Anais da Academia Brasileira de Ciências
versão impressa ISSN 0001-3765
versão On-line ISSN 1678-2690
FERREIRA, Ana C.M.; FERREIRA, Valderez P.; SOARES, Dwight R. e VILARROEL-LEO, Hugo S.. Chemical and mineralogical characterization of elbaites from the Alto Quixaba pegmatite, Seridó province, NE Brazil. An. Acad. Bras. Ciênc. [online]. 2005, vol.77, n.4, pp.729-743. ISSN 0001-3765. http://dx.doi.org/10.1590/S0001-37652005000400011.
The Alto Quixaba pegmatite, Seridó region, northeastern Brazil, is a 60º/80ºSW-trending subvertical dike discordantly intruded into biotite schists of the Upper Neoproterozoic Seridó Formation. It has three distinct mineralogical and textural zones, besides a replacement body that cuts the pegmatite at its central portion and in which occur, among other gem minerals, colored elbaites. Elbaites usually occur as prismatic crystals, elongate according to the c-axis, with rounded faces and striations parallel to this axis. Optically, crystals are uniaxial negative with strong pleochroism; refractive index extraordinary axis = 1.619-1.622 and ordinary axis = 1.639-1.643, birefringence between 0.019 and 0.021, average relative density of 3.07, and the following unit cell parameters: ao = 15.845 Å, co = 7.085 Å and V = 1540.476 Å. There is alkali deficiency in the X site of 12-17%. The elbaites are relatively enriched in MnO (1.69 to 2.87%) and ZnO (up to 2.98%).
Palavras-chave : elbaite; tourmaline; Seridó pegmatitic province.