Limousinite, Baca[Be4p4o16]Center Dot 6h(2)O, A New Beryllophosphate Mineral With A Phillipsite-Type Framework

Limousinite, ideally BaCa[Be4P4O16]center dot 6H(2)O, is a new beryllophosphate mineral discovered in the Vilatte-Haute pegmatite, Chanteloube near Razes, Limousin, Haute-Vienne, France. The new mineral is intimately associated with microcrystalline pale brown greifensteinite, black amorphous vitreous Mn-oxyhydroxide, triplite, and quartz. It forms isolated, partly corroded, colorless to snow-white crystals up to 0.9 mm long, showing rhombic cross sections. Limousinite is transparent with a vitreous luster, non-fluorescent, without cleavage planes; its calculated density is 2.58 g/cm(3). Optically, the mineral is biaxial negative, alpha = 1.532(2), beta = 1.553(3), gamma = 1.558(2) (measured under 589 nm wavelength light), 2V(calc.) = 18 degrees, non-dispersive, with Z parallel to the elongation of the prismatic crystals. Electron-microprobe analyses indicate an empirical formula of (Ba0.91K0.07)(Sigma 0.98)(Ca0.87Na0.05)(Sigma 0.92)[(Be3.87Al0.13)(Sigma 4)P4O16]center dot 5.56H(2)O, calculated on the basis of 4 P atoms per formula unit, assuming 4 (Be + Al) pfu and a water content calculated from refined site-occupancy factors. A single-crystal structure refinement was performed to R-1 = 4.90%, in the P2(1)/c space group, with a = 9.4958(4), b = 13.6758(4), c = 13.4696(4) angstrom, beta = 90.398(3)degrees, V = 1749.15(10) angstrom(3), Z = 4. The crystal structure is characterized by a beryllophosphate framework similar to that of phillipsite-group zeolites, based on corner-sharing BeO4 and PO4 tetrahedra forming interconnected four- and eight-membered rings. Large cages within this zeolite framework contain Ba, Ca, and water molecules. Limousinite is the third known natural zeolite-type beryllophosphate, together with pahasapaite and wilancookite; it is also the first phosphate with a framework identical to that of a natural zeolite silicate.