Perroudite is a new sulfo-halide of Hg and Ag from Cap-Garonne, Var, France; Broken Hill, New South Wales; and Coppin Pool, Western Australia, Australia. Averaged microprobe analyses gave the following formulae: Hg5.04Ag4.03S3.96(Cl1.55I1.55Br0.87)Σ3.97 (Cap-Gar-onne), Hg5.0Ag4.2S5.45(I1.6Cl1.55Br0.25)Σ3.4 (Broken Hill), and Hg5.7Ag3.5S5.4(Cl2.0I1.0Br0.5)Σ3.5(Coppin Pool). A structure determination of a Coppin Pool crystal gave the general formula Hg5-xAg4+xS5-x(Cl,I,Br)4+x(–1.4 < x < 1.4) (Mumme and Nickel, 1987). The mineral is orthorhombic, space group P21212, with a = 17.47(3), b = 12.23(2), c = 4.29(2) Å, V = 917.5 Å3, and Z = 2. The averaged calculated density is 6.92 g/cm3. The strongest seven lines in the X-ray diffraction pattern are (d in Å, (hkl), I/I0) 3.012,(140)(321),100; 2.965,(430)(411)(031),80; 3.945,(111),60; 2.638,(511)(331)(620),35; 3.694,(230)(211),30;2.740,(421),30; 2.446,(710),30 (Cap-Garonne data). Perroudite forms bright red, transparent prismatic crystals to 0.07 mm in length, elongated along [001] and often flattened on {100}. The main forms present are {100} and {001}, and some crystals from Broken Hill and Coppin Pool are fluted with hollow terminations. Luster vitreous to adamantine; streak orange-red; brittle with one perfect cleavage on (100); fracture irregular. The refractive indices are in the range 2.3-2.4; the mineral is biaxial positive; 2Vmeas = 70°; dispersion r > v (very strong); pleochroism marked from brownish-red to yellow to brownish-yellow.

Perroudite forms from the alteration of Hg- and Ag-bearing tennantite at the type locality (Cap-Garonne). At Broken Hill it occurs in massive white kaolinite associated with silver halides, native silver and native gold, and possibly other mercury-bearing minerals. At Coppin Pool, perroudite occurs with supergene minerals such as covellite, cerussite, anglesite, phosgenite, etc., derived by the weathering of galena in a quartz vein. Halide-rich solutions are implicated in the formation of perroudite at all three occurrences.

The mineral is named for Mr. Pierre Perroud, of Geneva, Switzerland. Type specimens are lodged in the Natural History Museum, Geneva, Switzerland. Material representative of the two Australian localities has been deposited in the Museum of Victoria and the Western Australian Government Chemical Laboratories, Perth, Western Australia.

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