Laphamite, an Arsenic Selenide Analogue of orpiment, from Burning Anthracite Deposits in Pennsylvania

Pete J. Dunn, Donald R. Peacor, Alan J. Criddle and Robert B. Finkelman
Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109
Department of Mineralogy, British Museum (Natural History), London, England, SW7 5BD
Exxon Production Research Company, PO Box 2189, Houston, Texas 77001

Abstract: Laphamite, ideally As2(Se,S)3, is monoclinic, P21/n, with a = 11.86(1), b = 9.756(9), c = 4.265(9) Å, β = 90.17°, V = 493.5 Å3, Z = 4, D (calc.) = 4.60, D (meas.) = 4.5(1) g/cm3. Microprobe analysis yielded: As 47.0, Se 43.7, S 8.7, sum = 99.4 wt. %. In plane polarized light in polished section at about 3200 K laphamite is moderately bireflectant from white to grey. Reflection pleochroism is absent but fiery red internal reflections are characteristic as are golden yellow reflections along scratches. Anisotropy is moderate with grey rotation tints, masked in some sections by internal reflections. Reflectance spectra in air and oil between 400 and 700 nm are given. Colour values for the CIE illiminant C in air are, for R1 followed by R2: Y% 29.0, 33.5, λd 480, 478; Pe% 9.1, 8.7. Laphamite forms dark red crystals, up to 5.0 mm, associated with orpiment, arsenolite, and an unnamed phase, NH4AlF4, in a burning coal waste-dump at Burnside, Northumberland County, Pennsylvania.

Keywords: laphamite • new mineral • orpiment • Burnside Pennsylvania

Mineralogical Magazine; June 1986 v. 50; no. 356; p. 279-282; DOI: 10.1180/minmag.1986.050.356.11
© 1986, The Mineralogical Society
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