The crystal chemistry of roméite

Roméite (Ca, Fe, Mn, Na)2(Sb5+, Ti4+)2(O, OH, F)7 is a rare mineral found in metamorphic iron-manganese deposits and in hydrothermal Sb-bearing veins. It is isostructural with the pyrochlore-group minerals of the general formula A2–mB2X6–wY1–n · pH2O. The pyrochlore-group minerals are important Nb and Ta ores, and are also used as an actinide host phase in␣radioactive waste. The crystal chemistry of roméite from the type locality Praborna (Italy), from Massiac (France), and from four newly discovered localities in␣the Swiss Alps, and of “lewisite”, a questionable species related to roméite from Tripuhy (Brazil), is compared to that of pyrochlore. A wide range of substitutions has been observed including (1) independent substitutions on the A- and B-sites, and (2) coupled substitutions between the A- and B- and between the A- and Y- sites. Only the roméite from Massiac, derived from weathering of stibnite, contains significant H2O (up to 14 wt %). The A-site vacancies in roméite appear to be controlled by the primary conditions of crystallization, and not by post-crystallization alteration. The Y-site chemistry of roméite varies from locality to locality; it can be dominated by F, OH, or be fully vacant. The “lewisite” octahedral crystals studied are a sub-microscopic mixture of roméite with a mineral structurally related to pyrochlore, which grows at the expense of roméite.