Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Résumé Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Bindi, L. Articles by Spry, P. G. Search for Related Content GeoRef GeoRef Citation

EFFECTS OF SULFUR-FOR-SELENIUM SUBSTITUTION ON THE STRUCTURE OF LAPHAMITE, As₂(Se,S)₃

Luca Bindi^1,, Paola Bonazzi² and Paul G. Spry³

¹ Museo di Storia Naturale - sez. di Mineralogia e Litologia, Università degli Studi di Firenze, via La Pira 4, I–50121 Firenze, Italy
² Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira 4, I–50121 Firenze, Italy, and C.N.R. Istituto di Geoscienze e Georisorse - sez. di Firenze, via La Pira 4, I–50121 Firenze, Italy
³ Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011–3210, U.S.A.

E-mail address: luca.bindi{at}unifi.it

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) ų (LAP1) and a 11.934(4), b 9.804(4), c 4.268(2) Å, β 90.32(4)°, V 499.4(4) ų (LAP2). The refinement converged to R1 = 5.46% (LAP1, assuming merohedral twinning) and 5.15% (LAP2). Quantitative analysis led to the chemical formulae: (As_1.97Sb_0.03)(Se_2.28S_0.64As_0.08) and (As_1.99Sb_0.01)(Se_2.25S_0.71As_0.04) 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.

Keywords: laphamite, chemical composition, structure refinement, selenium, S-for-Se substitution.