Laphamite: structure determination

Reference:
• Bindi, L., Bonazzi, P., Spry, P.G. (2008): Effects of sulfur-for-selenium substitution on the structure of laphamite, As2(Se,S)3. Canadian Mineralogist, 46, 269-274.

Abstract:
To evaluate the effects of the S-for-Se substitution in the As2X3 phases (X = S, Se), the crystal structure and chemical composition of two crystals of laphamite (LAP1, LAP2) from the type locality, the Western Middle Anthracite field, Northumberland County, Pennsylvania, U.S.A., were investigated. The unit-cell parameters are: a 11.891(7), b 9.742(5), c 4.274(3) Å, β 90.03(4)°, V 495.1(5) Å3 (LAP1) and a 11.934(4), b 9.804(4), c 4.268(2) Å, β 90.32(4)°, V 499.4(4) Å3 (LAP2). The refinement converged to R1 = 5.46% (LAP1, assuming merohedral twinning) and 5.15% (LAP2). Quantitative analysis led to the chemical formulae: (As1.97Sb0.03)(Se2.28S0.64As0.08) and (As1.99Sb0.01)(Se2.25S0.71As0.04) for LAP1 and LAP2, respectively. The crystal structure of laphamite was found to be topologically identical to that of orpiment. It is based on AsX3 pyramids linked by the sharing of X atoms to form As2X3 layers parallel to (010). The refinement of the site-occupancy factor in both LAP1 and LAP2 crystals indicates Se to be disordered over the three anion positions (i.e., X1, X2, and X3), with a marked preference for the X3 site and, to a lesser extent, for the X2 site. Both a and b parameters increase linearly as a function of the ratio Se/(S + Se), whereas the cparameter remains substantially unchanged. No clear trend can be observed for the angle β.