Abstract

The new mineral anorpiment, As2S3, the triclinic dimorph of orpiment, has space group P1İ 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)°, V = 488.38(4) Å3 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 1. 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 = 35–40° 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) As1.96S3.04. The strongest powder X-ray diffraction lines are [d (hkl) I] 4.867(002) 97, 4.519 (110,11İ1) 77, 3.702 (1İ1İ1) 46, 3.609 (022,11İ2) 82, 2.880 (201,02İ2,1İ2İ1,023) 75, 2.552 (1İ13,1İ31,132) 100, 2.469 (114,130,13İ1) 96. The structure of anorpiment [R1 = 0.021 for 1484 reflections with Fo > 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 104.15°. Each As atom is strongly bonded to three S atoms at S–As–S angles between 91.28 and 103.59°, forming an AsS3 pyramid with As at its apex. The As–S linkages within the layers form rings of six AsS3 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 As2S3 macromolecules, but arranged in a different stacking sequence.

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