Abstract

The Serra Branca pegmatite in northeastern Brazil is unique because of its megacrysts of bluish-green amazonite currently exploited for ornamental purposes and as high-quality gemstones. Here we analyze microtextural, spectroscopic, and geochemical data on specimens from two magmatic generations of amazonite from this pegmatite in order to characterize their mineralogy and propose a schematic evolution diagram for twinning during their development. The amazonite consists of intergrowths of the low microcline and low albite XRD varieties that evolved according to a twin coarsening process, in which the first generation (I-tg) of ± A/P twinning developed at the monoclinic-triclinic transformation, whereas the second generation (II-tg) gave rise mainly to ± A twins. The bluish-green color of the amazonite was preserved during the process of microcline twinning. In addition to the irrational II-tg twinning, which is typically found in microcline from anorogenic settings, the serra branca amazonite crystals also show elevated contents of Rb, Pb, Fe, Cs, and Tl in both generations, whereas albitic intergrowths present high concentrations of Fe, Pb, Sr, and Ga. Finally, our chemical and spectroscopic results are compatible with the hypothesis that Pb-water centers account for the bluish green color in amazonite.

Keywords:
Serra Branca amazonite pegmatite; anorogenic pegmatite; twin evolution; spectroscopy; geochemistry