Abstract
Radtkeite, Hg3S2CII, 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-μm grains and as finely dispersed grains, coatings, and prismatic crystals as long as 30 μm, some of which are hollow, in altered tuffaceous sediments. Radtkeite is orthorhombic, with possible space groups Fmmm, F222, or Fmm2; a = 16.85(1) Å, b = 20.27(2) Å, and c = 9.133(2) Å; V = 3119.4(5) Å3 and Z = 16. The four strongest lines in the X-ray powder diftaction pattern of synthetic material are (d Å, I/I0, hkl) 2.54, 100, 080; 2.65, 85, 313; 3.89, 48, 420; 2.71, 44, 620. 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. Luster is almost adamantine. Hardness is 2–3. Cleavages are: pinacoidal {010} perfect, {001} and {100} very good; prismatic {0kl}, {h0l}, and {hk0} good. Fracture is splintery, conchoidal, and hackly. Dmeas = 7.0 ± 0.1 g/cm3, and Dmeas = 7.05 ± 0.1 g/cm3. Preliminary optical measurements indicate that the mineral is biaxial (+), with 2Vz, approximately 35°. Refractive indices are greater than 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; color pale gray with yellow cast; strong anisotropism commonly masked by abundant canary yellow internal reflections. The polishing hardness is less than that of cinnabar; 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.
