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Anorpiment, As_(2)S_(3), the triclinic dimorph of orpiment

Kampf, A. R. and Downs, R. T. and Housley, R. M. and Jenkins, R. A. and Hyršl, J. (2011) Anorpiment, As_(2)S_(3), the triclinic dimorph of orpiment. Mineralogical Magazine, 75 (6). pp. 2857-2867. ISSN 0026-461X.

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The new mineral anorpiment, As_(2)S_(3), the triclinic dimorph of orpiment, has space group P1 and cell parameters a = 5.7577(2), b = 8.7169(3), c = 10.2682(7) Å, α = 78.152(7), β = 75.817(7), γ = 89.861(6)º, V = 488.38(4) Å^3 and Z = 4. It occurs at the Palomo mine, Castrovirreyna Province, Huancavelica Department, Peru. It is a low-temperature hydrothermal mineral associated with dufrénoysite, muscovite, orpiment, pyrite and realgar. It occurs in drusy crusts of wedge-shaped, transparent, greenish yellow crystals. The streak is yellow. The lustre is resinous on crystal faces, but pearly on cleavage surfaces. The Mohs hardness is about 1½. The mineral is sectile with an irregular fracture and one perfect and easy cleavage on {001}. The measured and calculated densities are 3.33 and 3.321 g cm^(-3), respectively. All indices of refraction are greater than 2. The mineral is optically biaxial (—) with 2V = 35-40º and no observed dispersion. The acute bisectrix (X) is approximately perpendicular to the {001} cleavage. Electron microprobe analyses yielded the averages and ranges in wt.%: As 58.21 (57.74-59.03), S 38.72 (38.33-39.00), total 96.94 (96.07-97.75), providing the empirical formula (based on 5 atoms) As_(1.96)S_(3.04). The strongest powder X-ray diffraction lines are [d (hkl) I] 4.867(002) 97, 4.519 (110,111) 77, 3.702 (111) 46, 3.609 (022,112) 82, 2.880 (201,022,121,023) 75, 2.552 (113,131,132) 100, 2.469 (114,130,131) 96. The structure of anorpiment [R_1 = 0.021 for 1484 reflections with F_o > 4σ(F)] consists of layers of covalently bonded As and S atoms. Each S atom bonds to two As atoms at As-S-As angles between 100.45 and 104.15º. Each As atom is strongly bonded to three S atoms at S-As-S angles between 91.28 and 103.59º, forming an AsS_3 pyramid with As at its apex. The As-S linkages within the layers form rings of six AsS_3 pyramids. Interlayer bonding forces are interpreted as van der Waals. The structure of anorpiment is similar to that of orpiment in that it is composed of layers of As2S_3 macromolecules, but arranged in a different stacking sequence.

Item Type:Article
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URLURL TypeDescription
Kampf, A. R.0000-0001-8084-2563
Downs, R. T.0000-0002-8380-7728
Additional Information:© 2011 The Mineralogical Society. Received 12 September 2011; Accepted 27 October 2011. Luca Bindi and an anonymous reviewer provided helpful comments on the manuscript. The microprobe analyses were supported by a grant to Cal tech from the Northern California Mineralogical Association. Support from the Arizona Science Foundation and the RRUFF project is acknowledged by RTD. The remainder of this study was funded by the John Jago Trelawney Endowment to the Mineral Sciences Department of the Natural History Museum of Los Angeles County.
Funding AgencyGrant Number
Northern California Mineralogical AssociationUNSPECIFIED
Arizona Science FoundationUNSPECIFIED
Natural History Museum of Los Angeles County John Jago Trelawney EndowmentUNSPECIFIED
Subject Keywords:anorpiment; new mineral; crystal structure; orpiment; arsenic sulphide; van der Waals forces; Palomo mine; Peru
Issue or Number:6
Record Number:CaltechAUTHORS:20120120-152007630
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:28898
Deposited By: Jason Perez
Deposited On:23 Jan 2012 19:48
Last Modified:09 Mar 2020 13:18

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