CRYSTAL CHEMISTRY OF MINERALS OF THE WÖHLERITE GROUP FROM THE LOS ARCHIPELAGO, GUINEA

As part of the SYNTHESYS European project, a suite of samples from the nepheline syenites of the Los Archipelago, Guinea, was studied in order to characterize the crystal chemistry of the minerals belonging to the wöhlerite group. Chemical and X-ray studies permitted the identification of four different mineral species: låvenite, normandite, wöhlerite, and hiortdahlite I. The identification of hiortdahlite II reported by Robles et al. (2001) seems to be questionable. Låvenite and normandite occur in the agpaitic suite, in association with analcime, catapleiite, fluorite, mosandrite, sérandite, sphalerite, and villiaumite; wöhlerite and hiortdahlite I are associated with britholite-(Ce), magnetite, titanite, and zircon. All the studied låvenite and normandite crystals are intermediate terms in the låvenite-normandite series. Wöhlerite shows the highest Mn content ever recorded for this species. Hiortdahlite I is the poorest in terms of both Zr and Na content and the richest in Ca and F among the known occurrences of "hiortdahlite". The crystal structures of these four phases were refined, with R factors ranging from 1.9% (for Mn-rich wöhlerite) to 7.3% (for hiortdahlite I). The wide range in Zr-Ti ratios observed in the låvenite-normandite series using new data and all available chemical data, not reported in other Zr-Ti minerals, seems to be related to the contemporaneous substitution of Mn by Ca. The comparison of the geometry of (Nb,Ti)- and Zr-centered polyhedra, including all the available structural studies of the wöhlerite group, indicates it is possible to make a distinction between (Nb,Ti)-dominant and Zr-dominant sites in this mineral family.