EDGARITE, FeNb_3S_6, FROM THE KHIBINA ALKALINE COMPLEX, RUSSIA: SOLUTION OF THE CRYSTAL STRUCTURE

摘要

The crystal structure of edgarite, a rare Fe-Nb sulfide mineral, was solved using intensity data collected from a crystal from the Khibina alkaline massif, Russia. The study revealed that the structure is hexagonal, space group P6_3, with cell parameters a 5.912(1), c 12.675(2) A, and V 383.7(1) A3. Refinement of an anisotropic model led to an R index of 0.0392 for 580 independent reflections. One Fe and three Nb sites occur in the crystal structure of edgarite. Edgarite is shown to have the formula FeNb_~(3+)Nb~(4+)S_6. Niobium can be expected to be chalcophile in unusually reducing crustal assemblages and to be trivalent and tetravalent in the mantle.