Refinement of the crystal structures of realgar, AsS and orpiment, As2S3*

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Abstract
The crystal structures of realgar, AsS and orpiment, AS 2 S 3 were refined using diffractometer data. The cell dimensions of realgar are: a = 9.325 ±.003 Å, b = 13.571 ± .005 Å, c = 6.587 ± .003 Å, β = 106° 23′ ± 5′, and those of orpiment are: a = 11.475 ± .005 Å, b = 9.577 ± .004 Å, c = 4.256 ± .002 Å, β = 90° 41′ ± 5′. In both cases the space group is P 2 1 / n ( C 2 h 5 ). A total of 1525 independent reflections were measured for realgar, and the final R factor was 0.045 (based on 1205 reflections). For orpiment 836 independent reflections were obtained, and the final R factor was 0.064 (based on 586 reflections). The structures are essentially the same as in the original determinations 1,2 . Realgar has four separate AS 4 S 4 molecules per unit cell, with cradle-like molecular configurations, van der Waals forces being operative between the molecules. The structure of orpiment consists of layers of covalently bonded sulfur and arsenic atoms, lying normal to the (010) direction. Van der Waals forces are responsible for inter-layer cohesion. Bond distances and angles are closely similar in the two structures, the mean As–S bond distances being 2.237(2) Å in realgar and 2.283(5) Å in orpiment.

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