The role of isomorphous substitutions in natural selenides belonging to the pyrite group

The present paper reports chemical and structural data of selenide minerals belonging to the pyrite group. Eighteen samples of minerals in this group with variable chemical composition (7 samples of penroseite, NiSe2; 10 samples of krutaite, CuSe2; 1 sample of trogtalite, CoSe2) were studied by means of X-ray single-crystal diffraction and electron microprobe. On the basis of information gained from the chemical characterization, we can conclude that a complete solid solution between NiSe2 and CuSe2 exists in nature with the absence of pure end-members. Although verified only for the Ni-rich members, we also infer a solid solution between NiSe2 and CoSe2. The unit-cell parameters were modeled using a multiple regression method as a function of the Co, Ni, and Cu contents