Limusinite, BaCa[Be4P4O16]*6H2O, a new beryllophosphate mineral with a phillipsite-type framework

Limousinite, ideally BaCa[Be4P4O16]6H2O, is a new beryllophosphate mineral discovered in the Vilatte-Haute pegmatite, Chanteloube near Raz`es, 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/cm3. Optically, the mineral is biaxial negative, a ¼ 1.532(2), b ¼ 1.553(3), c ¼ 1.558(2) (measured under 589 nm wavelength light), 2Vcalc. ¼ 188, non-dispersive, with Z parallel to the elongation of the prismatic crystals. Electron-microprobe analyses indicate an empirical formula of (Ba0.91K0.07)R0.98(Ca0.87Na0.05)R0.92[(Be3.87Al0.13)R4P4O16]5.56H2O, calculated on the basis of 4 P atoms per formula unit, assuming 4 (Be þ Al) pfu and a water content calculated from refined site-occupancy factors. A single-crystal structure refinement was performed to R1¼4.90%, in the P21/c space group, with a¼9.4958(4), b¼13.6758(4), c¼13.4696(4) A? , b¼ 90.398(3)8, V¼1749.15(10) ?A3, Z¼4. 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.