Polhemusite, a new Hg-Zn sulfide from Idaho

Polhemusite, empirically Hg_0.15Zn_0.87Fe_0.01S_0.97, is a microscopic constituent of mercury–bearing antimony ore from the B and B deposit, Big Creek district, Valley County, Idaho. Polhemusite occurs as stubby tetragonal prisms, dipyramids, and irregular grains associated with stibnite, cinnabar, mercurian sphalerite, and zincian metacinnabar in a quartz lode. Polhemusite is black, resinous to adamantine, locally with dark red internal reflection.

X–ray diffraction data for polhemusite can be indexed according to a primitive tetragonal cell having a = 8.71, c = 14.74A. A pseudocubic subcell, a = 5.33A, is also present. Possible space groups are P4/n, P4₂/n, P4/nbm, P4/nmm, P4₂nnm, and P4₂/ncm. Single–crystal data and density are lacking, owing to the minute size of the particles. The cell content, Z, is probably about 24 to 32. Stronger lines of Debye–Scherrer and Gandolfi photographs are 3.16 ms, 3.08 vs, 1.888 s, 1.608 ms, 1.222 m, 1.086 m, 1.024 m.

Polhemusite is variable in composition. Microprobe analysis gives, in weight percent, Hg 18.0–34.7, Zn 42.6–54.7, Fe 0.3–0.7, S 24.0–28.9, sum 101.3–102.8. The mean of 15 analyses, each on a separate grain, is Hg 25.8 ± 5.5, Zn 49.1 ± 3.8, Fe 0.5 ± 0.1, S 26.7 ± 1.6, sum 102.1 ± 0.7. Compositional variation results from zoning, as well as from grain–to–grain variation. Limiting compositions are equivalent to Hg_0.10Zn_0.92S_0.99 and Hg_0.22Zn_0.83S_0.95.

In polished section, polhemusite looks gray in air, much darker in oil. Against stibnite, it is lavender gray to bluish gray. Polishing hardness > stibnite. Reflection pleochroism in aggregates: distinct in air, very slightly brownish gray to slightly lavender gray; conspicuous in oil, light brownish gray (almost yellow) to purplish gray, but partly obscured by internal reflection. Where unobscured by internal reflection, anisotropism is distinct in air, strong in oil. Internal reflection, not everywhere detectable in air, is strong—lavender, pink, reddish orange, dark red; in oil it is strong, red–orange, not visible in all grains. Reflectance at 470, 546, 589, and 650 nm is (R₁, R₂; (R₁ + R₂)/2, in parentheses) 20.1–24.4 (21.8), 17.1–18.3 (17.6), 16.5–17.7 (16.9), ∼17.9 percent. Bireflectance decreases with increasing wavelength and is effectively absent at 650 nm. HV₂₅ = 220–333, mean 262. Knee–shaped twins resembling those of rutile are common; other simple twins and some lamellar twins are present. Cleavage or parting is rare. Zoning is conspicuous in some areas.

Polhemusite, except for its reflection pleochroism and anisotropism, somewhat resembles sphalerite, but its quantitative ore;microscopic properties, twin habit, and X;ray diffraction data are distinctive. Its composition overlaps that of the associated mercurian sphalerite. Polhemusite is inferred to be a metastable, disordered form of (Zn,Hg)S—a polymorph of mercurian sphalerite.