Weissbergite: a Rare Mineral...

... and thallium, a rare, toxic metal

Here are two photomicrographs taken in reflected light, depicting a grain of the rare mineral weissbergite. The left image is in plane-polarized, and the right-hand in offset cross-polarized light.

weissbergite in PPL [78 kb] weissbergite in XP [67 kb]
1. Left: A coarse grain of weissbergite, unusual in displaying a thin rind of dull, brownish-grey fibrous material. Magnification 80x, long-axis field-of-view 1.4 mm.

2. Right: The same grain, displaying more clearly the alteration rim and apophyses of strongly-anisotropic, thallium-rich material, probably a second thallium mineral. Scale as above.

"Rock of the Month # 19, posted January 2003" --- sample 1812 (ex: Paragon Minerals, Tempe, AZ).

This sample is from Lookout Pass, Tooele County, Utah, southwest U.S.A. It contains weissbergite as steel-grey, fine- grained metallic laths in a banded grey to massive black host. This mineral is a Tl-Sb sulphide, triclinic TlSbS2, the Sb equivalent of lorandite, TlAsS2. The latter was the first Tl mineral to be described, from its type locality of the prolific Allchar area of Macedonia (Rieck, 1993). The host rock seems to be a black, silicified, decalcified and brecciated limestone, with silica and Tl minerals in low-volume vuggy fractures between host-rock clasts. Cream, white, brown and sulphur-yellow secondary minerals include limonite and probably oxidized Tl-rich species. A similar sample (1843) displays silvery crystals in dull grey host rock. It is said to contain weissbergite, pink avicennite (Tl2O3) in voids, parapierrotite and pierrotite (Tl-Sb-As sulphides) and black vrbaite (a Hg-Tl-As-Sb-S phase). The crystals occur in a fine-grained silica-dominated host rock, darkened by organic matter. They are creamy-white in reflected light, bireflectant and intensely anisotropic in grey to bluish-grey tints. Anhedral, ragged-sided grains, soft, up to 2.8x1.0 mm in size. Near the extinction position, they may show a crinkly (?) pressure twinning, reminiscent of stibnite. The Tl-mineral content and grain size vary throughout the rock, apparently as a function of degree of silicification and the organic matter content.

Thallium minerals

Most Tl minerals, all rare, have been described from a few localities, such as Lengenbach, Switzerland; Allchar (a.k.a. `Alsar'), Macedonia; the Hautes-Alpes of France; sediment-hosted gold deposits in the U.S. Southwest (e.g., Carlin, Nevada; Mercur, Utah); the Noril'sk-Talnakh ore field, Siberia; Hemlo gold deposit (Ontario, Canada); and Ilimaussaq (Greenland).

Mineral species Ideal formula Crystal system
Avicennite Tl2O3 Cubic
Bernardite TlAs5S8 Monoclinic
Carlinite Tl2S Rhombohedral
Chabourneite Tl21-xPb2x(Sb,As)91-xS147 Triclinic
Christite TlHgAsS3 Monoclinic
Chalcothallite (Cu,Fe,Ag)6.3(Tl,K)2SbS4 Tetragonal
Crookesite Cu7TlSe4 Tetragonal
Edenharterite TlPbAs3S6 Orthorhombic
Ellisite Tl3AsS3 Rhombohedral
Fangite Tl3AsS4 Orthorhombic
Galkhaite (Cs,Tl)(Hg,Cu,Zn)6(As,Sb)4S12 Cubic
Gillulyite Tl2(As,Sb)8S13 Monoclinic
Hatchite AgPbTlAs2S5 Triclinic
Hutchinsonite (Pb,Tl)2As5S9 Orthorhombic
Imhofite Tl6As15S25 Monoclinic
Jankovicite Tl5Sb9(As,Sb)4S22 Triclinic
Jentschite TlPbAs2SbS6 Monoclinic
Lanmuchangite TlAl(SO4)2.12H2O Cubic
Lorandite TlAsS2 Monoclinic
Parapierrotite Tl2(Sb,As)10S16 Monoclinic
Picotpaulite TlFe2S3 Orthorhombic
Pierrotite Tl2(Sb,As)10S16 Orthorhombic
Raguinite TlFeS2 Orthorhombic
Rathite I (Pb,Tl)3As5S10 Monoclinic
Rebulite Tl5Sb5As8S22 Monoclinic
Routhierite TlHgAsS3 Tetragonal
Sicherite TlAg2(As,Sb)3S6 Orthorhombic
Simonite TlHgAs3S6 Monoclinic
Stalderite TlCu(Zn,Fe,Hg)2As2S6 Tetragonal
Thalcusite Cu3FeTl2S4 Tetragonal
Thalfenisite Tl6(Fe,Ni,Cu)25S26Cl Cubic
Vaughanite TlHgSb4S7 Triclinic
Vrbaite Hg3Tl4 As8Sb2S20 Orthorhombic
Wallisite CuPbTlAs2S5 Triclinic
Weissbergite TlSbS2 Triclinic

Thallium minerals have been described from a number of mineral deposits in the American Southwest, notably gold deposits such as Carr Fork and Mercur in Utah (Cameron and Garmoe, 1987; Wilson et al., 1991, 1993; Foit et al., 1995). Most are sulphides and sulphosalts although, analogous to Sb, oxidized minerals do occur in supergene alteration zones. A table of 34 Tl minerals is provided. The abundant weissbergite in the sample is consistent with a tremendous enrichment of the rock in Tl, as much as 5 wt.% Tl, equivalent to the richest Tl ore described by Xiao (2001) in Guizhou province, China. Weissbergite was first found in grains up to 0.5 mm in size at the east pit of the Carlin gold deposit, Nevada, USA (Dickson and Radtke, 1978). The weissbergite occurs in silicified, brecciated dolomitic rocks, and microprobe work on the type samples indicated near-pure TlSbS2 (although emission spectrography revealed minor arsenic, 300 ppm, and Fe, 200 ppm). Consistent with this finding, qualitative energy-dispersive analysis of the featured sample did not detect any elements besides Sb, Tl and S. A range of hydrothermal Tl minerals has been described at Carlin (e.g., Radtke et al., 1977) in mineralized silty argillaceous dolomites of the Roberts Mountains Formation. Other sites in Nevada and Utah, particularly gold deposits, are known to host Tl minerals. In addition to these rare, discrete phases with essential Tl, many ore minerals may contain appreciable Tl (Nowacki et al., 1982), e.g., up to 3.2% (stibnite), 1.56% (jordanite) and 0.2% (orpiment)

Thallium, the metal

Thallium is a heavy metal, atomic number 81, atomic weight 204.38, with two stable isotopes, 203Tl (29.524%) and 205Tl (70.476%). As such, it is one of the largest elements with stable isotopes, its immediate neighbours in the periodic table being Au - Hg - (Tl)- Pb- Bi. World production is very small, of the order of 16 tonnes per year, with at least 9 tonnes produced in Japan (Trueb, 1994). Tl is one of a family of metals (such as cadmium, indium and germanium) that occur in Zn-(Cu,Pb,Ag) ores and can be recovered from the flue dusts of Zn smelters. Tl is highly toxic, and indeed has been used in rat poisons, a factor which complicates its other potential uses, and its future utility probably lies with stable, non-toxic forms. Tl nitrate is used for producing optical glass with a very high index of refraction (higher than lead glass), for use in fax machines and photocopiers. There is interest in complex metal oxides, some of them incorporating Tl, which may exhibit superconductivity at elevated temperatures (Pool, 1989), and which may have new electronic uses in the future.

The mean abundance of thallium in the bulk continental crust is estimated at 360 parts per billion by weight (ppb), compared to lead (Pb, 8,000 ppb), bismuth (Bi, 60 ppb), antimony (Sb, 200 ppb), silver (Ag, 80 ppb) and uranium (910 ppb: all values from Taylor and McLennan, 1995). Returning to the featured sample, weissbergite has an ideal composition of 52.37 wt.% Tl, 31.20% Sb, 16.43% S, and the rock in question contains roughly 100,000 times the crustal average content! This is a degree of "natural refining" 100 times more than that typically seen in modern ores of precious metals such as gold and platinum, which are often economic to recover at only 1,000 times or so their typical crustal abundances.

Thallium is one of a number of rare metals that can in principle be recovered via natural concentration by certain metal-tolerant plant species ("phytomining"). This neat harnessing of the power of plants to "hyperaccumulate" specific metals may provide a double reward: the economic value of the rare metal plus the "phytoremediation" of old mine sites, in which mine tailings and smelter wastes may contain chemically unstable forms of one or more potentially hazardous metals (Anderson et al., 1999). The converse possibility, that Tl may build up in the human food chain and cause illness and death, has been documented in detail in a small region of southern China by Xiao (2001) and Xiao et al. (2002,2004a,b,c). It is now well-established that Tl may present a threat via its presence in the food chain (e.g., concentrated into cabbage), in the water supply, and in fuel (Tl-enriched coal).


ANDERSON,CWN, BROOKS,RR, CHIARUCCI,A, LACOSTE,CJ, LEBLANC,M, ROBINSON,BH, SIMCOCK,R and STEWART,RB (1999) Phytomining for nickel, thallium and gold. J.Geochem.Explor. 67, 407-415.

CAMERON,DE and GARMOE,WJ (1987) Geology of skarn and high-grade gold in the Carr Fork Mine, Utah. Econ.Geol. 82, 1319-1333.

DICKSON,FW and RADTKE,AS (1978) Weissbergite, TiSbS2, a new mineral from the Carlin gold deposit, Nevada. Amer.Mineral. 63, 720-724.

FOIT,FF, ROBINSON,PD and WILSON,JR (1995) The crystal structure of gillulyite, Tl2(As,Sb)8S13, from the Mercur gold deposit, Tooele County, Utah, U.S.A. Amer.Mineral. 80, 394-399.

NOWACKI,W, EDENHARTER,A, ENGEL,P, GOSTOJIC,M and NAGL,A (1982) On the crystal chemistry of some thallium sulphides and sulphosalts. In `Ore Genesis, the State of the Art' (Amstutz,GC, El Goresy,A, Frenzel,G, Kluth,C, Moh,G, Wauschkuhn,A and Zimmermann,RA editors), Springer-Verlag, 804pp., 689-697.

POOL,R (1989) Superconductivity: is the party over? Science 244, 914-916, 26 May.

RADTKE,AS, DICKSON,FW, SLACK,JF and BROWN,KL (1977) Christite, a new thallium mineral from the Carlin gold deposit, Nevada. Amer.Mineral. 62, 421-425.

RIECK,B (1993) Famous mineral localities: Allchar, Macedonia. Mineral.Record 24, 437-449.

TAYLOR,SR and McLENNAN,SM (1995) The geochemical evolution of the continental crust. Reviews of Geophysics 33, 241-265.

TRUEB,L (1994) Controversy surrounds thallium production in Japan and elsewhere. Northern Miner 80 no.9, 12-13, 02 May.

WILSON,JR, ROBINSON,PD, WILSON,PN, STANGER,LW and SALMON,GL (1991) Gillulyite, Tl2(As,Sb)8S13, a new thallium arsenic sulfosalt from the Mercur gold deposit, Utah. Amer.Mineral. 76, 653-656.

WILSON,JR, SEN GUPTA,PK, ROBINSON,PD and CRIDDLE,AJ (1993) Fangite, Tl3AsS4, a new thallium arsenic sulfosalt from the Mercur Au deposit, Utah, and revised optical data for gillulyite. Amer.Mineral. 78, 1096-1103.

XIAO,T (2001) Environmental Impact of Thallium Related to the Mercury-Thallium-Gold Mineralization in Southwest Guizhou Province, China. PhD Thesis, Université du Québec à Chicoutimi, 246pp.

XIAO,T, BOYLE,D, GUHA,J, ROULEAU,A, HONG,Y and ZHENG,B (2002) Groundwater-related thallium transfer processes and their impacts on the ecosystem: southwest Guizhou province, China. Applied Geochemistry 18, 675-691.

XIAO,T, GUHA,J, BOYLE,D, LIU,C and CHEN,J (2004a) Environmental concerns related to high thallium levels in soils and thallium uptake by plants in southwest Guizhou, China. Science of the Total Environment 318, 223-244.

XIAO,T, GUHA,J, BOYLE,D, LIU,C-Q, ZHENG,B, WILSON,GC, ROULEAU,A and CHEN,J (2004b) Naturally-occurring thallium: a hidden geoenvironmental health hazard? Environment International 30, 501-507.

XIAO,T, GUHA,J and BOYLE,DR (2004c) High thallium content in rocks associated with Au-As-Hg-Tl and coal mineralization and its adverse environmental potential in SW Guizhou, China. Geochemistry: Exploration, Environment, Analysis 4, 243-252.

Graham Wilson, 02 January 2003, updated 16 October 2004, web attributes adjusted 04 February 2005

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