The crystal chemistry of romeite

Romeite (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 A(2-m)B(2)X(6-w)Y(1-n). pH(2)O. 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 romeite 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 romeite 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 romeite from Massiac, derived from weathering of stibnite, contains significant H2O (up to 14 wt%). The A-site vacancies in romeite appear to be controlled by the primary conditions of crystallization, and not by post-crystallization alteration. The Y-site chemistry of romeite 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 romeite with a mineral structurally related to pyrochlore, which grows at the expense of romeite.