Pezzottaite, Cs(Be_2Li)Al_2Si_6O_(18), A spectacular new beryl-group mineral from the Sakavalana pegmatite, Fianarantsoa Province, Madagascar

摘要

Pezzottaite is a new mineral from the Sakavalana pegmatite, located 25 km south of the village of Mandosonoro, southwest of the town of Aritsirabe, 140 km southwest of Ambatofinandrahana, in Fianarantsoa province, central Madagascar. It usually occurs as isolated crystals that can have three distinct habits: (1) irregularly shaped flat masses that fill cavities between "cleavelandite", quartz and tourmaline; (2) subhedral-to-euhedral hexagonal tabular crystals up to 10 cm in diameter; and (3) small flat to equant to elongated crystals attached to faces of large tourmaline crystals. The form {001} is dominant, with minor {100} and {101}; no twinning was observed. Pezzottaite is moderate red (Munsell #15) to pink, with moderate dichroism in polarized light in hand specimen: omega = pink-orange and epsilon = purplish pink to pinkish purple. The streak is colorless to white, crystals are transparent to translucent with a vitreous luster and no observable fluorescence in long-and short-wave ultraviolet light. Crystals are brittle with both conchoidal and irregular fracture, have an imperfect cleavage parallel to {001}, and no observable parting. Mohs hardness is 8, the observed density is 2.97-3.14 g/cm~3 and the calculated density is 3.06 g/cm~3. Pezzottaite is uniaxial negative with epsilon = 1.601-1.611 and omega = 1.612-1.620, depending on Cs content. In transmitted plane-polarized light, it is strongly pleochroic, orange-red |epsilon and purple-violet ||omega. Pezzottaite is rhombohedral, space group R3c, with the following unit-cell parameters refined from X-ray powder-diffraction data: a 15.946(4), c 27.803(8) A, V6122(2) A~3, Z = 18. The ten strongest lines in the X-ray powder-diffraction pattern are as follows: d (A), I, (hkl): 3.271, 100, (036); 2.871, 52, (T53); 3.027, 41, (146); 3.09, 29, (T50); 2.215,14, (270); 1.636,14, (0.6.12); 2.229,12, (T2.12); 1.749, 12, (36.12); 1.743, 12, (390); 1.518, 11, (399). Chemical analysis by electron microprobe and ICP (Li) gave SiO_2 55.55, A1_2O_316.00, Sc_2O_3 0.03, MnO 0.04, Na_2O 0.19, K_2O 0.04, Rb_2O 0.64, Cs_2O 16.12, Li_2O 2.16, BeO_(calc) 7.95, H_2O 0.28, sum 99.00 wt. percent, where the amount of H_2O was determined by crystal-structure analysis. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is (Cs_(0.74)Rb_(0.04)K_(0.01)Na(0.04)_(SIGMA0.83)(Be_(2.06)Li_(0.94))Al_(2.04)Si_(6.00)O_(18-) (H_2O)_(0.10). Chemical analysis by LA-ICP-MS gave SiO_2 54.58, TiO_2 0.01, A1_2O_3 16.88, FeO 0.02, MnO 0.02, CaO 0.22, Nap 0.46, K_2O 0.14, Rb_2O 0.44, Cs_2O 18.23, Li_2O 2.12, BeO 8.14 sum 101.26 wt. percent. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is (Cs_(0.84)Rb_(0.03)K_(0.02)Na_(0.10))_(SIGMA0.98)(Be_(2.10)Li_(0.92))_(SIGMA_3.02)Al_(2.00)(Si_(5.86)Al_(0.14))O_(18). The end-member formula of pezzottaite is Cs(Be_2Li)Al_2Si_6O_(18). The mineral is named for Dr. Federico Pezzotta of the Museo Civico, Milano, Italy, for his major role in characterizing the granitic pegmatites of Madagascar. The new mineral and mineral name have been approved by the Commission of New Minerals and Mineral Names of the International Mineralogical Association (2003-022). Pezzottaite is related to the minerals of the beryl group, but differs in having essential Cs and a superstructure that arises from ordering of Be and Li in tetrahedral coordination.