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Pararsenolamprite, a new polymorph of native As, from the Mukuno mine, Oita Prefecture, Japan

¹ Department of Geology, National Science Museum, 3-23-1 Hyakunincho, Shinjuku, Tokyo 169-0073, Japan
² Department of Earth Sciences, Faculty of Science, Toyama University, Gofuku 3190, Toyama 930-8555, Japan
³ Chuo-cho 2-5-20, Nakatsu, Oita 871-0024, Japan

^* E-mail: matubara{at}kahaku.go.jp

Pararsenolamprite, the third polymorph of native As, is found at the Mukuno mine, Oita Prefecture, Japan. It is orthorhombic, Pmn2₁ or P2₁nm, a = 3.633(2), b = 10.196(2), c = 10.314(2) Å, Z = 18. The seven strongest lines in the X-ray powder diffraction pattern are: 5.17 (100) (002), 4.60 (24) (012), 3.259 (58) (013), 2.840 (27) (032), 2.580 (22) (004), 2.299 (23) (024), and 1.794 (26) (105). Electron microprobe analysis gives As 91.89, Sb 7.25, S 0.48, total 99.62 wt.% (mean of 8), and lead to the empirical formula, As_0.96Sb_0.03S_0.01. It is lead grey in colour and opaque with metallic lustre and black streak. It is sectile and brittle with perfect cleavage on {001}. The VHN₂₅ is 66–91 kg/mm², corresponding to 2–2.5 in Mohs’ hardness scale. The measured and calculated densities are 5.88(5) g/cm³ and 6.01 g/cm³, respectively. In reflected plane-polarized light in air, it is white with a slightly greenish blue tint. Anisotoropy is strong, dark brown to dark greenish grey. Bireflectance is distinct; parallel to elongation it is creamy; perpendicular to elongation it is brown, grey and green. Internal reflections are absent. The reflectance spectra are tabulated in the text.

Pararsenolamprite occurs as euhedral crystals in close association with arsenic, stibnite and quartz in a Sb-As-Ag-Au-bearing quartz vein cutting altered Neogene andesite from the Mukuno mine. It forms radial or parallel aggregates of bladed cystals up to 0.8 mm in length.

KEYWORDS: pararsenolamprite, arsenic, polymorph, new mineral, Mukuno mine, X-ray, chemistry, optical data

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