VILLAMANINITE, A CASE OF NONCUBIC PYRITE-TYPE STRUCTURE

This paper reports the results obtained in a study of the crystal stru cture of two villamaninite samples from Villamanin (Leon, Spain), labe led (1) and (2). Villamaninite, (Cu,Ni,Co,Pe)(S,Se)(2), is a pyrite-ty pe disulfide. Different long-period elements, including Au, in ionic s ubstitution are also observed. Previous authors have assumed a cubic P a(3) over bar symmetry for this mineral. The result of our single crys tal study shows a deviation from the cubic symmetry Pa(3) over bar pyr ite-type to a pseudocubic symmetry, which is in agreement with the obs erved optical anisotropy shown by both samples. The structural refinem ent process leads to a monoclinic model, space group P12(1)1. with a = 5.709(2), b = 5.707 (2), c = 5.708 (2) Angstrom, beta = 90.01(1)degre es for sample (1), and a = 5.704(3), b = 5.703 (3), c = 5.704(3)Angstr om, beta = 89.99(2)degrees for sample (2), with Z = 4. Previous Mossba uer spectroscopic studies stating two different cation sites for Au su pport the monoclinic model.