FOLD 
Reconnaissance Study of the Geology of 

U.S. Vermiculite Deposits—

Are Asbestos Minerals Common Constituents?

By Bradley S. Van Gosen, Heather A. Lowers, Alfred L. Bush, 

Gregory P. Meeker, Geoffrey S. Plumlee, Isabelle K. Brownfield, 

and Stephen J. Sutley

U.S. Geological Survey Bulletin 2192

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

Version 2.0, 2005

This publication is only available online at:

http://pubs.usgs.gov/bul/b2192/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not

imply endorsement by the U.S. Government

Manuscript approved for publication May 7, 2002

Published in the Central Region, Denver, Colorado

Graphics by the author

Photocomposition by Carol A. Quesenberry

Edited by L.M. Carter
Contents

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

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

The Vermiculite-Asbestos Issue .................................................................................................................1

Regulatory Definitions of Amphibole Asbestos ........................................................................................2

Study Goals.....................................................................................................................................................3

The Geology and Uses of Vermiculite ........................................................................................................3

Vermiculite Geology .............................................................................................................................3

Vermiculite Uses and Production.......................................................................................................3

Amphibole Mineralogy of the Libby Vermiculite Deposit........................................................................4

USGS Reconnaissance Study......................................................................................................................4

Conclusions.....................................................................................................................................................6

References Cited............................................................................................................................................6

Figures

 1. Map showing location of the Libby (Rainy Creek, Zonolite) mine near Libby, Mont. ................2

 2. Photograph showing examples of raw vermiculite and processed

 (heat-expanded) material....................................................................................................................4

3–5. Scanning electron microscope photographs showing:

 3. Vermiculite ore material collected from an open pit of the Libby mine..................................5

 4. Vermiculite-rich material collected from Gem Park Complex, Colo........................................6

 5. Sample collected from Smith mine deposit, Wyoming..............................................................7

Table

 1. Ideal compositions for the fibrous amphibole minerals found in U.S. vermiculite deposits....5

iii
Introduction    

Abstract

Unusually high incidences of asbestos-related mortality 

and respiratory disease in the small town of Libby, Montana, 

have been linked to amphibole mineral fibers intergrown with 

the vermiculite deposits mined and milled near the town from 

1923 to 1990. A study conducted by the U.S. Agency for 

Toxic Substances and Disease Registry concluded that mortality due to asbestosis in Libby mine and mill workers and residents during 1979 to 1998 was much higher than expected for 

a similar Montana or United States population group. Recent 

medical testing of past and present mineworkers and residents 

of Libby showed lung abnormalities in nearly one-fifth of the 

adult study participants. The U.S. Environmental Protection 

Agency, under Superfund authority, is completing sampling 

and cleanup of asbestos-bearing materials in the mine, mill, 

and town sites. The U.S. Geological Survey is conducting a 

study, reviewed herein, to investigate the mineral content of 

other U.S. vermiculite deposits and to determine if the amphibole asbestos minerals like those found in the Libby deposits 

are common in other vermiculite deposits.

Introduction

Vermiculite is a general term applied to a group of platy 

minerals that form from the weathering of micas by ground 

water. Their distinctive characteristic is a prominent accordion-like unfolding and expansion when heated to between 

800° and 1,100°C, depending on the composition and content 

of the vermiculite-bearing material. After processing (heat 

expansion), the vermiculite material is very lightweight and 

possesses fire- and sound-insulating properties. It is thus well 

suited for many commercial applications. The Libby (also 

called “Rainy Creek” or “Zonolite”) mine was the world’s 

largest producer of vermiculite during its operation.

Several varieties of the amphibole mineral group occur 

within the vermiculite ore deposit that was mined near Libby. 

A study by Meeker and others (2003) showed that the fibrous 

and asbestiform amphiboles in the Libby vermiculite deposit 

have compositions characteristic of winchite, richterite, and 

tremolite, as defined by the International Mineralogical Association (Leake and others, 1997). U.S. Occupational Safety 

and Health Administration (OSHA) asbestos standards include 

the regulation of five amphibole varieties, when these particular mineral compositions occur as particles that fit the regulatory definition of “asbestos fiber” (slender mineral particles 

with a specific length:diameter ratio). As explained below 

(Regulatory Definitions of Amphibole Asbestos), winchite 

asbestos and richterite asbestos are not currently (2005) listed 

in the OSHA asbestos standards.

The U.S. Geological Survey (USGS) has recently studied 

the composition of 101 vermiculite-rich, archived samples 

collected from 62 vermiculite mines and deposits in 10 U.S. 

States. The samples were collected as part of a survey of 

the Nation’s vermiculite resources in 1947, 1966-1967, and 

1975-1976. At each mine or deposit that was visited, a limited 

number of hand samples were collected to represent the crude 

vermiculite material (“ore”). Thus, these samples likely do not 

fully represent all the materials present at a given site. In the 

current study, portions of each sample were examined by Xray diffraction (XRD), scanning electron microscopy (SEM), 

energy-dispersive spectroscopy (EDS), and electron probe 

microanalysis. Despite the reconnaissance nature of the original sampling, the mineralogical characterization of the vermiculite samples has so far yielded consistent results. These preliminary results indicate that amphibole fibers (consistent with 

“amphibole asbestos” as defined by OSHA) are not common 

in some types of vermiculite deposits, but the amphibole 

asbestos mineralogy similar to that found in the Libby deposit 

is not unique. Initial results suggest that vermiculite deposits that formed within geologic settings similar to the Libby 

deposit—relatively quartz deficient, potassium-, sodium- and 

calcium-rich igneous intrusions, typically zoned—may contain 

amphibole fibers with chemical compositions similar to that of 

the Libby deposit minerals. Also, vermiculite deposits found 

where masses of ultramafic rocks are cut by granite and (or) 

pegmatite can contain amphibole fibers. These relationships 

may help guide priorities for sampling, monitoring, permitting, 

and reclamation of active and historic vermiculite mines.

The Vermiculite-Asbestos Issue

The association of amphibole asbestos with mined vermiculite has received significant nationwide attention since 

November 1999, when the Seattle Post-Intelligencer newspaper 

reported that unusually high incidences of asbestos-related 

Reconnaissance Study of the Geology of

U.S. Vermiculite Deposits—

Are Asbestos Minerals Common Constituents?

By Bradley S. Van Gosen, Heather A. Lowers, Alfred L. Bush, Gregory P. Meeker,

Geoffrey S. Plumlee, Isabelle K. Brownfield, and Stephen J. Sutley
open-pit

mine

tailings

Rainy

Creek

Kootenai River

Libby

115°37'30" 115°30' 115°22'30"

48°30'

48°22'30"

2 37

0

0

1 MILE

1 KILOMETER

MONTANA

Kalispell

Great

 Falls

HELENA

Butte

Missoula

Area of

figure 1

Figure 1. Location of the Libby (Rainy Creek, 

Zonolite) mine near Libby, Mont., site of open-pit 

mining of vermiculite from 1923 to 1990.

    Reconnaissance Study of the Geology of U.S. Vermiculite Deposits

mortality and respiratory disease occur in the small, 

vermiculite-mining town of Libby, Mont. (Schneider, 1999). 

Asbestosis and other respiratory diseases in Libby residents are 

thought to be directly linked to amphibole-asbestos minerals 

found in the vermiculite ore deposit, which was mined about 

6 miles northeast of the town (fig. 1) from 1923 until mine 

closure in 1990 (106th Congress, 2000). The U.S. Agency for 

Toxic Substances and Disease Registry (ATSDR) reviewed 

death certificate data for Libby mineworkers and residents 

for the 20-year period of 1979 to 1998 (Dearwent and others, 

2000). Their report concluded that “mortality in Libby resulting from asbestosis was approximately 40 to 60 times higher 

than expected” compared “with mortality statistics for the state 

of Montana and the U.S. population” (Dearwent and others, 

2000). The ATSDR conducted medical tests of the mineworkers and past and present residents of Libby. Included were sets 

of chest X-rays, pulmonary function lung tests, and face-to-face 

interviews with each person to review his or her medical history. Results released in August 2001 reportedly showed lung 

abnormalities (such as pleural thickening or scarring of the 

lungs) in about 18 percent (994) of the 5,590 adults participating in the testing (McLaughlin, 2001; Smith, 2001). The U.S. 

Environmental Protection Agency (EPA), under Superfund 

authority, is sampling and performing remediation in the town 

of Libby, the mines and mill sites, and the surrounding area.

The Libby (also known as the “Rainy Creek” or “Zonolite”) vermiculite mine was an open-pit operation that began 

as a modest producer in 1923 but grew to be a major part of 

the world vermiculite market. The Libby mine is estimated to 

have supplied more than 50 percent of the world’s vermiculite output based on available U.S. and world production data 

between 1924 and 1990 (DiFrancesco and Potter, 2001). It 

processed an estimated 20,000 tons (18,000 metric tons (t)) 

in 1940, 150,000 tons (136,000 t) in 1950, and 200,000 tons 

(180,000 t) in 1970 (McDonald and others, 1986). 

Regulatory Definitions of

Amphibole Asbestos

The term “asbestos” is a commercial term that refers to a 

group of silicate minerals that will easily separate into strands 

of thin, strong fibers that are flexible, heat resistant, and chemically inert, and thus well suited for applications such as heat 

insulation (Cossette, 1984; Ross, 1981; Ross and others, 1984; 

Skinner and others, 1988; Zoltai, 1981). In a mineralogical 

sense, the two groups of asbestos minerals are (1) serpentines, 

with the only asbestiform variety called chrysotile, and (2) 

specific amphiboles, including crocidolite (riebeckite asbestos), cummingtonite-grunerite asbestos (commercially called 

amosite), and the asbestiform varieties of tremolite, anthophyllite, and actinolite. According to OSHA asbestos regulation 

standards, in order to determine the “asbestos” content of a 

particular mineral assemblage, an analyst should count mineral particles that
The Geology and Uses of Vermiculite    

1. Are 5 μm (micrometers) or longer in length and display asbestos growth habit (length:diameter aspect ratios of at 

least 3:1, often exceeding 100:1);

2. Lack longitudinal striations that suggest the particle 

is a “cleavage fragment” (Campbell and others, 1979; Zoltai, 

1981), which are currently not regulated by OSHA based on a 

1992 OSHA ruling (Occupational Safety and Health Administration, 1992, p. 24320); and

3. Are composed of one of the six asbestos “mineral” 

phases—chrysotile, crocidolite (riebeckite), cummingtonitegrunerite (“amosite”), tremolite, anthophyllite, and actinolite. 

These are the mineral compositions and morphologies that were 

historically encountered in occupational exposures associated 

with asbestosis.

The OSHA asbestos regulation also notes: “Asbestos 

fibers exist in bundles that are easily parted, show longitudinal 

fine structure and may be tufted at the ends showing ‘bundle 

of sticks’ morphology” (Title 29, Code of Federal Regulations, 

1999 (29 CFR 1910.1001)). Another guideline, although 

apparently unofficial in its application, is that asbestos fibers 

are flexible, typically indicated by long, thin fibers that appear 

to bend but not break. True asbestos is thought to be 

extremely flexible and not brittle (Zoltai, 1981).

For this study, we applied the terms “fiber” or “fibrous” 

in accordance with OSHA asbestos analysis guidelines, which 

define asbestos fiber as: “A particle that is 5 μm or longer, with 

a length-to-width ratio of 3 to 1 or longer” (Title 29, Code of 

Federal Regulations, 1999 (29 CFR 1910.1001)). We further 

considered “fibers” to include only those mineral particles 

with diameters visually estimated to be 3 μm or less, because 

regulatory agencies worldwide have generally agreed that at 

about 3 μm or less the mineral fibers are most hazardous to the 

human respiratory system. (See Cossette, 1984, p. 34–36.)

Study Goals

Careful analyses and characterization of the Libby asbestos 

minerals will (1) complement toxicological studies that examine 

the effects of these minerals on the human respiratory system, 

and (2) assist regulatory agencies in the identification of potentially dangerous asbestiform amphiboles relative to less hazardous amphiboles. For example, the U.S. OSHA and the U.S. 

EPA asbestos regulations (Title 40, Code of Federal Regulations, 

Part 61 and Part 763; Title 29, Code of Federal Regulations, 

Part 1910 and Part 1926) do not specify asbestiform richterite 

and asbestiform winchite, which are common constituents of the 

Libby deposits, as regulated asbestos minerals. (See Wylie and 

Verkouteren, 2000; Verkouteren and Wylie, 2000.)

A study is underway by the USGS to analyze the mineralogy of vermiculite-rich samples collected from 62 U.S. vermiculite mines and deposits. The study is initially being conducted 

by reconnaissance sampling and analyses using XRD, SEM, 

EDS, and electron probe microanalysis. The purpose of the 

study is to determine how common the amphibole asbestos minerals, like those found at Libby, are in other vermiculite deposits, 

and if they occur in similar morphologies and compositions.

The Geology and Uses of Vermiculite

Vermiculite Geology

Vermiculite is a general term applied to a group of platy, 

mica-like, hydrated silicate minerals with the general formula 

(Mg,Fe,Al)3(Al,Si)4O10(OH)2•4H2O. Most vermiculite group 

minerals are the products of aqueous alteration of micas, primarily the biotite subgroup (annite-phlogopite); they pseudomorph the platy morphology of the replaced mica. Typically, 

the biotite grains alter to hydrobiotite or chlorite, then to vermiculite (Bush, 1976). Vermiculites encompass a wide range 

of chemical compositions, vary in color from light yellow to 

green to brown to black, and generally have a bronze hue. 

Their distinctive characteristic is a prominent exfoliation when 

heated from 800° to 1,100°C. When heated, the vermiculite 

plates expand at right angles to the cleavage (accordion-like) 

as the contained water is converted to steam. This forms 

elongated, worm-like, light-weight particles that trap air. Individual particles can expand from 6 to as much as 30 times their 

original volume in the longest dimension (fig. 2). 

Vermiculite forms from the low-temperature, weathering 

alteration of micaceous minerals in the zone of ground-water 

circulation (Bush, 1976). Thus, commercial vermiculite 

deposits require an igneous or metamorphic host rock that contains an abundance of large mica crystals, especially biotite or 

phlogopite, which have interacted with ground water and (or) 

surface waters. Biotite forms in a wide range of igneous and 

metamorphic environments, and phlogopite occurs in metamorphosed carbonate and ultramafic rocks. Bassett (1959) 

attributed the formation of biotite and asbestiform amphiboles 

in the Libby deposits to the alteration of augite (in pyroxenites) by high-temperature silica-rich solutions. Vermiculite 

formation clearly postdates the high-temperature rock-forming 

processes (Bassett, 1959; Bush, 1976).

Bush (1976) categorized vermiculite deposits into three 

broad groups, as follows: (1) deposits within large ultramafic 

intrusions, such as pyroxenite plutons, many of which are 

zoned and are cut by syenite or alkalic granite and by carbonatitic rock and pegmatite; (2) deposits associated with small 

to large ultramafic intrusions, such as dunite and unzoned 

pyroxenite and peridotite, cut by pegmatite and syenitic or 

granitic rocks; and (3) deposits in ultramafic metamorphic 

rocks. The Libby deposits are of the first deposit type.

Vermiculite Uses and Production

Processed (heat-expanded) vermiculite material is very 

light weight and has fire- and sound-insulating properties. 

Therefore, it is useful material for many commercial applications. Processed vermiculite is commonly used as a lightweight aggregate in concrete or plaster to make low-weight, 

sound-deadening, fire-resistant walls, boards, panels, and 

coatings. Other common applications are in horticulture and 

agriculture, as an additive to mulch, potting soils, and growing 

mixes, and as a carrier and extender for fertilizers, pesticides, 

and herbicides (Potter, 2001).
0

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1 INCH

1 2 CENTIMETERS

    Reconnaissance Study of the Geology of U.S. Vermiculite Deposits

In 1999, South Africa led the world in production of vermiculite (218,000 t), followed by the United States (175,000 

t), with much smaller production (each with 40,000 t or less) 

from People’s Republic of China, Russia, Brazil, and several 

other countries (Potter, 2001). Three operations currently 

(2002) produce vermiculite in the United States—one operated 

by W.R. Grace & Co. near Enoree, S.C., one operated by Virginia Vermiculite Ltd. in Louisa County, Va., and one operated 

as a Virginia Vermiculite subsidiary near Woodruff, S.C.

Amphibole Mineralogy of the Libby

Vermiculite Deposit

The Libby deposit contains amphiboles of several compositions that form intergrowths with the vermiculite and 

gangue rocks (fig. 3). A detailed sampling and analyses of 

amphibole-rich rock from the Libby (Rainy Creek) vermiculite deposit was performed by Meeker and others (2003). 

They found that: 

“The range of amphibole compositions, determined from 

electron probe microanalysis and X-ray diffraction analysis, indicates the presence of winchite, richterite, tremolite, and magnesioriebeckite.”

They show that nearly complete solid solution occurs in 

the Libby amphiboles between the ideal end-member compositions, as defined by Leake and others (1997) and discussed 

by Wylie and Verkouteren (2000) and Verkouteren and Wylie 

(2000). Also, the Libby amphibole particles display a continuum of morphologies, ranging from prismatic crystals to 

asbestiform fibers; most of the Libby amphiboles have a shape 

between prismatic and asbestiform. Meeker and others (2003) 

observed that Libby winchite, richterite, tremolite, and possibly magnesioriebeckite can occur in fibrous or asbestiform 

habit. Fibrous winchite and richterite at other localities, as at 

Libby, are found associated with altered alkaline igneous rocks 

(Wylie and Verkouteren, 2000).

USGS Reconnaissance Study

The USGS is assembling a growing set of currently more 

than 100 samples of vermiculite. Most were collected from 

site visits to deposits throughout the U.S. by A.L. Bush in 

1947, 1966-1967, and 1975-1976, and mostly during active 

mining. Additional samples were collected in 2001 at abandoned mine sites. The study samples are grab samples of vermiculite-bearing material (“ore”) and are not fully representative of the materials mined or processed, or found in outcrop, 

at each particular deposit. Also, gangue and bedrock materials 

generally are not represented. Splits from 101 samples have 

been analyzed using SEM, EDS, and XRD methods. The 

samples analyzed are from deposits in Georgia (1 sample), 

South Carolina (27), North Carolina (14), Texas (21), Arizona 

(2), California (3), Wyoming (15), Idaho (4), Colorado (10), 

and Montana (4 non-Libby samples). The analytical results 

are compared to those for a representative suite of 30 samples 

that were collected from former mining operations at Libby.

Of the 101 vermiculite samples analyzed by SEM, 13 

samples representing five vermiculite-rich districts were found 

to contain numerous fibrous amphibole particles. Two Colorado vermiculite districts—the Gem Park Complex, which 

straddles part of the Custer and Fremont County boundary, 

and the Powderhorn district in Gunnison County—contain 

an abundance of fibers semiquantitatively determined to be 

winchite, richterite, and riebeckite (fig. 4). A deposit in the 

Gold Hill district in Latah County, Idaho, contains abundant fibers of actinolite on the surface of vermiculite grains. 

Samples from the Smith mine vermiculite deposit in Converse 

County, Wyo., were found to contain fibrous tremolite and 

Figure 2. Examples of raw vermiculite (left) 

and processed (heat-expanded) material (right). 

When heated to 800°–1,100°C, the water naturally contained between the layers of the vermiculite structure is converted to steam. The 

steam pushes the crystal layers apart, expanding the vermiculite material from 6 to as much as 

30 times its original thickness.
50 MICROMETERS

Mineral Ideal cation ratios

winchite (CaNa)Mg4 (Al, Fe3+)Si8O22(OH)2

richterite Na(CaNa)Mg5Si8O22(OH)2

tremolite Ca2(Mg, Fe)5 Si8O22(OH)2

Mg/(Mg+Fe2+) = 1.0 – 0.9

actinolite Ca2(Mg, Fe)5 Si8O22(OH)2

Mg/(Mg+Fe2+) = 0.5 – 0.89

riebeckite Na2(Mg, Fe2+)3Fe3+

2 Si8O22(OH)2

(crocidolite) Mg/(Mg+Fe2+) < 0.5

anthophyllite (Mg, Fe2+)7Si8O22(OH)2

Mg/(Mg+Fe2+) = 1.0 – 0.5

USGS Reconnaissance Study    

anthophyllite particles within a serpentine groundmass (fig. 

5). The Addie district in Jackson County, N.C., contains abundant fibrous anthophyllite and tremolite associated with the 

vermiculite and talc. Amphibole fibers were found in minute 

amounts (a few fibers each) in 11 other samples from nine 

deposits scattered throughout the U.S. The remaining vermiculite samples analyzed either lacked amphiboles or contained 

amphiboles that appear nonfibrous; these samples correspond 

to 48 deposits that generally fit the geologic characteristics of 

Bush’s (1976) “type-2” or “type-3” deposits.

Thus far in the study, four of the six vermiculite districts 

that show abundant amphibole fibers associated with the 

vermiculite—the Libby vermiculite deposit, two Colorado 

areas, and one Idaho deposit—all have similar geologic settings. The Libby (Rainy Creek or Zonolite) deposit formed by 

alteration of a large zoned pyroxenite pluton having a central 

biotite-rich pyroxenite core. A younger mass of syenite cuts 

the outer zones of the pluton, and alkalic syenite dikes cut the 

biotitite core. A nearby small mass of nepheline syenite is 

present. Fenitization of the metasedimentary rocks surrounding the pluton suggests that a carbonatitic mass occurs at depth 

(Boettcher, 1967). One of the vermiculite-rich Colorado districts containing fibers is hosted by a zoned pluton—the Gem 

Park Complex—consisting mostly of pyroxenite and gabbro, 

cut by abundant carbonatite dikes and irregular masses with 

associated fenite (Parker and Sharp, 1970). The Gem Park 

Complex is cut further by minor dikes and bodies of lamprophyre, syenite porphyry, and nepheline syenite pegmatite. The 

complex is interpreted to contain a large carbonatite body at 

depth (Parker and Sharp, 1970; Armbrustmacher, 1984). The 

other Colorado vermiculite district with fibers is hosted by the 

Iron Hill stock (or “complex”), also described as a carbonatite 

intrusion. This stock is zoned, composed of mostly pyroxenite in the core. The stock’s core is cut by nepheline-pyroxene- garnet-rich rock. Sodic syenite and nepheline syenite 

intruded along the borders of the stock, and late dikes of nepheline gabbro and quartz gabbro cut the entire stock complex 

(Larsen, 1942; Temple and Grogan, 1965; Nash, 1972; Olson, 

1974; Hedlund and Olson, 1975; Olson and Hedlund, 1981). 

The Idaho vermiculite deposit that contains actinolite fibers 

is hosted by a zoned hypabyssal stock—the Gold Hill stock. 

This stock has a core of mainly hornblende syenite with subordinate hornblende monzonite and hornblende syenodiorite 

(Tullis, 1944). Pyroxenite and syenite dikes are spatially associated with the stock and vermiculite deposit (Tullis, 1944).

The Smith mine vermiculite deposit in Wyoming contains 

tremolite and anthophyllite fibers formed at the contact zone 

between a granitic intrusion and massive serpentinite (Hagner, 

Figure 3. Scanning electron microscope (SEM) photograph of 

vermiculite ore material collected from an open pit of the Libby 

mine, Montana. The platy minerals have been identified as vermiculite, hydrobiotite, biotite, and phlogopite. Fibrous particles 

are amphibole phases with compositions identified as winchite, 

richterite, and tremolite (table 1).

Table 1. Ideal compositions for the fibrous amphibole minerals 

found in U.S. vermiculite deposits.

[Cation ratios from Leake and others, 1997]
50 MICROMETERS

    Reconnaissance Study of the Geology of U.S. Vermiculite Deposits1944). The amphibole fibers and biotite grains (which converted to vermiculite) apparently formed in a zone of metasomatic chemical exchange along the contact of the intruding 

granite with serpentine-rich country rocks. The North Carolina district—the Addie district—is associated with the Day 

Book dunite deposit and contains alteration zones composed 

of vermiculite (weathered phlogopite), fibrous tremolite and 

anthophyllite, and talc along serpentine-rich contacts between 

dunite masses and intruding pegmatites (Murdock and Hunter, 

1946; Kulp and Brobst, 1954). The general geologic characteristics of these Wyoming and North Carolina vermiculite 

deposits correspond to Bush’s (1976) “type-2” deposits. El 

Shazly and others (1975) described asbestos-vermiculite 

deposits in Egypt, and summarized similar deposits elsewhere, 

whose geologic characteristics are very similar to the Wyoming and North Carolina “type-2” vermiculite deposits.

Conclusions

The results of this mineralogical survey are preliminary, 

and the study sample set was collected in a reconnaissance 

fashion; therefore, additional, more representative sampling 

is planned. The results of this preliminary survey are consistent and suggest that fibrous amphiboles, in more than trace 

amounts, may not be common in the ore zones of some types 

of vermiculite deposits. However, the asbestiform amphibole 

mineralogy of the Libby deposit is not unique. The initial 

results suggest that vermiculite deposits associated with zoned, 

alkalic/calcic, quartz-poor plutons, especially those with characteristics of carbonatite intrusions, may be likely to contain 

fibrous amphiboles. Also, vermiculite deposits associated with 

ultramafic bodies cut by granite and (or) pegmatite deserve 

scrutiny, because contact metamorphic reactions along these 

contact zones often form serpentine-rich bodies that contain an 

abundance of amphibole fibers. These relationships may help 

guide priorities for sampling, reclamation, permitting, and 

monitoring of active and inactive vermiculite mines.

References Cited

106th Congress, 2000, Federal, State, and local response to public 

health and environmental conditions from asbestos contamination 

in Libby, Montana (full committee field hearing): Statements from 

Hearings Held at the 106th Congress, Second Session, U.S. Senate 

Committee on Environment and Public Works, February 16, 2000; 

available on the worldwide web at http://www.senate.gov/~epw/

stm1_106.htm#02-16-00

Armbrustmacher, T.J., 1984, Alkaline rock complexes in the Wet 

Mountains area, Custer and Fremont Counties, Colorado: U.S. 

Geological Survey Professional Paper 1269, 33 p.

Bassett, W.A., 1959, The origin of the vermiculite deposit at Libby, 

Montana: American Mineralogist, v. 44, nos. 3 and 4, p. 282–299.

Boettcher, A.L., 1967, The Rainy Creek alkaline-ultramafic igneous 

complex near Libby, Montana—I, Ultramafic rocks and fenite: 

Journal of Geology, v. 75, no. 5, p. 526–553.

Bush, A.L., 1976, Vermiculite in the United States, in Eleventh Industrial 

Minerals Forum: Montana Bureau of Mines and Geology Special 

Publication 74, p. 145–155.

Campbell, W.J., Steel, E.B., Virta, R.L., and Eisner, M.H., 1979, 

Relationship of mineral habit to size characteristics for tremolite 

cleavage fragments and fibers: U.S. Bureau of Mines Report of 

Investigations 8367, 18 p.

Code of Federal Regulations, 1999, Title 29, part 1910, section 1001, 

Appendix B, p. 34, and Appendix J, p. 66 (July 1, 1999 edition).

Figure 4. SEM photograph of vermiculite-rich material collected from Gem Park Complex, Colorado. Platy minerals visible 

are vermiculite and hydrobiotite. Fibrous particles are amphibole phases with compositions identified as winchite, richterite, 

and riebeckite (table 1).
50 MICROMETERS

References Cited    

Cossette, Marcel, 1984, Defining asbestos particulates for monitoring 

purposes, in Levadie, Benjamin, ed., Definitions for asbestos and 

other health-related silicates: Philadelphia, Pa., American Society 

for Testing and Materials, ASTM Special Technical Publication 834, 

p. 5–50.

Dearwent, Steve, Imtiaz, Rubina, Metcalf, Susan, and Lewin, Michael, 

2000, Health consultation—Mortality from asbestosis in Libby, 

Montana [report dated December 12, 2000]: Agency for Toxic 

Substances and Disease Registry; available on the worldwide web 

at http://www.atsdr.cdc.gov/HAC/pha/libby/lib_toc.html

DiFrancesco, C.A., and Potter, M.J., 2001, Vermiculite statistics, in

Kelly, Thomas, Buckingham, David, DiFrancesco, Carl, Porter, 

Kenneth, Goonan, Thomas, Sznopek, John, Berry, Cyrus, and 

Crane, Melissa, Historical statistics for mineral commodities in the 

United States, version 5.4: U.S. Geological Survey Open-File Report 

01-006; available on the worldwide web at http://minerals.usgs.gov/

minerals/pubs/of01-006/vermiculite.html

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