LIMOUSINITE, BaCa[Be4P4O16].6H2O, A NEW BERYLLOPHOSPHATE MINERAL WITH A PHILLIPSITE-TYPE FRAMEWORK

Limousinite, ideally BaCa[Be 4 P 4 O 16 ]Á6H 2 O, is a new beryllophosphate mineral discovered in the Vilatte-Haute pegmatite, Chanteloube near Razès, 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, a ¼ 1.532(2), b ¼ 1.553(3), c ¼ 1.558(2) (measured under 589 nm wavelength light), 2V calc. ¼ 188, non-dispersive, with Z parallel to the elongation of the prismatic crystals. Electron-microprobe analyses indicate an empirical formula of (Ba 0.91 K 0.07) R0.98 (Ca 0.87 Na 0.05) R0.92 [(Be 3.87 Al 0.13) R4 P 4 O 16 ]Á5.56H 2 O, 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 R 1 ¼ 4.90%, in the P2 1 /c space group, with a ¼ 9.4958(4), b ¼ 13.6758(4), c ¼ 13.4696(4) Å, b ¼ 90.398(3)8, V ¼ 1749.15(10) Å3 , Z ¼ 4. The crystal structure is characterized by a beryllophosphate framework similar to that of phillipsite-group zeolites, based on corner-sharing BeO 4 and PO 4 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.