Alkali-Deficient elbaite from pegmatites of the serido region, Borborema Province, Ne Brazil

Abstract

The chemical composition and structural formulae of elbaite from pegmatites of the Serido Pegmatitic Province, northeastern Brazil, indicate important vacancies in the X structural site, with values varying from 12 up to 44%, Na being the dominant component in this site (55-85%), Cain low amounts (<1-11%) and K in insignificant amounts (<1%). There is a linear positive correlation between vacancy in the X site and Al in the Y site, a behavior observed in tourmalines elsewhere in the world cited in the literature, and believed as due to compensation of the additional of Al in the Y-octahedral site by deficiency of cations in the X site.