area (Noble, 

Mansfield, and others, 1922).

Because Pleistocene Lake Tecopa is no longer a closed basin, but has been drained and eroded, it is unlikely 

that the present-day saline minerals would approach 

the composition of the salts present at the time of the 

formation of the authigenic minerals. Rather, they 

would reflect modern conditions.

OTHER MINERALS

There were a few nonclay minerals in the mudstones 

which were so widely scattered or present in such small 

amounts that they will be listed here together 

although there may be no relationship among them. 

The most widespread of these minerals were the amphiboles which usually are present only in trace 

amounts in any particular location. Only one or two of
NONCLAY MINERALS 19

FIGURE 10.—Transmission electron micrograph of sample T-93; suspended crystals of magadiite, Pleistocene Lake Tecopa basin.

FIGURE 11.—Transmission electron micrograph of suspended crystals of magadiite from Trinity County, Calif.
20 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

the identifiable, diagnostic, amphibole peaks were visible on the X-ray diffractometer charts because the 

amounts of the minerals were so small, and therefore 

the type of amphibole usually was not determined. The 

only identified species of amphibole was tremolite.

Searlesite was inconclusively identified in 10 

samples, most of which were located in the west side of 

the lake beds. In one or two samples, only tentative 

identification was made of pyrite, magnetite, and 

anhydrite because of the small amounts present. No 

conclusions could be drawn about them.

CLAY MINERALS

The amounts of nonclay minerals were subtracted 

from the total mineral content, and the remaining clay 

mineral content was recalculated to 100 percent. From 

these data the stratigraphic distribution of clay 

minerals was plotted (fig. 12). In this diagram certain 

relationships become apparent. Sepiolite is found, with 

few exceptions, close to the tuffs. Montmorillonite, 

when present, is also found close to the tuffs. Mica, illite, saponite, and mixed-layer clays are found at 

almost all stratigraphic levels. The reasons for these 

distributions will become apparent in the discussions 

of the individual minerals.

Data from representative X-ray diffraction patterns 

of randomly oriented mounts of saponite and sepiolite 

are shown in table 4. Frequently clay minerals in saline 

sediments are poorly crystallized (Grim, 1968); this is 

especially true of the authigenic minerals. This poor 

crystallmity, noted in most of our samples, together 

with small particle size which made it impossible to obtain pure samples accounts for the fact that some 

smaller reflections, usually found on X-ray diffraction 

patterns, are missing from these data.

CHLORITE AND CHLORITE-MICA

Chlorite and mixed-layered chlorite-mica are found 

in the stream beds outside the margins of the lake and, 

to a lesser extent, in the basin areas close to the 

mouths of the incoming streams. There is very little 

chlorite in the central or southern parts of the basin except close to the Amargosa River.

Where the amount of chlorite or chlorite-mica is 

given as "?", the identification is based on the 

presence of a small diffraction peak appearing at 14° 

26 for chlorite or to one appearing between 10° and 14° 

26 for chlorite-mica. Because of the small amounts present these peaks are not seen on heating to 400 °C or on 

glycolation; neither are the higher order reflections

TABLE 4.—Comparison of X-ray diffraction spacings of 

sepiolite and saponite from Pleistocene Lake Tecopa 

with previously reported occurrences

[Leaders (——), not found or not reported]

Sepiolite Saponite

Lake Tecopa, Apandandrava, 

Calif. India 1

d I/I l 3 d

12.37 100 12.3 

———— —— 7.56

6

-7 o

——— —— 5.05

/ QT

4.50 25 4.54

3.75 13 3.77 

——— —— 3.55

3.36 10 3.36 

——— —— 3.20

3

f\£-

2 Q /

——— —— 2.70

— 2 63

2.57 25 2.57

I/Il

100 

8 

4 

6

14 

13 

14 

3 

15 

8

3

2b 

6b 

6 

18

Lake Tecopa, 

Calif.

d

14.5

4.52 

3.78 

3.67

3.21

2.59

1.71 

1.52

I/Il

100

50 

12 

10

10

40

10 

30

Milford, 

Utah2

d

16.6 

4.94 

4.51 

3.70

3.21 

2.89 

2.58 

2.26 

2.06 

1.72

1.52

I/Il

100 

20 

80 

40

10 

40 

50 

10 

10 

30

90

1 Fahey, Ross, and Axelrod (1960).

2 Cahoon (1954).

3Plus 25 more peaks with intensities less than 10.

seen. Therefore, we recognize the possibility that some 

of these peaks may be due to chlorite-montmorillonite 

although this is not likely because montmorillonite 

itself is present in minor amounts.

MICA AND ILLITE

Detrital mica or illite is present in all samples except 

one of the limestone samples taken from the Dublin 

Hills.

There appears to be no significant stratigraphic 

variations in the amounts of mica or illite present (fig. 

12). The variations shown apparently are due to the 

dilution effects of the clay minerals. The micas are 

predominantly biotite, although occasional flakes of 

muscovite were found. In the sand-sized fraction, most 

of the micas are well crystallized and easily visible to 

the naked eye and their 001 X-ray diffraction peaks are 

sharp. In the silt-sized fraction, the mica becomes 

somewhat illitic with the 001 diffraction peak becoming asymmetrical on the low-angle side; and in the claysized fraction, the micaceous material becomes very illitic and often grades into a mixed-layered combination 

with saponite or montmorillonite (fig. 13).
NONCLAY MINERALS 21

en

cc

LU

—10

— 5

P 9_

LU

§

CD

en

2—

en Q 

cczz

2' 2 

CO

u. 2

2 —

— 25

— 30

LU CC

O

Q. 

LU

cn

cc 

O

O

O

Q.

en

O

cc

LU

LUCC

O

cc 

O

O

Q. LU -} 

< N^ 

CC >^

cc 

en

-TUFF A-

-TUFF B

TUFF C

0 50 100 0 50100 0 50 0 50100 0 50 

CLAY MINERALS, AS PERCENT OF CLAY FRACTION

0 50

FIGURE 12.—Vertical distribution of clay minerals, in percent. If more than one sample was taken at a given level, the data from the 

number of samples shown in the second column were averaged. No samples were taken between 9.45 and 18.29 m below tuff B.
22 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

RANDOM 

MOUNTS

ORIENTED CLAY 

AGGREGATE MOUNTS

11 10 9876543 

DEGREES 29 COPPER Ko

11 10 9 8 7 6 5 4 

DEGREES 20 COPPER Ka

FIGURE 13.—X-ray diffraction curves showing typical gradation from mica to mixed-layer mica-saponite.
CLAY MINERALS 23

LITHIAN SAPONITE

The most widespread clay mineral in the area is 

lithian saponite, which is found not only within the 

Lake Tecopa basin but also in Chicago Valley and 

along the Amargosa River upstream.

Electron micrographs (fig. 2) of some of the purified 

clays from the Lake Tecopa clay beds show a fibrous 

character. There may be some sepiolite in the sample 

which was otherwise undetected, but as can be seen in 

figure 8, the sepiolite fibers are about twice as long as 

the fibers in the clay which has been identified as 

saponite. Saponite, as well as sepiolite, may be fibrous 

as reported by Midgely and Cross (1956), and Caillere 

and Henin (1957). The chemical analysis (table 5) 

shows that the mineral is high in magnesium, has a little more iron and aluminum than is usual for saponite, 

and contains lithium. Although the sample was 

purified as much as possible (no impurities detected by 

X-ray diffraction) there could be a slight excess of 

silica due to the amorphous material which is common 

in the area. The amount of lithium is less than has been 

reported for hectorite from Hector, Calif. (Ross and 

Hendricks, 1945; Ames and others, 1958), and the 

alumina content is greater than in hectorite; therefore, 

we term this clay a saponite, recognizing the possibility that instead of being a lithian saponite, the clay actually may be a mixture of saponite and hectorite. The 

position of the 060 diffraction peak near 1.51 A indicates that almost all the smectite minerals in the 

Lake Tecopa mudstones are trioctahedral. Lithium, 

magnesium, and fluorine analyses of 12 samples from 

various parts of the lake beds (table 6) demonstrate 

that these elements are present throughout the area. 

Fluorite was found in none of the samples: small 

amounts would have been easily detected by X-ray diffraction. Fluorine was not determined in the purified 

sample because the purified sample was depleted by 

other analyses.

Lithium in clays has been found in other locations in 

the arid southwestern part of the United States as 

reported by Kesler (1960), Norton (1965), and Shawe, 

Mountjoy, and Duke (1964).

Foshag and Woodford (1936) reported that a clay 

from 5 km south of Hector, Calif., was a hydrous 

magnesium silicate containing appreciable lithia and 

related to saponite. They stated that similar clays were 

widespread in the Mojave Desert region of California 

and Nevada. Strese and Hofmann (1941) named the 

mineral hectorite.

TABLE 5.—Chemical comparison of Pleistocene Lake 

Tecopa saponite with other smectites

[Values in percent. Leaders (——), not found; 

n.d., not determined]

Sample——~

Lake San 

Tecopa, Hector, Bernadino, Clay Spur, 

Calif. Calif. Calif. Wyo. 

123 4

O -i /"\

A120 3 —— ——

Fe20 3 —— ——

Fed — — — —

MnO ——————

MgO ——————

CaC\ —

v c\

Na20 -— — —

Li20 —————

T-i C\

F __________

P2o5 ————

H

r\ i

TT f\

TotalCD = F —— —— 

Total — —— _

55.82 

4.61 

2.04 

.21 

.04

21.11 

1.42 

1.50 

.47 

.34

.19 

n.d. 

.03 

5.76 

5.26

98.80

98.80

55.86 

.13 

.03

None

25.03 

Trace 

.10 

2.68 

1.05

None 

5.96

2.24 

9.90

102.98

-2.51 

100.47

44.00 

10.60 

Trace

24.30 

2.00

6.20 

12.60

99.70

99.70

60.96 

18.27 

2.83 

.14

2.96 

.10 

.31 

1.44

.08

6.56 

6.78

100.43

100.43

2.

3.

4.

SAMPLE DESCRIPTIONS

Lithian saponite, acetic acid-leached clay

fraction.

Hectorite (Ross and Hendricks, 1945). 

Saponite (Ross and Hendricks, 1945). 

Montmorillonite (Kerr and others, 1950).

Ames, Sand, and Goldich (1958) described hectorite 

as being formed by the alteration of silica tuffs by solutions containing lithium and fluorine in the last stages 

of hot-spring activity and by lake waters that furnished magnesium. Sand and Ames (1958) said, in their investigation of altered silica volcanics for a source of 

refractory clay, that saponite is often formed as a 

result of hot-springs activity where pyroclastics are 

deposited in alkaline lakes. Their investigation included the Hector, Calif., area.

Later, Sand and Regis (1960), reporting on a lithiumand fluorine-bearing montmorillonite from Tooele 

County, Utah, questioned whether the magnesium in 

hectorite was furnished by lake waters.

Although hot springs are found in the area at the 

present time, it does not appear that the lithium and 

fluorine in the saponite were due to hot-springs activi-
24

TABLE 6.—Weight percent of magnesium, 

lithium and fluorine in total samples 

from scattered locations in clay beds 

of Pleistocene Lake Tecopa

[Mg and Li determined by atomic 

absorption by Violet Merritt. 

F determined volumetrically 

by (Mrs.) Johnnie Gardner]

CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

V. "'

Sample No, Mg Li

8A ———— -

9 ————— .

12C ———— -

D ———— -

16B ———— -

17A ———— -

B ———— -

25A ———— -

32C ———— -

36B ———— -

36E ———— -

41 ———— .

10.1

3.00

13.4

7.00

8.80

8.80

13.4

4.00

10.1

10.6

13.6

8.80

0.082

.021

.080

.041

.042

.044

.058

.010

.034

.056

.078

.060

0.50

.11

.74

.29

.10

.37

.35

.04

.31

.43

.78

.50

ty. Analyses of the clay-sized fractions (table 7) of sample 53 from Chicago Valley and of sample 61A from the 

bed of the Amargosa River (fig. 14) about 29 km 

upstream from Lake Tecopa show that lithium and 

fluorine are both present outside the lake basin as well 

as within the basin, and in the same order of 

magnitude. The presence of lithium and fluorine in the 

present-day drainage suggests that these elements 

came from some other source than hot springs within 

the lake basin.

Noble, Smith, and Peck (1967) presented fluorine 

analyses of 164 volcanic rocks of late Miocene or 

Pliocene age from southern Nye County, Nev., about 

160 km north of the area under present study. The 

largest amount of fluorine in any sample was 1.40 percent with the average value being about 0.13 percent. 

Noble (Written Commun., 1970) supplied the authors 

with analyses of glasses from the same area that contained about 10-40 ppm lithium. Livingston (1963) 

reported 0.16 ppm lithium in the waters of the 

Amargosa River at Beatty, Nev., which is approximately 120 km upstream from Lake Tecopa.

The occurrence of lithium in such a wide area 

demonstrates that there is sufficient lithium available

FIGURE 14.—Polygonal mud cracks in bed of the Amargosa River at 

sample locality 57. The clay mineral crusting the entire surface is 

saponite.

without relying on hot springs as a source. Lithium 

could have been introduced into the lake by the waters 

of the Amargosa River.

Although hectorite has been thought to be of 

hydrothermal origin (Ames and others, 1958), 

ghassoulite from Morocco, which Faust, Hathaway, 

and Millot (1959) equated with hectorite, was said by 

Millot (1949) and Jeannette (1952) to be authigenically 

formed in a lake environment.

Papke (1969, 1970), in studies of the 

montmorillonite-type clay deposits in Nevada, 

reported that montmorillonite is the predominant clay 

mineral in all the deposits studied except at Ash 

Meadows where saponite, formed by the sedimentation and alteration of volcanic ash in lakes, is present. 

He did not determine if lithium was present in the 

samples.

The presence of other detrital minerals, such as 

quartz, plagioclase feldspar, and mica, in the clays plus 

the presence of saponite in the drainage area outside 

the basin indicates that the saponite is detrital. 

However, we cannot rule out the possibility that some 

saponite may have formed authigenically in the basin 

sediments under conditions similar to those described 

by Papke (1969, 1970) in the Ash Meadows area north 

of Shoshone in the Amargosa River drainage.
CLAY MINERALS 25

MONTMORILLONITE

Montmorillonite, in the Lake Tecopa area, appears 

as an authigenic mineral that has two types of origin. 

"Amargosite," found just west of Shoshone, is a soft, 

sticky, white montmorillonite which, despite the desiccating effect of the present climate, is moist. This clay 

was formed by the hydrothermal alteration of a 

volcanic ash by warm springs laden with salts 

(Melhase, 1926). The warm springs found in the 

Shoshone area today may be relicts of the springs that 

produced the alteration.

The other type of montmorillonite is found in or very 

close to the tuffs deposited in the lake itself and was 

formed by the action of the saline lake waters on the 

siliceous glass of the tuff. Sheppard and Gude (1968) 

reported that they observed glass shards, which had 

been altered to zeolites, that were coated with montmorillonite. This montmorillonite is restricted and 

composes only a small fraction of the clay minerals. 

Montmorillonite is readily formed by the alteration of 

volcanic ash when alkalis and alkaline earths, particularly magnesium, are present (Papke, 1969). A 

possible reason for the small amount of montmorillonite could be that it was formed immediately 

after deposition of the ash (Grim, 1968) when the 

Na+ +K+:H+ activity ratio was low. Alteration of 

silica glasses to montmorillonite was viewed by Hay 

(1963) as a hydrolysis reaction releasing silica and 

alkali ions into solution and raising the pH, producing 

conditions more favorable to formation of clinoptilolite 

than to formation of more montmorillonite.

Several samples taken high above tuff A contained 

montmorillonite even though the site was not near any 

known tuff. These montmorillonites are usually 

associated with clinoptilolite. Inasmuch as these two 

minerals are commonly derived from volcanic ash, 

their presence indicates that an ash fall may have been 

deposited after that which formed tuff A. This 

evidence supports the conjecture of an ash fall as a 

source of silica for the sepiolite found above tuff A.

SEPIOLITE

TECOPA BASIN OCCURRENCE

The Tecopa basin sepiolite is most prevalent in a narrow band near the upper limit of the lake beds at about 

488-m elevation. This elevation coincides with the 

stratigraphic level of tuff A (fig. 15); most of the 

sepiolite is found within a meter or two of the tuff, 

usually just below its base. A few other, scattered, 

sepiolite-bearing sediments were detected near tuff B 

(fig. 16), and occasionally throughout the stratigraphic 

column above tuff B (fig. 12), especially near the lake 

margins (fig. 15). One sample (25 SB), located near an

intermediate tuff deposited 5 m below tuff B, also contained sepiolite, as did several samples in an 

anomalous occurrence 7.6-12 m above tuff A.

Sepiolite is one of the most abundant of the possibly 

authigenic clay minerals found in the mud and 

siltstone beds adjoining the tuffs in the Tecopa basin. 

Little or no sepiolite has formed directly in the tuff formations, except occasionally in clay partings which 

were deposited in the tuffs. The only other probable 

authigenic clay mineral identified in or near the tuffs is 

the small amount of montmorillonite mentioned in the 

preceding section.

The sepiolite of Tecopa basin does not occur in bulk 

form or in pure beds. Electron micrographs of the 

suspended material, and X-ray diffraction patterns of 

the total samples show the sepiolite fibers to be 

dispersed among the detrital and zeolitic minerals.

As shown in the electron micrograph (fig. 6) of the 

clay fraction of sample 21, most sepiolite fibers are 

about 1 \ji or less long. However, some sepiolite fibers 

more than 2 \JL long have been observed in sample 1A 

(fig. 7). The electron micrograph of a surface replica of 

some of the sepiolite-bearing sediment (fig. 8) shows 

that the fibers may occur in very small randomly 

oriented bundles.

An attempt was made to separate and purify the approximately 50 percent of sepiolite from sample 21, 

but we were unable to remove all the saponite, illite, 

and poorly crystalline feldspar. An X-ray diffraction 

pattern of the impure sepiolite displays the broad 

peaks typical of many poorly crystalline sedimentarytype sepiolites. The diffraction pattern is compared in 

table 4 to that of a well-crystallized sepiolite. Apparently no rapid, large-scale accumulation of sepiolite 

has taken place at any time in this area, but rather a 

localized deposition in small amounts.

Some sepiolites have been found in arid, alkaline environments similar to the environment of the Lake 

Tecopa beds. Grim (1953) stated that sepiolite is particularly prevalent in sediments accumulating in 

desert lakes containing alkaline waters with slight circulatory movement. Alternatively, Parry and Reeves 

(1968), after noting that others (Guven and Kerr, 1966; 

Grim and others, 1960; Eardley and Gvodestsky, 1960; 

and Droste, 1961) had reported no sepiolite in their 

various reports on desert lakes and playas, suggested 

that sepiolite apparently was not as common as Grim 

(1953) had previously indicated. They did conclude, 

however, that the sepiolite found at Mound Lake, Tex., 

was formed in a saline, lacustrine environment by the 

alteration of preexisting montmorillonite. Others who 

have reported occurrences of sepiolite in saline, 

lacustrine environments include Bradley (1930), Yar-
26 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

TABLE 7.—Lithium and fluorine in samples 

outside the boundaries of Pleistocene 

Lake Tecopa

[F determined by electrode method by 

D. R. Norton and (Mrs.) Johnnie 

Gardner; Li determined by atomic 

absorption by Wayne Mountjoy]

Sample

No.

Localities

(percent)

Li 

(ppm)

53——— Chicago Valley 

6 LA—— Amargosa River, 

29 kilometers 

upstream of 

Pleistocene 

Lake Tecopa.

0.48 

.58

330

440

zhemshkii (1949), Hardie (1968), and Papke (1972). Inasmuch as sepiolite is found in some saline, lacustrine 

environments but not in others, apparently a particular set of circumstances other than the basic environment is necessary for its formation.

PREVIOUS WORK ON SEPIOLITE

Siffert and Wey (1962) produced at room 

temperature a fibrous clay of the sepiolite type by reacting Mg*2 ions with silica in solution. The initial pH of 

the silica-magnesia solution was 11.20 and the final pH 

was 8.73. Wollast, Mackenzie, and Bricker (1968) 

reacted sea water with aqueous silica and produced a 

hydrated magnesium silicate compositionally and 

structurally similar to sepiolite. They also 

demonstrated that with a rise in pH of alkaline lake 

waters that have been concentrated by evaporation 

sepiolite may be precipitated if sufficient sources of 

magnesium and silica are available and if the dissolved 

silica is not removed from the waters by diatoms 

through biochemical processes. However, even if the 

silica is available, if the pH doesn't rise above 8.0, 

sepiolite doesn't form, even matastably, but the 

waters tend to become saturated with amorphous 

silica.

Conversely, if silica activity is below the sepiolite 

saturation level and the pH is near 10, the magnesium 

may precipitate as brucite or hydromagnesite, or, 

under the proper conditions, even as dolomite. At a 

later time, if silica is reintroduced to the interstitial 

waters by dissolution of diatoms or nearby volcanic 

ash and dispersion of the resulting silica by circulating

waters, then sepiolite may form as an authigenic 

mineral when the silica combines with the earlier formed magnesium mineral or with magnesium in solution.

Another determinative factor is the presence or 

absence of reactive aluminous phases in the waters or 

sediments. If ionic alumina is present, the silica and 

magnesium are more likely to combine with it to form 

alumino-silicates, such as chlorite or montmorillonite, 

rather than sepiolite.

In an attempt to synthesize sepiolite, Mumpton and 

Roy (1958) experimented with gel mixtures of the approximate composition of the average sepiolite using 

MgO (38.1 percent), SiO2 (59.6 percent), A12O3 (2.3 percent), and H2O. In about 100 runs at various 

temperatures and water pressures, they produced only 

montmorillonoids, talc, silica, or chloritic phases, but 

no sepiolite. They had started with the essential ingredients with which Hast (1956) and later Wollast, 

Mackenzie, and Bricker (1968) synthesized sepiolite, 

except that Mumpton and Roy added A12O3 to the 

system. This would indicate that the presence of 

alumina in the mixture inhibits the formation of 

sepiolite from solution. Millot (1960), Isphording 

(1973), and Heron and Johnson (1966) have proposed 

an authigenic origin by direct precipitation for 

sepiolite in the natural sediments that they have 

studied. Precipitation could occur where the pH and 

MgO contents were high but A12O3 was low or absent. 

In addition, Wollast, Mackenzie, and Bricker (1968) 

pointed out that sepiolite was commonly associated 

with carbonate sediments where alumina was low at 

the time of their sedimentation but not associated with 

shales where alumina was high.

Therefore, it is evident that in order to form sepiolite 

authigenically, we need a source of magnesium, a 

source of silica, a pH higher than 8.0, and little or no 

reactive alumina present.

MODES OF SEPIOLITE FORMATION

Several possible sources were considered for the 

sepiolite in the Tecopa basin sediments:

1. Clastic deposition was ruled out because most of the 

sepiolite was concentrated in the vicinity of the ash 

beds; a more uniform distribution throughout the 

strata would be expected if the sepiolite were of 

clastic origin. Also, no sepiolite was found in the 

sediments surrounding the basin.

2. Direct diagenesis from the volcanic ash is unlikely. 

Little or no sepiolite was found within the tuff formations. It occurred most frequently within a few 

feet above or below the ash beds. Furthermore, none 

of the optically examined volcanic glass shards 

showed any alteration to sepiolite.
CLAY MINERALS 27

3. A transformation of a parent smectite clay to a 

sepiolite, as suggested by Parry and Reeves (1968) 

for sediments in pluvial Mound Lake, Tex., was also 

considered. The dominant smectite mineral, a highmagnesian trioctahedral saponite, is found in all sections and stratigraphic levels of the sediments of 

Tecopa basin whereas sepiolite is not, the sepiolite 

being principally detected near the tuff beds. 

Although we do not believe that the saponite was 

transformed directly to sepiolite, we do not rule out 

the possibility that slight dissolution of the mineral 

in a high-pH environment may have supplied some 

of the silica and magnesium necessary to 

authigenically form the sepiolite.

4. The sepiolite may have been precipitated directly 

from solution. We contend that the principal mode of 

sepiolite precipitation involved postdepositional factors as well. Silica, in solution or in the form of a 

natural gel, may have combined with a previously 

formed magnesium mineral in solid or colloidal form, 

or may have possibly reacted with magnesium in 

solution in a high-pH, postdepositional environment. 

The latter possibility seems the most probable.

One way in which sepiolite precipitation might occur 

involves the direct combination of silica and 

magnesium as they mix in solution in the lake waters 

after being brought in from the lake's drainage basin. 

Papke (1972) suggested a sequential precipitation of 

dolomite and sepiolite in the Amargosa Flat, north of 

the study area. In this playa basin the magnesium in 

solution probably increased in concentration through 

evaporation until it began to precipitate out as 

dolomite. Then slowly increasing silica in solution combined with the magnesium and also precipitated 

sepiolite to form a combined bed about 1.2 m thick. A 

small percentage of detrital minerals was also included 

in the bed, and this, combined with a lack of internal 

bedding in the formation, indicates "rapid accumulation or, at least, constant conditions during 

deposition." Apparently this did not occur in the Lake 

Tecopa basin as no massive beds of this sort, containing sepiolite, were identified in its sediments.

Jones and VanDenburgh (1966) have shown that 

even though the waters of Lake Abert, a closed basin 

lake in southern Oregon, contained high concentrations of magnesium and silica in solution, no sepiolite 

or even dolomite was spontaneously precipitated 

therein. Droste (1961) has also found that many of the 

playa and closed basin lakes of southern California 

contain no sepiolite in their sediments. Therefore, even 

though the ingredients may be there, sepiolite isn't 

necessarily formed in these lakes in the manner 

described by Papke. Because of the formation in Lake

Tecopa of only small quantities of sepiolite, most of 

which was adjacent to the tuff beds, widespread 

precipitation in open water seems unlikely.

SOURCES OF MAGNESIUM AND SILICA

Sheppard and Gude (1968) have reported the 

presence of thin beds of dolomite in the Tecopa 

sediments: one below tuff B and one between tuffs B 

and A, both toward the central part of the basin. If 

there were any dolomite beds in the younger strata 

they have since been removed by erosion. The presence 

of the two thin beds would indicate that at various 

times the lake waters had a high concentration of 

magnesium in solution and a pH of well over 8.0 in 

order to precipitate the dolomite (Peterson and others, 

1963, and Skinner, 1963), or to convert calcite to 

dolomite. In addition, chemical analyses of the 

unaltered shards of the tuffs A and B showed a 

magnesium oxide content of 0.5 to 0.8 percent, which 

could become available on dissolution of the ash (Sheppard and Gude, 1968). A magnesium content of 22 ppm 

was also determined in the spring water of the basin 

(Sheppard and Gude, oral commun., 1976). Additional 

magnesium in solution or as detritus was probably 

brought in from the north through the Amargosa 

River drainage areas which contains dolomite formations and significant quantities of the high-magnesium 

smectite, saponite. When the ash and clay beds were 

deposited, this magnesium from all these sources was 

incorporated into the sediments and interstitial water, 

possibly as brucite or hydromagnesite in solid or colloidal form if the pH became high enough, or as solution with a high concentration of Mg ions and a fairly 

high pH. Further concentration of magnesium would 

occur during periods of evaporation of the lake waters. 

It is evident that a sufficient supply of magnesium was 

available for the precipitation of sepiolite if other environmental conditions were propitious.

An adequate source of silica in solution is also readily available in the ash beds which were deposited at intervals in the lake basin and surrounding countryside. 

Alexander, Heston, and Her (1954) have shown in 

laboratory experiments that different forms of amorphous silica, including colloidal silica, will approach a 

constant solubility concentration in water in the range 

of 100 to 140 ppm at a temperature of 25 °C. The soluble silica is in the form of the monosilicic acid Si (OH)*. 

The solubility of the silica is almost unaffected by pH's 

below 9 but increases rapidly as the pH climbs, 

reaching about 400 ppm at a pH of 10 and about 3,600 

ppm at a pH of 11. An increase in temperature will also 

increase the amount in solution and the rate of solubility of silica. They also noted, as did Krauskopf (1956),
28 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

116°20'

EXPLANATION

Clay mineral 

I Sepiolite

Mica

Montmorillonite 

Saponite 

Mixed-layer clay

Sample taken 

In tuff A 

Above tuff A 

Below tuff A

Measured section

.74-66 B 

? T4-70 C

Edge of Pleistocene Lake Tecopa

234 5 KILOMETERS

I I I

FIGURE 15.—Locality map of sample taken within 1 m of tuff A. Clay mineral content expressed as proportion of length of bar.
116°20'

CLAY MINERALS

15'

29

EXPLANATION

Clay mineral 

] Sepiolite

Mica 

Saponite 

Mixed-layer clay 

Chlorite

Sample taken 

In tuff B 

Above tuff B 

Below tuff B

Measured section

Edge of Pleistocene Lake Tecopa

01234 5 KILOMETERS

I I I I I I

FIGURE 16.—Samples taken within 1 m of tuff B. Clay minerals expressed as proportion of length of bar.
30 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

that at a pH over 9.0, colloidal, low-molecular-weight 

polysilicate ions are in equilibrium with the soluble 

monosilicic acid. As the soluble silica is removed from 

the solution by precipitation, the colloidal polysilicate 

spontaneously reverts to the soluble form and 

replenishes the supply of available silica in solution. 

Therefore, we postulate a constantly renewable source 

of soluble silica available in the vicinity of the ashfalls 

in the beds of Lake Tecopa when the pH was high owing to evaporative concentration. We do not know the 

ultimate concentration of the silica in the lake but 

some recent studies of alkaline carbonate brines in 

closed basins have determined natural waters to contain as much as 2,700 ppm silica (Jones and others, 

1967).

SEPIOLITE FORMATION IN LAKE TECOPA

As previously indicated, sepiolite is not found within 

the tuffs of the Tecopa basin but is most prevalent in 

sediments immediately adjacent to both the unaltered 

ash and the zeolitized ash. The question arises as to 

why sepiolite is absent from the ash but present in the 

other two environments.

The mudstones, siltstones, and volcanic ash were 

probably deposited in waters with a pH of 9 or higher. 

Some intermixing of the ash and the previously 

deposited detritals occurred at their interface. Subsequent evaporation and concentration tended to increase the salinity and pH in a downdip direction 

toward the center of the basin. However, owing to rain, 

snow, or the inflowing waters of the Amargosa River, 

incursions of fresher water into the lake basin flushed 

out the excess alkaline cations and lowered the pH of 

the outer, more-exposed, edges of the ash beds. This 

left the outer edges unaltered even as zeolitization 

started in downdip parts of the bed. In addition, there 

were periods of time when evaporation of the lake 

limited the waters to the central part of the basin so 

that the edges of the ash bed were not immersed in 

high-pH waters. Authigenic sepiolite, however, was 

still able to form beneath the unaltered ash.

Undoubtedly, in times of fairly high pH (about 8) 

some dissolution of the ash occurred and silica and the 

alkalis were disseminated, by circulating waters, 

downdip and into the detrital beds below and above 

the ash bed. The concentration of silica increased to 

saturation in the ash but the pH was not high enough 

nor the concentration of the magnesium great enough 

to precipitate sepiolite. With an influx of fresh water 

the pH dropped throughout the system. The soluble 

silica was then in a supersaturated state so it began to 

precipitate out as a colloid or as sepiolite when it came 

in contact with the magnesium-saturated, high-pH

pore waters in adjacent detrital beds. Some sepiolite 

also may have formed over long periods of time when 

initially admixed ash dissolved in the high-pH, original 

pore waters of the adjacent beds and combined there 

with magnesium in solution to precipitate out.

Inasmuch as alumina does not readily go into solution at a pH of less than 9.5, probably insufficient 

alumina was present to precipitate zeolites either in 

the unaltered tuff or in any significant quantity in the 

adjacent beds where sepiolite occurred, as seen in the 

table of mineral analyses (table 2). What small 

amounts of zeolite were formed with the sepiolite probably were precipitated from the original pore waters 

which dissolved the admixed volcanic glass to provide 

the necessary alumina, silica, and the alkali cations. 

Phillipsite was most commonly found in the zeolitized 

tuffs or at their interface with the siltstones and 

claystones of the adjacent beds. Clinoptilolite was the 

zeolite most often found with sepiolite, sometimes with 

and sometimes without phillipsite.

Downdip in the basin the ash beds were zeolitized 

owing to more constant exposure to high-pH waters. 

Dissolution of the ash (and concentration of the 

waters) had produced a pH of 9-10 and the necessary 

soluble ingredients, silica, alumina and the alkalis, for 

formation of the various zeolites; but sepiolite didn't 

form in the ash bed owing to the presence of alumina. 

All or most of the alumina was used in forming zeolites 

or other aluminosilicates within the tuffs, or at the interface of the tuffs and the adjacent beds, as shown by 

Sheppard and Gude (1968). After removal of the 

alumina the waters disseminating into the adjacent 

beds were still supersaturated with silica. A drop in pH 

then caused the silica to form a colloid or to combine 

with the available magnesium and precipitate out as 

sepiolite in the pore waters near the zeolitized tuff. An 

alternative method to direct precipitation of the 

mineral from solution is as follows: Silica has a great 

affinity for magnesium which would tend to adsorb on 

the colloidal silica surface to form a hydrated 

magnesium silicate. Later desiccation could remove 

some of the water to form sepiolite.

Sepiolites found in sediments near other tuffs in the 

basin sequence probably were formed in the same manner as those described above. However, some sepiolite 

detected in other parts of the basin may have a different genesis.

Several samples, taken from sediments approximately 7.6-9.1 m, and one at 12 m above tuff A, also contained sepiolite. Sample 4, containing approximately 

40 percent sepiolite was taken about 1.3 km southwest 

of Shoshone, in the northern part of the basin and 7.6 

m above the tuff A. It is possible that a thin ash bed
CONCLUSIONS 31

might have been deposited in the lake waters at that 

time, to act as a silica supply for sepiolite. No trace of 

one was found by the authors at this location or at the 

other locations around the basin at that altitude where 

the samples containing sepiolite were collected. 

However, several nonsepiolite-bearing samples taken 

at those altitudes, 2, 20A, 20B, and 24, contained dioctahedral montmorillonite and clinoptilolite, both of 

which are products of ash devitrification.

Under the proper conditions of pH, magnesium concentration, and silica saturation, it is possible that at 

the locations listed in the preceding paragraph and 

other anomalous locations far from ash beds a direct 

precipitation from solution may have occurred as 

described by Papke (1972). However, no real evidence 

of this was found. An alternative possibility for a silica 

source for sepiolite precipitation would be the presence 

of abundant diatom colonies in the lake. Sheppard and 

Gude (1968) have stated that 42 species of diatoms 

were identified in their fossil locality 5 about 1.6 km 

northeast of the site of sample 4, at about 488+ m 

altitude. Some of the diatoms were identified as "fresh 

water" types which would correlate with their location 

in the north end of the basin where the Amargosa 

River entered, supplying fresh water from a considerable drainage area to the north.

A flourishing colony of diatoms might remove a considerable quantity of silica from solution in the lake 

waters (Phillips and VanDenburgh, 1971). If the supply of fresh water diminished radically, the salinity 

would rise and kill off the colony causing decay of the 

organisms and a rapid return of silica to the environment. With a high enough pH and source of 

magnesium, a localized precipitation of sepiolite might 

occur wherever a colony had been located. Sample 37 in 

the same approximate area as diatom fossil locality 5, 

and altitude as sample 4 (7.6 m above A tuff), has an 

ostracode bed that indicates a fresh-water environment. Sample area 39 on the east side of the basin at 

approximately the same altitude as sample area 4 also 

contains fresh water ostracodes, as well as a small 

amount of sepiolite. Two other sample sites, 50 and 56, 

containing sepiolite, are in an area where fresh water 

would intermittently incur and are at about the same 

altitude as the site of sample 4. Therefore, although 

diatoms were not actually found at the sepiolite sites 

at that lake level, 7.6-9.1 m above tuff A, we cannot 

rule out the possibility that they were a source of the 

necessary silica for sepiolite precipitation in the highpH, magnesium-concentrated pore waters of a later 

lake phase.

On the basis of our laboratory experimentation and 

field evidence we conclude that the sepiolite found in

the Tecopa basin is of authigenic origin. The volcanic 

ash beds, and possibly in some cases fossil diatoms, 

supplied the necessary silica in solution or colloidal 

form to combine with magnesium in a high salinity, 

high-pH lake-water environment. Sepiolite may have 

precipitated directly from solution in open lake waters 

at various times after the deposition of tuff A but no 

proof is available.

DISCUSSION 

SEQUENCE OF EVENTS

During the middle to late Pleistocene, the Amargosa 

River was dammed by alluvial fan deposits south of 

the present-day site of Tecopa, Calif. (Sheppard and 

Gude, 1968). Waters began to accumulate behind the 

dam with simultaneous deposition of fine-grained 

detritus. Figure 12 shows that the clay minerals 

deposited below the tuff C were mostly micas (or 

illites), saponite, and mixed layered material. Some 

small beds of volcanic ash were laid down at this time 

also.

Later, the ash fall which subsequently produced tuff 

C was laid down, followed by the deposition of more 

detrital mudstones. The deposition of these mudstones 

was also interrupted periodically by light ash falls, one 

of which was large enough that it has been termed the 

"intermediate tuff".

The ash fall which produced tuff B was next 

deposited. By this time the concentration of the 

magnesium and the pH of the lake waters had increased to the point that some sepiolite was being formed by 

the reaction of the magnesium-rich waters and the 

silica from the ash falls which had been taken into solution by the lake waters. The salinity of the water would 

not have increased very rapidly at first but as the lake 

grew and covered a greater area, the enlarged surface 

area would have increased evaporation to produce 

greater salinity.

More detrital material was deposited as mudstones, 

followed by another ash fall which produced tuff A. 

Again, magnesium-bearing waters of the lake combined with the plentiful silica to produce even more 

sepiolite than was produced in associaton with tuff B. 

This was the last major ash fall although the presence 

of montmorillonite and sepiolite above tuff A indicates 

that there later was at least one minor ash fall. Detrital 

material was deposited as long as the lake existed.

CONCLUSIONS

Most of the minerals which make up the mudstones 

of the Tecopa basin are detrital in origin. They consist 

of quartz, plagioclase feldspar, some of the calcite and 

dolomite, and the clay minerals mica, illite, chlorite,
32 CLAY MINERALOGY OF PLEISTOCENE LAKE TECOPA

the lithian saponite and mixed layered combinations of 

these. All these minerals are found areally throughout, 

and at all stratigraphic levels of, the Tecopa sediments 

as well as in the basin drainage area. This would tend 

to preclude an authigenic origin for them in the basin.

The clay minerals brought into the lake as detritus 

from the drainage area were not affected by diagenesis. 

No differences can be detected between them and the 

clay minerals found along the tributaries. If there were 

any dissolution or alteration of the clays, the amount 

was negligible. This corroborates the findings of 

Droste (1961) who, after comparing the clay minerals 

from 45 playas in the Mojave Desert with the clay 

minerals from surrounding areas, concluded that clays 

deposited in alkaline lakes reflected the source rocks 

rather than diagenesis.

Possibly some of the lithian saponite was formed by 

diagenesis of montmorillonite, as suggested by Papke 

for the Amargosa Flat sediments; however, most of it 

is considered to be detrital.

The minerals that we consider to be authigenic include much of the calcite and dolomite, sepiolite, 

magadiite, potassium feldspar, montmorillonite, 

zeolites, and the saline minerals.

During this entire process, the calcium and CO2 in 

the lake waters were combining to precipitate the 

calcite which is almost ubiquitous. It is especially 

abundant at the north end of the lake where the 

calcium-bearing waters of the Amargosa River came 

into contact with the carbonate waters of the lake. The 

calcite is less abundant in the southwestern part of the 

lake near the entrance of Greenwater valley.

Only small amounts of dolomite are found in the lake 

beds although magnesium was probably present in its 

waters throughout the life of the lake and became more 

concentrated as the salinity of the waters increased. 

Dolomite is not readily precipitated from magnesiumrich solutions (Peterson and others, 1963, Peterson and 

others, 1966, Clayton and others, 1968), although 

calcite is readily precipitated if calcium is present. Any 

dolomite that was precipitated would have been formed very slowly. The thin beds of finely crystalline 

dolomite recognized in the basin by Sheppard and 

Gude (1968) probably are authigenic in origin. Their 

presence would imply water conditions of high salinity 

and pH and a good concentration of magnesium.

The formation of authigenic sepiolite also requires a 

special environment as shown in experimentation by 

Siffert and Wey (1962), Wollast, Mackenzie, and 

Bricker (1968), and Mumpton and Roy (1958). It needs 

a source of silica and of magnesium in high-pH waters 

but with the absence of reactive aluminous phases. In 

the closed basin of Tecopa, the pH of the lake waters 

reached at least 8.0, owing to evaporation and the concentration of magnesium and silica and various salts. 

The dissolution of volcanic ash, which periodically was 

deposited in the lake, then supplied the necessary silica 

to combine with magnesium in forming the sepiolite. 

The reactive alumina, which was also released by 

dissolution of the ash, served a twofold purpose. First, 

it was essential in the formation of the zeolites within 

the tuff beds and at their interface with the detrital 

beds. Second, the alumina prevented the precipitation 

of sepiolite within the tuffs but did not inhibit the 

dissemination of the silica-supersaturated, and by then 

alumina-depleted, waters into the surrounding 

sediments where sepiolite formed in the high-pH, 

magnesium-saturated pore waters. The dissolution of 

diatoms may have supplied the necessary silica for 

some sepiolite formation where volcanic ash was not 

available.

Grim, Kulbicki, and Carozzi (1960) stated that the 

clay minerals rich in magnesium, such as sepiolite, probably would not form in an environment of highsodium content. However, in the mudstones of Lake 

Tecopa the sepiolite content is well established, 

dolomite is present in small quantities, and yet 

magadiite, a hydrous sodium silicate which is 

precipitated from silica-rich sodium waters (Eugster, 

1969), is found in one area at about the same 

stratigraphic level as the sepiolite.

Determination of how widespread the magadiite formation was in the basin because of subsequent erosion 

and leaching action is impossible; but, because of the 

magnesium content of the lake waters and because of 

the occurrence of magadiite only in one part of the 

basin, we conclude that this was a local occurrence 

caused by the presence of sodium-rich springs in this 

area making contact with the silica-bearing lake 

waters.

Authigenic, dioctahedral montmorillonite is not as 

widespread in basin sediments as detrital, trioctahedral smectite saponite. It is found in a few 

localities where diagenesis of the volcanic ash has occurred to form clinoptilolite and montmorillonite. 

Sheppard and Gude (1968) also reported some montmorillonite coatings on shards associated with other 

zeolites such as phillipsite. The other occurrence of 

montmorillonite, "amargosite," is caused by the 

hydrothermal alteration of volcanic ash by hot-spring 

waters. This unique mineral formation was found in 

the vicinity of the town of Shoshone.

Other authigenic minerals, such as the zeolites and 

potassium feldspar, are found scattered throughout 

the basin in small quantities in the detrital sediments, 

and larger more concentrated amounts are found in or 

near the tuffs. Sheppard and Gude (1968) have described the formation of these minerals in their paper on the
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basin.

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