Radtkeite, Hg (sub 3) S (sub 2) ClI, a new mineral from the McDermitt mercury deposit, Humboldt County, Nevada

Radtkeite, Hg (sub 3) S (sub 2) ClI, is a new mineral that occurs in small amounts with cinnabar and corderoite at one locality in the McDermitt mercury mine, Humboldt County, Nevada. The mineral occurs mostly as 2 mu m grains and as finely dispersed grains, coatings and prismatic crystals as long as 30 mu m, some of which are hollow, in altered tuffaceous sediments. Radtkeite is orthorhombic with possible space groups Fmmm, F222, or Fmm2; a 16.92(2), b 20.25(2), c 9.110(3) Angstrom, Z = 16. The strongest lines in the powder XRD pattern are 2.64(100), 2.71(44), 3.90(41), 2.58(29), 2.53(28) Angstrom . Radtkeite is opaque in thick sections but translucent in thin grains. The mineral and streak are yellow-orange, and both blacken on exposure to sunlight. Lustre almost adamantine; H. 2-3. Cleavages are: pinacoidal 010 perfect, 001 and 100 very good; prismatic 0kl, h0l, and hk0 good. Fracture is splintery, conchoidal, and hackly. D 7.0 +- 0.1 g/cm (super 3) , D (sub calc) 7.05 +- 0.1 g/cm (super 3) . The mineral is biaxial with 2V (sub gamma) approx 35 degrees ; refr. ind. > 2.0. Pleochroism of thin plates is orange, orange with red tint, and orange with green tint. Reflected light properties of natural and synthetic radtkeite are: medium reflectance, slightly lower than cinnabar; colour pale grey with yellow cast; strong anisotropism commonly masked by abundant canary yellow internal reflections; VHN 65-75. Radtkeite formed during late stages of mineralization by reaction of cinnabar and corderoite with halide- rich hydrothermal solutions in a surface-dominated environment; it exists in several textures as a result of fluctuating saturation levels.