Coldwellite, ideally Pd3Ag2S, is a new mineral species discovered in a heavy-mineral concentrate from the Marathon deposit, Coldwell Complex, Ontario Canada. It is cubic, crystallizing in space group P4332 (#212) with a 7.2470(8) Å, V 380.61(1) Å3, Z = 4. The five strongest lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 2.427(100)(221), 2.302(38)(310), 2.195(38)(311), 1.4280(44)(510,431), 0.9294(24)(650,643), and 0.9208(20)(732,651). Associated minerals in the discovery sample include Au-Ag alloy, hollingworthite, isoferroplatinum, keithconnite, kotulskite, mertieite-II, michenerite, palladoarsenide, sobolevskite, sperrylite, stillwaterite, vysotskite, Ti- and Cr-rich magnetite and ilmenite, pyrrhotite, chalcopyrite, and pentlandite, with trace amounts of bornite, cobaltite, and galena. Coldwellite is white with a light pinkish brown tint, and it has a metallic luster. No streak or microhardness could be measured. The mineral shows no internal reflections, pleochroism, bireflectance, or anisotropy. The reflectance values (%) in air for the standard COM wavelengths are: 41.9 (470 nm), 44.9 (546 nm), 44.0 (589 nm), and 45.0 (650 nm). The density, 9.90(1) g/cm3, was calculated using the empirical formula and the unit-cell parameters from the refined crystal-structure. The average result (n = 23) of electron-microprobe analyses is: Pd 56.1, Fe 0.16, Ag 38.2, S 5.63, total 100.09 wt.%, which corresponds to (Pd2.99Fe0.02)∑3.01Ag2.00S0.99, based on 6 apfu. The name recalls the type locality.

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