Christite, a new thallium mineral from the Carlin gold deposit, Nevada

Christite, TlHgAsSr, occurs with realgar, orpiment, and lorandite in barite veins and with realgar, lorandite, and getchellite in mineralized carbonaceous ilty dolomite in the Carlin gold deposit, north-central Nevada. The mineral is named for Dr. Charles L. Christ of the U.S. Geological Survey. The color is crimson or deep red, but varies to bright orange in thinner plates and crystals; the streak is bright orange, and the luster is adamantine. The mineral is monocl in ic, space group P2, /n,a-- 6.113 ( l) , b = 16.188(4) , c = 6.1I l ( l) A, wi th 0: 96.71(2)', Z = 4, and cell volume: 600.6 A8. Strongest X-ray powder diffraction lines, in A, and thei r re lat ive in tensi t ies are2.98 (10),3.62 (8),3.49 (6),2.692(6),2.216 (5),4.03 (6), and 3.36 (5). Electron microprobe analyses gave Tl 35.2, Hg 35.1, As 13.1, S 16.6, sum 100.0 weight percent. The mineral occurs in small subhedral to anhedral grains which usually lack well-developed forms but may show a bladed or flattened habit. Synthetic crystals are tabular, show {010) and {T0l} pinacoids, and {1 l0} and {01 l} prisms, and have perfect {010}, excellent {l l0} and {001}, and good {T0l} cleavages. Vickers hardness varied from 28.3-34.6 and averaged 31.5 kg mm- ' (10 determinations). Density of synthetic TlHgAsSs is 6.2(2) (meas) and6.37 g cm-e (calc). In reflected light christite is grayish-white with a faint blue tint, lacks visible bireflectance, is anisotropic, and has a brilliant red-orange internal reflection. Reflec-tances in air are: Rsco.- = 23,7-23.9; Rsrs.-: 24.9-25.2; Rsre,- = 26.5-26.9: and Ra7s. ' = 29.6-30.0