Diagenesis of Tuffs in the 

Barstow Formation, Mud 

Hills, San Bernardino 

County, California

GEOLOGICAL SURVEY PROFESSIONAL PAPER 634

Diagenesis of Tuffs in the 

Barstow Formation, Mud 

Hills, San Bernardino 

County, California

By RICHARD A. SHEPPARD and ARTHUR J. GUDE 3d

GEOLOGICAL SURVEY PROFESSIONAL PAPER 634

Physical properties^ chemistry , and origin of 

silicate minerals formed in silicic tuffs of a 

lacustrine deposit

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog-card No. 79-650237

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

Washington, D.C. 20402 - Price 50 cents (paper cover)
CONTENTS

Abstract-____-_-_-_______--_---_--_--__-__-__---___

Introduction________________________________________

Previous work_____-______-----________-_-__-_

Scope of investigation.____-_---_-_-_-______-____

Laboratory methods.___-____-_--_-___--_-_..-___

Acknowledgments..___ _ __________________________

Regional geology.__---_____-_-___-_____.______-____

Stratigraphy and lithology of the Barstow Formation __ 

Conglomerate, sandstone, and siltstone-___________

Mudstone.._ _ __-____--___--_____--____---___-

Carbonate rocks________________________________

Tuff ----------------_-------------------_-_

Authigenic minerals.________________________________

Analcime- _-__-__-__-_-.-_--_--_________________

Chabazite__ _ _________________________________

Clay minerals.__________________________________

Clinoptilolite- ___________________________________

Erionite.__-__-__-___-___-_-_____-_-___________

Mordenite--_-_-_-_________---_-__--.-_-___-____

Opal. __-_---__---_-_-_-__---____-_-_-__-_-_--_

Phillipsite__ __________________________________

Potassium feldspar____________________________

Quartz_ ______________________________________

Page 

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5

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Page

Altered tuffs..______.___..________._____ 17 

Field description _____________________________ 17

Petrography.. .. ______-_.-__-.- _____ 18

Nonanalcimic zeolitic tuff_-_--_----___--_____ 19

Analcimic tuff_--_-_---_-__--_-----_--_-____ 20

Potassium feldspar-rich tuff.-____--__--______ 20

Clinoptilolite-analcime-potassium feldspar relationships in the Skyline tuff._______________________ 21

Relationship of authigenic silicate mineralogy to 

thickness of tuffs..________-___----_________- 26

Genesis of the authigenic silicate minerals. ______________ 26

Interpretation of a saline, alkaline depositional

environment for parts of the Barstow Formation _ _ _ 27 

Correlation between salinity of depositional environment and authigenic silicate mineralogy_ _ _ _ 27

Solution of glass to form alkali- and silica-rich 

zeolites.______________-__-_--_-___-_-----__-_ 28

Reaction of alkalic, silicic zeolites to form analcime.. __ 28 

Reaction of zeolites to form potassium feldspar- _____ 30

Metasomatism during diagenesis of tuff.________________ 32

References. ____________-_____.--_---_-----------_--- 32

Index -_-____-__-___-------------------------- 35

ILLUSTRATIONS

Page

FIGURE 1. Index map showing location of the Mud Hills._.________________-____--_---_---------..------------- 2

2. Diagrammatic sketch showing X-ray diffractometer patterns of authigenic silicate minerals._____________ 3

3. Generalized geologic map of the Mud Hills and vicinity.________________-__---_--------------------- 4

4. Photograph of the Barstow Formation along the axis of the Barstow syncline.___________-__--_-------- 5

5. Generalized columnar section of the upper part of the Barstow Formation______________-__-__---_----- 6

6. Photographs of typical lacustrine mudstone of the Barstow Formation________________-_---------_--- 7

7. Photograph of casts of mud cracks on bottom of Dated tuff--__-------------------------------------- 9

8. Photomicrograph of altered tuff, showing analcime crystals____________-_-_-----_------------------ 10

9. Determinative curve for the anhydrous composition of analcime-______________----------------------- 11

10. Histogram of the silicon content of analcime._________________________----------_------------------ 12

11. Photomicrograph of zoned clinoptilolite crystal______-____________-___-_---------------------------- 13

12. Photomicrograph of oscillatory zoning in clinoptilolite-_________________-.____----------------------- 13

13. Photomicrograph of spherulitic phillipsite___-_-___---___--_-__----.__--_----.-----_---------------- 16

14. Electron micrograph of potassium feldspar. ________________-_____-_---__-_--__--__-_--------------- 17

15. Photomicrograph of altered Skyline tuff, showing segregations of quartz in finely crystalline potassium

feldspar.----_--_-_-___________________._______________________-.___--_------_--_---------- 17

16. Photograph of hogbacks formed by altered tuffs_--------------_------------------------------------ 18

17. Photograph of clinoptilolite-rich part of Skyline tuff. _______________-_____--_---_--__----------------- 18

18-22. Photomicrographs:

18. Zeolitic tuff, showing pseudomorphs of shards___________________-_-_____-_--_--_----------- 19

19. Zeolitic tuff, showing pseudomorphs of fibrous clinoptilolite-____________-____--_--_---------- 19

20. Zeolitic tuff, showing pseudomorphs of clinoptilolite and mordenite.______-_--__----_---------- 19

21. Plagioclase with potassium feldspar overgrowth._____________--_____-__-____---_-_---------- 21

22. Potassium feldspar pseudomorph after plagioclase__----___--------__--_---_----------------- 21

23. Geologic sketch map showing the sampled locations of the Skyline tuff-___-__-__---_-----_------------ 22
IV CONTENTS

Page 

FIGURE 24. Photograph of Skyline tuff, showing remnants of clinoptilolitic tuff surrounded by analcimic tuff. __________ 25

25. Photograph of Skyline tuff, showing remnants of clinoptilolitic tuff surrounded by analcimic tuff and potassium feldspar__ _ ___________________________________________________________________________ 26

26. Plot showing the correlation between the silicon content of analcime and the precursor zeolite-_-______--___ 29

27. X-ray diffractometer traces of synthetic potassium feldspar, natural potassium feldspar, and natural clinoptilolite-_.._ . . . - . .-- 31

TABLES

Page 

TABLE 1. Mineral content of the finer than 2n fraction of five mudstones.______________________________________ 8

2. Chemical analysis of mudstone._________________-______--__-___----------__---------------------_ 8

3. Chemical analysis, molecular norm, and semiquantitative spectrographic analysis of shards from Hemicyon

tuff--_-_-------_--__--__________________________-_______--__------__--------------------__ 9

4. Formulas of selected alkalic zeolites.___________________.-_______________--_____--_-_---_--__----__ 10

5. Chemical analyses and composition of unit cell of analcime and chabazite_-_________--_-_-__-____--____ 11

6. Chemical analyses and composition of unit cell of clinoptilolite_ ______________________________________ 14

7. Chemical analyses and composition of unit cell of mordenite and phillipsite____________________________ 15

8. Mineralogic composition of the Skyline tuff as estimated from X-ray diffractometer patterns of bulk samples. 24
DIAGENESIS OF TUFFS IN THE BARSTOW FORMATION, MUD HILLS, 

SAN BERNARD1NO COUNTY, CALIFORNIA

By RICHARD A. SHEPPAED and ARTHUR J. GUDE 3o

ABSTRACT

The Barstow Formation of Miocene age consists chiefly of 

lacustrine and fluviatile clastic rocks. Lacustrine rocks are 

mainly mudstone with interbedded tuff and carbonate rocks. 

The tuffs make up about 1-2 percent of the stratigraphic section 

and are the most conspicuous and continuous strata. Most tuff 

layers are less than 1 foot thick, but some are as much as 7 feet 

thick. All tuffs were originally silicic, and most were vitric and 

fine to very fine grained. This report summarizes the physical 

properties, chemistry^ and genesis of those silicate minerals 

that formed in the tuffs during diagenesis.

Zeolites, monoelinic potassium feldspar, silica minerals, and 

clay minerals now compose the altered tuffs; parts of some tuffs, 

however, contain as much as 90 percent relict glass. The zeolites 

are chiefly analcime and clinoptilolite, but local concentrations 

of chabazite, erionite, mordenite, and phillipsite have also been 

found. The zeolites and potassium feldspar occur in nearly 

monomineralic beds or associated with other authigenic silicate 

minerals. The zeolites, except analcime, are locally associated 

with relict glass; analcime commonly is associated with the 

other zeolites; and potassium feldspar is associated with analcime as well as some of the other zeolites. Nowhere in tuffs of 

the Barstow Formation is either analcime or potassium feldspar 

associated with relict glass.

Solution of silicic glass by moderately alkaline and saline 

pore water provided the materials necessary for the formation 

of zeolites and, subsequently, the potassium feldspar. The zeolites, except analcime, formed directly from the glass by a solution-precipitation mechanism. The observed paragenesis of 

zeolite minerals is attributed to variations in the activity of 

SiO2, the activity of H2O, and the proportion of cations in the 

pore water during diagenesis. Much, if not all, of the analcime in 

the tuffs seems to have formed from alkalic, silicic zeolite precursors. Analcime in the Barstow Formation ranges in its Si: Al 

ratio from about 2.2 to 2.8 and thus falls at the silica-rich end 

of the analcime series. The silicon content of analcime is believed to be inherited, at least in part, from the precursor zeolite relatively siliceous analcime formed from a zeolite such 

as clinoptilolite, whereas relatively aluminous analcime formed 

from a zeolite such as phillipsite. Potassium feldspar, like analcime, has apparently not formed directly from the silicic glass. 

The formation of potassium feldspar from analcime and clinoptilolite is documented; however, feldspar may have also formed 

from the other precursor zeolites.

Tuffs such as the Skyline tuff of informal use show a lateral 

gradation in authigenic mineralogy of nonanalcimic zeolites to 

analcime, and then to potassium feldspar. The chief factor responsible for this lateral gradation is probably a variation in

the salinity of the pore water trapped in the tuff during 

tion. Highly saline pore water during diagenesis favors' the 

ultimate formation of analcime or potassium feldspar ir the 

silicic tuffs, rather than the formation of zeolites sue1! as 

clinoptilolite and phillipsite.

INTRODUCTION

The Mud Hills are in west-central San Bernardino 

County (fig. 1), Calif., mainly in T. 11 N., Rs. 1 and 2 

W. The nearest principal town is Barstow, about 10 

miles to the south. The Fort Irwin Eoad, a paved highway between Barstow and Fort Irwin, provides access 

to the Mud Hills by way of the graveled Fossil Bed 

Road. The area is shown on the 15-minute topographic 

maps of the Lane Mountain and Opal Mountain quadrangles by the U.S. Geological Survey.

The Mud Hills are in the western part of the Mojave 

Desert, which is characterized by isolated mountain 

ranges surrounded by broad alluvial plains. The Mud 

Hills range in elevation from about 2800 to 4200 feet, 

Numerous canyons radiate southward from the northcentral part of the Mud Hills and provide accers to 

nearly continuous exposures of the Barstow Formation.

PREVIOUS WORK

The Barstow Formation contains rich vertebrate 

faunas that have received much attention since the 

pioneer work of Merriam (1911). Lewis (1964, 1968) 

reviewed the nomenclature of the Barstow Formrtion 

and discussed the faunas. As these topics will not be 

considered here, the reader is referred to Lewis' reports.

The rocks of the Barstow Formation, unlike the fossils, have received only cursory study except in recent 

years. Baker (1911, p. 342-347), Pack (1914, p. 145- 

150), and Knopf (1918, p. 257-259) briefly described 

the Tertiary deposits of the Mud Hills. These deposits 

include the Barstow Formation as well as the underlying formations. Durrell (1953) studied the stratigraphy of the Tertiary deposits during an investigation 

of strontianite deposits at the eastern end of the Mud 

Hills. Like most earlier workers, Durrell (1953, p. 24)

1
DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

117° 116.

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SAN BERNARDINO COUNTY

China Lake

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CALIFORNIA

FIGURE 1. Index map of the Mojave Desert in western San Bernardino County, Calif., showing the location of the Mu*l Hills.

Base from U.S. Geological Survey, State of California, 1: 500,000,1953.

assigned the Tertiary strata to the Rosamond Series, a 

name abandoned by the U.S. Geological Survey (Dibblee, 1958). T. W. Dibblee, Jr. (1967, p. 88-89), mapped 

in detail the geology of the Mud Hills. Several short 

reports on the Barstow Formation (Sheppard and 

Gude, 1965a; Gude and Sheppard, 1966; Sheppard, 

1967) have been published as part, of the present study. 

R. P. Steinen (1966) recently mapped the geology of 

the Mud Hills and studied the stratigraphy of the Barstow Formation.

SCOPE OF INVESTIGATION

This investigation of the Barstow Formation is primarily a study of the genesis and distribution of zeolites

and other associated authigenic silicate minerals in the 

tuffaceous rocks. Zeolites are common authigenic minerals in tuffaceous rocks of Cenozoic age throughout the 

desert regions of southeastern California (Sheppard 

and Gude, 1964). The tuffs of the Barstow Formation in 

the Mud Hills were chosen for detailed study because 

reconnaissance in 1963 showed an abundance and 

variety of authigenic silicate minerals, and because the 

tuffs are well exposed. The common occurrence of authigenic potassium feldspar in the tuffs also provided 

the opportunity to study the genetic relationships of the 

feldspar to vitric material and zeolites. Although authigenic clay minerals are common in the tuffs, their min-
INTRODUCTION

eralogy received only a cursory examination in this 

study.

Sampling was confined to surface outcrops and 

weighted heavily in favor of the tuffs, although the other 

rock types were sampled sufficiently to obtain representative material. Weathered surface outcrops were avoided. 

No cores were available to this investigation.

LABORATORY METHODS

X-ray diffractometer patterns were made of all bulk 

samples of tuffs. The samples were first ground to a 

powder and packed in aluminum sample holders and 

then exposed to nickel-filtered copper radiation. Relative abundances of authigenic minerals were estimated 

from the diffractometer patterns by using peak intensities. Estimates are probably less reliable for mixtures

containing glass or opal because these materials yield a 

rather poor X-ray record. Artificial mixtures of glass 

and zeolites were prepared and used as standards to 

obtain more reliable estimates of these constituents.

Optical studies using immersion oil mounts and thin 

sections supplemented the abundance data obtained by 

X-ray diffraction and provided information on the age 

relationships of the authigenic minerals. All measi^reinents of the indices of refraction are considered accurate to ±0.001.

Most samples of altered tuff contained more than one 

authigenic mineral. To identify each mineral in the diffractometer patterns of bulk samples, the patterns were 

compared with a "sieve" (fig. 2) prepared from pnre 

mineral separates at the same scale as the patterns of 

the bulk samples. One mineral at a time can be "sieved"

40 30

26, IN DEGREES 

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Analcime

Chabazite

Clinoptilolite

Erionite

Mordenite

Phillipsite

Potassium feldspar

2.0 2.5 3.0 3.5 4.0

 I i L_i I i I i I i I i I i I i I i I i I i i i i i i

5.0 6.0 7.0 8.0 9.010 15 20 30 50 100

11111111.1111 iiiiiniliiiiiiiiil i I i I i i i i 111111 I LJ I i

CuKa, IN ANGSTROMS

FIGURE 2. X-ray diffractometer patterns of authigenic silicate minerals. Copper radiation with nickel filter. Relative intensities

indicated by height of lines above base line.
4 DIAGENESIS OF TUFFS, BARSTOW FORMATION. SAN BERNARDINO COUNTY, CALIF.

from the bulk patterns until all lines are identified. This 

procedure made the identifications routine and helped 

the analyst recognize minor or trace amounts of 

constituents.

Mordenite in samples that are chiefly clinoptilolite 

is difficult to detect on X-ray diffractometer patterns because these zeolites have many coincident peaks. The 

technique described by Sudo, Nisiyama, Chin, and 

Hayashi (1963, p. 10) was found useful for the discrimination. Powdered tuff is treated with GN HC1 for 1 

hour. This treatment causes decomposition of the 

clinoptilolite, but the mordenite persists.

The "pure" mineral separates were prepared for 

chemical analysis from nearly monomineralic tuffs. The 

zeolites were separated by disaggregation followed by 

flotation in a heavy liquid mixture of bromoform and 

acetone, utilizing the equipment and technique described 

by Schoen and Lee (1964).

ACKNOWLEDGMENTS

Grateful appreciation is expressed to those in the 

Geological Survey who provided technical assistance 

during this investigation. Ellen S. Daniels, Harriet 

Neiman, and Elaine L. Munson performed the chemical 

analyses. Paul D. Blackmon determined the clay mineralogy of representative mudstones and altered tuffs. 

Melvin E. Johnson prepared the thin sections.

We are especially grateful to Robert G. Schmidt, who 

suggested this investigation and who had recognized 

the occurrence of authigenic zeolites and potassium feldspar in the Barstow Formation as early as 1954. Thomas 

W. Dibblee, Jr., kindly provided us with his unpublished 

geologic maps of the Mud Hills and adjacent areas.

REGIONAL GEOLOGY

The generalized geology of the Mud Hills anc1 vicinity is shown in figure 3, which is modified from Dibblee

116°45'

EXPLANATION

34°45'

10 

I

15 MILES

I

Plutonic and hypabyssal rocks

Includes gneiss and metavolcanic rocks .

Contact

Fault, showing relative movement

Dashed where approximately located; dotted

where concealed

FIGURE 3. Generalized geologic map of the Mud Hills and vicinity. (Modified from Dibblee, 1963, pi. 11.)
STRATIGRAPHY AND LITHOLOGY OF THE BARSTOW FORMATION

(1963, pi. 11). Other generalized geologic maps of the 

area have been published by Dibblee (1961, p. B198) 

and the California Division of Mines and Geology 

(1963).

The Tertiary deposits of the Mud Hills are about 

6,000-7,000 feet thick and include in ascending order 

the following: Jackhammer Formation, Pickhandle 

Formation, and Barstow Formation. Both the Jackhammer Formation and the Pickhandle Formation consist of sedimentary and volcanic rocks. The Tertiary 

deposits are folded into the Barstow syncline, whose 

axis trends nearly east. The Barstow syncline is nearly 

symmetrical, and both limbs generally dip 20°-40° (fig. 

4). Numerous faults, trending mainly northwest, cut 

the deposits. Movement along some of these faults has 

been predominantly right lateral (Dibblee, 1961). The 

Tertiary deposits are locally unconformably overlain 

by Quaternary alluvial deposits. Renewed movement on

some faults apparently continued into Holocene time, 

because the Quaternary deposits are displaced.

The hills north of the Mud Hills are underlain chiefly by granitic rocks and intermediate to basic plutonic 

rocks of Mesozoic age (McCulloh, 1960). The Calico 

Mountains, east of the Mud Hills, are underlain, chiefly by sedimentary rocks, volcanic rocks, and silicic to 

intermediate intrusive rocks, all of Tertiary age (McCulloh, 1960, 1965). Paleozoic metasedimentary and 

metavolcanic rocks flank the Calico Mountains on the 

north. The hills south of the Mud Hills are underlain 

mainly by granitic rocks of Mesozoic age and a complex 

sequence of metamorphic rocks of probable Precambrian age (Miller, 1944; Bowen, 1954; Dibblee, 1960; 

McCulloh, 1965). In the vicinity of Opal Mountain 

and Black Mountain, northwest of the Mud Hills, the 

rocks are chiefly Miocene silicic volcanic rocks and 

Pleistocene basalt.

FIGURE 4. View of the Baxstow Formation eastward along the axis of the Barstow synclijie at Rainbow Basin. The upper 

part of the Barstow Formation, shown in the photograph, is chiefly lacustrine mudstone and interbedded tuff. Gently 

dipping Pleistocene alluvial deposits unconformably overlie the Barstow Formation.

STRATIGRAPHY AND LITHOLOGY OF THE 

BARSTOW FORMATION

The Barstow Formation, of Miocene age (Lewis, 

1964,1968), consists chiefly of fluviatile and lacustrine 

rocks that are about 3,000-4,000 feet thick. The formation unconformably overlies the Pickhandle Formation 

(Bowen, 1954) and is unconformably overlain by sand 

and gravel of Quaternary age. An unknown thickness 

of the Barstow Formation was eroded prior to deposition of the Quaternary alluvium.

The lacustrine rocks of the formation interfinger with 

relatively coarse clastic rocks of fluviatile origin. The

336-374 O 69 2

lacustrine rocks are chiefly mudstones and interbedded 

carbonate rocks and tuffs. Fluviatile rocks include conglomerate, sandstone, and siltstone. Some sandstone and 

siltstone are tuffaceous. Sandstone and conglomerate 

have locally channeled into mudstone and, more rarely, 

into tuff. Areas underlain by thick mudstone units are 

commonly badlands; and differential erosion of the 

more resistant rocks, such as sandstone, conglomerate, 

carbonate rock, and altered tuff, has produced hogbacks. 

A generalized columnar section for the upper part 

of the Barstow Formation is shown in figure 5. Detailed measured sections of the formation have been
6 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

Hemicyon tuff

Dated tuff 

Camel track tuff

Skyline tuff

EXPLANATION

Mudstone with local sandstone and conglomerate

FEET 

r 0

- 50

- 100

- 150

L 200

Yellow tuff

FIGURE 5. Generalized columnar section of the upper part of 

the Barstow Formation. Five of the most persistent tuffs are 

assigned informal field names.

given by Steinen (1966) and Sheppard (1967). Five 

of the most persistent tuffs in the upper part of the 

formation are assigned informal field names, in ascending order: Yellow tuff, Skyline tuff, Camel track tuff,

Dated tuff, and Hemicyon tuff. The Skyline and Dated 

tuffs crop out the length of the Mud Hills, but the other 

tuffs are less extensive. The Yellow tuff crops out eastward from the western part of Rainbow Basin; the 

Camel track tuff crops out between Rainbow Basin 

and Owl Canyon; and the Hemicyon tuff crops out in 

about the western third of the Mud Hills.

CONGLOMERATE, SANDSTONE, AND SILTSTONE

Conglomerate, sandstone, and siltstone occur throughout the formation and throughout the Mud Hills; however, the thick accumulations are in the southwestern, 

south-central, and northeastern parts of the area. These 

coarse fluviatile rocks interfinger with mudstone of 

lacustrine origin. Highlands both north and south of 

the Mud Hills probably supplied detritus for these 

coarse clastic rocks.

The conglomerate is green, brown, or red; medium to 

thick bedded; and poorly to moderately well indurated. 

The framework constituents of the conglomerate range 

in size from pebbles to boulders up to 3 feet in diameter. 

The pebbles generally are angular to subrounded, 

whereas the cobbles and boulders generally are subrounded to rounded. The composition of the fragments 

also is variable, but generally the fragments are medium- to coarse-grained granitic rocks, or granitic rocks 

with minor volcanic and metamorphic rocks. Fragments 

of quartz and pegmatitic, granitic rock also are present. 

Some conglomerates contain only volcanic fragments 

that include pink and green silicic or intermediate lavas 

and, rarely, pink lapilli tuff.

The sandstone and siltstone are green, brown, gray, or 

red and generally poorly to moderately well indurated. 

Local cementation by brown-weathering calcite makes 

these rocks very hard and resistant. The sandstone is 

thin to thick bedded and fine to coarse grained; the siltstone is generally laminated or thin bedded. In addition to calcite, clay minerals and clinoptilolite locally 

are cements in the sandstone and siltstone. Primary 

sedimentary structures other than bedding are rare in 

the sandstone; however, the siltstone is commonly 

ripple laminated. The sandstone is locally crossbedded 

and, more rarely, ripple marked. The siltstone generally contains abundant spherical or irregular calcareous 

concretions.

The framework constituents of the sandstone and siltstone consist of varying amounts of mineral grains and 

rock fragments. Sorting is generally poor; and the 

clasts have an estimated roundness of 0.2-0.6, although 

most are in the lower part of this range. The matrix 

is chiefly clay minerals and is less than 15 percent of 

the rock. The detrital minerals are mainly quartz and 

feldspar and lesser amounts of biotite, hornblende, epi-
STRATIGRAPHY AND LITHOLOGY OF THE BARSTOW FORMATION

dote, clinopyroxene, muscovite, chlorite, zircon, magnetite apatite, and sphene. Generally quartz exceeds feldspar; and plagioclase exceeds alkali feldspar. Much of 

the plagioclase is sericitized. Biotite is very abundant in 

some of the sandstone. The rock fragments are chiefly 

granitic and volcanic rocks and lesser amounts of sedimentary and low-grade metamorphic rocks. Volcanic 

rock fragments include hyalopilitic and spherulitic 

lavas and sand-sized pumice or pseudomorphs of clay 

minerals and clinoptilolite. Following the classification 

of Petti John (1957, p. 291), these sandstones and siltstones are termed arkose or subgraywacke, depending 

on whether feldspar or rock fragments predominate.

MUDSTONE

Mudstone, or a silty or calcareous variant, is the predominant rock in the lacustrine deposits of the Barstow 

Formation. The mudstone has an earthy luster and is 

pastel green, gray, brown, and, rarely, red. Calcareous 

concretions are common, particularly in the green mudstone. Most mudstone is even bedded and medium to 

thick bedded. Fresh mudstone breaks with a subconchoidal or conchoidal fracture (fig. 6^4.), and the fractures 

are commonly filled or partly filled with gypsum. 

Where weathered, the mudstone has a characteristic 

punky "pop corn" surface coating that is several inches 

thick (fig. 6B). A white efflorescence of thenardite 

(Na2SO4 ) commonly coats weathered mudstone in the 

eastern part of Eainbow Basin and much of the mudstone to the east.

In addition to clay minerals, most of the mudstone 

contains calcite, detrital silt and sand, authigenic silicate minerals, and, locally, angular fragments of charcoal. The silt and sand fractions make up as much as 15 

percent of the rock, although they generally are less 

than 5 percent. Most of these relatively coarse grains 

are angular to subangular and are chiefly mineral fragments and rarely granitic rock fragments. Quartz, 

plagioclase, biotite, and hornblende compose most of the 

silt and sand fractions; and sanidine, zircon, muscovite, 

and epidote are minor constituents locally. Neither 

vitric material nor pseudomorphs after vitric material 

were recognized in the mudstone.

The mineral content of the finer than 2/* fraction of 

five mudstones is given in table 1. Standard X-ray techniques were used to identify the clay minerals. After 

disaggregation in Avater and complete removal of soluble salts, the finer than 2/* fraction AVRS sedimented on 

tiles and oriented by suction. X-ray data were then obtained from untreated, glycolated, and heated (400°C 

and 550°C) samples. Montmorillonite and illite occur 

in all samples, and two samples also contain mixedlayer illite-montmorillonite. Neither chlorite nor kaoFIGURE 6. Typical lacustrine mudstone of the Barstow Formation. A, Small excavation showing contrast of fresh and 

weathered mudstone. Fresh mudstone breaks with a subconchoidal to conchoidal fracture, but weathered mudstone has a 

punky "pop corn" coating several inches thick. Bedding dips 

toward right of photograph. B, Characteristic "pop corn" coating on weathered mudstone.

linite was recognized. The clay mineral suite is 

presumably detrital, although the possibility that some 

of the clay minerals are authigenic cannot be rejected. 

Analcime and potassium feldspar in the mudstones 

are probably authigenic, although petrographic evidence is lacking. X-ray diffraction data of bulk samples 

indicate that the mudstone generally contains 10-20 

percent aiialcime and 10 percent or less potassium feldspar. The analcime is anhedral to subhedral and ranges 

in size from less than 0.002 mm to about OA mm. Clay 

minerals adjacent to large analcmie crystals are alined 

parallel to the surface of the analcime. Possibly the 

clay minerals were pushed aside during gro\vth of the 

analcime.
8 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

TABLE 1. Mineral content of the finer than 2n fraction of five 

mudstones

[Belative abundance expressed in parts of 10; Tr., trace; _, looked for but not found. 

Analyst: Paul D. Blackmon]

Minerals

Montmorillonite.- .-_... _..._.-._..-_.._ 2+ Tr. 4 3 1+ 

Illite - --.-- -..- .-..--..--... 3 6 3+ 4+ + 

Mixed-layer illite-montmorillonite---------- 2 1 .............._.---.--

Analcime___________...._.___ 2 2 <1 <1 1+

Potassium feldspar..........._........... Tr. Tr. 1 <1 <1

Calcite-_.__........................................__ Tr. Tr. Tr.

Ferroan dolomite_______._____.. Tr. ___. Tr. ................

Quartz__-..--_.....___...................................... Tr. Tr.

Goethite__._.___._............... Tr. ........ Tr. Tr. Tr.

A chemical analysis of a typical mudstone is given 

in table 2. The K2O and Na2O contents of this mudstone 

are higher than generally reported for pelitic rocks 

(Shaw, 1956). However, the abundance of K2O and 

Na2O in this mudstone is similar to that of lacustrine 

mudstone in the Triassic Lockatong Formation of New 

Jersey (Van Houten, 1965). Authigenic analcime and 

potassium feldspar in the Barstow mudstone probably 

account for the high alkali contents.

Some mudstone contains numerous disseminated cavities, 0.5-3.0 mm long, that are filled or nearly filled with 

calcite. Inasmuch as many cavities are bounded by 

planar surfaces, they may be crystal molds. The shape 

of some cavities resembles the morphology of crystals 

of gaylussite (CaCCVNaaCOs'SHaO). Calcite occurs 

in the cavities as clusters of anhedral and subhedral 

crystals. Apparently the calcite precipitated in cavities 

that formed by solution of a readily soluble saline 

mineral.

TABLE 2. Chemical analysis of mudstone

[Lab. No. D100507; field No. SM-1-31C. Locality: Rainbow Basin, BE 1/4 sec. 24' 

T. 11 N., B. 2 W., San Bernardino County, Calif. Analyst: Ellen S. Daniels]

8iQ» - ---- - 47.95

Al2O3 ___-___ ___ __._ 14.97

 _ _ 4.78 

_-_-_-___-___- .90 

MgO_. _-_-_-___-____ 5.08

CaO______--____-____ 5.00

BaO__- _ _ __ _ -__ .05

Na2O___-_--___-____- 2.96

K2O_________________ 4.54

H2O+ _____-_________- 3.97

H2O-_.__ ____________ 4.53

P205 __ 

MnO__ 

CO2 __ 

SO3 __- 

Cl _

. 13 

. 11 

3.70 

. 30 

. 03 

. 14

Subtotal. 

Less O__

3.72 

.07

TotaL 3.65

Ti02. .58

CARBONATE BOCKS

Carbonate rocks are locally common in the Barstow 

Formation and include limestone and dolomite, although the former is much more abundant. The limestone is light gray to light brown, finely to coarsely 

crystalline, and thin to thick bedded. Much of the 

limestone weathers dark brown. Some of the limestone

is wavy bedded or vuggy or contains irregular segregations of dark-green to black opal. Most of the limestone contains some clastic grains, locally as much as 

40 percent of the rock. Thin interbeds of green calcareous mudstone are common in the thick limestone 

units. Ostracodes are locally common in thin beds of 

finely crystalline limestone. Apparently some of the 

limestone is of algal origin (Knopf, 1918, p. 258-259). 

The dolomite is light gray, finely crystalline, and 

thin bedded. The beds characteristically weather light 

brown. Clastic material locally makes up as much as 20 

percent of the rock.

TUFF

Tuffs in the Barstow Formation of the Mud Hills 

make up about 1-2 percent of the stratigraphic section 

and are the most conspicuous and continuous strata. At 

least 35 tuffs are recognized; most occur in the upper 

half of the formation, and most were deposited in lakes. 

The tuffs range in thickness from 0.25 inch to 7.0 feet, 

but most are less than 1 foot thick. The thicker tuffs generally are more continuous than the thinner ones. Thin 

tuffs commonly are single beds, but tuffs thicker than 6 

inches generally are multiple bedded. Individual beds 

of multiple-bedded tuffs are 1/16 inch to 3.5 feet thick 

but generally are less than 6 inches thick.

The original textural and structural features of the 

tuffs generally are preserved even though no vitric material remains. Most tuffs are even bedded, but beds of 

some thick multiple-bedded tuffs channel into lower beds 

as much as several inches. Individual beds of both 

single-bedded or multiple-bedded tuffs are commonly 

graded, being coarser at the base. The lower contact of 

a tuff generally is sharp, but the upper contact commonly is gradational into the overlying rock. Crossbedding and ripple marks or ripple laminations are 

present but not common. Some tuffs show contorted 

laminations that probably formed after deposition by 

internal flowage. Casts of mud cracks (fig. 7) or vertebrate 'animal tracks are locally common on the 'bottoms 

of some tuffs.

Accretionary lapilli are common in the lower part of 

basal beds of two thick tuffs, the Skyline tuff and the 

Hemicyon tuff. The lapilli are spherical or flattened 

slightly in the plane of the bedding and are 2-5 mm in 

diameter. Broken lapilli are very rare. Each accretionary lapillus characteristically consists of a structureless 

core surrounded by one or more concentric layers. The 

diameter of the core is generally more than half of the 

total diameter of the lapillus. Lapilli and matrix consist chiefly of vitric shards or pseudomorphed shards, 

and minor angular crystal fragments. The grain size of 

shards in the cores of the lapilli and in the matrix is
STRATIGRAPHY AND LITHOLOGY OF THE BARSTOW FORMATION 9

Tfc^.i^-^ifrjl

**f

FIGUEE 7. Casts of mud cracks on bottom of Dated tuff at 

Rainbow Basin.

similar, but the shards of the concentric layers are distinctly smaller. Each concentric layer is graded outward 

from relatively coarse to fine material. In contrast to the 

shards of the structureless core, those shards of the concentric layers are oriented with their long dimension 

tangential to the core or concentric layers. This preferred orientation and fine grain size of shards in the 

concentric layers enables the lapilli to be recognized in 

thin section.

Accretionary lapilli apparently form by accretion of 

moist ash in eruptive clouds and then fall as mudpellet rains (Moore and Peck, 1962). The lapilli in the 

Barstow Formation probably fell directly into 'a lake, 

because prolonged, or perhaps any, transport by streams 

would disaggregate them. The source of the ash in the 

Barstow Formation is unknown, but the occurrence of 

accretionary lapilli suggests that the vent was nearby. 

Moore and Peck (1962, p. 191) found that, with rare 

exceptions, accretionary lapilli occur within a few miles 

of the vent.

Most of the tuffs are vitric and fine or very fine 

grained. The vitric material commonly is of two types: 

platy bubble-wall shards that formed from the walls of 

relatively large broken bubbles; and pumice shards that 

contain small elongated bubbles. Most tuffs are composed of both types, but the platy shards are predominant. The index of refraction of unaltered shards ranges 

from 1.495 to 1.498.

Crystal and rock fragments in the tuffs are generally 

angular and range from less than 1 percent to about 60 

percent of the rock. Most tuffs, however, contain less 

than 5 percent crystal and rock fragments. The crystals 

are chiefly plagioclase (An21_3G ), quartz, sanidine, biotite, and hornblende. Minor amounts of any or all of 

the following are also present: Zircon, clinopyroxene,

apatite, magnetite, and, rarely, hypersthene. All these 

crystals are presumably pyrogenic.

Rock fragments are chiefly volcanic and granitic but 

also include quartzite, chert, and schistose and gneissic 

metamorphic rocks. The volcanic rock fragments, 

mainly spherulitic and hyalopilitic lava, could have 

been torn from the vent area during the eruption of the 

ash. Although the other rock types could also have been 

derived from the vent during an eruption, some probably are epiclastic and were carried to the basin by 

streams or wind.

The upper part of some tuffs, particularly thick tuffs, 

probably contains reworked ash mixed with epiclastic 

grains derived from highlands surrounding the basin. 

This part generally has an abundance of nonvolcanic 

rock fragments as well as grains of epidote, muscovite, 

chlorite, mici*ocline, perthite, and altered plagioclase. 

Commonly, these constituents are subangular to 

subrounded.

The only available chemical analysis of vitric material from the Barstow Formation is the one prepared 

for this study (table 3). The shards were separated 

from the other constituents of a relatively fresh part of

TABLE 3. Chemical analysis, molecular norm, and semiquantitative spectrographic analysis of shards from Hemicyon tuff

[Lab. No. D100550; field No. M4-32B. Locality: tributary to Fossil Canyon, NEJ£ 

SEJi sec. 15, T. 11 N., E. 2 W., San Bernardino County, Calif. Chemical analysis 

by Ellen S. Daniels; semiquantitative spectrographic analysis by Harriet Neiman]

Chemical analysis

SiO2 -----

AljOs ---

FejOs - 

FeO -----

MgO- .

CaO-. 

Na20 ....

----- 71,26

----- 12.44

.65

...... .31

--__- .35

----- .65

...... 4.47

KjO 

H20+ 

H2O- .

Ti02 

"P-T*.

MnO.--

C02 -

 ---- 2.68

 . 6.02

.75

------ .15

------- .02

.06

 -- .01

Cl. .... ... 0.11

F_-_ ----- .05

Subtotal.... 99.98 

LessO -.- .05

Total.. -- 99.93

Molecular norm

Q .....

all

B 

Ba 

Be -

Cr ....

Cu .

...... 33.14

...... 17.09

.-__.- 43.15

 . 3.45

C .

11 -----

Semiquantitative

...... 0.002

...... .07

------ .0003

----- .0001

...... .0007

Ga 

La.. ..

Nb .

Pb .

ST.. ......

------ 1.20

...... 1.08

..... . .18

-- - .24

hm 0.42

Total.. --. 100.00

spectrographic analysis 1

----- 0.003

.... .... . .005

001

------ .002

------- .01

V--. -- ... 0.001

Y ------- .002

Yb ------- .0002

Zr .............. .01

1 Results are reported in percent to the nearest number in the series 1, 0.7, 0.5, 0.2, 

0.15, 0.1, and so forth, which represent approximate midpoints of group data on a 

geometric scale. The original group for semiquantitative results will include the 

quantitative value about 30 percent of the time. The following elements were looked 

for but not found: Ag, As, Au, Bi, Cd, Ce, Co, Ge, Hf, Hg, In, Li, Mo, Ni, Pd, Pt, 

Re, Sb, Sc, Sn, Ta, Te, Th, Tl, II, W, and Zn.
10 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

the Hemicyon tuff by disaggregation followed by flotation in a heavy-liquid mixture of bromoform and acetone. The shards were then "scrubbed" in an ultrasonic 

bath in an attempt to remove adhering authigenic 

chabazite and montmorillonite. This treatment was only 

partly successful, so the vitric material that was analyzed included about 3-4 percent impurities, chiefly 

montmorillonite. The analysis shows that the shards are 

silicic and hydrated. The K2O :Na2O ratio of the glass 

is lower than expected from the SiO2 content, but the 

ratio may have been modified during the secondary 

hydration (Noble, 1967). This hydrated glass could be 

classified as rhyodacite if the normative minerals (table 

3) are applied to the rock classification of Nockolds 

(1954, p. 1008). Inasmuch as the original composition 

of the glass is unknown, the shards are considered rhyolitic in further discussions. On the basis of phenocryst 

content, the other tuffs in the formation probably also 

were rhyolitic.

AUTHIGENIC MINERALS

ANALCIME

Analcime, commonly referred to as analcite, is one 

of the more abundant zeolites in sedimentary rocks. 

Since its discovery in the Green River Formation 

(Bradley, 1928) and in lacustrine tuffs near Wikieup, 

Ariz. (Eoss, 1928), analcime has been reported in rocks 

that are diverse in age, lithology, and sedimentary environment (Hay, 1966). Analcime has an ideal formula 

of NaAlSi2O6 'H2O, but the analcime of sedimentary 

rocks is generally more siliceous (table 4).

Analcime is a very common zeolite in the tuffs of the 

Barstow Formation, where it ranges from trace amounts 

to nearly 100 percent. The analcime is associated with 

each of the other zeolites, but the association with chabazite is rare. Analcime is also commonly associated with 

authigenic clay minerals and potassium feldspar and

TABLE 4. Formulas of selected alkalic zeolites

[Formulas are standardized in terms of a sodium end member that has one aluminum 

atom. Formulas are taken from Hay (1966, p. 7); however, the formulas for chabazite, clinoptilolite, and erionite are modified from the writers' unpublished 

data]

Name Dominant 

cations

Formula

Analcime____ Na 

Chabazite__- Na, Ca 

Clinoptilolite- Na, K, Ca 

Erionite.____ Na, K, Ca 

Mordenite___ Na, Ca, K 

Phillipsite_ _ Na, K

NaAlSi2 .o-2 . 8O6 . 0_7 .6.1.0-1.3H2O

NaAlSii.7-3.8O5.4-9.e-2.7-4.lH2O

NaAlSi4.2-5.oO10 .4-i2.o-3.0-4.0H2O

NaAlSi3 .o-3.7O8 . 0_9 . 4 -3.0-3.4H2O

NaAlSi4.s-5.oOn.o-u.o-3.2-3.5H2O

NaAlSiL3-3.4O4.6-8.s-l.7-3.3H2O

less commonly associated with authigenic quartz. Unlike 

the other zeolites, analcime is not associated with relict 

glass.

Analcime occurs in the tuffs as anhedral to euhedral 

crystals, although most are subhedral to euhedral 

(fig. 8). The crystals range from 0.006 to 0.3 mm in 

size, but most are 0.01-0.05 mm. The analcime is isotropic and could easily be overlooked in thin section 

except that it is generally pale tan in transmitted light 

but milky in reflected light due to abundant inclusions 

of opal. Commonly, the outer part of the analcime 

crystals is free of inclusions and is colorless and clear.

The index of refraction of analcime from the Barstow Formation ranges from 1.484 to 1.487. Saha (1959, 

p. 302-303) showed that the index of refraction of 

synthetic analcimes decreases with increasing silicon 

content. The above indices are consistent with high 

silicon contents.

A chemical analysis of an analcime from the Barstow 

Formation is given in table 5. The analcime was sepFIGURE 8. Photomicrograph of altered tuff, showing subhedral 

and euhedral crystals of analcime. Unpolarized light.
AUTHIGENIC MINERALS 11

arated from a nearly monomineralic tuff; however, the 

analcime contained abundant inclusions of opal. Even 

so, the analysis shows that the analcime contains only 

minor amounts of cations other than sodium.

TABLE 5. Chemical analyses and composition of unit cell of 

analcime and chabazite

[a, in column head, uncorrected analysis; b, analysis corrected for opal impurities]

Analcime Chabazite

Chemical analyses -

[Analyst of analcime: Elaine L. Munson. Analyst of chabazite: Ellen S. Daniels]

SiO2 _ _ _- _ ___ _ _

Al2O3----------------__----_-_

Fe2O3 --. ----_----_-__---____

FeO____________. _____________

MgO_______._________________.

CaO_ ________________________

Na2O-_-___-_________________.

K2O_--_-_-___________________

H2O+____--_-_-_______________ 

H2O-_-__-__-___________._____

Ti02 _______-_________________.

P205 _________-_______________

MnO____________________ _ __

CO2 __-__------____________ __.

Cl__ __________________________

F

Subtotal _

Less O

Total __ _ ___ _ ___ .

Molecular SiO2 :Al2O3+Fe2O3

___ 67.68

___ 14. 10

.21

.00

. 12

. 10

.__ 8.38

. 14

6.12 } 

.__ 2.24 J

.06

.01

.00

.04

.01

.__ 99.21

. 01

99.20

60.24

19.75

.28

.00

. 16

. 13

11. 15

. 18

> 8. 95 |

.08

.01

.00

.05 __

. 01 _

99. 99

. 01

99.98

5.40

59.68

13. 11

. 13

. 02

.79

1.13

5.30

.62

10.25 

8.76

. 04

.02

.00

.01

99.86

.00

99. 86

7. 68

Composition of unit cell

[Fe+2, Ti, P, and Mn were omitted in calculation of the unit cell]

Si . ------__-______________________ 35.00

Al__._ __ _____ __ ___________ 12.84

Fe+3 __________ __ ______________ . 12

Mg ____________ _ ___________________ . 14

Ca-. ------- ______ _ ___ _ ______________ .08

Na ______ __ _ _____ __ _________ _ 12.56

K_______________________________________ . 13

17.34

O ______________ _ ___________________ 96.00

Si+Al + Fe+3 _____________________________ 47.96

Si:Al+Fe+3 ____________ _______ _ ___ 2.70

28.53

7.39

.05

.56

.58

4.91

.38

16. 34

13.97

72.00

35.97

3.83

Analcime. Lab. No. D100242; field No. M3-105A; separated from a thin tuff about 

1,330 ft stratigraphically below the Skyline tuff. Locality: south part of Rainbow 

Basin, SW1/4SE1/4 sec. 24, T. 11 N., R. 2W., San Bernardino County, Calif.

Chabazite. Lab. No. D100593; field No. M4-32F; separated from the upper part of 

the Hemicyon tuff. Locality: tributary to Fossil Canyon, NE1/4SE1/4 sec. 15. 

T. 11 N., R. 2W., San Bernardino County, Calif.

An attempt to correct for the opal impurities was 

made by utilizing the data of Saha (1959; 1961) on 

synthetic analcimes. Saha showed that the (639) peak 

of analcime falls at higher angles (degrees 20, CuKa± 

radiation) for analcimes of higher silicon content. Displacement of this (639) peak was measured against 

the (331) peak of a silicon internal standard. The 

compositions and corresponding displacements of the 

(639) peak for the synthetic analcimes (Saha, 1961, 

p. 865) were replotted and are given in figure 9. The 

displacement of the (639) peak for the analyzed specimen was then measured and the SiO2 content calculated 

from figure 9. Excess SiO2 in the original analysis 

is assumed to be in the opal inclusions. Subtraction of 

this excess SiO2 from the original analysis and recalculation of the analysis to 100 percent resulted in the 

corrected analysis that is given in table 5. This corrected 

analysis was recalculated into atoms per unit cell, on 

the basis of 96 oxygen atoms, and is also given in 

table 5.

The composition of analcime from 24 other tuffs was 

determined by measurement of the displacement of the 

(639) peak (figs. 9 and 10). These analcimes show a 

compositional range of (NaAl^s^Sis^sOste'^H^O to 

(NaAl) 12. 6Si35.4O9 6- H2O. Thus, the analcimes from 

tuffs of the Barstow Formation range in Si: Al ratio 

from about 2.2 to 2.8. Coombs and Whetten (1967) 

studied analcimes from many sedimentary environments and determined a range in Si: Al ratio of about 

2.0-2.7.

2.20

- 13.66

1.70

32

16

33 34 35

15 14 13

ATOMS PER UNIT CELL (0=96)

36 Si 

12 NaAl

FIGURE 9. Determinative curve for the anhydrous composition 

of analcime from X-ray diffractometer data. Dots are 

replotted from the data of Saha (1961) for synthetic analcimes. A20 is the 2e of the (639) peak for analcime minus the 

26 of the (331) peak for the silicon internal standard. Radiation is CuKeti. The brackets show the compositional range 

of analcime from tuffs of the Barstow Formation.
12 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

40-,

32.5 33.0 33.5 34.0 34.5

SILICON ATOMS PER UNIT CELL

35.0 35.5

FIGURE 10. Histogram showing the distribution of the silicon 

content of analcime in 41 samples from 25 tuffs of the Barstow Formation. Composition of analcime determined from 

X-ray diffractometer data by measurement of the displacement of the (639) peak of analcime.

CHABAZITE

Chabazite was unknown in sedimentary deposits prior 

to its discovery by Hay (1964, p. 1377) in tuffs and 

tuffaceous clays at Olduvai Gorge, Tanzania. Since 

then, authigenic chabazite has been recognized in tuffs 

from Arizona (Sand and Eegis, 1966), Nevada (Hoover 

and Shepard, 1965), and Oregon (Gude and Sheppard, 

1966, p. 914), as well as in the Barstow Formation. The 

physical properties and chemistry of these chabazites 

differ from that of the common chabazite that occurs in 

cavities of mafic igneous rocks. Chabazite has an ideal 

formula of Ca2Al4Si8O24 12H2O, but natural chabazites show considerable variation in cation content and 

Si: Al ratio (table 4).

The chabazite of the Barstow Formation rarely occurs in monomineralic beds. Generally it is associated 

with clay minerals and the following zeolites: Clinoptilolite, erionite, mordenite, and, rarely, analcime. Authigenic quartz or opal are rarely associated with the 

chabazite in altered tuffs. Most of the chabazite-bearing tuffs in the formation also contain relict glass; however, the other authigenic zeolites of the formation are 

rarely associated with relict glass. The chabazite occurs 

as aggregates of anhedral crystals that are 0.002-0.05 

mm in size.

The mean index of refraction ranges from 1.461 to 

1.468, and the birefringence is about 0.002. This compares with a range of 1.470-1.494 given by Deer, Howie, 

and Zussman (1963, p. 387) for chabazite from nonsedimentary environments.

An analysis of chabzite from the Barstow Formation 

is given in table 5. The chabazite was separated from 

a nearly monomineralic part; of the Hemicyon tuff. The 

analysis was recalculated into atoms per unit cell, on 

the basis of 72 oxygen atoms, and this is also given in 

table 5. Monovalent cations exceed divalent ones, and 

the Si: Al + Fe'3 ratio (3.83) is much higher than that 

for ideal chabazite. Gude and Sheppard (1966, p. 914) 

attributed the relatively low indices of refraction 

(a=1.460, y=1.462) and small cell dimensions (#= 

13.712±0.001 A, c = 14.882±0.002 A) of this analyzed 

specimen to its high silicon content.

CLAY MINERALS

Most altered tuffs in the Barstow Formation contain authigenic clay minerals, and some consist predominantly of clay minerals. The clay mineralogy of the 

finer than 2/u, fraction for seven samples was determined 

by Paul D. Blackmon using the standard X-ray techniques described previously. Montmorillonite occurs in 

all samples, commonly with trace to minor amounts of 

mixed-layer illite-montmorillonite. A sample of the 

Yellow tuff also contains illite as well as montmorillonite and mixed-layer illite-montmorillonite. Montmorillonite is generally the predominant clay mineral 

reported in altered tuff; however, illite and mixed-layer 

illite-montmorillonite are known from tuffs subjected 

to diagenesis (Schultz, 1963).

CLINOPTILOLITE

Clinoptilolite is a member of the heulandite structural group (Hay, 1966, p. 11). Although there is still 

some disagreement on the distinction between these 

closely related zeolites, most workers agree that clinoptilolite is the silicon-rich (Hey and Bannister, 1934; 

Mumpton, 1960) and alkali-rich (Mason and Sand, 

1960) member. Indices of refraction (Mason and Sand, 

1960, p. 350) and reaction to thermal treatment (Mumpton, 1960, p. 359-361; Shepard and Starkey, 1964) have 

also been used to distinguish clinoptilolite from 

heulandite.
ATJTHIGENIC MINERALS 13

Clinoptilolite is a very common zeolite in tuffs of the 

Barstow Formation, where it ranges in content from 

trace amounts to nearly 100 percent. The clinoptilolite 

is associated with each of the other zeolites and with one 

or more of the following authigenic silicate minerals: 

Clay minerals, potassium feldspar, opal, and quartz. 

The clinoptilolite occurs as prismatic or platy crystals 

that are 0.005-0.25 mm long; however, most clinoptilolite is 0.01-0.02 mm long.

The mean index of refraction ranges from 1.471 to 

1.482 and is within the range considered by Mason and 

Sand (1960, p. 350) to be characteristic of clinoptilolite. 

Most clinoptilolites from tuffs of the Barstow Formation have a mean index of refraction less than 1.476. The 

birefringence is low, about 0.003. All clinoptilolites 

examined have parallel extinction and, except for the 

rims of some large zoned crystals, are length slow.

Large crystals, 0.04-0.25 mm long, in some tuffs are 

zoned. The zoned crystals generally consist of a uniform core and a progressively zoned rim (fig. 11). The 

core is length slow, but the rim is length fast. The index 

of refraction of the rim is as much as 0.010 higher than

FIGURE 11. Photomicrograph of large zoned clinoptilolite crystal, showing uniform core and progressively zoned rim. 

Crossed nicols.

that of the core. Electron microprobe analyses of two 

large zoned clinoptilolites by R. C. Surdam (written 

comm., 1967) showed that the rim contains 2.9-3.5 

weight percent less SiO2 than the core. Some large 

crystals show oscillatory zoning (fig. 12), but no attempt was made to study the differences in the index of 

refraction across the zones.

Three chemical analyses of clinoptilolite from the 

Barstow Formation are given in table 6. The analyses 

were recalculated into atoms per unit cell, on the basis 

of 72 oxygen atoms, and these are also given in table 6. 

Monovalent cations exceed divalent ones, and the 

Si: Al+Fe+3 ratio is 4.20-4.37. Sodium exceeds potassium, as is generally true for clinoptilolite; however, 

potassium-rich clinoptilolites have been reported from 

other areas (Minato, 1964; Sheppard and others, 1965, 

p.247).

The BaO content of clinoptilolite 2 is unusual, and 

the barium may be present as barite impurities. The 

analyzed separate contained minor barite that was 

recognizable in immersion oil grain mounts. An attempt

FIGURE 12. Photomicrograph of zeolitic tuff, showing oscillatory zoning in relatively large crystals of clinoptilolite. 

Crossed nicols.
14 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

TABLE 6. Chemical analyses and composition of unit cell of 

clinoptilolite

[a, in column head, uncorrected analysis; b, analysis corrected for COs plus equivalent 

CaO to make calcite or for SOs plus equivalent BaO to make barite]

Chemical analyses

[Analyst: Ellen S. Daniels]

SiO2 _ --------------- 63. 69 64.38 64.70 64.23 64.71

A12O3 -

MgO

BaO

____________ 12. 47 12.37 12.43 12.20 12.29

.--_-____-_-_ .62 .44 .44 .41 .41

_____________ .02 .02 .02 .03 .03

_____________ 1.42 .34 .34 .74 .75

.________--__ 2.25 1.25 1.26 1.83 1.64

.----____----.______ .78 .53___________

._-_-____-_-_ 2.46 4.30 4.32 3.17 3.19

K|O__ ______________ 1.80 2.27 2.28 3.97 4.00

H.O+ ______________ 8.93 7.85 7.89 6.60 6.65

H.O- _____________ 5.97 5.64 5.67 6.11 6.16

TiO2______________ .07 .07 .07 .08 .08

P2O5 _____________ .02 .01 .01 .02 .02

MnO____ ____________ .01 .01 .01 .00 .00

CO____ _-----_-_-_---_---_-_-_------_-- .16 _--__-

S03 13

.03 .01 .01 .01 .01 

.02 .01 .01 .05 .05

Subtotal.-.______ 99. 78 99.88 99.99 99.61 99.99

Less O__-____-_- .02 .00 .00 .02 .02

TotaL________ 99.76 99.

Molecular SiO2 :Al2O3+

______________ 8.40 ____

99. 99 99. 59 99. 97

8.64

Composition of unit cell

[Fe+2, Ti, P, and Mn were omitted in calculation of the unit cell]

Fe+3 

Mg_

28.99 _______ 29. 19

6.69 _-__ _ 6. 61

.21 _______ . 15

.96 

1. 10

23

61

09

29. 09

6.51

. 14

.50

.79

H20+- 

H2O-_

2. 17

1.04

13.56

3.78

1. 31

11.87

9. 06

72. 00

8. 53

72. 00

Si+Al+Fe+3 ___________ 35.89 _______ 35.95

Si:Al+Fe+3 __________ 4.20 _____ 4.32

2.78

2.30

9.97

9.24

72. 00

35.74

4. 37

1. Lab. No. D100555; field No. SM-6-5C; separated from the Hemicyon tuff. Locality: tributary to Fossil Canyon, NW1/4NE1/4 sec. 15, T. 11 N., R. 2 W., San 

Bernardino County, Calif.

2. Lab. No. D100554; field No. SM-4-4A; separated from the Skyline tuff. Locality: 

about 1 mile east of mouth of Owl Canyon, NW1/4NE1/4 sec. 29, T. 11 N., R. 1 W., 

San Bernardino County, Calif.

3. Lab. No. D100594; field No. SM-4-11-I; separated from a tuff about 525 ft stratigraphically below the Skyline tuff. Locality: about I mile east of Owl Canyon, 

SW1/4SE1/4 sec. 20, T. 11 N., R. 1 W., San Bernardino County, Calif.

was made to correct the chemical analysis for the barite 

by subtracting the S03 content plus an equivalent 

amount of BaO to make barite. The excess BaO, 0.53 

per cent, is presumed to be in the clinoptilolite. If the 

SO3 content of the analysis is in error and is low, some, 

or perhaps all, of the excess BaO may be in additional 

barite.

ERIONITE

Erionite was considered an extremely rare mineral 

prior to the work of Deffeyes (1959a, b) and Regnier 

(1960, p. 1207), who showed it to be a common authigenic zeolite in altered rhyolitic vitric tuffs in northcentral Nevada. Since then, erionite has been recognized 

in altered tuffs from many of our Western States.

Erionite in the altered tuffs of the Barstow Formation 

is associated with each of the other zeolites but is more 

commonly associated with clinoptilolite. The erionite 

is also commonly associated with authigenic clay minerals but rarely associated with authigenic quartz or 

opal. The erionite content of an altered tuff is generally 

20 percent or less. Like most erionite of sedimentary 

deposits (Hay, 1966, p. 10), erionite of the Barstow 

Formation is acicular or prismatic. Individual crystals 

are 0.006-0.04 mm long. Erionite rarely occurs in aggregates of radiating crystals.

Indices of refraction for erionite are o>= 1.461-1.465 

and e==1.465-1.469. These indices are lower than those 

reported by Deer, Howie, and Zussman (1963, p. 398) 

and may be due to a relatively high silicon content of 

the Barstow erionite. Birefringence is 0.004, higher than 

that of the other associated zeolites. The erionite shows 

parallel extinction and is length slow.

Inasmuch as erionite occurs in small amounts and 

invariably is associated with some other zeolite in the 

altered tuffs, no attempt was made to separate material 

for chemical analysis. Erionite is alkali-rich (table 4) 

and generally has a Si: Al + Fe+3 ratio range of 3.0-3.5 

(Hay, 1966, p. 9).

MORDENITE

Mordenite has been confused with clinoptilolite or 

heulandite in sedimentary rocks because of their similar indices of refraction and chemistry (Coombs and 

others, 1959, p. 69; Shumenko, 1962). X-ray diffractometer techniques, fortunately, are adequate for positive 

identification (fig. 2). In recent years, mordenite has 

commonly been identified in Cenozoic tuffaceous rocks 

from several areas of the Western United States (Moiola-,1964; Sheppard and Gude, 1964; Curry, 1965; 

Hoover and Shepard, 1965; Surdam and Hall, 1968).

Mordenite in the altered tuffs of the Barstow Formation is associated with each of the other zeolites, but it
AUTHIGENIC MINERALS 15

is more commonly associated with clinoptilolite. Mordenite is also associated with authigenic clay minerals 

and rarely with authigenic quartz, opal, or potassium 

feldspar. The mordenite content of an altered tuff generally is 20 percent or less and only rarely is as much 

as 40 percent. Mordenite crystals are acicular or prismatic and generally 0.01-0.04 mm long. They commonly are larger than the crystals of other associated 

zeolites.

Indices of refraction for m'ordenite are fairly constant and are within the lower part of the range (1.472- 

1.487) given by Deer, Howie, and Zussman (1963, p. 

401). The mean index ranges from 1.474 to 1.478 and the 

birefringence is low, about 0.003. All mordenites examined have parallel extinction and are length fast.

An analysis of mordenite from the Barstow Formation is given in table 7. The mordenite was separated 

from a nearly monomineralic tuff about 4 feet above the 

Skyline tuff in the western part of the Mud Hills. The 

analysis was recalculated into atoms per unit cell, on the 

basis of 48 oxygen atoms and is given in table 7. Monovalent cations exceed divalent ones and the Si:Al + 

Fe+3 ratio is 4.71. Sodium exceeds potassium as is true 

for most mordenites regardless of genesis. However, a 

potassic mordenite was recently found in tuffs of the 

Miocene Obispo Formation of Surdam and Hall (1968) 

in southern California.

OPAL

Opal is difficult to recognize in altered tuffs because 

of its isotropic and nondescript character. Opal is colorless to pale brown in thin section and has an index of 

refraction near 1.46. Most identifications of opal from 

the Barstow Formation are based on X-ray diffractometer powder data of bulk samples. The opal has characteristically broad peaks at the following d spacings: 

4.26 A, 4.09 A, and 3.82 A. Opal is commonly associated 

with clinoptilolite and clay minerals and rarely associated with chabazite, mordenite, and erionite. Segregations containing as much as 70 percent opal are locally 

common.

PHILLIPSITE

Phillipsite has long been known to occur in deposits 

on the sea floor (Murray and Renard, 1891, p. 400-411). 

Since the discovery by Deffeyes (1959a) of phillipsite 

in altered rhyolitic tuffs of Nevada, this zeolite has been 

commonly reported as a rock-forming constituent of 

tuffs deposited in saline lakes (Hay, 1964).

The phillipsite in tuffs of the Barstow Formation is 

commonly associated with clinoptilolite and analcime 

and less commonly associated with erionite and mordeTABLE 7. Chemical analyses and composition of unit cell of 

mordenite and phillipsite

[a, in column head, uncorrected analysis; b, analysis corrected for COj plus equivalent 

CaO to make calcite, and for SOs plus BaO to make barite]

Mordenite Phillipsite

Chemical analyses

[Analyst: Ellen S. Daniels]

SiO2 66. 31 55. 18

A12O3 ------------------------- 11. 52

Fe2O3 

FeO_. 

MgO. 

CaO_ 

BaO.

.67 

.02

.54 

2.58

14.97

.41

.00

.26

.20

1.47

56.60

15.35

.42 

.00 

.27 

.07

H2O-. 

TiO2 ._ 

P20S -_ 

MnO__ 

CO2___ 

SO3 --

3.09

.96

8.46

5.50

. 12

.00

.00

6.77

3.39

9.40

6.73

.25

.01

.00

. 10

6.95

3.48

9.64

6.90

.26

.01

.00

09

02

78

04

01

04

01

Subtotal___________________ 99.88 99.97 100.00

LessO_._____________ .03 .01 .01

Total______.__________ 99.85 99.96 99.99

Molecular SiO2 :A12O3 + 

Fe2O3 ____________ 9.42 6. 15

Composition of unit cell

[Fe+2, Ti, P, and Mn were omitted in calculation of the unit cell]

SL ___________________________ 19.78

Al______________________________ 4.05

Fe+3 _________________________ . 15

Mg_____________________________ .24

Ca________-__________________-__ .82

Na______________________________ 1.79

K ____________________________ .37

H2O+___-___________-____-____-__ 8.42

H_O-_______--____ ____________ 5.47

O___-___-_-_---____-__---___-___ 48.00

Si+Al+Fe+3__-__________________ 23.98

3 _________________-_-__ 4.71

12.06

3.85

.07

.09

.02

2.87

.94

6.85

4.90

32.00

15.98

3.08

Mordenite. Lab. No. D100551; field No. M4-54B; separated from a tuff 4 ft stratigraphically above the Skyline tuff. Locality: about 1 mile north of Fossil Canyon, 

NW1/4SE1/4 sec. 9, T. 11 N., K. 2 W., San Bernardino County, Calif.

Phillipsite. Lab. No. D100592; field No. M4-5A; separated from the lower part of 

the Yellow tuff. Locality: west wall of Owl Canyon, NW1/4SW1/4 sec. 19, T. 

11 N., K. 1 W., San Bernardino County, Calif.
16 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

nite. The association of phillipsite with chabazite has 

not been recognized in these tuffs, although this association has been noted in tuffs from other areas (Hay, 

1964, p. 1377; Sheppard and Gude, 1968). Phillipsite is 

also commonly associated with authigenic clay minerals 

and potassium feldspar and rarely associated with 

authigenic quartz.

Most of the phillipsite occurs as spherulites in the 

altered tuffs, although prismatic crystals 0.01-0.02 mm 

long occur locally. The spherulites range in diameter 

from 0.04 to 1.0 mm, but most are 0.1-0.3 mm (fig. 13).. 

Clusters of several mutually interfering spherulites are 

common, and many spherulites are malformed.

Indices of refraction for phillipsite vary considerably, much more than for any other zeolite in the Barstow Formation. The mean index ranges from 1.448 to 

1.476, and the birefringence is very low, about 0.002. 

Most of the phillipsites have a mean index between 

1.460 and 1.470. The spherulites generally are zoned; 

the outer part of the spherulite invariably has a higher 

index. A difference in index of as much as 0.018 has

FIGURE 13. Photomicrograph of spherulitic phillipsite in altered tuff. Matrix consists of clay minerals and minor potassium feldspar. Arrow indicates relict shard outlined by clay 

mineral. Crossed nicols.

been measured between the interior and exterior parts. 

All phillipsites have parallel or nearly parallel extinction, and except for one specimen, are length slow. The 

length-fast phillipsite has the highest (1.476) mean 

index of refraction that was measured.

These indices of refraction for phillipsite from the 

Barstow Formation are similar to those of phillipsites 

from saline lakes (Hay, 1964, p. 1374), but are much 

lower than those of phillipsite from other environments. 

Deer, Howie, and Zussman (1963, p. 386) reported a 

range of 1.483-1.514 for presumably nonsedimentary 

phillipsites.

A chemical analysis of spherulitic phillipsite that was 

separated from the Yellow tuff is given in table 7. The 

analysis was corrected for barite and calcite impurities 

associated with the phillipsite. The corrected analysis 

was then recalculated into atoms per unit cell, on the 

basis of 32 oxygen atoms, and is given in table 7. Monovalent cations greatly exceed divalent cations and sodium is greatly in excess of potassium. The Si :Al+Fe+3 

ratio is 3.08 and is much higher than the range of 1.3-2.2 

given by Deer, Howie, and Zussman (1963, p. 393) for 

phillipsite. The high potassium plus sodium and silicon 

contents of this phillipsite distinguish it from phillipsites in nonsedimentary deposits. However, the analysis 

is very similar to analyses of phillipsites from altered 

rhyolitic tuffs in lacustrine deposits of southern California (Hay, 1964, p. 1375; Sheppard and Gude, 1968), 

and southeastern Oregon (Regis and Sand, 1966). Phillipsite from the Barstow Formation does differ from 

other similar phillipsites by its high sodium content.

POTASSIUM FELDSPAR

Potassium feldspar occurs as an authigenic mineral 

in sedimentary rocks that are diverse in lithology, depositional environment, and age (Hay, 1966). The 

authigenic feldspar is a pure or nearly pure potassium 

variety and may occur as authigenic crystals or overgrowths or replacements of plagioclase. The authigenic 

potassium feldspar in tuffs of the Barstow Formation 

occurs chiefly as authigenic crystals and only rarely as 

overgrowths or replacements of plagioclase. The feldspar occurs in monomineralic beds or associated with 

one or more of the following authigenic minerals: 

Quartz, clay minerals, analcime, clinoptilolite, mordenite, and phillipsite. The association of potassium 

feldspar with analcime and clay minerals is especially 

common. Nowhere in the Barstow Formation has authigenic feldspar been found associated with relict glass.

Potassium feldspar occurs in the altered tuffs as lowbirefringent aggregates of crystals that range in size 

from less than 0.002 mm to about 0.01 mm. Paul D. 

Blackmon, using electron microscopy, has shown that
ALTERED TUFFS 17

the crystals are subhedral to euhedral and generally less 

than 0.005 mm in size (fig. 14). The feldspar is identical 

with authigenic potassium feldspar from sediments of 

Searles Lake, Calif. (Hay and Moiola, 1963, p. 323), 

and altered tuffs of Pleistocene Lake Tecopa, Inyo 

County, Calif. (Sheppard and Gude, 1968). The mean 

index of refraction is 1.518 which suggests nearly pure 

potassium feldspar. Other optical data could not be obtained because of the small size of the crystals.

The feldspar is monoclinic inasmuch as X-ray diffractometer patterns of powders show only the (131) 

peak rather than the pair, (131) and (131), the criterion 

suggested by Goldsmith and Laves (1954, p. 3) for distinguishing monoclinic from triclinic potassium feldspar. The following cell constants were refined on a 

digital computer by least-squares analysis of X-ray 

diffractometer powder data:

a= 8.593 ±0.002 A 

~b= 12.967 ±0.003 A 

c=7.165 ±0.001 A 

y3=116° 3.3'±1.0' 

Cell volume=717.25±0.19 A 3

The structural state estimated from the & and c cell 

edges is that of sanidine, but the cell seems to be distorted (D. B. Stewart, 1965, written commun.).

QUARTZ

Authigenic quartz is a common constituent in the 

altered tuffs and is associated with all the other authigenic silicate minerals except opal. Quartz is commonly 

associated with analcime and potassium feldspar, less 

commonly associated with clay minerals and clinoptilolite, and only rarely associated with the other zeolites.

FIGURE 14. Electron micrograph of potassium feldspar, showing 

subhedral to euhedral outlines of crystals. Electron micrograph by Paul D. Blackmon.

Some bulk samples contain as much as 80 percent quartz. 

The quartz occurs as veinlets and spheroidal or irregular segregations of anhedral crystals 0.005-0.2 nun 

in size. The spheroidal segregations are 0.25-2.0 mm in 

diameter (fig. 15). Some quartz is evidently chalcedonic, 

because it occurs in fibrous aggregates and has indices 

of refraction below 1.54. The X-ray diffractometer powder pattern of this material is identical with that of 

quartz.

FIGURE 15. Photomicrograph of sample from upper part of the 

Skyline tuff. Tuff has been replaced by finely crystalline 

potassium feldspar (gray) and spheroidal segregations of 

anhedral quartz (white). Unpolarized light.

ALTERED TUFFS 

FIELD DESCRIPTION

All tuffs in the Barstow Formation of the Mud Hills 

are altered, at least in part. Relict glass is rare, but parts 

of some tuffs contain as much as 90 percent relict glass. 

The altered tuffs are generally white to pale gray or 

pastel shades of yellow, brown, or green with a dull or 

earthy luster. Eelict vitric material, if present, can be 

recognized by its distinct vitreous luster. Altered tuffs 

are resistant and form ledges (fig. 16). Original textures
18 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

FIGURE 16. View looking west along the north limb of the Barstow syncline at Rainbow Basin, showing resistant hogbacks 

formed by altered tuffs. Tuff near the center of the photograph 

is the Skyline tuff.

and sedimentary structures, such as crossbedding and 

ripple marks, are generally preserved in the altered 

tuffs. Preservation of these features is convincing evidence that the present differences in composition and 

mineralogy of the tuffs are due to postdepositional 

processes.

Authigenic silicate minerals in the altered tuffs generally cannot be positively identified in the field because 

of the very small size of the crystals. If the altered tuff 

is nearly monomineralic, certain gross physical properties of the rock may aid field identification. On this basis, 

then, the following tuffs generally can be recognized: 

Tuffs that contain a zeolite other than analcime, 

tuffs that are analcime rich, and tuffs that are potassium feldspar rich. Certain of the nonanalcimic zeolitic 

tuffs commonly can be further differentiated.

Those tuffs that are nonanalcimic are relatively hard 

and generally show good preservation of the original 

vitroclastic texture. Tuffs rich in clinoptilolite commonly are white to pale gray, very well indurated, and 

low in porosity, and they generally break with a blocky 

to conchoidal fracture (fig. 17). Phillipsite-rich tuffs are 

generally yellow and characteristically are spherulitic. 

Phillipsite is the only authigenic silicate mineral in the 

altered tuffs that is spherulitic. Tuffs rich in mordenite 

generally cannot be distinguished from clinoptiloliterich ones; however, the former tend to be more porous 

and break with an irregular or blocky fracture rather 

than a conchoidal fracture. Tuffs rich in erionite or 

chabazite cannot be distinguished from one another; 

however, they commonly can be distinguished from 

clinoptilolite- or mordenite-rich tuffs by their greater 

porosity and irregular fracture.

FIGURE 17. Clinoptilolite-rich part of the Skyline tuff, showing 

typical blocky and conchoidal fractures.

Tuffs that contain mostly analcime are porous, friable, 

and generally a pastel shade of green, yellow, or brown. 

Perhaps the most characteristic features are their sugary 

texture and poor preservation of the original vitroclastic 

texture. Individual euhedra of analcime commonly can 

be seen with a hand lens. Analcimic tuffs break with an 

irregular fracture or, more rarely, a blocky fracture.

Tuffs replaced mainly by potassium feldspar are 

porous, very friable, punky, and white to pale green. 

If only minor authigenic quartz is associated with the 

feldspar, the tuffs can be disaggregated with the fingers. 

The feldspar-rich tuffs generally break with an irregular fracture and show poor preservation of the original 

vitroclastic texture.

PETROGRAPHY

The authigenic mineralogy of the altered tuffs was 

determined chiefly by study of X-ray diffractometer 

powder data of bulk samples, supplemented by thinsection and oil-immersion study. Thin sections were 

especially useful for determining the age relationship 

of the authigenic minerals; however, they generally 

were not examined until the mineralogy of the altered 

tuffs was known by X-ray methods. Optical identification of the zeolites is particularly difficult because of 

their small crystal size and similar optical properties 

and habits.

Crystal fragments, unlike vitric material, in the 

altered tuffs generally are unaltered. However, plagioclase locally is replaced by analcime or potassium feldspar ; and plagioclase, biotite, and rarely hornblende are 

locally replaced by calcite. Hornblende and apatite 

locally show marginal solution.
ALTERED TUFFS 19

NONANALCIMIC ZEOLITIC TUFF

The vitroclastic texture is preserved in most tuffs 

that are chiefly zeolites other than analcime. Some zeolitic tuffs lack relict texture or have only vague ghosts 

of shards where the zeolite is coarsely crystalline or 

where authigenic clay minerals are absent. Typcial 

pseudomorphs of bubble-wall shards consist of a thin 

marginal film of montmorillonite that is succeeded inward by crystals of one or more zeolites (fig. 18). The 

pseudomorphs may be either solid or hollow, and commonly both occur in the same specimen. The larger 

pseudomorphs commonly are the hollow ones. Hollow 

pseudomorphs characteristically consist of the marginal 

clay film succeeded inward by a single layer of zeolite 

(commonly clinoptilolite) crystals that are oriented 

perpendicular to the shard wall (fig. 19). The solid 

pseudomorphs are of two types: those in which the 

single layer of zeolite grew until crystals from opposite 

walls joined in the central part of the shard, and those 

which resemble the hollow pseudomorphs except that 

what was the central cavity is now filled with another 

zeolite, quartz, or calcite (fig. 19). The zeolite of the 

central filling is generally more coarsely crystalline 

than the marginal zeolite and is randomly oriented. 

Some pseudomorphs still have a small central cavity 

(fig. 20) even though a second zeolite grew on the 

marginal layer of zeolite crystals.

Spherulites of pliillipsite are characteristic of the 

Yellow tuff. The spherulites (fig. 13) resemble oolites 

except that they have a radial internal structure rather 

than a concentric structure that is so characteristic of

FIGURE 18. 'Photomicrograph of zeolitic tuff, showing pseudomorphs of platy bubble-wall shards that consist of an outer 

film of montmorillonite (light) and a filling of clinoptilolite 

(dark). Matrix is finely crystalline clinoptilolite and montmorillonite. Crossed nicols.

oolites. The spherulites lack nuclei, but some have 

engulfed scattered pyrogenic crystals during their 

growth. Most spherulites are malformed. The spherulites commonly enclose ghosts of shards that are outlined by a continuous or discontinuous film of montmo-

'ft

* '"**. t - ifv -'C1 

"V." s-J*. *'w»=,>8 ":* !.

.

t f«.

--,, . * Ulr |

*i*L* i v--, '. 'r\ ,'^L'^ * £>& *' : **£,-

r '{* .< 

-;- , '.V i , '*' i. * * U

 v.. < - - .

0.0? ram

FIGURE. 19. Photomicrograph of zeolitic tuff, showing pseudomorphs of fibrous clinoptilolite (cl) after shards. The bands 

in the clinoptilolite are due to variations in the index of 

refraction along the length of the fibers. The large pseudomorph in the center of the photograph has a hollow interior. Other pseudomorphs are filled with calcite (ca). A 

thin outer film of montmorillonite surrounds the pseudomorphs 

but is not obvious. Matrix is finely crystalline clinoptilolite 

and montmorillonite. Unpolarized light.

j

FIGURE 20. Photomicrograph of zeolitic tuff, showing pseudomorphs of shards that contain a prismatic layer of clinoptilolite and an interior filling of randomly oriented mordenite. Matrix is chiefly clinoptilolite with minor montmorillonite. Large crystal in lower part of photograph is a 

pyrogenic feldspar. Unpolarized light.
20 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

rillonite. Individual fibers of phillipsite pass undistorted through these relict shards. The same alinement 

of relict shards persists within the spherulites of a bed 

and attests to the formation of the spherulites during 

diagenesis rather than by direct precipitation on the 

lake floor.

The stages of alteration of fresh vitric tuff to zeolitic 

tuff can be observed by thin-section study of the Hemicyon tuff. The earliest recognizable alteration is the 

development of a thin film of montmorillonite that 

envelops the shards. An advanced stage shows only 

the large vitric particles that are enclosed by a montmorillonite film and set in a matrix of finely crystalline 

zeolite and montmorillonite. The finer glass particles 

apparently alter before the coarser ones. Pumice shards 

commonly are altered to a greater degree than platy 

bubble-wall shards of comparable size, probably because 

of their greater porosity and surface exposure. A more 

advanced altered variant consists of ragged remnants 

of large glassy shards "floating" in a matrix of zeolite 

and montmorillonite. The small shards are pseudomorphed by zeolite and montmorillonite. A final stage 

shows zeolite and montmorillonite pseudomorphs after 

shards in a finely crystalline matrix of the same minerals. The smaller pseudomorphs generally are solid, 

but the larger ones commonly are hollow.

Thin-section study indicates that at least some of the 

zeolite pseudomorphs formed by crystallization of the 

zeolite(s) in cavities from which the glass had been 

dissolved. The hollow pseudomorphs, such as those illustrated in figures 19 and 20, are difficult to explain 

in any other way. Other zeolite replacements probably 

were formed contemporaneously with solution of the 

glass, but convincing evidence is lacking.

The paragenesis of the authigenic silicate minerals 

in the zeolitic tuffs can be ascertained by studying the 

sequence of filling of the shard cavities and by contrasting the mineralogy of the matrix with the mineralogy 

of the pseudomorphed shards, particularly the larger 

shards. Presumably, the mineral in the interior of a 

pseudomorph formed later than the minerals nearer the 

periphery of the pseudomorph. Those minerals in the 

finely crystalline matrix are presumed to have crystallized prior to the minerals that compose the large pseudomorphs. The following sequences of crystallization 

were determined using the above criteria:

Paragenesis of autliigenic silicate minerals 

[Earliest mineral listed on, left]

Montmorillonite cha'bazite clinoptilolite 

Montmorillonite chabazite erionite 

Montmlo rillonite phillipsite clinoptilolite 

Montmorillonite clinolite mordenite 

Montmorillonite clinoptilolite quartz

These relationships indicate that montmorillonite consistently was the earliest silicate mineral to form in the 

tuffs. Chabazite and phillipsite are early zeolites; but 

clinoptilolite, erionite, and mordenite are relatively late 

zeolites. The age relationship between erionite and 

clinoptilolite and between erionite and mordenite is 

unknown. According to X-ray diffractometer study, 

erionite occurs with each; but the critical textural relations were not recognized in thin sections.

ANALCIMIC TUFF

Analcime in the altered tuffs is characteristically subhedral to euhedral and isotropic. Most analcime is pale 

tan in transmitted light and milky in reflected light because of abundant minute inclusions of opal. Vitroclastic texture in analcimic tuffs is vague and is spotty if at 

all recognizable. Most nearly monomineralic analcimic 

tuffs lack any evidence of a relict vitroclastic texture.

X-ray diffractometer study has shown that analcime 

is associated with each of the other zeolites, but paragenetic relationships could be observed in thin sections 

for only analcime and clinoptilolite, and analcime and 

phillipsite. Several specimens of the Skyline tuff 

clearly show clinoptilolite pseudomorphs after shards 

that have been replaced by analcime. In the Yellow tuff 

and a tuff about 750 feet stratigraphically below the 

Yellow tuff, analcime has replaced spherulites of phillipsite. Remnants of the spherulites can be seen in some 

thin sections. Analcime has also replaced pyrogenic 

plagioclase locally in the Dated tuff.

POTASSIUM FELDSPAR-RICH TUFF

Authigenic potassium feldspar occurs in trace to 

major amounts and is associated with each of the zeolites except chabazite and erionite. The association of 

potassium feldspar with analcime in the altered tuffs is 

especially common. Similar to the analcime occurrences, 

the potassium feldspar is not associated with relict glass. 

These occurrences contrast with those of the other zeolites which are locally associated with relict glass.

Potassium feldspar generally occurs as aggregates 

of minute subhedral to euhedral crystals. Relict vitroclastic texture is vague or nonexistent in tuffs or those 

parts of tuffs where potassium feldspar is abundant. The 

vitroclastic texture is recognizable in the form of quartz 

pseudomorphs after shards, remnants of clinoptilolite 

pseudomorphs after shards, remnants of a clay mineral 

film that vaguely outline shards, and, rarely, hollow 

molds of shards.

Thin-section study of tuffs that contain analcime and 

potassium feldspar shows that the feldspar has formed 

from the analcime. All stages of the replacement are 

recognizable: from marginal replacement, that commonly gives the analcime crystals a ragged appearance,
ALTERED TUFFS 21

to complete pseudomorphs of finely crystalline potassium feldspar. Locally, the crystal outlines of the 

pseudomorphed analcime are noticeable because of clay 

minerals or irresolvable opaque material. Analcimic 

tuffs generally have a vague vitroclastic texture, and replacement by potassium feldspar causes further degradation or obliteration of the texture.

Formation of potassium feldspar from zeolite precursors is clearly shown by the feldspar replacements of 

analcime and the occurrence of remnants of clinoptilolite pseudomorphs after shards in a finely crystalline 

matrix of potassium feldspar., The absence of associated 

relict glass and potassium feldspar in tuffs of the Barstow Formation strongly suggests that most, if not all, 

potassium feldspar formed from zeolite precursors 

rather than directly from glass. Feldspathic tuffs that 

have a vague vitroclastic texture probably formed from 

a tuff rich in a zeolite such as clinoptilolite which generally preserves the texture, whereas feldspathic tuffs 

that lack vitroclastic texture probably formed from 

analcimic tuffs.

Authigenic potassium feldspar also occurs as overFIGTJRE 21. 'Photomicrograph of plagioclase grain (P) with a 

partial overgrowth of potassium feldspar (K). The overgrowth is untvvinned. Matrix is chiefly finely crystalline authigenic potassium feldspar. Crossed nicols.

growths on plagioclase or as replacements of plagioclase 

in some feldspar-rich tuffs. The overgrowths (fig. 21) 

are thin continuous or discontinuous sheaths that characteristically have an irregular outline but are in sharp 

contact with the plagioclase host. Replacement of plagioclase by potassium feldspar generally is complete, 

although rare partial replacements have been observed. 

These partial replacements consist of potassium feldspar surrounding isolated but optically continuous remnants of plagioclase. The complete replacements are 

generally hollow pseudomorphs; solid pseudomorphs 

are rare. The hollow pseudomorphs (fig. 22) retain the 

general shape of the plagioclase grain; however, the 

outline is very irregular rather than smooth. The hollow pseudomorphs consist of a boxwork of potassium 

feldspar that extinguishes uniformly. Potassium feldspar in the pseudomorphs, like that of the overgrowths, 

is untwinned.

CLINOFTILOLITE-ANALCIME-FOTASSIUM FELDSPAR 

RELATIONSHIPS IN THE SKYLINE TUFF

The Skyline tuff is a relatively thick tuff that crops 

out from one end of the Mud Hills to the other (fig. 23).

FIGURE 22. Photomicrograph of a potassium feldspar pseudomorph -after plagioclase, showing typical boxwork structure. 

Matrix consists of finely crystalline potassium feldspar, clay 

minerals, and calcite. Crossed nicols.
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ALTERED TUFFS 23

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24 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

In the central part of the Mud Hills, the tuff is chiefly 

potassium feldspar or potassium feldspar and analcime 

(Sheppard and Gude, 1965a). The tuff at either end 

of the Mud Hills is zeolitic and consists chiefly of

clinoptilolite (table 8). Minor erionite or mordenite, and 

rare phillipsite or chabazite, are locally associated with 

the clinoptilolite. As much as 30 percent relict glass 

occurs in parts of the tuff at the westernmost exposure.

TABLE 8. Mineralogic composition of the Skyline tuff as estimated from X-ray diffractometer patterns of bulk samples

[_, looked for but not found; Tr., trace]

Locality 

(fig. 23)

2 

3

6 

8 

13 

16

17 

18

10

21

22 

24 

26

33

34

36

37 

38 

39 

42

43 

44

45 

50 

51

52 

53

54

55

58 

60

62

63 

65

X-ray analysis (parts in 10)

Sample

CT_-_._

CB___ 

B_________

A.........

D.. ...... 

C_. .___.__

A-_-._____

B_. .......

A.........

A_ .

A........

B. ........

A.......

A.......

B.

A.......

SM-4-iA... 

4B._ 

4C._ 

A.. .

B.. .

B

A...

B...

A... .

C...__ 

B.....

A....

A.. .

D 

C___._

B. . _

A.........

C_____

B...

A... .

B..

A...

B

B

A..

SM-6-2D 

2C2B 

2A_.

B.

A..

B...

A.._

B

A.....

B.

A.._

C...

B...... .

A.........

A_.

C.._

B....

A.........

SM-1-8A..

A.. .......

XT.

XB. ......

SM-1-8C 

D... .

C. ________

A-._____._

B...

E.

X._ .......

A.....

X.........

A...

SM-3-9F. 

9C_. 

9B__ 

9A_. 

9E..

Sample taken Clay, Clay, Chaba- Phillip- Clinopti- Morden- Potassi10 A i 14 A 2 zite site Erionite lolite ite Analcime urn feld- Quartz » Calcitc Other * 

spar

22 in. above base _

Atbase ____ ....... _ .

At top ___ . __ . __ .. _ 

Nearmiddle _ _______ .

Near middle, 4 ft west of C _ 

Atbase ____ . __ . _ ..

Sin. above base.. ____ .. _

6 in. above base _ .___ __ ...

6 in. above base.. ....... __ .

At top. _________ ... _ ..

6 in. above base

At upper part.. .

Near top.......... _ Near base... __________ ...

Attop ____ ..__ __ . _

Nearmiddle __ ....... _

Atbase _ ...... __ ..

Near top __ .... __ .....

Nearmiddle.- __ ... ..

New top. _

Near middle __

Nearmiddle.--.. _ ... _ ..

Near base... .

At top..- _ .............. _ .

Near middle. _ Atbase. _ _ ._

Sin. above base... _ ... __ ....

Attop _______ . __ ....- 

72 in. above base. _ __

40 in. above base. .

Atbase _ __

42 in. above base __ ....... .....

5in. above base __ . __ _ __ .

Atbase-- _...__

10 in. above base

Atbase. ___ . .

Nearmiddle- _ .. . .....

At upper part of clinoptilolite 

remnant. 

At lower part of clinoptilolite 

remnant.

Atbase.... __ _______ .........

Atbase.. _ .... _____ ........

Tr. ._

Tr. ..

Tr.

Tr. ...

Tr.

1 ...

1 ...

1 

Tr. 

Tr.

2

1 ..

Tr. ..

Tr. 

Tr. 

Tr.

Tr.

Tr. 

1 

1 

1

Tr.

Tr. 

1 

Tr.

Tr.

Tr. ..

Tr.

Tr. __

Tr.

Tr. ..

1

1 ..

Tr.

Tr.

1

1 ..

Tr. ...._.____....

..Tr. _

5 ..............

1 - _-__--__..

Tr. ..............

Tr. ...___....._._

1 ______________

Tr. .......__.._..

1 .-.-._-._-.

1 ______________

Tr. . ______ ...

Tr. ______________

1 .-..--.-..-.

1 ______________

1 __-_-___.___-.

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

Tr. ..............

1

Tr. .... . .......

1

1 ______________

Tr. _......__.....

Tr. ._.._.........

Tr. _.____..__....

Tr. ..............

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

2 ._......_..__.

Tr. ..............

Tr. ..............

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

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

Tr. ..............

1 _._.._.___

3 ..........

2 .

5 ..

Tr.

1 ..._......

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

Tr. . ... . .

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

2 5 ....

Tr. __ _._ _ .

1 . ...

Tr. ..............

1

Tr. ..............

1 --._..-. 

Tr. ._ ....

Tr. ..............

Tr. .............

Tr. .... .........

Tr. .............

._- _ 8 ...

 -______ _- Tr. ...

.- - ____ 9 ...

 ... 8 __.

-_-__. _ 10 ...

-- . - 7 ...

..____-__- 10 ...

8 ..

7 ...

.._____ Tr. 10 ...

.-..- ...... 7 __.

.. __-_.__ 9 _..

_______ Tr. 2 ...

_ _ _ 1

. __-__-_ __- 9 ...

. _ 8 ...

 ... 9 ...

-- .. - 8 ...

_-.--_. . - 5 ...

. _ _- . 8 ...

...... Tr. 10 _..

._ - 9 ...

.________._-__- 10 ...

....... _.._.____ 9

. _ 10

.__ -________. 6 ...

.----..--.-.--._ 9

.... -.--.-... 3 ._

........ .-- __.. 8

...--_--..-...-.. 7

...... 1 7

....... Tr. 10

2 .......... 7

2 .......... 3 _.

..____. 2 6 _.

....... 1 9 _.

.._ . 1 4 .

10

..---.--.-...___. 9

2 _......___ 5

................ 8 ...

...... ____.____- 10

.__-.-.-.__..___. 10

..-.- 1 1

.._- _-..__ 3

..-.--.--........ 6

.___- _-_ . 10

..___ _- 10 ...

...-.--.--.-..... 9

._ ..__ __ 10 ...

.____ ._. Tr. ...

._ -_-___. 8 ...

..__-_.__ _____ 3

.__ _._____ _ Tr. ...

..-__ ._____ Tr. .

._________.___... 4 ...

. _-_- 8

.-.- - 4

. __-_ 10 __.__-. ._ . _ 

. 10 __-. - _-- 

.- ..-.-_- - 10 .....__.-__-. -.--

.____.__ 4 1 ....._.. 

. _._ _- 10 -__ 

.--...-. 8 Tr. 2 _.--....

1 __________

. _ ____ _____ 10 - -----

 10 __ _ -_-

Q

Q

.__ _ _.._-___ 10 -._ 

Q

Q

.-_-__-______-_-________ 1 ..-.. .....

- - .. 1 ....__.......

1 Q

. .._._____... 1 ..........

TV

----- 3 ._ 11

.... ... . Tr. 9 ...

. ___ .._.... Tr.

----- 2 .. _.__.. 

.... Tr. 1 

.... __- Tr. ...... ____ _ 

 ...___ 2 __________

1 Tr

----------- 1 . 

2

------ i -. .. ..- - 

----- 7 ___________________ _ _ ------

----- 1 __ .__.___- 

..-.--. Tr. _ _ _______ --

-.- . 2 _________________ - 

________ 1 --.--._-- -------

 .. 2 ,_____ ..... . 

Tr ------

.-__.___ Tr. _. ___. . 

1 -_-.. _______

Tr

.-_-_.. Tr. ........ ..,.... - 

.... . ... 1 9 .__. 

 __ 8 .... ... 1 _-_ - 

.... _ 5 5 ___._____ - - 

.--- .. .. ... 10 __.__ - -- 

._ ______.___-_ 10 _._---_ -- 

4 Tr. 5 ..-- .--

 .... 8 .......... 2 ... 

1

6 _. ___ . __ ----

...... 5 Tr. 5 _____ ________

 _ ___ .____. 10 ___ ____- - -

.... 8 1 1 

3 Tr. 7 __ _ _

________ 6 4 -- -

4 5 ____.__ __ - -

_. . 5 _ - ----

2 _ __ . __... -- 

Tr. 6 ............. __ 

... . _ 1 9 _________ 

Tr.

1

1

Tr.

1

3

4

Tr.

Tr.

Tr

See footnotes at end of table.
ALTERED TUFFS 25

TABLE 8. Mineralogic composition of the Skyline tuff as estimated from X-ray diffractometer patterns of bulk samples Continued

[__, looked for but not found; Tr., trace]

Locality 

(fig. 23)

66 

68 

69

70

95 

96

99

100 

101

103

Sample

A.........

A.... ..

B. ........

A 

X. .....

C 

c.. .......

B 

A 

A.. .

B... ......

A ... 

B. ...... ..

A-.-.. .

A..... .

B 

A.. .

C...... ... 

B...... ...

A. .. 

X-ray analysis (parts in 10)

Sample taken Clay, Clay, Chaba- Phillip- Clinopti- Morden- Analcime Potassi10 A i 14 A a zite site Erionite lolite ite umfeld- Quartz 3 Calcite Other * 

spar

6 in. above base ...........

Near middle _______ ,. ....

30 in. above base ..............

Near middle in clinoptilolite 

remnant. 

Near middle, 7 ft north of A. . 

At lower part. _________ . Near top ___ _. .............

Near middle __ .. ............

12 in. above base... _ - ......

At top. .. ....... .....

Tr, 

1

2 ...

2 ...

2

1 ...

2 ...

Tr. ...

Tr. ...

Tr. ...

Tr.

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

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

Tr. ..............

Tr. ..............

Tr

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

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

Tr. ..............

5 ..............

Tr. ..............

................. Tr. .................... 9 ..............................

............. .... ... . - 7 .......... 1 ..... - Tr.

. . 6 .......... 2 ._..-.-__... . 

..... .._. .. . 314 ....................

....... ...... . ._. - 10 ..............................

.. . ...- .--.. _ - 9 . - .-.......

., ..... .- Tr. .............. ...... 8 .......... ....... -....-....

......... -. -...-. .. 10 -- . . .. 

. -.. .. . 10 .......... ....... -.. 

....... 1 8 -_...-- --.. . - 

q

.-. . . . 1 9 .......... ....................

.. . ...- .. . -. ... . . .. 10 .- .-.---.. .-. .

.-- ....- .-. -. - 9 1 

.,......- .-.....-. -. . . . 10 ...... . . . -.....

... -.-......-.-. ............ 2 3 

......................... ........ .. 10 ..............................

......... .. -. .... .. 10 ........... .. .. 

.... - --. -..- 10 -. -- .. 

» Authigenic illite and pyrogenic biotite.

2 Chiefly clay minerals with a basal spacing of 12-14 A.

3 Includes authigenic, pyrogenic, and detrital quartz.

Parts of the zeolitic Skyline tuff between Fossil Canyon and Rainbow Basin commonly are analcimic. The 

analcime occurs in irregular discontinuous zones at the 

top and 'bottom of the tuff. The content of analcime 

ranges from trace amounts to about 90 percent. Locally, 

the Skyline tuff is analcimic throughout its entire thickness. Analcimic tuff can readily be distinguished from 

the predominantly clinoptilolitic tuff because it is light 

brown or yellowish green and granular in contrast to 

the white conchoidal-fracturing clinoptilolitic phase. 

Vitroclastic texture is obvious in the clinoptilolitic part 

but vague or absent in the analcimic parts.

Remnants of zeolitic tuff occur locally in the predominantly feldspathite Skyline tuff at Rainbow Basin 

and the western wall of Owl Canyon. The remnants are 

in the middle part of the tuff and are generally analcimic. Contacts between the remnants and the potassium 

feldspar-rich tuff are irregular and gradational. At several localities, notably on the north limb of the Barstow 

syncline in the northwestern part of Rainbow Basin 

and on the south limb of the syncline near the top of 

the west wall of Owl Canyon, the remnants consist of 

one or more irregular clinoptilolitic cores (figs. 24 and 

25) that are enclosed or nearly enclosed by an irregular 

sheath of analcimic tuff. The contacts are gradational 

and are not controlled by lidding within the tuff.

The analcimic and clinoptilolitic remnants in the Skyline tuff are interpreted as relicts of a diagenetic alteration that preceded the formation of potassium feldspar. This interpretation is consistent with the paragenetic relationship observed in thin sections of the gradational zones. The sequence of formation was (1) rhyolitic glass to clinoptilolite to analcime to potassium

4 Chiefly plagioclase. Sample A from locality 26 contains a trace of halite; sample A 

from locality 53 contains 4 parts opal; and sample C from locality 53 contains 3 parts

feldspar, or (2) rhyolitic glass to clinoptilolite to potassium feldspar. Nowhere is there any evidence that analcime or potassium feldspar formed directly from the 

rhyolitic glass.

Other tuffs of the Barstow Formation show a similar gradation of clinoptilolitic or phillipsitic tuff to 

analcimic tuff, and zeolitic (chiefly clinoptilolite and 

analcime) tuff to feldspathi'c tuff. The complete 

sequence from nonanalcimic zeolitic tuff, to analcimic

FIGURE 24. Skyline tuff, showing irregular remnants of clinoptilolitic tuff (white) surrounded by analcimic (gray) tuff. 

Uppermost part of tuff is not visible in photograph but consists chiefly of authigenic quartz and potassium feldspar. 

Base of tuff is generally analcimic; but locally the clinoptilolitic remnants extend to the base, or the base is potassium feldspar-rich. The tuff overlies brownish mudstone. 

Locality : south limb of syncline in southeastern part of Rainbow Basin; SE%NE% sec. 24, T. 11 N., R 2 W.
26

FIGURE 25. Skyline tuff, showing irregular remnants of 

clinoptilolitic (cl) tuff that are surrounded by analcimic 

(A) tuff. Remainder of tuff consists chiefly of potassium feldspar (K). Note that potassium feldspar-rich tuff is locally 

in contact with the clinoptilolitic remnant. Locality : north 

limb of syncline in northern part of Rainbow Basin ; 

sec. 24, T. 11 N, R. 2 W.

DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

altered rhyolitic vitric tuffs of Cenozoic age. The zeolites, except analcime, formed during diagenesis by 

reaction of the vitric material with interstitial water 

(Deffeyes, 1959a), which may have originated as either 

connate water of a saline lake (Hay, 1964; Sheppard 

and Gude, 1968) or meteoric water (Hay, 1963). Most 

analcime and feldspar in the altered tuffs formed during diagenesis by reaction of precursor zeolites with 

the pore water (Sheppard and Gude, 1965a, p. 4; Hay, 

1966, p. 90-98).

Hay (1966) discussed those factors that may control 

the formation and distribution of zeolites and associated authigenic silicate minerals in sedimentary rocks. 

The authigenic mineralogy correlates with temperature, pressure, and chemistry of the pore water, and 

composition, permeability, and age of the host rock. 

Inasmuch as all the authigenic silicate minerals of the 

Barstow Formation occur within certain tuffs, differences in the composition, permeability, and age of the 

host rock cannot explain their distribution. The Barstow Formation was probably subjected to only shallow 

burial, and there is no evidence of hydrothermal activity ; thus, the temperature and pressure must have been 

relatively low during the formation of the silicate 

minerals. Comparison of the authigenic mineralogy of 

these tuffs in the Barstow Formation with the 'authigenic mineralogy of tuffs in relatively young salinelake deposits suggests that the mineralogy reflects 

differences in the chemistry of the pore water during 

diagenesis.

Experimental work by others indicates that the 

activity ratio of alkali ions to hydrogen ions and the 

activity of silica are the major chemical parameters 

of the water that control whether clay minerals, zeolites, or feldspars will form at conditions that approximate surface temperatures and pressures (Hemley, 

1959, 1962; Garrels and Christ, 1965, p. 359-370; Hess, 

1966). Zeolites and feldspars are favored over clay 

minerals by relatively high alkali ion to hydrogen ion 

activity ratios and by relatively high silica activities. 

The high alkali ion to hydrogen ion activity ratio necessary for the formation of zeolites in a tuff can be 

attained in the depositional environment of a saline, 

alkaline lake (Hay, 1964) or in the postdepositional 

environment by solution and hydrolysis of rhyolitic 

vitric material by subsurface water (Hay, 1963, p. 237- 

242).

There is no direct indication of the chemical environment that existed during the deposition of the tuffs 

of the Barstow Formation. The chemistry of the water 

must be inferred from the sedimentary rocks of the 

basin. Most tuffs were deposited in lakes which probably varied in number, location, and size. The abundance

tuff, and then to f eldspathic tuff, however, has not been 

recognized in these other tuffs, probably because of 

their limited distribution.

RELATIONSHIP OF AUTHIGENIC SILICATE 

MINERALOGY TO THICKNESS OF TUFFS

In addition to the previously described vertical and 

lateral variations in authigenic mineralogy of the tuffs, 

there seems to be a relationship between the authigenic 

mineralogy and the thickness of the tuffs. Thin tuffs 

generally consist of analcime and (or) potassium feldspar, whereas thick tuffs consist of zeolites other than 

analcime. Analcime and (or) potassium feldspar occurs 

in 98 percent of the tuffs that are 6 inches or less thick. 

Ninety-four percent of those zeolitic tuffs that lack 

analcime or potassium feldspar are more than 6 inches 

thick.

A similar relationship between the authigenic 

mineralogy and thickness of tuffs was recognized by 

Hay (1966, p. 43) at Olduvai Gorge, Tanzania. Pleistocene tuffs less than 1 inch thick generally are chiefly 

potassium feldspar, but thicker tuffs contain phillipsite 

or phillipsite and potassium feldspar. The tuffs were 

originally similar in composition and were deposited 

in a saline lake.

GENESIS OF THE AUTHIGENIC SILICATE MINERALS

Zeolites and feldspars of authigenic origin occur 

throughout the world in sedimentary rocks that are 

diverse in lithology, depositional environment, and age. 

Zeolites and feldspars are especially abundant in
GENESIS OF THE AUTHIGENIC SILICATE MINERALS 27

of mammal tracks in lacustrine mudstones suggests that 

the lakes were, at least locally, very shallow. Mud cracks 

further suggest that the lakes occasionally desiccated. 

Inasmuch as relatively coarse clastic rocks locally intertongue with the lacustrine rocks, some lakes or some 

parts of the lakes were relatively fresh. Other lakes 

or parts of the lakes probably were alkaline and moderately to highly saline.

INTERPRETATION OF A SALINE, ALKALINE DEPOSITIONAL ENVIRONMENT FOR PARTS OF THE BARSTOW FORMATION

The obvious evidence for a saline lake, bedded saline 

minerals, has not been found in the Barstow Formation 

at the Mud Hills, although bedded colemanite 

Ca2B6Oii-5H2O) occurs in contemporaneous lacustrine 

rocks on the southern flank of the Calico Mountains, 

about 10 miles southeast of the Mud Hills (Campbell, 

1902, p. 12-13; Baker, 1911, p. 349-353; Noble, 1926, 

p. 59-60; McCulloh, 1965). Disseminated crystal molds 

that are filled or partly filled with calcite occur in mudstone and tuff of the upper part of the formation at the 

northeastern part of Rainbow Basin. The molds resemble the morphology of gaylussite (CaCO3 Na2CO3 

5H2O) and suggest saline conditions during deposition. 

The interbedded tuffs at this locality consist chiefly of 

analcime and (or) potassium feldspar. The common efflorescence of thenardite (Na2SO4 ) on the surface of 

weathered mudstone, particularly at Rainbow Basin 

and eastward, suggests a depositional environment of 

moderate salinity. Although the conditions under 

which dolomite precipitates at nearly room temperature 

is not known, the beds of dolomite and dolomitic mudstone in the eastern part of the Mud Hills suggest a 

depositional or diagenetic environment of high pH and 

moderate salinity (Smith and Haines, 1964, p. P52- 

P53; Jones, 1965, p. A44). Dolomite occurs at Searles 

Lake in muds mainly near contacts with salines.

A saline, alkaline depositional environment for parts 

of the Barstow Formation can be inferred from the occurrence of certain authigenic silicate minerals that 

have been found by other mineralogists to indicate 

saline waters. Zeolites such as clinoptilolite and niordenite occur in altered silicic tuffs that were deposited in 

either fresh or saline waters; however, zeolites such as 

erionite and phillipsite in altered silicic tuffs are found 

almost exclusively in saline-lake deposits (Hay, 1964, 

p. 1384; 1966, p. 67). The common occurrence of analcime and potassium feldspar in the apparently nontuffaceous mudstones of the Barstow Formation may 

also indicate a saline-lake environment, because these 

authigenic minerals are rare in nontuffaceous mudstones 

deposited in fresh water (Hay, 1966, p. 67).

CORRELATION BETWEEN SALINITY OF DEPOPITIONAL ENVIRONMENT AND AUTHIGENIC SILICATE MINERALOGY

Studies of tuffs deposited in relatively young saline 

lakes where water analyses are available have shown a 

strong correlation between salinity and the authigenic 

silicate mineralogy (Hay, 1964, 1966). Tuffaceous sediments deposited in fresh water still contain unaltered 

glass, but those deposited in saline water are altered 

and contain zeolites, potassium feldspar, or searlesite. 

Hay (1966, p. 68) found that potassium feldspar is a 

major constituent of tuffs saturated with sodium carbonate brine in Searles Lake, but potassium feldspar is 

absent and zeolites are common in equivalent deposits 

that contain mildly saline water at China Lake.

Older lacustrine deposits that contain interbedded 

saline minerals also show a correlation between the inferred salinity of the depositional environment and the 

authigenic mineralogy of rhyolitic tuffs. In the Pleistocene deposits of Lake Tecopa, Calif. (Sheppard and 

Gude, 1968), glass is unaltered in tuff deposited in frosh 

water; however, the tuffs consist chiefly of phillips'te, 

clinoptilolite, and erionite where they were deposited 

in moderately saline water, and potassium feldspar and 

searlesite where they were deposited in highly saline 

water. Miocene tuffs at Krainer, Calif., contain pot^-ssium feldspar and analcime where they are associated 

with high concentrations of sodium borate but contain 

clinoptilolite and phillipsite where they are associated 

with rocks indicative of lower salinity (Hay, 1966, p. 

68). Tuffs in the Eocene Green River Formation of 

Wyoming1 are altered to montmorillonite where tl 3y 

were deposited in fresh water; to clinoptilolite and 

mordenite (Goodwin and Surdam, 1967), in slightly 

saline water; to analcime, in moderately saline water; 

and to potassium, feldspar and albite, in highly saline 

water (Hay, 1965; 1966, p. 44-52).

The diverse authigenic mineralogy of the tuffs in 

the Barstow Formation seems to have resulted fr->m 

differences in pH and salinity of the lake water trapped 

in the tuffs during deposition. The lake water probably 

ranged from fresh water to water with a pH of 9 or 

higher and a moderate to high salinity. Those tuffs that 

consist of unaltered glass with minor zeolite and montmorillonite probably were deposited in relatively fresh 

water; whereas those tuffs tha/t consist of zeolites exclusive of analcime, or of only analcime, or of potassium feldspar were deposited in water of relatively low, 

moderate, and high salinity, respectively.

The gradational change in the areal distribution of 

authigenic silicate minerals in the Skyline tuff (fig. 

23 and table 8) from chiefly clinoptilolite with minor
28 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

relict glass to clinoptilolite plus analcime to analcime 

plus potassium feldspar to chiefly potassium feldspar 

was probably due to a chemical zonation of the pore 

water during diagenesis. This zonation may have been 

inherited from the chemical zonation that existed in the 

lake during deposition of the Skyline tuff. Fresh water 

along shore may have been succeeded basinward by 

increasingly saline water. Such a mechanism seems the 

best explanation for a similar distribution of authigenic silicate minerals in tuffs of Pleistocene Lake 

Tecopa, Calif. (Sheppard and Gude, 1968). At Lake 

Tecopa, however, the margin of the lake is readily discernible. An alternate explanation for the chemical 

zonation of the pore water in the Skyline tuff is postdepositional "freshening" by meteoric water. Uniformly 

alkaline and saline water trapped as pore water in the 

tuff during deposition may have been freshened along 

shore by the encroachment of fresh water after deposition (Eugster and Smith, 1965, p. 518).

SOLUTION OF GLASS TO FORM ALKALI- AND 

SILICA-RICH ZEOLITES

Solution of silicic glass by moderately alkaline and 

saline pore water provides the materials necessary for 

the formation of the zeolites. Deffeyes (1959a, p. 607) 

emphasized that zeolites form during diagenesis not 

by devitrification of the shards but by solution of the 

shards and subsequent precipitation of zeolite from the 

solution.

Tuffs in the Barstow Formation are generally interbedded with relatively impermeable mudstones and 

after deposition may have behaved as closed systems 

consisting of silicic glass and the connate lake water. 

The early formation of montmorillonite probably was 

favored by <a relatively low Na+ + K+ : H+ activity ratio 

(Hemley, 1962). This activity ratio would have been 

at its lowest value at the time of deposition. Subsequent 

solution of glass or the formation of montmorillonite 

by an initial marginal hydrolysis of the glass would 

cause an increase in the pH and the concentration of 

alkali ions (Hay, 1963, p. 240), thereby increasing the 

NV + K+ : H+ activity ratio of the pore water and providing an environment more suitable for the formation of 

zeolites rather than additional montmorillonite.

Inasmuch as the zeolites of the Barstow Formation 

differ noticeably in chemical composition, the following 

factors may influence which zeolite will form: Activity 

of SiO2 , activity of H2O, and the proportion of cations 

in the pore water. Where relationships are clear, the 

petrographic evidence indicates that phillipsite and 

chabazite formed before clinoptilolite, and clinoptilolite formed before mordenite. Phillipsite and chabazite 

contain more H2O and less SiO2 than clinoptilolite. 

Thus, the formation of phillipsite or chabazite should 

be favored over clinoptilolite by a relatively Hgh activity of H2O and (or) low activity of 'SiO2 . 'Such conditions may have prevailed in the silicic tuffs during 

early diagenesis.

Continued solution of the glass coupled with the 

early formation of phillipsite or chabazite in a tuff 

probably has the effect of enriching the pore water in 

SiO2 and cations. The activity of SiO2 may thus increase and the activity of H2O decrease to levels suitable for the formation of clinoptilolite rather than 

phillipsite or chabazite. Mordenite rather than clinoptilolite probably formed when the activity of SiO2 

reached a maximum.

That the SiO2 concentration can effect whicl zeolite 

will form was demonstrated experimentally bv Ciric 

(1967). At temperatures of 80°-90°C, Ciric synthesized 

zeolites with a range of Si: Al ratios in a sealed Pyrex 

tube into which sodium aluminate had been added at 

one end and sodium metasilicate added at the other. A 

relatively siliceous zeolite formed at the silica-rich end, 

an aluminous zeolite formed at the aluminate end, and 

a zeolite of intermediate Si:Al ratio formed in the 

middle of the tube.

REACTION OF ALKALIC, SILICIC ZEOLITES TO 

FORM ANALCIME

Analcime in some nontuffaceous lacustrine rocks apparently formed during diagenesis by reaction of aluminosilicate minerals with the pore water. For example, 

solution of plagioclase and quartz in sediments of 

Searles Lake probably supplied some of the silicon and 

aluminum for the formation of analcime, searlesite, and 

potassium feldspar (Hay and Moiola, 1963). Montmorillonite and kaolinite may also have reacted to form 

analcime in certain nontuffaceous sediments (Hay and 

Moiola, 1963, p. 330; Pipkin, 1967).

Analcime in other nontuffaceous saline lacustrine deposits probably formed by direct precipitation from the 

lake water. The analcime in the Triassic Lockatcng Formation either precipitated directly or formed at an early 

stage of diagenesis from a colloidal precursor o^ aluminosilicate mineral (Van Houten, 1960; 1965, p. 835- 

836). At Lake Natron, Kenya, analcime in nontuffaceous 

clays was precipitated from a sodium carbonate brine 

(Hay, 1966, p. 36-38).

Ever since the discovery of analcime in tuffaceous 

sedimentary rocks, most workers have assumed that the 

analcime formed directly from vitric material. The
GENESIS OF THE ATJTHIGENIC SILICATE MINERALS 29

presence of vdtroclastic texture and pyrogenic crystals 

in some analcimic tuffs seemed sufficient evidence; however, Hay (1966, p. 91) showed that these criteria do 

not necessarily prove that the glass altered directly to 

analcime. Hay (1966, p. 90-93) concluded from a comparison of the authigenic mineralogy of tuffs in modern 

and ancient saline-lake deposits that analcime commonly 

formed at low temperatures by reaction of alkalic, silicic 

zeolite precursors. Furthermore, relict fresh glass has 

not been confirmed in analcimic tuff; thus, there is 

doubt that analcime ever has formed directly from glass.

Much, if not all, of the aualcime in tuffs of the Barstow Formation seems to have formed from alkalic, 

silicic zeolite plrecursors. These alkalic zeolites, because 

of their open structure (Smith, 1963) and large internal 

surface area, would seem to be particularly susceptible to 

alteration in the diagenetic environment.

Experimental low-temperature work by other mineralogists, and theoretical considerations, indicate that 

the formation of analcime is favored over an alkalic, 

silicic zeolite such as clinoptilolite by a high Na+ : H+ 

ratio (Hess, 1966), relatively low activity of SiO2 

(Coombs and others, 1959; Senderov, 1963; Campbell 

and Fyfe, 1965), and, perhaps, relatively low activity 

of H2O. A comparison of chemical analyses of rhyolitic 

glass and clinoptilolite (Hay, 1963, p. 230; Sheppard 

and Gude, 1965b) suggests that sodium is lost from the 

glass during the formation of clinoptilolite. Perhaps 

this sodium, plus that originally in the pore water, was 

sufficient to provide an environment suitable for crystallization of analcime some place else. A relatively high 

salinity of the pore water would decrease the activity of 

H2O and thus favor the formation of the less hydrous 

analcime over any of the other zeolites in the Barstow 

Formation, all of which are much more hydrous than 

analcime. An increase in pH during diagenesis would 

decrease the activity of SiO2 (Coombs and others, 1959; 

Senderov, 1963) and increase the Na+ : H+ ratio. Both 

conditions should favor the formation of analcime over 

clinoptilolite. Quartz is more common in the analcimic 

tuffs than in the other zeolitic tuffs of the Barstow Formation. Crystallization of quartz in the tuffs would have 

lowered the activity of SiO2 .

These arguments are based on the assumption that 

chemical factors alone are responsible for the formation 

of analcime; however, kinetic factors may be equally, or 

perhaps more, important. Analcime may simply form 

later than zeolites such as clinoptilolite and phillipsite.

Studies of the composition of analcime in sedimentary 

rocks have shown a range in Si: Al ratios of about 2.0- 

2.9 (Coombs and Whetten, 196T; lijima and Hay, 1968). 

Coombs and Whetten (196T) studied analcime from

rocks that are diverse in age, lithology, depositional environment, and geographic location and concluded that 

(1) analcime in the high part of the above compositional 

range formed by the reaction of silicic glass with saline 

water, (2) analcime in the intermediate part of the range 

formed by "burial metamorphic reactions" due to increased temperature and pressure, and (3) analcime in 

the low part of the range formed either by direct precipitation from alkaline water or by reaction of clay 

minerals and other materials with alkaline water.

Analcime in tuffs of the Barstow Formation ranges in 

Si: Al ratio from about 2.2 to 2.8 and, thus, nearly spans 

the compositional range known for sedimentary rod's. 

Unlike the analcimes studied by Coombs and Whetten 

(1967), these analcimes in the Barstow Formation 

formed in rocks that originally were similar in composition and were deposited in a similar environment. 

None of the tuffs were deeply buried; furthermore, there 

is no correlation between analcime composition and 

stratigraphic position.

Analcime in the tuffs of the Barstow Formation 

formed from zeolite precursors and not directly frcm 

rhyolitic glass. Petrographic study showed that analcime formed from clinoptilolite and phillipsite; however, analcime probably formed from the other autHgenic zeolites as well. Inasmuch as the zeolites have a 

wide range of Si: Al ratios (table 4), it is tempting to 

suggest that the silicon content of analcime was affected 

by the silicon content of the precursor. To check the 

validity of this idea, the composition of analcime associated with phillipsite was compared with the composition of analcime associated with clinoptilolite. 

Figure 26 shows that analcime associated with phillipsite has 33.1-34.4 silicon atoms per unit cell, whereas 

analcime associated with clinoptilolite has 34.5-35.1 

silicon atoms per unit cell. Inasmuch as phillipsite and 

clinoptilolite are relatively low- and high-silicon zeolites, respectively, a correlation between the compositions of the precursor and analcime is suggested. 

Additional determinations, particularly from another 

formation, are needed for confirmation.

A

A AA

33.0 33.5 34.0 34.5

SILICON ATOMS PER UNIT CELL

35.0 35.5

FIGURE 26. Plot showing the correlation between the silicon 

content of analcime and the precursor zeolite. Composition 

of analcime determined from X-ray diffractometer data by 

measurement of the displacement of the (639) peak of aralcime. A, composition of analcime associated with phillipsite; 

 , composition of analcime associated with clinoptilolite.
30 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

REACTION OF ZEOLITES TO FORM POTASSIUM 

FELDSPAR

The formation of potassium feldspar by the reaction 

of zeolite precursors in tuffs that were never buried 

deeply was well documented by the studies of Hay 

(1966, p. 93-98) and Sheppard and Glide (1968). 

Analcime is replaced by potassium feldspar and albite 

in tuffs of the Green Eiver Formation in Wyoming 

(Hay, 1965, 1966; lijima and Hay, 1968) where the 

formation of feldspar can be correlated with high salinities of the depositional environment. Phillipsite is replaced by potassium feldspar and searlesite in rhyolitic 

tuffs of Pleistocene Lake Tecopa, Calif. (Sheppard and 

Gude, 1968), where high salinities prevailed. Some zeolitic tuffs of Lake Tecopa contain abundant clinoptilolite and erionite as well as phillipsite; therefore, these 

zeolites could also have been precursors for the potassium feldspar.

Petrographic study of the feldspathic tuffs in the 

Barstow Formation has shown that potassium feldspar 

replaced analcime and clinoptilolite. Other zeolites such 

as chabazite, erionite, mordenite, and phillipsite may 

also have been precursors for the feldspar, but the textural evidence was not observed. Relict glass is nowhere 

associated with the potassium feldspar; thus, the direct 

formation of feldspar from rhyolitic glass seems unlikely in these tuffs.

Zeolites, because of their open structure (Smith, 

1963), apparently are particularly susceptible to alteration in the cliagenetic environment. The initially high 

salinity of the pore water trapped in the tuffs during 

deposition probably was the major factor that controlled the alteration. A relatively high salinity would 

lower the activity of H2O and favor the formation of 

anhydrous potassium feldspar from the hydrous zeolites, including analcime.

Other factors such as the K+ : H+ ratio and the activity 

of SiO2 also affect, the formation of potassium feldspar 

(Hemley and Jones, 1964; Hess, 1966). Hydrolysis experiments by Garrels and Howard (1959, p. 87) suggested that potassium feldspar will form at 25°C in 

environments with a K+ : H+ ratio greater than about 

109- 5 . Potassium feldspar may form at near surface 

conditions where the K+ : H+ ratio is as low as about 

105"6 and the pore water is saturated with amorphous 

SiO2 (Orville, 1964).

The zeolitic tuffs of the Barstow Formation that are 

rich in clinoptilolite commonly contain opal which may 

have contributed a favorable chemical environment for 

the formation of potassium feldspar. However, feldspathic tuffs in the formation now contain quartz or 

chalceclonic quartz but lack opal. If the activity of 

SiO2 was controlled by quartz rather than by opal, relatively high concentrations of K> would have bem necessary for the formation of potassium feldspar. Unfortunately, the relative ages of potassium feldspar and 

quartz are unknown.

Studies by lijima and Hay (1968) on the composition of 'analcime in tuffs of the Green River Fcrmation 

showed that analcime associated with authigenic potassium feldspar is less siliceous than that not associated 

with potassium feldspar. lijima and Hay concluded 

that siliceous analcime became partly desilicated during 

the reaction to form feldspar. Analcime in the tuffs of 

the Barstow Formation show albout the same compositional range whether associated with authigenic potassium feldspar or not. Unlike the analcime in t\*<$ Green 

River tuffs, the Barstow analcime seems to be crowded 

with inclusions of opal. Thus, a highly reactive form 

of SiO2 was nearby when the analcime reacted to form 

potassium feldspar. Desilication of analcime was unnecessary.

Although silicate reactions are generally regarded 

as sluggish at low temperatures, potassium feldspar has 

been synthesized from aluminosilicate precursors at 

temperatures no higher than 250°C. Gruner (If 36) synthesized potassium feldspar from montmorillomte in a 

10 percent solution of KHCO3 heated to 245°C for 42 

days. Barrer and Hinds (1950) synthesized potassium 

feldspar from leucite in a solution saturated with 

K2CO3 and Na2CO3 and heated to 195-200°0 for 16 

hours. The leucite had been prepared from analcime 

by cation exchange in a solution saturated with KC1 

at 150°C. Nemecz and Varju (1962, p. 425) reported 

the synthesis of potassium feldspar from clinoptilolite 

in a solution containing potassium ions and 1^-ated to 

not more than 250°C for 12 hours.

The authors, unaware of the work by Neriecz and 

Varju (1962), independently synthesized potassium 

feldspar from clinoptilolite. Clinoptilolite from the 

Barstow Formation was ground finer than 100 mesh and 

then heated over a steam bath in a saturated solution 

of KOH at about 80°C for 44 hours. After repeated 

washings followed by drying, the material was X-rayed. 

The diffractometer pattern (fig. 2Y) showed that the 

clinoptilolite was converted to potassium feldspar. This 

synthetic feldspar is compared in figure 2Y with the natural potassium feldspar that occurs in the Barstow 

Formation, and the similarity is obvious. Alt! ough the 

synthetic feldspar was prepared at a higher temperature 

than that which probably prevailed during diagenesis 

and was prepared in a chemical environment that probably did not even closely approximate the diagenetic environment, the simple experiment demonstrates the 

rapidity of the reaction of clinoptilolite to form potassium feldspar in a favorable environment.
GENESIS OF THE AUTHIGENIC SILICATE MINERALS 31

50 40 30 20 10

26 CuKa, IN DEGREES

FIGURE 27. Comparison of X-ray diffractometer traces of synthetic potassium feldspar with those of natural clinoptilolite and 

natural potassium feldspar from the Barstow Formation. A, Clinoptilolite. B, Synthetic potassium feldspar prepared from 

clinoptilolite (A) in a saturated solution of KOH at about 80 °C for 44 hours; peak marked by query is unidentified. C, 

Natural potassium feldspar from the Skyline tuff. Radiation is CuKa.
32 DIAGENESIS OF TUFFS, BARSTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

METASOMATISM DURING DIAGENESIS OF TUFF

The transition of rhyolitic glass to an alkalic silicic 

zeolite, then to analcime, and finally to potassium feldspar is well documented for the Barstow Formation and 

other Cenozoic formations (Hay, 1966). Formation of 

alkalic silicic zeolites from rhyolitic glass results mainly 

in losses of silicon and potassium, and gains of calcium 

and H2O (Hay, 1963, p. 230-231; 1964, p. 1382-1383; 

Sheppard and Gude, 1965b). The reaction of alkalic 

silicic zeolites to form analcime involves mainly losses 

of silicon, calcium, potassium, and H2O, and a gain of 

sodium. The final transformation of analcime to potassium feldspar results in losses of sodium and H2O, and 

gains of potassium and silicon.

These metasomatic changes apparently are mostly restricted to the tuff bed and take place betweer the solid 

phases and the pore fluid. Except for H2O, a chemical 

balance is probably maintained for a given tuff if the 

entire extent of the tuff is considered. Gains and losses 

of constituents between the tuff and the enclosing strata 

probably are slight; however, the metasomatic changes 

within the tuff bed must be considerable. Th^. changes 

that involve silicon are not too difficult to understand 

because most altered tuffs, whether zeolitic or feldspathic, contain free silica. The most puzzling aspect of 

the metasomatic changes is the "plumbing" that permits 

the drastic removal of potassium to form anaJeime, and 

then the subsequent addition of potassium to form potassium feldspar.

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34 DIAGENESIS OF TUFFS, BAESTOW FORMATION, SAN BERNARDINO COUNTY, CALIF.

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INDEX

[Italic page numbers indicate major references]

Page

Acknowledgments _ .. ____ . _ ..... ___ 4

Alteration, stages, fresh vitric tuff to zeolitic

tuff.. ............. ...... ......... 20

Analcime ____ ....... _ ................. 10

composition _ . __ . _ ........ _ ...... 11

formation ___ . _ .. __ ..... _ . _ . 26

formed from reaction of zeolites with

water _ ..... _ --.__ ___ . __ . 28

in mudstones. --..---.... _ ............. 7

indicator of saline-lake environment...... 27

replacement by potassium feldspar -.-... 20 

Analcite. See Analcime.

Analyses, chemical, analcime.- _- _. -._. 11

chemical, chabazite.. .......___. _ ....... 11

clinoptilolite --___- _. 14

mordenite.-.. _ ...... _ . _ . __ 15

mudstones _ . __ .................. 8

phillipsite. -..-.. .................... 15

mineral content, mudstones.-..-----..-.- 8

mineralogic, Skyline tuff ............... 24

shards, Hemicyon tuff. . __ ...... ....... 9

unit cell composition, analcime .......... 11

chabazite.-...-.-.........-.--...-.-.. 11

clinoptilolite-...-. ................... 14

mordenite--.-----..--.--...--.-.--..- 15

phillipsite............................ 15

AnimaltrackS 8

Barite __ 13

Barstow Formation, chemical environment- . 26

depositional environment _ ...._ __ 27

fossils. _ - . _. 1,8

stratigraphy and lithology. ..--.-....-.... 5

topographic features-..-. ----.-..-_-----._ 5

Barstow syncline... _-.-....__--._.-...-..-._. 5

Blackmon, Paul D., analyst..- ..-.---.-.-.... 8

Calcite, in cavities in mudstone. . 

Camel track tuff, outcrop extent. 

Carbonate rocks... _-..---... .....

Chabazite... .....................

composition.. _ . _ ....

- 8

...... 6

...... 8

...... 12

..... 11

Chemical parameters controlling formation of

silicates-.-.-.-...-...... __ ... 26

Clay minerals. . _ ..... _ .................. 12

in mudstones.-. -----.-...--.-----..-_.-.- 7

Clinoptilolite... ........-._.--_-._.._......... 12

analyses-..-.... ...-............_.-_..._.. 14

zoning.... ................................ 13

Colemanite. _ .............................. 27

Conglomerate....- _ ..-......._._._........ 6

Crystals in tuffs.. ........--..--.-..-_-.__.... 9

Daniels, Ellen S., analyst ................ 8,9,11,14

Dated tuff, outcrop extent _ ................. 6

Dolomite.. ................................... 8,27

Erionite. 14

Feldspar, formation__-.-..........---...... 26

Formulas of selected alkalic zeolites ... _ 10

Fossils in Barstow Formation.

Page 

. 1,8

Qaylussite____................._...._ 8,27

Geology, regional.. 4

Glass, solution to form zeolites.___....__ 28

Gypsum-._----...-....---.----..-......---.- 7

Hay, R. L., zeolite formulas --....---------- 10

Hemicyon tuff, accretionary lapilli........... 8

analyses of chabazite-__....._._... 11

analyses of clinoptilolite--___._..... 14

analyses of shards.----..---..----.-..---- 9

outcrop extent.._..........._......... 6

states of alteration..-----.-.---._.. -.. 20

Laboratory methods_........

Lapilli, accretionary, in tuffs... 

Limestone..--...----.-.-----.-.

Lithology, Barstow FormationLocation of area..-._-_.-..-._-.

Mammal tracks_ _ - - 27

Metasomatism during diagenesis.............. Sg

Mineral associations, analcime-.------.------- 10

chabazite................................. 12

clinoptUolite...---....---.--.-.-..---.. 13

erionite.---.--. - - - 14

mordenite.__...... _...__..__.... 14

opal...................................... 15

phillipsite-.......---.----.-...--------- 15

potassium feldspar_.......- .. .- . 16,20

quartz..---------..----.-..------------ 17

relationship to depositional environment.. 27

Montmorillonlte .- - - --- 20

Mordenite__.._. .... . ... - - 14

detection method_..-........--- .- . 4

Mudcracks ---.--...-- -------- 8,27

Mudstone... - .---. .--.__- -------.----- 7

Munson,ElaineL., analyst..--.. ....- - . 11

Neiman, Harriet, analyst. 9

Opal--.-.--.--.--....-...-.----------- /5,30

in analcime_.-........-.-.-. -..-..---- 11

Ostracodes..----- --.-----.----.-- . --.- 8

Paragenesis, authigenic silicate minerals.. 20

zeolites-.--..---------..-------------.. 28

Petrography, altered tuffs. ___.. ... 18

pH, relation to mineral formation-. ._._ 27,28

PhfflipBite------.------.----------- 15

spherulites...__ 16

in Yellow tuff........ ......... 19

Plagioclase feldspar, replacement by potassium feldspar.. 21

Potassium feldspar. _.. .. - . . 16

factors affecting formation_ ._.. . £0

formation from zeolitic precursors . 21

formed by reaction of zeolites with water. SO

in mudstones__ 7

indicator of saline-lake environment------ 27

synthesized...---------------------------- 30

Previous studies.

Quartz----.---------------.----.--------- 17,30

References . .... 

Regional geology . 

Rock fragments in tuffs.

32

9

Salinity, correlation with silicate minerals.... #7

Sampling -..-..-.. ---..- --.---- 3

Sandstone..---------------------------------- 6

Scope of study....-------.-----.-------.- 2

Searlesite. . 27

Shards--.--- ------.---- -------------- 9

Silicate minerals, authigenic, paragenesis.. 20

SUtstone_.. .-. ...--..-------.----. 6

Skyline tuff, accretionary lapilli...-- _..-.. 8

analyses of clinoptilolite ___..._...- 14 

effect of chemical zonation of pore water

during diagenesis-- .. 87

mineralogic composition--------------- 24

outcrop extent ..... ... 6

sequence of formation.------- 25

variations in authigenic mineralogy...... 21

Spherulites, phillipsite........ L ........... 16

Stratigraphy, Barstow Formation.--.- . 5

Stronttanite------------------------------- 1

Thenardite 7,27

altered, fleld description ------- . 17

analcimic ---------------------------- SO

composition. ._ _.- .... 8

distinguishing characteristics of types -. 18

metasomatism during diageneis.... ... S2

nonanalcimic zeolitic 19

original texture and structure.. .-.. . 8

petrography .. . ------ 18

potassium feldspar-rich.. .... . 20

relationship between authigenic mineralogy and thickness.__.- ----- 26

salinity-silicate mineralogy correlation- «7

Vitric material in tuffs.

Water, pore, relation to analcime formation-... 

pore, relation to potassium feldspar formation.- - 

9

29

30

Yellowtuff, analyses of phillipsite.---------- 15

clay minerals-.-.-. - 12

outcrop extent...-...-- _. 6

phillipsite spherulites--- . 19

Zeolites, alkalic, formulas..- ------------- 10

factors controlling type formed-. 28

formation.. .... . .. .- 26

formed from solution of glass. _ ----------- 28

paragenesis------..---------- -- 28

reaction with water to form analeime. ----- 28

reaction with water to form potassium

feldspar-_ __ .. SO

Zoning, in cUnoptilolites------------------- 13

35

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