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Home , Pezzottaite

Pezzottaite, Cs(Be Li)Al Si O , 2 2 6 18 ASpectacularNewMineral Related to the group, from Madagascar

Frank C. Hawthorne and Mark A. Cooper Geo l og i ca l Sc i ences , Un i ve r s i t y o f Man i t oba W i nn i peg , Man i t oba , Canada R3T 2N2

William B. (Skip) Simmons and Alexander U. Falster Geo l ogy and Geophys i cs , Un i ve r s i t y o f New Or l eans New Or l eans , LA 70148 , USA

Brendan M.Laurs Gemo l og i ca l I ns t i t u t e o f Ame r i ca , Ca r l sbad , CA 92008 , USA

Thomas Armbruster Lab . Chem . M i ne r . Kr i s t a l l og r . , Un i ve r s i t y o f Be r n , F r e i es t r . 3 CH - 3012 Be r n , Sw i t z e r l and

George R. Rossman

D i v i s i on o f Geo l og i ca l and P l ane t a r y Sc i ences Ca l i f o r n i a I ns t i t u t e o f Techno l ogy Pasadena , CA 91125 , USA

Adi Peretti

GRS Gemr esea r ch Sw i ss l ab L t d . , P i l a t uss t r . 17 , P . O . Box 4028 CH - 6003 Luce r ne , Sw i t z e r l and Detlef Günter

Labo r a t o r y o f I no r gan i c Chem i s t r y , ETH Hohgge r be r g , HC I , G113 8093 Zu r i ch , Sw i t z e r l and

Bernard Grobety

Depa r t men t o f Geosc i ences , Un i ve r s i t y o f F r i bou r g , Pe r o l l es CH - 1700 F r i bou r g , Sw i t z e r l and

ABSTRACT

Pezzottaite is a newmineral from theSakavalanamine3 km southof the village of Mandosonoro, close to the town of Antsirabe, 140 km west of Ambatofinandrahana, central Madagascar. It usually occurs as isolated that can have three distinct habits: (1) irregularly shaped flat masses that fill cavities between cleavlandite, quartz and ; (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: =pink-orangeand =purplishpinktopinkishpurple.The 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 , have an imperfect 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.Pezzottaiteisuniaxialnegativewith = 1.608 and =1.616,both±0.001.Itisstronglypleochroic,orange-red|| and purple-violet || .

Pezzottaite is hexagonal, spacegroup R3c, with thefollowingunit-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.Thetenstrongestlinesinthe 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 70);1.636,14,(0612);2.229,12,(-1212);1.749,12,(-3612);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, Na2O0.52,K2O0.15,Rb2O0.76,Cs2O13.57,Li2O2.16,BeOcalc 8.05, H2O0.28,sum97.74wt.%,wheretheamountofH2Owasdetermined by -structure analysis. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is (Cs0.62 Rb0.05 K0.02 Na0.11)=0.80 (Be2.07 Li0.93)(Al1.97 Mn0.01) =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, Na2O0.46,K2O0.14,Rb2O0.44,Cs2O18.23, Li2O2.12,BeO8.14sum101.27wt.%.Theresultingempiricalformula, calculated on the basis of 18 structural O atoms, is (Cs0.84 Rb0.03 K0.02 Na0.10) =0.98 (Be2.10 Li0.92) =3.02 Al2.00 (Si5.86 Al0.14)O18.Theend- member formula of pezzottaite is Cs (Be2 Li) Al2 Si6 O18.Themineralis named for Dr. Federico Pezzotta of the Museo Civico, Milano, Italy, for his major role in characterizing the granitic of Madagascar. Pezzottaite has been approved by the Commission of New and 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.