Abstract

The crystal structure of getchellite, As0.98Sb1.02S3.00, from the type-locality (the Getchell mine, Humboldt County, Nevada), monoclinic, P21/a, a = 11.949(3), b = 9.028(1), c = 10.130(2) Å, β = 116.15(1)°, V = 980.9(4) Å3, Z = 8, was solved by direct methods and refined with full-matrix leastsquares techniques to R = 0.058 and Rw = 0.062 for 505 observed reflections [I0> 5σ (I0)] collected using Mo radiation. The structure is made up of (As,Sb)S3 trigonal pyramids forming an eight-membered (As,Sb)8S8 ring; the sheets formed by the eight-membered rings are connected to each other by S1 and S6 atoms are parallel to (001). The sheet structure is responsible for the cleavage and twin planes of getchellite. The cations predominantly occupy the M2 site in natural getchellite, because the M2S6 polyhedron has the most distorted (irregular) geometry compared with other metal polyhedra in the structure. The amount of As incorporated at the M1 sites correlates strongly with M2 site composition. An important feature of As3+ sulfide minerals is that the crystal structures are composed of a 3-dimensional network consisting not only of AsS3 pyramids but also containing trigonal pyramids of other constituent cations because the structures are governed strongly by the stereochemical requirement of As3+ cations for MS3 trigonal pyramids.

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