Villamaninite, a case of noncubic pyrite-type structure

Abstract

This paper reports the results obtained in a study of the crystal structure of two villamaninite samples from Villamanín (León, Spain), labeled (1) and (2). Villamaninite, (Cu,Ni,Co,Fe)(S,Se)2, is a pyrite-type disulfide. Different long-period elements, including Au, in ionic substitution are also observed. Previous authors have assumed a cubic Pa{\bar 3} symmetry for this mineral. The result of our single-crystal study shows a deviation from the cubic symmetry Pa{\bar 3} pyrite-type to a pseudocubic symmetry, which is in agreement with the observed optical anisotropy shown by both samples. The structural refinement process leads to a monoclinic model, space group P1211, with a = 5.709 (2), b = 5.707 (2), c = 5.708 (2) Å, β = 90.01 (1)° for sample (1),° and a = 5.704 (3), b = 5.703 (3), c = 5.704 (3) Å, β = 89.99 (2)° for sample (2), with Z = 4. Previous Mössbauer spectroscopic studies stating two different cation sites for Au support the monoclinic model.