DEANESMITHITE, HG21+HG32+CR6+O5S2, A NEW MINERAL SPECIES FROM THE CLEAR CREEK CLAIM, SAN BENITO COUNTY, CALIFORNIA

Deanesraithite, idealized as Hg21+Hg32+Cr6+O5S2, is triclinic, space g roup P1BAR, with unit-cell parameters refined from powder data: a 8.11 6 (6), b 9.501 (8), c 6.891 (9) angstrom, alpha 100.43 (8)-degrees, be ta 110.24 (8)-degrees, gamma 82.80 (8)-degrees, V 489 (2) angstrom3, a :b:c = 0.8542:1: 0.7253, Z = 2. The strongest eight lines in the X-ray powder pattern [d in angstrom(I)(hkl)] are: 5.72(90)(110BAR), 3.373(6 0)(112BAR,102BAR,121BAR), 8(100)(122BAR,212BAR,131BAR,112BAR,202BAR,03 1BAR), 2.864(50b)(012,201,220BAR,022BAR), 2.774(50)(221BAR), 2.536(50) (132BAR), 2.486(50)(310), and 2.425(60)(221,032BAR,302BAR). The minera l is a rare constituent in a small prospect pit near the long-abandone d Clear Creek mercury mine, New Idria district, San Benito County, Cal ifornia. It is most closely associated with cinnabar and edoylerite in a host rock principally composed of quartz and magnesite. Deanesmithi te typically occurs as fan-shaped aggregates of elongate crystals and, less commonly, as isolated clusters of bladed to acicular radiating c rystals. Tabular crystals also are present but rare. The largest known crystal is 0.5 mm in length, but the average crystal length is approx imately 0.1 mm. Individual crystals are subhedral to euhedral, with bo th a bladed and a tabular habit. Crystals are flattened on {100}; blad ed crystals are elongate [001]; tabular crystals are elongate [010]; s triations on {100} are parallel to [001]; cleavage {110BAR} well-devel oped, {001} fair. Forms observed are: {100}, {320}, (001), {510BAR}, { 011BAR}, {010}, {210}, (310), {410}, (510), {610}, {11O0BAR}, {320BAR} , {210BAR}, (023), {032BAR}, and {101BAR}. The mineral is orange-red w ith a less intense orange-red streak. Physical properties include: ada mantine luster; transparent; nonfluorescent; brittle to friable; irreg ular to subconchoidal fracture; hardness less than 5; calculated densi ty 8.06 (for empirical formula), 8.14 g/cm3 (for idealized formula). I n polished section, deanesmithite is weakly bireflectant and weakly pl eochroic. In reflected plane-polarized light, it is dark bluish grey t o light grey with bright yellow-orange to orange-red internal reflecti ons. Measured reflectance values are tabulated, in air and in oil, for two crystals. Electron-micro-probe analyses yielded Hg2O 34.9, HgO 54 .4, CrO3 8.6, S 5.3, sum 103.2, less O=S 2.6, total 100.6 wt.%, corres ponding to Hg1.981+Hg2.972+Cr1.026+O5.05S1.95 based on O+S=7. The orig inal value for HgO, 90.7 wt.%, was partitioned in a ratio of 2 Hg2O : 3 HgO after the crystal structure was determined. The crystal structur e was solved by direct methods and refined to R = 2.92% from 1748 obse rved X-ray reflections [I > 2.5 sigma(I)] out of a total of 2808 uniqu e reflections obtained from measuring and averaging the whole sphere o f data to a 2theta of 60-degrees collected with MoKalpha radiation. Im portant features of the crystal structure are summarized. The name is for Professor Deane K. Smith, Pennsylvania State University, for his m any contributions to structural and experimental mineralogy, and for h is long time service to the International Centre for Diffraction Data.