PEZZOTTAITE
Cs(Be2Li)Al2Si6O18
A Spectacular New Beryl-Group Mineral
From The Sakavalana Pegmatite, Fianarantsoa Province, Madagascar

Authors:
FRANK C. HAWTHORNE & MARK A. COOPER
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

WILLIAM B. (SKIP) SIMMONS & ALEXANDER U. FALSTER
Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148

BRENDAN M. LAURS
Gemological Institute of America, Carlsbad, CA 92008

THOMAS ARMBRUSTER
Lab. Chern. Mineralogie und Kristallographie, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

GEORGE R. ROSSMAN
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125

ADOLF PERETTI
GRS Gernresearch Swisslab AG, Hirschmattstrasse 6, P.O. Box 4028, CH-6003 Lucerne, Switzerland

DETLEF GÜNTER
Laboratory of Inorganic Chemistry, ETH Hõhggerberg, HCI, G113, 8093 Zürich, Switzerland

BERNARD GROBETY
Department of Geosciences, University of Fribourg, Pérolles, CH-1700 Fribourg, Switzerland


- Part 1 -
ABSTRACT
Webmaster's Note: All Figures and Tables will appear in new windows.

Pezzottaite is a new mineral from the Sakavalana pegmatite, located 25 km south of the village of Mandosonoro, southwest of the town of Antsirabe, 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: ω = pink-orange and ε = 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/cm3 and the calculated density is 3.06 g/cm3. Pezzottaite is uniaxial negative with ε = 1.601-1.611 and ω = 1.612-1.620, depending on Cs content. In transmitted plane-polarized light, it is strongly pleochroic, orange-red ||ε and purple-violet ||ω.

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)Å, V 6122(2) Å3, Z = 18. The ten strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I, (hkl): 3.271, 100, (036); 2.871, 52, (¯153); 3.027,41, (¯146); 3.09, 29, (¯150); 2.215, 14, (¯270); 1.636, 14, (0.6.12); 2.229,12, (¯12.12); 1.749, 12, (¯36.12); 1.743, 12, (¯390); 1.518, II, (¯399). Chemical analysis by electron microprobe and ICP (Li) gave SiO2 55.55, Al2O3 16.00, Se2O3 0.03, MnO 0.04, Na2O 0.19, K2O 0.04, Rb2O 0.64, Cs2) 16.12, Li2O 2.16, BeOca1c 7.95, H2O 0.28, sum 99.00 wt.%, where the amount of H20 was determined by crystal-structure analysis. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is:

(Cs0.74Rb0.04K0.01Na0.04)Σ0.83(Be2.06Li0.94)Al2.04Si6.00O18- (H2O)0.10.

Chemical analysis by LA-ICP-MS gave SiO2 54.58, TiO2 0.01, AI2O3 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 101.26 wt.%. The resulting empirical formula, calculated on the basis of 18 structural O atoms, is:

(Cs0.84Rb0.03K0.02Na0.10)Σ0.98(Be2.10Li0.92)Σ3.02Al2.00(Si5.86Al0.14)O18.

The end-member formula of pezzottaite is Cs(Be2Li)Al2Si6O18.

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.

Paper First Published in
"The Mineralogical Record",volume 35, September-October 2004.

TopAbstract, Part 2, Part 3, Part 4, References
Back to Publications


Last Updated: August 22, 2006
Webmaster: Raymond Sprague
For more E-Mail information & Inquiry: Skip Simmons, Ph.D.
URL: http://www.uno.edu/pegmatology/
Copyright - ©2005 Raymond Sprague - All Rights Reserved of layout, design & revisions
Copyright - ©2005 MP2 Group- All Rights Reserved of Information, Name & Logo's