Abstract
Two columbite samples, space group Pbcn, with a = 14.233, b = 5.724, c = 5.104 Å, Z = 4 and a = 14.221, b = 5.727, c = 5.102 Å, of similar chemistry (average composition Fe0.65Mn0.30Ti0.05Nb1.5Ta0.5O6) but from different rock types of the Kings Mountain pegmatite, North Carolina, were investigated. One columbite specimen (NCP5) originates from a primary spodumene pegmatite, the other (NCP1) from a rock composed mainly of sugary albite formed by subsequent hydrothermal activity. Single-crystal X-ray structure refinements indicate that both columbite samples show strong disorder between (Nb,Ta) and (Fe,Mn) on octahedral A and B positions. This is in agreement with a Mössbauer spectrum (NCP5), which was fitted with two doublets characteristic of two distinct [6]Fe2+ sites. Both samples show strong chemical zoning with respect to Nb and Ta, but weak zoning of Fe and Mn. The similar chemical composition and cation ordering indicate that columbite in both rocks is formed by the same process. Late hydrothermal alteration of the original spodumene pegmatite did not affect the composition of the earlier crystallized columbite. The primary columbite (NCP5) shows slightly stronger disorder of two- and five-valent cations than columbite (NCP1) found in the hydrothermally altered part of the pegmatite.
