George R. Rossman
Division
of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125, USA
GRS Gemresearch
Swisslab Ltd., Pilatusstr. 17, P.O. Box 4028
CH-6003 Lucerne, Switzerland
Laboratory
of Inorganic Chemistry, ETH Hohggerberg, HCI, G113
8093 Zurich, Switzerland
Department
of Geosciences, University of Fribourg, Perolles
CH-1700 Fribourg, Switzerland
Pezzottaite is a new mineral from the Sakavalana mine 3 km south of the village of Mandosonoro, close to the town of Antsirabe, 140 km west of Ambatofinandrahana, central Madagascar. It usually occurs as isolated crystals that can have three distinct habits: (1) irregularly shaped flat masses that fill cavities between cleavlandite, quartz and tourmaline; (2) subhedral-to-euhedral hexagonal tabular crystals up to 7 cm in diameter; (3) small euhedral-to-flat-to-elongated crystals attached to faces of large tourmaline crystals. The form {001} is dominant, with minor {100} and {110}; no twinning was observed. Pezzottaite is raspberry red to pink, with moderate dichroism: ε = pink-orange and ω = 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 g/cm3 and the calculated density is 3.06 g/cm3. Pezzottaite is uniaxial negative with ε = 1.608 and ω = 1.616, both ± 0.001. It is strongly pleochroic, orange-red || ε and purple-violet || ω.
Pezzottaite is hexagonal, space group R3c, with the following unit-cell parameters refined from X-ray powder-diffraction data: a 15.973(4), b 15.973(4), c 27.803(8) Å, V 6122(2) Å3, Z = 18. The ten strongest lines in the X-ray powder-diffraction pattern are as follows: d(), I, (hkl): 3.272, 100, (0 3 6); 2.871, 52, (-1 5 3); 3.027, 41, (-2 4 6); 3.019, 29, (-1 5 0); 2.215, 14, (-2 7 0); 1.636, 14, (0 6 12); 2.229, 12, (-1 2 12); 1.749, 12, (-3 6 12); 1.743, 12, (-3 9 0); 1.518, 11, (-3 9 9). Chemical analysis by electron microprobe and ICP (Li) gave SiO2 56.52, Al2O3 15.60, Sc2O3 0.03, FeO 0.01, MnO 0.09, Na2O 0.52, K2O 0.15, Rb2O 0.76, Cs2O 13.57, Li2O 2.16, BeOcalc 8.05, H2O 0.28, sum 97.74 wt.%, where the amount of H2O was determined by crystal-structure analysis. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is (Cs0.62 Rb0.05 K0.02 Na0.11)Σ=0.80 (Be2.07 Li0.93) (Al1.97 Mn0.01) Σ=1.98 Si6.05 O18 (H2O)0.10. Chemical analysis by LA-ICP-MS gave SiO2 54.58, TiO2 0.01, Alg2O3 16.88, FeO 0.02, MnO 0.02, CaO 0.22, Na2O 0.46, K2O 0.14, Rb2O 0.44, Cs2O 18.23, Li2O 2.12, BeO 8.14 sum 101.27 wt.%. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is (Cs0.84 Rb0.03 K0.02 Na0.10) Σ=0.98 (Be2.10 Li0.92) Σ=3.02 Al2.00 (Si5.86 Al0.14) O18. The end-member formula of pezzottaite is Cs (Be2 Li) Al2 Si6 O18. 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. Pezzottaite has been approved by the Commission of New Minerals and Mineral Names of the International Mineralogical Association. 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.