LA MORGANITA DE LA PEGMATITA LAS TAPIAS, CÓRDOBA (ARGENTINA): UN CASO DE BERILO NO HEXAGONAL

COLOMBO, FERNANDO; PANNUNZIO MINER, ELISA VICTORIA; CARBONIO, RAÚL E.; SFRAGULLA, JORGE; LIRA, RAÚL

Río Cuarto

Congreso; 7º Congreso de Mineralogía y Metalogenia; 2004

Departamento de Ciencias Geológicas (UBA) y Departamento de Geología (Univ. Nac. de Río Cuarto)

The Las Tapias pegmatite (31º56´32´´ S, 65º05´08´´ W) is an LCT-type body which hosted one of the largest beryl concentrations in the world (over 3,200 tones). Pink beryl (morganite) occurs very infrequently as palest to light pink, translucent fillings or replacements related to cracks in greenish to off-white beryl masses. EDS analyses show that the morganite contains Na and Cs, while the greenish beryl contains traces of Fe in addition to Na and Cs. Refractive indices and c/a = 1.001 (hexagonal setting) point to an alkali-bearing beryl, as morganite commonly is. Departure from the hexagonal symmetry is indicated by biaxial optic figures and the presence of small peaks (at 8.314, 8.209, 8.106, 4.652, 3.600, 3.338 and 3.171 Å) in the powder X-ray diffraction pattern, which could not be indexed in space group P/6mcc with a unit cell similar to that reported for truly hexagonal beryl. The observed reflections (except for a very minor peak at 8.83 Å) can be reproduced with a monoclinic cell a =18.5569(8), b = 16.1021(8), c = 9.1991(2), beta 90.500(3), V = 2748.6(2). Heating to 900ºC for 16 hours resulted in merging of some reflections. No definite conclusion can be drawn about the origin of biaxiality, but it seems to be rather widespread in beryl from pegmatites and warrants further investigation.