Dfstrfbo.tion, fharat::tefizcltkrh, and Oen~sfS of 

-Morelen·ite :·ln ~)Miocene~ Silicic Yuffs ·at 

0¥tJco.a_•-·•·· ··• ··· ~oL1ntain · · .. ~_-_-_· I Ny· · ·e">C<Juntt'/ · 1 I ~· Nevada 

Prepared in cooperation with 

U .5. Department of Energy 

Distribution, Characterization, and Genesis of 

Mordenite in Miocene Silicic Tuffs at 

Yucca Mountain, Nye County, Nevada 

By RICHARD A. SHEPPARD, ARTHUR j. GUDE, 3d, and 

JOAN j. FITZPATRICK 

Prepared in cooperation with 

U .5. Department of Energy 

U.S. GEOLOGICAL SURVEY BULLETIN 1777 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas l. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Sheppard, Richard A. 

Distribution, characterization, and genesis of mordenite in Miocene silicic tuffs 

at Yucca Mountain, Nye County, Nevada. 

(U.S. Geological Survey bulletin ; 1777) 

Bibliography: p. 

Supt. of Docs. no.: 119.3:1777 

1. Mordenite-Nevada-Yucca Mountain Region. 2. Volcanic ash, tuff, etc.-

Nevada-Yucca Mountain Region. 3. Geology, Stratigraphic-Miocene. 

4. Geology-Nevada-Yucca Mountain Region. 5. Radioactive waste disposal in 

the ground-Nevada-Yucca Mountain Region. I. Gude, Arthur j. (Arthur James), 

1917- . II. Fitzpatrick, Joan j. Ill. United States. Dept. of Energy. IV. Title. 

V. Series. 

QE75.B9 no. 1777 

[QE391.M8] 

557.3 s 

[621.48' 38] 

87-600028 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Geologic setting 3 

Stratigraphy 3 

Analytical methods 3 

Diagenetic mineralogy of the tuffs 6 

Distribution of mordenite 7 

Description of mordenite 13 

Chemical composition of mordenite 13 

X-ray powder diffraction data for mordenite 15 

Paragenesis 16 

Chemical changes during zeolitization 17 

Genesis of mordenite 18 

References cited 21 

FIGURES 

1. Location map showing Yucca Mountain, Nevada Test Site boundary, 

and major calderas 2 

2. Generalized geologic map of Yucca Mountain in vicinity of drill holes 

discussed in text 4 

3. Diagram showing diagenetic zonation in drill holes USW -G2, USW -G 1, 

USW -G4, and UE25b-1H 7 

4-8. Photomicrographs of tuff showing: 

4. Well-preserved vitroclastic texture 13 

5. Clinoptilolite that grew on a thin layer of compact fibers of 

mordenite 13 

6. Large pseudomorph of a vitric particle that consists of two layers of 

fibrous mordenite 14 

7. Hollow pseudomorph of a vitric particle that consists, from margin 

to center, of smectite, mordenite, and clinoptilolite 14 

8. Spherulitic mordenite 14 

9-15. Scanning electron micrographs of tuff showing: 

9. Filiform mordenite that projects into hollow interior of a 

pseudomorph of a shard 14 

10. Filiform mordenite draped across euhedral clinoptilolite 15 

11. Tangle of filiform mordenite draped across lepispheres of opal 15 

12. Filiform mordenite draped across authigenic alkali feldspar and 

quartz 15 

13. Tangle of filiform mordenite in a pseudomorph of a pumice 

fragment 15 

14. Filiform mordenite draped on etched clinoptilolite 18 

15. Crystals of mordenite and partially dissolved clinoptilolite 18 

16. Schematic diagram showing the relationships of clinoptilolite, mordenite, 

analcime, and albite as functions of temperature, pH, and sodium ion 

concentration 21 

Contents Ill 
TABLES 

1. Stratigraphy of Miocene volcanic rocks penetrated by drill holes 5 

2. Mineralogic composition of tuffs as estimated from X-ray diffractometer 

patterns of bulk samples of core 8 

3. Chemical analyses and composition of unit cell of mordenite 16 

4. X-ray powder diffraction data for mordenite 16 

5. Cell parameters for mordenite 17 

6. Chemical analyses and chemical comparison of unaltered glass and 

mordenite-rich tuffs 19 

IV Contents 
Distribution, Characterization, and Genesis of 

Mordenite 1n Miocene Silicic Tuffs at 

Yucca Mountain, Nye County, Nevada 

By Richard A. Sheppard, Arthur J. Gude, 3d, and Joan J. Fitzpatrick 

Abstract 

Tuffs at Yucca Mountain in the southwestern Nevada 

volcanic field are being investigated as a possible deep 

repository for high-level radioactive wastes. A sequence, as 

much as about 3,000 meters thick, of Miocene silicic ash-flow 

tuffs, bedded tuffs, lavas, and flow breccias was derived chiefly from the Timber Mountain-Oasis Valley caldera complex. 

Previous studies by others of core from several drill holes have 

shown that much of the original vitric material of the volcanic 

and volcaniclastic rocks was altered during diagenesis to clay 

minerals, silica minerals, zeolites, and feldspars. Unaltered glass 

still persists in the upper part of the sequence, but zones 

characterized by clinoptilolite and mordenite, analcime, and 

albite follow in succession with depth. 

Information on the distribution, characterization, and 

genesis of mordenite in selected samples of core from four drill 

holes was obtained by transmitted light microscopy, scanning 

electron microscopy, X-ray powder diffraction, and chemical 

analysis. In samples of tuff from the clinoptilolite-mordenite 

zone, mordenite makes up 0-90 percent of the tuff and is commonly associated with clinoptilolite. Pseudomorphs of 

mordenite after shards and pumice fragments are common. 

Mordenite occurs chiefly as a tangle of filiform crystals, as 

spherulites, and as layers of compact fibers. Individual mordenite crystals are <1-1 00 micrometers long and have lengthto-width ratios of about 20-100. The mean index of refraction 

for the mordenite is 1.466-1.474, and the birefringence is low, 

about 0.003. Cell parameters show the following ranges: 

a=18.074-18.114 A, b=20.435-20.528 A, C=7.489-7.546 A, 

and V = 2,771-2,803 A 3• Two new chemical analyses of relatively pure mordenite indicate Si:AI ratios of 4.78 and 5.06 and 

Na:K ratios of 2.19 and 0.57, respectively. 

Where associated with clinoptilolite, the mordenite was 

formed earlier, later, or both earlier and later than the clinoptilolite. The chemical constitutents for some mordenite were 

derived from dissolution of the original silicic vitric material 

by ground water, but the constituents for some late-formed 

mordenite came from dissolution of clinoptilolite. Scanning 

electron microscopic examination shows the growth of filiform 

mordenite on partially dissolved clinoptilolite and is convincing textural evidence that diagenetic reactions formed 

mordenite at the expense of clinoptilolite. This is the first unequivocal evidence for this reaction in tuff at Yucca Mountain 

and the adjacent Nevada Test Site or, perhaps, anywhere else. 

Results o'f hydrothermal syntheses of mordenite from natural 

clinoptilolite by others indicate that formation of mordenite is 

favored by higher temperature, pH, or sodium ion concentration. At least some of the mordenite in the silicic tuffs beneath 

Yucca Mountain seems due to diagenesis in a relatively 

high thermal regime near the Timber Mountain caldera to the 

north. 

INTRODUCTION 

Yucca Mountain is located in southern Nevada, 

about 24 km east of the town of Beatty (fig. 1). The thick 

sequence of Miocene ash-flow tuffs beneath Yucca 

Mountain has been intensively studied from drill core 

since 1978 to determine its potential as an underground 

repository for high-level radioactive waste. These studies 

are part of the Nevada Nuclear Waste Storage Investigations Project, managed by the Nevada Operations Office of the Department of Energy. Mineralogical and 

petrological investigations of the tuffs are necessary for 

adequate characterization of the potential waste repository. Zeolites are common alteration minerals in the tuffs 

of Yucca Mountain, and could provide important natural 

barriers to the migration of certain radionuclides. In addition, the mineralogical and petrological studies provide 

information on the thermal and alteration history of the 

tuffs. 

The mineralogical and petrological investigations 

of drill cores from Yucca Mountain have been undertaken 

chiefly by the Los Alamos National Laboratory, and 

Introduction 
some of the results have been published by Sykes and 

others (1979), Bish (1981), Bish and others (1981, 1982), 

Caporuscio and others (1982), Vaniman and others 

(1984), and Carlos (1985). Our present study is restricted 

to cores from drill holes USW -G 1, USW -G2, USW -G4, 

and UE25b-1H, and our investigations of the distribution, properties, and genesis of mordenite in the tuffs supplement other ongoing mineralogical studies at the Los 

Alamos National Laboratory. 

Acknowledgments 

We thank Richard W. Spengler for facilitating our 

sampling of core at the core library in Mercury, Nev., 

and for providing unpublished descriptions of the core. 

We greatly appreciate the guidance and patience James 

M. Nishi provided during our use of the scanning electron microscope. We also thank Barbara A. Lockett for 

preparing excellent thin sections from difficult materials. 

11~7_0 ______________________________________________________________________________ ~116° 

10 

~ 

~ 

I 

L.-·-·-· 

I ~ L·---·---·---·--- · 

20 30 KILOMETERS 

NEVADA 

TEST 

SITE 

Figure 1. Location map showing Yucca Mountain, Nevada Test Site (NTS) boundary, and major calderas. Modified from 

Scott and Castellanos (1984). 

2 Mordenite in Tuffs at Y~ -: a Mountain, Nevada 
Work was performed in cooperation with the U.S. Department of Energy for the Nevada Nuclear Waste Storage 

Investigations Project (Interagency Agreement 

DE-AI08-78ET44802). 

GEOLOGIC SETTING 

Yucca Mountain is within the southwestern Nevada 

volcanic field, which is a faulted, dissected volcanic 

plateau composed mainly of silicic ash-flow tuffs and 

minor lava flows and volcaniclastic rocks of Miocene age. 

Detailed geologic maps of Yucca Mountain and adjacent 

areas have been published by Christiansen and Lipman 

(1965), Lipman and McKay (1965), and Byers, Carr, Christiansen, and others (1976). Several calderas, including the 

Timber Mountain-Oasis Valley caldera complex, the 

Crater Flat caldera complex, the Tram caldera(?), and the 

Prow Pass and Bullfrog caldera(?) (Byers, Carr, Orkild, 

and others, 1976; Christiansen and others, 1977; Carr, 

1982), lie just north and west of Yucca Mountain and are 

the sources for the volcanic rocks. Yucca Mountain consists of many north- to northwest-trending, eastward-tilted 

structural blocks that are repeated by westward-dipping, 

high-angle normal faults. The strata of Yucca Mountain 

dip mainly 5° -20° eastward. 

A generalized geologic map of Yucca Mountain in 

the vicinity of the drill holes that will be discussed in this 

report is shown in figure 2. Except for remnants of the 

Timber Mountain Tuff, the exposed bedrock in the area 

of the drill holes is the Paintbrush Tuff. Stratigraphic units 

older than the Paintbrush Tuff crop out north of this area 

and occur in the subsurface at the drill hole sites. The core 

material used in our study came from drill holes USW02, USW-01, USW-04, and UE25b-1H, located from 

north to south. Drill holes USW-03 and J-13 are located 

to the south of these four holes. Although cores from 

USW-03 and J-13 were not sampled in our study, the 

distribution of mordenite in the tuffs of these two holes 

will be discussed briefly in the section, "Genesis of 

mordenite:' in this report. 

Stratigraphy 

The general stratigraphy of the Miocene volcanic 

rocks penetrated by the drill holes at Yucca Mountain is 

summarized in table 1. The volcanic rocks are at least 

1,830 m thick and perhaps as much as 3,050 m thick, and 

they range in age from 12.5 to 15.0 m.y. (Caporuscio and 

others, 1982). Detailed descriptions of the stratigraphy, 

lithology, and mineralogy of cores from the four drill 

holes have been given by Spengler and others (1979, 1981), 

Maldonado and Koether (1983), and Spengler and Chornack (1984). The Paintbrush Tuff is commonly the uppermost volcanic unit encountered in the holes, and 

consists, in descending order, of the Tiva Canyon, Yucca 

Mountain, Pah Canyon, and Topopah Spring Members. 

The Paintbrush Tuff consists chiefly of ash-flow tuffs that 

are moderately to densely welded and are devitrified. A 

densely welded, devitrified part of the Topopah Spring 

Member has received prime consideration as the host rock 

for a radioactive waste repository. The tuffaceous beds 

of Calico Hills underlie the Paintbrush Tuff and consist 

of a sequence of interbedded nonwelded, ash-flow tuffs 

and air-fall and reworked bedded tuffs. Beneath the tuffaceous beds of Calico Hills is the Crater Flat Tuff, which 

consists, in descending order, of the Prow Pass, Bullfrog, 

and Tram Members. The Crater Flat Tuff consists chiefly of ash-flow tuffs that are nonwelded to densely welded and locally devitrified. Relatively thin units of air-fall 

and reworked bedded tuff occur at the top and bottom 

of the Crater Flat Tuff and between the various members 

of the formation. Beneath the Crater Flat Tuff, in descending order, are an unnamed dacitic to rhyodacitic lava 

and flow breccia, the Lithic Ridge Tuff, and an older, unnamed sequence of ash-flow tuff, lava, flow breccia, and 

bedded tuff of rhyolitic to dacitic composition. 

All stratigraphic units from the Pah Canyon 

Member of the Paintbrush Tuff down through the Lithic 

Ridge Tuff were sampled for our study. Because mineralogical studies by Bish and others (1982) had shown 

mordenite to be most abundant in the tuffaceous beds 

of Calico Hills and in the Crater Flat Tuff, we concentrated our sampling to core from these two units. 

ANALYTICAL METHODS 

The mineralogy of the core samples was determined 

by X-ray diffractometer analysis, using a Picker X-ray diffractometer (series 6240) 1• Samples were first ground to 

a powder, and then packed in aluminum sample holders, 

exposed to copper radiation, and scanned at 2 ° 28 per 

minute over the range of 2 Q.40° 28. Relative abundances 

of the minerals were visually estimated from the diffractometer patterns following the procedure outlined by 

Sheppard and Oude (1968, p. 2-3). Estimates are less 

reliable for samples containing glass or opal because these 

materials yield rather poor X-ray records. Unit-cell 

parameters for mordenite were determined by X-ray 

powder diffraction using a fully automated Siemens 

D-500 powder diffractometer equipped with a graphite 

monochromator that produced CuKa radiation. Data 

were collected over a range of 5 Q. 70 ° 28 at a step size of 

0.02 ° 28 and a count time o( one second. Peak positions 

and cell parameters were refined using the algorithm of 

Appleman and Evans (1973) with constrained indexing. 

1 Any use of trade names in this report is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey. 

Analytical Methods 3 
52' 

EXPLANATION 

Quaternary alluvium and colluvium 

Miocene Timber Mountain Tuff 

Miocene Paintbrush Tuff 

o Drill site 

3 KILOMETERS 

NEVADA 

Area of 

f 

I 

I 

I 

\ 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 ~) 

iJ. 

~I I 

\ 0 J-13 

I 

' ' 

Figure 2. Generalized geologic map of Yucca Mountain in vicinity of the drill holes. Modified from 

Vaniman and others (1984). 

Textures and paragenetic relationships of the 

diagenetic minerals were examined in thin sections of the 

core by transmitted-light petrographic microscopy and 

by scanning electron microscopy of freshly broken 

fragments of the core. A Cambridge Stereoscan 250 MK2 

scanning electron microscope, equipped with a Tracor 

Northern energy dispersive X-ray unit, was used to study 

4 Mordenite in Tuffs at Yucca Mountain, Nevada 

the fragments of core, which had been coated with a goldpalladium alloy. 

The chemical compositions of bulk samples and 

mordenite separates were determined by standard X-ray 

fluorescence analyses. The H20 content was determined 

by weight loss on ignition at 900 °C. The chemical composition of mordenite was also determined with an ARL 
Table 1. Stratigraphy of Miocene volcanic rocks penetrated by drill holes at Yucca Mountain, Nevada 

[Compiled from Spengler and others (1979, 1981), Maldonado and Koether (1983), and Spengler and Chornack (1984)] 

Stratigraphic unit 

Paintbrush Tuff 

Tiva Canyon Member 

Yucca Mountain Member 

Unnamed tuff 

Pah Canyon Member 

Topopah Spring Member 

Tuffaceous beds of Calico 

Hi 11 s 

Crater Flat Tuff 

Prow Pass Member 

Thickness 

(meters) 

70-163 

0-30 

0-47 

0-80 

288-363 

27-289 

82-193 

Description 

Ash-flow tuff; compound cooling unit of 

high-silica rhyolite to quartz latite; 

nonwelded base, but most is moderately to 

densely welded. 

Ash-flow tuff, rhyolitic, single cooling 

unit; nonwelded in upper and lower parts 

but moderately to densely welded in 

middle. 

Bedded and ash-flow tuff, nonwelded. 

Ash-flow tuff, rhyolitic; single cooling 

unit having upper and lower nonwelded to 

slightly welded zones and a middle 

moderately to densely welded zone. 

Ash-flow tuff, multiple; compound cooling 

unit; compositionally zoned from rhyolite 

to quartz latite; nonwelded upper and 

lower parts but moderately to densely 

welded middle. 

Ash-flow tuffs, rhyolitic, nonwelded; and 

numerous, thin, interbedded, reworked and 

ash-fa 11 tuffs. 

Ash-flow tuff, rhyolitic; nonwelded to 

slightly welded upper and lower parts but 

moderately to densely welded middle; 

abundant lithic fragments of mudstone. 

Analytical Methods 5 
Table 1. Stratigraphy of Miocene volcanic rocks penetrated by drill holes at Yucca Mountain, Nevada-Continued 

Stratigraphic unit 

Bull frog Member 

Tram Member 

Unnamed dacite lava and 

flow breccia 

Lithic Ridge Tuff 

Unnamed volcanic rocks 

and volcaniclastic 

sedimentary rocks 

Thickness 

(meters) 

89-193 

154-372 

0-249 

185-303 

>345 

electron microprobe on polished thin sections that had 

been coated with carbon. 

DIAGENETIC MINERALOGY OF THE TUFFS 

Hoover ( 1968) recognized and described a vertical 

zonation of diagenetic minerals in the Tertiary tuffs of 

the Nevada Test Site, north and east of Yucca Mountain. 

Based on mineralogical data from drill holes, tunnels, and 

outcrops, Hoover briefly described a downward succession of authigenic mineral assemblages characterized by 

glass, chabazite, clinoptilolite, mordenite, and then 

analcime. Later, Moncure and others (1981) delineated 

three vertical diagenetic zones in the Tertiary tuffs 

beneath Pahute Mesa in the northwestern part of the 

Nevada Test Site, north of Yucca Mountain. Based on 

6 Mordenite in Tuffs at Yucca Mountain, Nevada 

Description 

Ash-flow tuff, rhyolitic; variable welding 

from nonwelded to densely welded. 

Ash-flow tuff, rhyolitic; compound cooling 

unit with variable welding from nonwelded 

to densely welded; abundant lithic 

fragments of silicic volcanic rocks in 

upper part. 

Lava, flow breccia, and bedded tuff of 

rhyodacitic or dacitic composition. 

Ash-flow tuff, rhyolitic; single cooling 

unit; nonwelded to moderately welded; 

abundant lithic fragments of silicic 

volcanic rocks near middle. 

Lava, flow breccia, ash-flow tuff, bedded 

ash-fall and reworked tuff, and 

conglomerate; rhyolitic, quartz latitic, 

and dacitic in composition. 

mineralogical studies of drill core, Moncure and others 

(1981) demonstrated a downward succession of zones 

characterized by glass, clinoptilolite, and then 

analcime+ albite. Actually, on the basis of the 

mineralogical descriptions provided by them (p. 388-389), 

the lowest zone could be subdivided into an upper 

analcime zone and a lower albite zone. 

Preliminary petrologic studies of core from drill 

hole USW-01 at Yucca Mountain led Bish and others 

(1981) to recognize four diagenetic zones in the silicic 

tuffs. These zones, named Zone I through Zone IV and 

adapted from Iijima (1978), are characterized downhole 

by glass, clinoptilolite + mordenite, analcime, and then 

albite, respectively. Subsequent mineralogical studies of 

all other drill holes in the silicic tuffs at Yucca Mountain 

recognized all or a portion of the same general diagenetic 

zonation. 
The diagenetic mineral zonation for the four drill 

holes examined in our study is shown diagrammatically 

in figure 3. The four diagenetic zones were delineated on 

the basis of X-ray powder diffractometer data of bulk 

samples of core. Mineralogical data from more than 600 

samples studied at the Los Alamos National Laboratory 

and about 100 additional samples (table 2) examined during our investigation were used to construct the diagram. 

The diagenetic zones are labeled with their characteristic 

mineral(s) and are the same vertical zones that have been 

recognized elsewhere beneath Yucca Mountain. We prefer 

to use the characteristic mineralogy to name the zones 

rather than Roman numerals (1-IV) as used previously 

by the Los Alamos National Laboratory because Iijima's 

(1978) use of numerals for zonal identification was 

originally applied to thick (commonly 3-6 km thick) 

marine sequences of silicic tuffs in 1 a pan which are 

somewhat different from the terrigenous tuffs of Yucca 

Mountain, and because certain authigenic silicate 

minerals in the lower two zones in 1 a pan have not been 

recognized beneath Yucca Mountain. 

The zone boundaries shown in figure 3 were drawn 

at the first appearance in a downhole direction of a 

diagenetic mineral characteristic of the next lower zone. 

Thus, the top of the clinoptilolite-mordenite zone was 

placed at the first appearance of clinoptilolite or 

mordenite, the top of the analcime zone was placed at 

the first appearance of analcime, and the top of the albite 

zone was placed at the first appearance of albite. The 

zones vary in thickness and transgress stratigraphic 

boundaries, although the crosscutting relationships are 

not obvious in figure 3 because the members of the formations are not shown. The diagenetic zones rise in 

elevation from south to north, particularly between drill 

holes USW -01 and USW -02, a distance of only about 

2.5 km. 

In addition to the characteristic diagenetic mineral(s), 

each zone contains a variety of other diagenetic minerals 

as well as the original pyrogenic crystal (chiefly quartz, 

feldspars, and biotite) and lithic fragments. The glass 

zone commonly contains authigenic smectite and opal in 

addition to the glassy shards and pumice fragments. 

Associated authigenic minerals in the clinoptilolitemordenite zone are opal and smectite or illite/smectite, 

and, locally in the lower part of the zone, authigenic 

quartz and potassium feldspar. The analcime zone commonly contains authigenic smectite or illite/ smectite, 

quartz, potassium feldspar, and local relicts of clinoptilolite and mordenite. The albite zone commonly contains authigenic potassium feldspar, quartz, chlorite, and 

illite/smectite or illite as well as local relicts of analcime. 

Authigenic minerals that are common in shallower zones 

persist locally in the deeper zones. For example, clinoptilolite occurs in USW -01 about 780 m beneath the top 

of the analcime zone (Bish and others, 1981). 

ELEVATION 

METERS FEET 

1600 

1400 

1000 

800 

600 

400 

1000 

200 

SEA SEA 

LEVEL LEVEL 

200 

1000 

400 

USW-G2 

EXPLANATION 

Paintbrush Tuff 

Tuff of Calico Hills 

Crater Flat Tuff 

Older tuffs and flows 

Mordenite 

Figure 3. Diagram showing diagenetic zonation in drill holes 

USW-G2, USW-G1, USW-G4, and UE25b-1 H. Drill holes are 

plotted elevation true and are arranged from north (left) to south 

(right). Dashed lines represent boundaries between the 

diagenetic zones, including glass (GI), clinoptilolite-mordenite 

(Cp-Md), analcime (An), and albite (Ab). 

Distribution of Mordenite 

The distribution of mordenite in the four drill holes 

is shown diagrammatically in figure 3. Mordenite occurs 

chiefly in the Crater Flat Tuff and in the tuffaceous beds 

of Calico Hills, and ranges in abundance from trace 

amounts to more than 90 percent of the bulk tuff. The 

interval of mordenite-bearing tuff is thickest in drill hole 

USW -02 where the mordenite occurs from the middle 

of the Pah Canyon Member of the Paintbrush Tuff 

downward to the upper part of the Tram Member of the 

Crater Flat Tuff, an interval of about 930 m. The magnitude of this mordenite-bearing interval in USW -02 is 

Diagenetic Mineralogy of the Tuffs 7 
Table 2. Mineralogic composition of tuffs of Yucca Mountain, as estimated from X-ray diffractometer patterns of bulk samples 

of core 

[---,looked for but not found; Tr, trace. Mica includes pyrogenic biotite and authigenic illite. Quartz includes pyrogenic and authigenic varieties. 

Traces of calcite identified in samples Gl-3136.7, Gl-4402.7, and UE25-2887 .9. Sample number consists of two parts; abbreviated drill hole 

number preceding the dash and the depth (in feet to the nearest tenth of a foot) from which sample was collected following the dash. Stratigraphic 

units: Tpp, Pah Canyon Member of the Paintbrush Tuff; Tpt, Topopah Spring Member of the Paintbrush Tuff; Tht, tuffaceous beds of Calico 

Hills; Tcp, Prow Pass Member of the Crater Flat Tuff; Tcb, Bullfrog Member of the Crater Flat Tuff; Tct, Tram Member of the Crater Flat 

Tuff; Tlr, Lithic Ridge Tuff] 

Sample No. 

G1-1300.0 

G1-1307.7 

G1-1345.0 

G1-1360.7 

G1-1396.1 

G1-1409.9 

G1-1445.8 

G1-2202.6 

G1-2233.8 

G1-2240.5 

G1-2257.2 

G1-2269.0 

G1-2291.1 

G1-2297.8 

G1-3005.3 

Strati g rap hi c 

unit 

Tpt 

Tpt 

Tpt 

Tpt 

Tpt 

Tpt 

Tht 

Tcb 

Tcb 

Tcb 

Tcb 

Tcb 

Tcb 

Tcb 

Tct 

(./) 

(./) 

ttr 

,....-. 

~ 

10 

10 

10 

10 

10 

10 

8 Mordenite in Tuffs at Yl!cca Mountain, Nevada 

X-ray 

OJ 

+> 

+> 

u ro OJ 

0.. E 0 (./) 

Drill hole 

Tr. 

Tr. 

Tr. 

2 

2 Tr. 

1 Tr. 

Tr. 

4 Tr. 

Tr. 

analysis (parts of ten) 

~ 

OJ ro +> 0.. 

(/) 

r- -o OJ 

0 .....- (/) 

OJ OJ ro +> OJ '+- ....- +> E u 0.. s:: ...... 0 N 

0 OJ u r- +> ro s:: -o ....- ro 01 s... 

u s... ro ~ ro ro ...... ....- 0 s:: :;: r- ::::l 

:E u :E c::c: 0...... CY 

USW-G1 

Tr. 

Tr. 

Tr. 

Tr. 

Tr. 8 Tr. 

Tr. 5 3 Tr. 

1 3 4 1 

2 4 2 2 Tr. 

Tr. 4 2 

1 9 

Tr. 10 

10 Tr. 

Tr. Tr. 2 8 
Table 2. Mineralogic composition of tuffs of Yucca Mountain, as estimated from X-ray diffractometer patterns of bulk samples 

of core-Continued 

X-ray analysis (parts of ten) 

~ 

Q) tO 

+-> C-

(/) 

....- "'0 Q) 

0 ....- Vl 

Q) Q) tO 

Q) •r- +-> Q) 4- ....-

Sample No. Stratigraphic +-> +-> E u .,...... C- ~ •r- 0 N 

t/) +-> 0 Q) u ....- +-> 

unit II) u ra ~ "'0 ....- ra ~ s... ....- nif tO Q) u s... ra ~ tO ra ....- c.. E •r- ....- 0 ~ ....- 0: :::::s 

c.!:' 0 (/) ~ u :e:: c:( c:( cr 

Dri 11 hole USW-G1 

G1-3029.5 Tct Tr. 1 1 8 

G1-3044.0 Tct Tr. 4 3 3 

G1-3073.4 Tct 1 Tr. 1 8 

G1-3104.5 Tct 2 Tr. 2 6 

G1-3114.1 Tct Tr. Tr. 1 2 7 

G1-3136.7 Tct 2 Tr. 2 6 

G1-3172.8 Tct Tr. Tr. 2 8 

G1-3194.9 Tct 1 Tr. 1 2 6 

G1-4306.6 Tlr 5 Tr. Tr. 2 3 

G1-4344.7 Tl r 7 Tr. 2 1 

G1-4402.7 Tlr 5 Tr. 3 2 

Dri 11 hole USW-G2 

G2-551.6 Tpp 4 2 2 2 Tr. Tr. 

G2-560.5 Tpp 2 Tr. 1 Tr. 7 Tr. 

G2-580.3 Tpp 1 Tr. Tr. 8 1 

G2-597.3 Tpp 5 5 Tr. 

G2-616.7 Tpp 5 1 Tr. 4 Tr. 

G2-631.0 Tpp 3 1 5 1 

G2-641.9 Tpp 1 2 3 3 1 

G2-649.7 Tpp 2 4 4 

G2-1996.6 Tht Tr. 1 Tr. 8 1 

G2-2029.7 Tht 2 Tr. 4 3 1 

Diagenetic Mineralogy of the Tuffs 9 
Table 2. Mineralogic composition of tuffs of Yucca Mountain, as estimated from X-ray diffractometer patterns of bulk samples 

of core-Continued 

Sample No. 

G2-2049.6 

G2-2072.9 

G2-2090.3 

G2-2121.7 

G2-2137.5 

G2-2156.7 

G2-2204.0 

G2-3192.2 

G2-3228.2 

G2-3249.8 

G2-3259.0 

G2-3275.6 

G2-3322.4 

G2-3358.0 

G2-3383.9 

G2-3409.4 

G2-3454.6 

G2-3493.1 

Stratigraphic 

unit 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tcp 

Tcp 

Tcp 

Tcp 

Tcp 

Tcb 

Tcb 

Tcb 

Tcb 

Tcb 

Tcb 

Cl) 

Cl) 

IU 

r--

t!J 

10 Mordenite in Tuffs at Yu~~=a Mountain, Nevada 

,...... 

1'0 

c.. 

0 

Dri 11 

3 

1 

4 

2 

2 

X-ray analysis (parts 

Q) 

....., 

·~ ....., 

u 1'0 Q) u E .,.. V) ::E: 

hole USW-G2 

Tr. 

2 

Tr. 

1 

Tr. 

Tr. 

Tr. 

Tr. 

Tr. 

2 

Q) 

....., 

,.... 

0 

.,.. 

....., 

c.. 

0 

c: 

,...... 

u 

2 

2 

6 

2 

2 

Q) 

....., 

.,... 

c: 

Q) 

"'0 

s... 

0 

::E: 

8 

5 

6 

2 

6 

6 

8 

Tr. 

Tr. 

2 

Tr. 

2 

3 

of ten) 

Q) 

e 

.,.. 

u 

,.... 

1'0 

c: 

c:( 

3 

6 

1 

Tr. 

s... 

1'0 

c.. 

V) 

"'0 

,...... 

Q) 

~ 

,.... 

1'0 

~ 

,.... 

c:( 

Tr. 

1 

6 

2 

1 

2 

2 

4 

2 

4 

3 

3 

Q) 

V) 

1'0 

,.... 

u 

0 

Cl 

1'0 

,.... 

CL 

N 

....., 

s... 

1'0 

::::3 

0' 

2 

2 

4 

3 

7 

6 

6 

8 

6 

7 

3 

7 
Table 2. Mineralogic composition of tuffs of Yucca Mountain, as estimated from X-ray diffractometer patterns of bulk samples 

of core-Continued 

Sample No. 

G4-1506.4 

G4-1531.8 

G4-1546.3 

G4-1561.2 

G4-1584.6 

G4-1604.3 

G4-1625.4 

G4-1652.0 

G4-1667.7 

G4-1691.8 

G4-1709.6 

G4-1738.4 

G4-1759.0 

G4-1765.2 

G4-1788.9 

G4-1798.8 

G4-1995.2 

G4-2032.6 

G4-2061.2 

G4-2087.9 

Stratigraphic 

unit 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tht 

Tcp 

Tcp 

Tcp 

Tcp 

Tcp 

Tcp 

Tcp 

V) 

II) 

ttl 

rt!' 

....-

n:s 

0. 

c::> 

Drill 

3 

8 

1 

3 

1 

X-ray 

ClJ 

~ 

.,..... 

~ 

u 

Q) 

E 

(/') 

hole 

Tr. 

Tr. 

Tr. 

Tr. 

1 

1 

1 

Tr. 

2 

1 

2 

1 

1 

analysis (parts of ten) 

n:s 

u 

.,... 

:::£ 

USW-G4 

Tr. 

Q) 

~ 

....-

0 

....-

.,-. 

~ 

c.. 

0 

c 

....-

u 

10 

10 

8 

9 

6 

6 

6 

5 

6 

7 

5 

5 

1 

Tr. Tr. 

1 

3 

5 

ClJ 

~ 

.,... 

c 

ClJ 

-o 

s.. 

0 

:::£ 

Tr. 

2 

4 

3 

4 

4 

3 

3 

3 

1 

1 

5 

5 

9 

ClJ 

e 

.,... 

u 

....-

n:s 

c 

< 

~ 

tO 

c.. 

V) 

-o Q) 

....- V) 

ClJ tO ~ ....-

u 

0 ....-

n:s C") 

~ n:s ....- ....- < c... 

1 

Tr. 

2 

2 

4 

8 1 

5 

6 1 

Tr. 

Diagenetic Mineralogy of the Tuffs 

N 

~ 

s.. 

tO 

::::s 

CT 

2 

2 

11 
Table 2. Mineralogic composition of tuffs of Yucca Mountain, as estimated from X-ray diffractometer patterns of bulk samples 

of core-Continued 

X-ray analysis (parts of ten) 

s.... Q) lt1 

+> 0-

V'l 

.....- "'0 Q) 

0 .....- V'l 

Q) Q) lt1 

Q) +> Q) 4- .....-

Sample No. Strati g rap hi c +> +> E u 0- s:: .,... 0 N 

II) +> 0 Q) u .....- +> 

unit II) ...- u 10 s:: "'0 .....- 10 C") ~ 

IU ctf Q) u 5- 10 ~ lt1 10 

c:; c.. E .,... .....- 0 s:: .....- 'Cl: :::::1 

C> V> :::E: u :::E: <: <: CY 

Dri 11 hole USW-G4 

G4-2117.6 Tcp 7 3 

G4-2137.2 Tcp Tr. 9 1 

G4-2164.0 Tcp 6 4 

G4-2189.3 Tcp 5 5 

G4-2209.4 Tcp 1 4 5 

G4-2239.1 Tcp 1 7 2 Tr. 

G4-2252.0 Tcb Tr. 8 2 

G4-2709.6 Tcb 1 9 Tr. Tr. 

G4-2734.2 Tcb Tr. 5 5 

G4-2757.6 Tct 1 6 3 

G4-2764.7 Tct 2 2 6 

G4-2806.5 Tct 3 2 5 

G4-2818.4 Tct 5 3 2 

G4-2830.0 Tct 2 2 6 

G4-2849.2 Tct 1 Tr. 2 7 

Dri 11 hole UE25b-1H 

UE25-2805.1 Tcb 3 2 5 

UE25-2829.6 Tcb 1 5 4 

UE25-2846.7 Tcb 4 4 2 

UE25-2871.7 Tcb 2 8 

UE25-2887.9 Tct Tr. 8 2 

UE25-2900.4 Tct 9 1 

12 Mordenite in Tuffs at Yucca Mountain, Nevada 
misleading, however, because the mordenite does not occur throughout the entire interval. For example, much 

of the Topopah Spring Member of the Paintbrush Tuff 

(about 170 m) and the upper part of the Prow Pass 

Member of the Crater Flat Tuff (about 70 m) consist of 

moderately to densely welded tuff that was devitrified or 

vapor-phase altered shortly after emplacement 

(Caporuscio and others, 1982). Because the original glass 

had already been altered prior to the zeolitization, these 

two parts of the "mordenite-bearing" interval consequently lack not just mordenite, but any zeolite. In drill 

holes USW -G 1 and USW -G2, mordenite persists downward well into the upper part of the analcime zone. 

Description of Mordenite 

Mordenite can be readily distinguished from the 

commonly associated clinoptilolite by its characteristic 

fibrous habit that is observable in thin sections and in 

scanning electron micrographs. Individual mordenite 

fibers range in length from less than 1 to about 100 J.Lm. 

The fibers are very delicate and commonly have lengthto-width ratios of 20-100. The indices of refraction are 

difficult, if not impossible, to measure because of the 

fibrous nature of the mordenite and the extremely small 

crystal size; the mean index of refraction, however, is in 

the range of 1.466-1.474. The birefringence is low, about 

0.003. 

Mordenite in the tuffs at Yucca Mountain has three 

common modes of occurrence: (1) pseudomorphs of 

original vitric material, (2) spherulites or radial aggregates 

of fibers, and (3) filiform or threadlike fibers that occur 

singly or entwined. The pseudomorphs of original vitric 

particles commonly consist of a marginal film of smectite succeeded inwardly by a single layer (6-25 J.Lm thick) 

of compact fibers of mordenite that grew perpendicular 

to the wall of the shard. Many of the pseudomorphs are 

hollow (figs. 4 and 5), but others have a central cavity 

that is filled or partly filled by a second generation of 

mordenite (fig. 6), clinoptilolite (figs. 5 and 7), or opal. 

The second-generation mordenite commonly has a higher 

mean index of refraction than the earlier, marginal 

mordenite. Spherulites of mordenite (fig. 8) are as large 

as 250 J.Lm in diameter, although most are 50-100 J.Lm in 

diameter. Spherulitic mordenite seems to be most common in the larger vitric particles. Filiform mordenite, 

which is observable only in scanning electron micrographs 

(figs. 9-13), occurs as delicate, straight or curved, threadlike fibers that commonly drape across or bridge earlier 

diagenetic minerals such as clinoptilolite (fig. 10) and opal 

(fig. 11) or devitrification minerals such as alkali feldspar 

and quartz (fig. 12). More rarely, the filiform mordenite 

occurs as an indescribable tangle of mostly curved fibers 

(fig. 13). 

Figure 4. Photomicrograph of mordenite-rich tuff (sample 

G1-2257.2) from Bullfrog Member of the Crater Flat Tuff in drill 

hole USW-G1, showing well-preserved vitroclastic texture. 

Pseudomorphs of shards are defined by a layer of compact 

fibers of mordenite (M). Some pseudomorphs are hollow (H). 

Figure 5. Photomicrograph of mordenite-rich tuff (sample 

G1-2291.1) from Bullfrog Member of the Crater Flat Tuff in drill 

hole USW-G1, showing well-preserved vitroclastic texture. 

Pseudomorphs of original vitric materials are defined by a layer 

of compact fibers of mordenite (M). Large pseudomorph in the 

central part of the photo shows relatively large, blocky crystals 

of clinoptilolite (C) that grew on a thin layer of compact fibers 

of mordenite. 

Chemical Composition of Mordenite 

Mordenite was separated from mordenite-rich 

samples of the tuffaceous beds of Calico Hills and of the 

Bullfrog Member of the Crater Flat Tuff by first crushing 

the tuffs and then disaggregating them in an ultrasonic 

bath. The mordenite was then concentrated by repeated 

gravity separations in a heavy-liquid mixture of bromoform and acetone. The final mordenite concentrates contained about 2-5 percent smectite, clinoptilolite, and 

hydrous iron oxide impurities. 

Diagenetic Mineralogy of the Tuffs 13 
Figure 6. Photomicrograph of mordenite-bearing tuff (sample G2-3454.6) from Bullfrog Member of the Crater Flat Tuff 

in drill hole USW-G2, showing large pseudomorph of a vitric 

particle that is defined by a relatively thin layer of compact fibers 

of mordenite (M) followed inwardly by a second layer of coarser 

mordenite fibers (M2). A discontinuous film of smectite (S) outlines the pseudomorph. Matrix consists of authigenic quartz (Q). 

Figure 7. Photomicrograph of mordenite-bearing tuff (sample G2-2029.7) from the tuffaceous beds of Calico Hills in drill 

hole USW-G2, showing a hollow pseudomorph of a relatively large vitric particle. From margin to center, the pseudomorph 

consists of a discontinuous film of smectite (S), a thin layer of 

compact fibers of mordenite (M), and blocky crystals of clinoptilolite (C) that project into the hollow interior. 

The chemical analyses by X-ray fluorescence and 

the contents of the unit cell based on 96 oxygen atoms 

are given in table 3. Both mordenites that were sampled 

are siliceous; the Si:Al ratios are 4. 78 and 5.06. Monovalent cations exceed divalent ones for both samples. The 

Na:K ratio for the mordenite from the tuffaceous beds 

of Calico Hills is about 0.57, but the Na:K ratio for 

mordenite from the Bullfrog Member of the Crater Flat 

Tuff is about 2.19. The potassium content is unusually 

14 Mordenite in Tuffs at Yucca Mountain, Nevada 

Figure 8. Photomicrograph of mordenite-bearing tuff (sample G2-3454.6) from Bullfrog Member of the Crater Flat Tuff 

in drill hole USW-G2, showing spherulitic mordenite. Crossed 

nicols. 

Figure 9. Scanning electron micrograph of sample 

UE25-2900.4 from the tuffaceous beds of Calico Hills in drill 

hole UE25b-1 H, showing compact layer offiliform mordenite 

that projects into the hollow interior of a pseudomorph of a 

shard. 

high (Passaglia, 1975) in both samples, but especially so 

in the mordenite from the tuffaceous beds of Calico Hills 

where potassium represents about 41 percent of the exchangeable cations. 

Electron microprobe analyses of other mordenitebearing samples were attempted without much success. 

All the electron microprobe analyses have low totals, and 

the analyzed content of sodium seems especially low. The 

electron microprobe analyses do suggest, however, a 

range of Si:Al ratios of about 4.8-5.2. Three electron 

microprobe analyses of mordenite from the Prow Pass 

Member of the Crater Flat Tuff, reported by Caporuscio 

and others (1982, p. 90), show that the Si:Al ratio is 
Figure 10.. Scanning electron micrograph of sample 

G2-2029.7 from the tuffaceous beds of Calico Hills in drill hole 

USW-G2, showing filiform mordenite draped across euhedral 

clinoptilolite. 

Figure 11. Scann ing electron micrograph of sample 

G2-2029.7 from the tuffaceous beds of Calico Hills in drill hole 

USW-G2, showing a tangle of filiform mordenite draped across 

lepispheres of opal. 

4.86-5.11 and that sodium is greatly in excess of the other 

exchangeable cations. 

Thus, mordenite from the tuffs of Yucca Mountain has a range in Si:AI ratios of about 4.8-5.2 and is 

alkali rich. Some mordenite is potassic, but the stratigraphic or diagenetic significance of such is unknown. 

X-ray Powder Diffraction Data for Mordenite 

An indexed powder pattern for mordenite from the 

Bullfrog Member of the Crater Flat Tuff is given in table 

4, and cell parameters for five mordenites from the tuffaceous beds of Calico Hills and the Crater Flat Tuff are 

Figure 12. Scanning electron micrograph of sample 

G2-3454.6 from Bullfrog Member of the Crater Flat Tuff in drill 

hole USW-G2, showing filiform mordenite draped across 

authigenic alkali feldspar (F) and quartz (Q). 

Figure 13. Scanning electron micrograph of sample 

G 1-2291.1 from Bullfrog Member of the Crater Flat Tuff in drill 

hole USW-G1, showing a tangle of filiform mordenite in a 

pseudomorph of a pumice fragment. 

given in table 5. The cell parameters show the following 

ranges: a= 18.074-18.114 A, b=20.435-20.528 A, 

c=7.489-7.546 A, and V=2,771-2,803 A3• Except for 

slightly smaller and slightly larger c dimensions, these 

ranges for the Yucca Mountain mordenites are within the 

ranges reported by Passaglia (1975) for 35 mordenites 

from a variety of rock types and localities. 

Passaglia's (1975) crystal chemical study of mordenites demonstrated a negative correlation between the 

b dimension and the Si:Si +AI+ FeH ratio, such that 

the Si:Si+Al+FeH±0.0113=8.12391-0.35632 b. The 

two analyzed samples (01-2291.1 and 02-1996.6) from 

Yucca Mountain confirm this relationship. The 

Si:Si+Al+FeH ratio for sample 01-2291.1 from the 

Diagenetic Mineralogy of the Tuffs 15 
Table 3. Chemical analyses and composition of unit cell of Table 4. X-ray powder diffraction data for mordenite from 

mordenite from Yucca Mountain, Nevada Yucca Mountain, Nevada 

[Sample No. Gl-2291.1 from Bullfrog Member of the Crater Flat Tuff 

G1-2291.1l G2-1996.62 in drill hole USW-Gl. Chemical analysis for sample given in table 3] 

Chemical analyses hk 1 .9..calc(A) ~bs (A) 

[Analyses by x-ray fluorescence by A.J. Bartel. H20 

determined by loss on ignition at 900 °C] 110 13.57 13.60 2 

020 10.26 10.06 15 

Si02 68.2. 68.6 200 9.05 9.03 72 

A1 203 12.1 11.5 111 6.60 6.61 35 

130 6.40 6.41 14 

Fe2o3 0.92 0.67 

021 6.08 6.09 3 

MgO 0.13 0.44 310 5.79 5.81 8 

CaO 2.02 2.54 131 4.88 

311 4.59 

Na 2o 3.24 1.18 330 4.53 4.54 26 

K20 2.26 3.16 240 4.46 

Ti02 0.08 0.05 420 4.14 4.15 9 

150 4.004 3.991 58 

P205 <0.05 <0.05 241 3.842 3.841 12 

MnO 0.09 0.03 002 3. 773 3. 771 2 

H20 10.8 11.5 421 3.629 3.628 3 

Total 99.84 99.67 510 3.564 3.559 4 

022 3.541 

Composition of unit cell 202 3.482 3.479 100 

[Calculated on the basis of 96 oxygen atoms] 060 3.421 

Si 39.50 39.96 350 3.394 3.395 40 

Al 8.26 7.90 222 3.298 

511 3.223 3.229 77 

Fe3+ 0.40 0.30 530 3.199 3.194 47 

Mg 0.12 0.38 312 3.161 

Ca 1.26 1.58 061 3.116 

441 3.095 

Na 3.64 1.34 042 3.040 

K 1.66 2.34 600 3.016 

261 2.946 

H20 20.86 22.34 

402, 332 2.898 2.897 27 

Si :Al 4.78 5.06 152 2.746 

l G1- 2291.1--Lab. No. 0260299; separated from the Bullfrog 550 2.715 

171 2.702 2.702 2 

Member of the Crater Flat Tuff in d ri 11 hole USW-Gl. 370 2.637 2.626 1 

2G2-1996.6--Lab. No. 0260303; separated from the tuffaceous 

512 2.590 2.589 7 

beds of Calico Hills in drill hole USW-G2. 080 2.566 2.567 7 

352 2.523 2.523 17 

641 2.458 

chemical analysis (table 3) is 0.82, and by calculation from 532 2.440 

the b dimension is 0.81±0.01. The Si:Si+Al+FeH 

ratio for sample 02-1996.6 from the chemical analysis 262 2.441 

is 0.83 and by calculation from b is 0.83 ± 0.01. Inasmuch 622 2.296 

570 2.279 

as two mordenites in table 5 have a b dimension smaller 800 2.262 2.261 2 

than that for sample 02-1996.6, these mordenites prob- 480 2.232 

ably are more siliceous than sample 02-1996.6. 

661 2.167 2.168 4 

082 2.122 2.123 2 

Paragenesis 0"10"0 2.053 2.053 3 

732 2.036 2.037 3 

The relative age relationships of the diagenetic 443 2.021 

silicate minerals in the tuffs can be determined by the 841 1.996 1.995 2 

16 Mordenite in Tuffs at Yucca Mountain, Nevada 
Table 5. Cell parameters for mordenite from Yucca Mountain, Nevada 

Sample No. ~(A) .!?_(A) .£(A) J_(A 3) 

G1-2291. P 18.096(8) 20.528(9) 7.546{6) 2803{2) 

G2-1996.62 18.074( 27) 20.470(31) 7.489(13) 2771(6) 

G4-2137.23 18.095(12) 20.435(13) 7.527(6) 2783(2) 

G4-2709.64 18.087(15) 20.439(10) 7.521(5) 2780(3) 

UE25-2900.45 18.114(22) 20.497( 17) 7.521(8) 2792(4) 

lG1-2291.1--Bullfrog Member of the Crater Flat Tuff in drill hole USW-G1. 

2G2-1996.6--Tuffaceous beds of Calico Hills in dri 11 hole USW-G2. 

3G4-2137.2--Prow Pass Member of the Crater Flat Tuff in drill hole USW-G4. 

4G4-2709.6--Bullfrog Member of the Crater Flat Tuff in drill hole USW-G4. 

5UE25-2900.4--Tram Member of the Crater Flat Tuff in drill hole UE25b-1H. 

sequence of filling of voids and once-hollow 

pseudomorphs of vitric particles and by replacement relations. The mineral in the interior of a void or 

pseudomorph presumably formed later than the 

mineral(s) nearer the periphery. The following sequences 

of crystallization involving mordenite were compiled from 

thin-section examination and scanning electron 

microscopy: 

Paragenesis of diagenetic silicate minerals 

[Earliest mineral listed on left] 

Smectite - clinoptilolite - mordenite 

Smectite - mordenite - clinoptilolite 

Smectite - mordenite - clinoptilolite - mordenite 

Smectite - mordenite - clinoptilolite - opal 

Smectite - mordenite - quartz 

Mordenite - clinoptilolite - analcime+ quartz 

Smectite - mordenite - analcime+ quartz 

These relationships indicate that smectite was consistently 

the earliest mineral to form and that mordenite was commonly the earliest zeolite to crystallize in the mordenitebearing tuffs. 

Scanning electron micrographs of the tuffs that 

contain clinoptilolite and mordenite commonly show that 

filiform mordenite postdates the clinoptilolite (fig. 10). 

Unlike the clinoptilolite shown in figure 10, some clinoptilolite was affected by dissolution prior to the crystallization of the filiform mordenite as clearly indicated by the 

scanning electron micrographs in figures 14 and 15. 

Figure 14 is a micrograph from the tuffaceous beds of 

Calico Hills in drill hole USW -G2 and shows filiform 

mordenite draped on clinoptilolite that had been etched 

before the crystallization of the mordenite. More severe 

dissolution of clinoptilolite is evident in the figure 15 

micrographs from the Bullfrog Member of the Crater Flat 

Tuff in drill hole USW -G 1. Figure 15A shows a large 

clinoptilolite crystal, similar to that illustrated in figure 

5, that grew on a mat of compact fibers of mordenite. 

The clinoptilolite has obviously been affected by dissolution, and minute filiform crystals of mordenite grew on 

the dissolution surface (fig. 15B). Neither the dissolution 

of the clinoptilolite nor the second generation of 

mordenite could be seen in thin sections of the same 

sample. Similar dissolution of clinoptilolite followed by 

crystallization of filiform mordenite has also been recognized in the Pah Canyon Member of the Paintbrush Tuff 

in drill hole USW -G2. 

Thus, evidence from the study of thin sections and 

scanning electron micrographs indicates two generations 

of mordenite in some tuffs at Yucca Mountain. At least 

locally, the second generation of filiform mordenite 

crystallized after the associated clinoptilolite had been affected by dissolution. 

CHEMICAL CHANGES DURING ZEOLITIZATION 

Chemical analyses of bulk samples of mordeniterich tuff from tuffaceous beds of Calico Hills and the 

Crater Flat Tuff are given in table 6, along with a chemical analysis of fresh glass from the Topopah Spring 

Member of the Paintbrush Tuff. In addition to the 

obvious and significant increase in the H20 content of 

the mordenite-rich tuffs, other approximate changes in 

chemical composition from fresh glass to zeolitic tuff 

were inferred by assuming that the Al20 3 content remained constant during diagenetic alteration, although 

this assumption has not been rigorously tested. Gains 

and losses of chemical components are based on the 

Chemical Changes During Zeolitization 17 
Figure 14. Scanning electron micrograph of sample 

G2-2072.9 from the tuffaceous beds of Calico Hills in drill hole 

USW-G2, showing filiform mordenite draped on clinoptilolite 

that had been etched prior to the crystallization of the 

mordenite. 

assumption that the original compositiOn of the 

mordenite-rich tuff was similar to the composition of the 

glass from the Topopah Spring Member. Numerous 

analyses of glass by Vaniman and others (1984) from the 

various stratigraphic units at Yucca Mountain indicate 

that this is a valid assumption. The chemical gains and 

losses for the mordenite-rich tuffs given in table 6 were 

determined by (1) recalculation of all the analyses to the 

anhydrous form and normalizing to 100 percent, (2) then, 

recalculation of these normalized analyses of the zeolitic 

tuffs, assuming constant Al20 3 content of the fresh 

glass, and (3) finally, comparison of the Al2

0 3-constant 

analyses with the anhydrous analysis of the fresh glass. 

The mordenite-rich tuffs differ significantly in composition from the fresh glass. Relative to the unaltered 

glass, all the mordenite-rich tuffs show consistent gains 

of MgO and CaO and losses of Na20, K20, and Si02 • 

Most mordenite-rich tuff has also gained Fe20 3• The 

most obvious chemical changes in the mordenite-rich tuffs 

are the gains of alkaline earth elements, particularly 

calcium, and the losses of alkalis. Similar gains and losses 

have been noted for the diagenetic alteration of silicic 

glass to mainly clinoptilolite in the John Day Formation 

of Oregon (Hay, 1963) and the Ricardo Formation of 

California (Sheppard and Gude, 1965). 

GENESIS OF MORDENITE 

Previous studies by others indicated that the formation of zeolites and associated silicate minerals in the 

tuffs at Yucca Mountain and adjacent areas and the vertical arrangement of the diagenetic zones are due to the 

18 Mordenite in Tuffs at Yucca Mountain, Nevada 

Figure 15. Scanning electron micrographs of sample 

Gl-2291.1 from the Bullfrog Member of the Crater Flat Tuff 

in drill hole USW-G1. A, Large, blocky crystal of clinoptilolite 

that grew on a mat of compact filiform mordenite. Clinoptilolite 

shows obvious effects of dissolution. B, Close-up of clinoptilolite 

showing minute crystals of filiform mordenite that grew from 

the dissolution surface of the clinoptilolite. 

alteration of silicic glass in a ground-water ·system. 

Hoover (1968) attributed the diagenetic mineral zonation 

to differences in the chemistry of the pore water, but more 

recent investigators (Moncure and others, 1981; Bish and 

others, 1982; Caporuscio and others, 1982) have explained the zonation by a regime of changing pore-water 

chemistry during diagenesis in conjunction with increased 

burial and higher temperatures. 

The initial chemistry of the ground water and, 

hence, the pore water, was modified greatly by dissolution of the highly soluble silicic glass in the tuffs. Continued dissolution of the glass elevated the salinity and 

pH of the pore water and provided a favorable chemical 

environment for the crystallization of zeolites in the 

clinoptilolite-mordenite zone. Dissolution of the silicic 
C') 

~ 

= ~ 

Cll 

;;;· 

a 

Table 6. Chemical analyses and chemical comparison of unaltered glass and mordenite-rich tuffs from Yucca Mountain, Nevada 

[Analyses by X-ray fluorescence by A.J. Bartel, K.C. Stewart, and J .E. Taggart. H2

0 determined by loss on ignition at 900 OC. A, uncorrected analysis in weight percent; B, analysis recalculated 

to anhydrous and normalized to 100 percent; C, gains ( +) and losses (-) in weight percent assuming the constant Al20 3 value of the unaltered glass (analysis lB)] 

Si02 

Al 203 

Fe2o3 

MgO 

GaO 

Na 20 

K20 

Ti02 

P205 

MnO 

H20 

1 

A B 

74.8 77.70 

11.9 12.36 

0.80 0.83 

<0.10 

0.50 0.52 

3.31 3.44 

4.83 5.02 

0.07 0.07 

<0.05 

0.06 0.06 

3.49 

Total 99.76 100.00 

2 

A B c 

69.2 77.80 -3.96 

11.6 13.04 

0.71 0.80 -0.03 

0.39 0.44 +0.44 

2.46 2.76 +2.74 

1.21 1.36 -2.15 

3.24 3.64 -1.57 

0.06 0.07 0.00 

<0.05 --- 0.00 

0.07 0.08 +0.02 

11.0 

99.94 99.99 

3 

A B c 

67.6 75.62 -9.77 

12.3 13.76 

1.22 1.36 +0.39 

0.13 0.14 +0.13 

2.49 2.78 +1.98 

2.32 2.60 -1.10 

3.22 3.60 -1.79 

0.11 0.12 +0.04 

<0.05 --- 0.00 

<0.02 -0.06 

9.70 

99.09 99.98 

4 

A B c 

68.0 75.47 -10.45 

12.5 13.87 

1.21 1.34 +0.36 

0.15 0.17 +0.17 

2.04 2.26 +1.49 

3.40 3.77 -0.08 

2.38 2.64 -2.67 

0.12 0.13 +0.05 

<0.05 --- 0.00 

0.30 0.33 +0.23 

9.62 

99.72 99.98 

5 

A B c 

68.1 76.41 -7.54 

12.0 13.46 

1.20 1.35 +0.41 

0.13 0.15 +0.14 

2.75 3.08 +2.31 

2.78 3.12 -0.58 

2.02 2.27 -2.94 

0.12 0.13 +0.05 

<0.05 --- 0.00 

0.02 0.02 -0.04 

10.6 

99.72 99.99 

6 

A B c 

67.9 74.57 -12.66 

12.9 14.17 

1.48 

0.38 

2.35 

2.18 

3.57 

0.19 

0.05 

0.05 

8.46 

1.62 +0.58 

0.42 +0.37 

2.58 +1. 73 

2.39 -1.36 

3.92 -1.60 

0.21 +0.11 

0.05 +0.04 

0.05 -0.02 

99.51 99.98 

1. Unaltered glass; sample No. G1-1307.7; separated from the Topopah Spring Member of the Paintbrush Tuff from drill hole USW-G1. 

2. Mordenite-rich tuff; sample No. G2-1996.6; bulk sample of the tuffaceous beds of Calico Hills from drill hole USW-G2. 

~ 3. Mordenite-rich tuff; sample No. G4-2137.2; bulk sample of the Prow Pass Member of the Crater Flat Tuff from drill hole USW-G4. 

a 

~ 4. Mordenite-rich tuff; sample No. G1-2291.1; bulk sample of the Bullfrog Member of the Crater Flat Tuff from drill hole USW-G1. 

~ 

5. Mordenite-rich tuff; sample No. G4-2709.6; bulk sample of the Bullfrog Member of the Crater Flat Tuff from drill hole USW-G4. 

~ 6. Mordenite-rich tuff; sample No. UE25-2900.4; bulk sample of the Tram Member of the Crater Flat Tuff from drill hole UE25b-1H. 
glass certainly enriched the pore water in alkalis and silica. 

Inasmuch as the original compositions of the glass from 

the various stratigraphic units at Yucca Mountain are 

similar, the original glass composition probably played 

no role in the arrangement of the diagenetic zones or in 

the composition of the individual diagenetic minerals. The 

highly silicic composition of the original glass did, 

however, dictate the ultimate crystallization of the highly 

siliceous zeolites, clinoptilolite and mordenite. Mariner 

and Surdam (1970) showed experimentally that the pH 

of the pore water also influences the Si:Al ratio of the 

pore water and the Si:Al ratio of the zeolite precipitate. 

A pH in the range of 7-9 would probably favor crystallization of siliceous zeolites such as clinoptilolite or 

mordenite rather than an aluminous zeolite such as 

phillipsite. Experiments with natural silicic glasses led 

Phillips (1983) to conclude that mordenite is generally 

favored by mildly alkaline solutions. 

Much of the mordenite in the clinoptilolitemordenite zone at Yucca Mountain is a direct alteration 

product of the silicic glass, and dissolution of the glass 

provided the essential components for this mordenite. 

Petrographic evidence indicates that at least some of the 

pseudomorphs of vitric particles formed by crystallization of mordenite in cavities from which the glass had 

been dissolved. The still-hollow pseudomorphs (figs. 4, 

5, and 7) are difficult to explain in any other way. Other 

mordenite replacements probably were contemporaneous 

with dissolution of the glass, but convincing evidence is 

lacking. As suggested by some of the scanning electron 

micrographs (figs. 14 and 15), some relatively late 

mordenite postdates dissolution of clinoptilolite, and the 

chemical components for this mordenite may have been 

derived from the clinoptilolite. 

The following brief summary of the synthesis of 

mordenite provides a foundation for a discussion of the 

crystallization of mordenite versus clinoptilolite. 

Mordenite was apparently first synthesized by Barrer 

(1948), who established that this zeolite readily crystallized from sodium aluminosilicate gels in the temperature 

range of 265-295 °C. Since then, Barrer and other 

investigators synthesized mordenite from a variety of 

starting materials and chemical systems and over a 

temperature range of about 75-400 °C (Senderov, 1963; 

Seki, 1973; Barrer, 1982). The first mordenite synthesis 

from natural glass was accomplished by Ellis (1960) using powdered silicic obsidian that was suspended in the 

natural hydrothermal fluid of a bore hole at Wairakei, 

New Zealand. More recently, laboratory syntheses of 

mordenite from natural silicic glasses have been reported 

by Wirsching (1976), Hawkins and others (1978), Kirov 

and others (1979), and Phillips (1983). These experimental studies indicated that mordenite is favored over clinoptilolite by higher temperatures and higher Na:K ratios of 

the system. Using natural clinoptilolite as a starting 

20 Mordenite in Tuffs at Yucca Mountain, Nevada 

material in a sodium system, Kusakabe and others (1981) 

found that clinoptilolite reacted to form mordenite under 

conditions of increased temperature, pH, and (or) Na + 

concentration, as shown schematically in figure 16. The 

hydrothermal conversion of clinoptilolite to mordenite 

was accomplished by Kusakabe and others (1981) over 

ranges of temperature of 170-365 °C, pHs of 5.3-12.23, 

and Na + concentrations of 40-2,300 ppm. The conversion to mordenite shifted to relatively higher temperatures 

with a decrease in Na+ concentration or pH. 

We have no direct evidence as to which of the above 

factors evaluated by Kusakabe and others (1981) may 

have caused the conversion of clinoptilolite to mordenite 

in the tuffs of Yucca Mountain, but some circumstantial 

evidence points to elevated temperatures. Drill hole 

USW -02, the northernmost hole studied, is only about 

8 km south of the Timber Mountain caldera, which 

postdates all the stratigraphic units penetrated by the drill 

holes. Compared to the tuffs in the other drill holes we 

studied, tuffs in USW -02 show certain effects suggestive 

of exposure to higher temperatures. All the boundaries 

between the diagenetic zones rise in elevation from south 

to north (toward the Timber Mountain caldera), especially from drill hole USW -01 to USW -02 (fig. 3). In addition, the analcime zone is significantly thinner in 

USW -02 than in USW -01. The increase in the amount 

of interstratified illite in authigenic illite/smectite with 

depth and the occurrence of illite in the lower part of 

USW -02 are suggestive of higher temperatures 

(180-230 °C) in USW-02 than in USW-01 (Caporuscio 

and others, 1982). The thickness of the mordenite-bearing 

interval is greater in USW -02 than in any of the other 

drill holes to the south. In fact, mordenite occurs only 

rarely in the Bullfrog and Tram Members of the Crater 

Flat Tuff in drill hole USW-03 (Vaniman and others, 

1984), about 8 km south of USW -02, and has not been 

reported at all in drill hole J -13 (Heiken and Bevier, 

1979), about 11 km southeast of USW-02. Finally, 

minerals characteristic of hydrothermal alteration, such 

as barite, fluorite, pyrite, and epidote, occur in parts of 

the Crater Flat Tuff and in stratigraphically lower units 

in drill hole USW -02 (Caporuscio and others, 1982; 

Maldonado and Koether, 1983). 

Corroborative data relating to the role of temperature in the conversion of clinoptilolite to mordenite are 

provided by hydrothermal experiments that were designed 

to simulate repository conditions for high-level nuclear 

waste (Blacic and others, 1982). Samples of clinoptiloliterich tuff from Yucca Mountain showed the reaction of 

clinoptilolite to form mordenite after prolonged hydrothermal exposure at 120 and 180 °C (Bish and others, 

1982). 

Mineralogical studies of silicic volcanic rocks from 

other areas also seem to show a relationship between 

temperature and the distribution of clinoptilolite and 
mordenite. Drill hole Y -6 in Yellowstone National Park, 

Wyo. (Bargar and Beeson, 1984), penetrated the Pleistocene Plateau Rhyolite which has a variety of hydrothermal minerals along fractures and in cavities. Clinoptilolite 

occurs only in the upper 30 m of the core, but mordenite 

persists to a depth of about 130 m where a temperature 

of about 170 °C was measured. Surdam (1985) studied 

the alteration of rhyolitic tuffs in the Miocene Obispo 

Formation of California and noted that the distribution 

of clinoptilolite and mordenite is related to the proximity of intrusive bodies. Mordenite occurs in the tuffs 

closest to the intrusives (higher temperature), and clinoptilolite occurs more distant from the intrusives (lower 

temperature). 

Thus, at least some of the mordenite in the silicic 

tuffs beneath Yucca Mountain seems due to diagenesis 

in a relatively high thermal regime near the Timber Mountain caldera to the north. This conclusion does not, 

however, mitigate the importance of the pore-water 

chemistry during diagenesis of the tuffs. 

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I 

c. 

I 

Clinoptilolite 

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