Corderoite, Hg3S2CI2, (isometric; space group I213, ao = 8.94 Å, Z = 4, Zdcalc = 6.85 gm cm-3), a new mineral from the Cordero mine in Humboldt County, Nevada, occurs in late Miocene playa sediments in association with montmorillonite, α-quartz, α-cristobalite, and cinnabar. It is also found, along with cinnabar, within the “opalite” layer, which consists of silicified rhyolitic tuffs and sediments at the base of the lake beds, and within altered basement volcanics belowthe opalite layer. In all cases corderoite replaces cinnabar.

The powder diffraction pattern and chemical composition for corderoite are analogous to those for the material synthesized and described by Puff and coworkers (1962) and are the same as for α-Hg3S2CI2 synthesized by Carlson (1967) and described by Frueh and Grey(1968).

Fresh corderoite is light orange pink (5YR 8/2) but upon exposure to artificial light or sunlight becomes light gray to black (N6 to N2). Grain size is less than 2 μm in diameter for the natural corderoite specimens. Because the grains are so small, crystals of α-Hg3S2CI2 were synthesized by Carlson's (1967) method. For these synthetic crystals, refractive index n > 25. Reflectances in air are R650 nm = 15.1; R589 nm = 15.1; R546 nm = 15.5, and R470 nm = 16.7 percent VHN15. Vickers hardness numbers for 15 and 25 g loads are: VHN15 = 35-58 with a mean of 45; VHN25 = 28-61, mean 48. Optical properties are: isotropic and colorless-pale yellowish white.

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