Chrysoberyl from the Tablada I pegmatite, Pocho pegmatitic group, Altautina district (Córdoba province, Argentina)

Abstract

Chrysoberyl, ideally BeAl₂O₈, has been found at the Tablada I pegmatite (31º 26' 07.3” S - 65º 18' 11.5” W), belonging to the Pocho pegmatite group, Altautina district, Córdoba Province. It occurs in the middle intermediate zone, associated with quartz and (rarely) microcline, as tabular greenish yellow crystals reaching 40 mm ´ 2 mm. In addition to Al, electron microprobe analyses showed Fe (2.31-3.88 wt.% Fe₂O₃, 0.037-0.062 apfu Fe³⁺) and Ti (<0.04-0.71 wt.% TiO₂, up to 0.011 apfu Ti). The Fe content is unusually high for chrysoberyl. Most Fe is assumed to be trivalent; however, a small fraction could be divalent and enter associated to Ti according to the scheme 2Al³⁺ « Ti⁴⁺ + Fe²⁺. Gallium, Sn and Ta are present at low levels (up to 599, 207 and 345 ppm, respectively) consistent with the geochemical signature of the pegmatite. There is a good correlation between Sn and Ta. Unit cell dimensions of chrysoberyl (in space group Pbnm) are a = 4.4312(4), b = 9.4216(7), c = 5.4843(4) Å, V = 228.97(4) ų. The paragenesis chrysoberyl + quartz is magmatic and formed under P_H2O<P_total. Textural evidences suggest that chrysoberyl does not derive from metamorphism of magmatic beryl. Chrysoberyl crystal rims are replaced by fine-grained muscovite and beryl, reflecting an open- system reaction. Scarce bertrandite has also been confirmed by WDS, included in quartz.