BERYL FROM MIAROLITIC POCKETS OF GRANITIC PEGMATITES, ELBA, ITALY: CHARACTERIZATION OF CRYSTAL CHEMISTRY BY MEANS OF EMP AND SIMS ANALYSES

Crystals of beryl from miarolitic pockets, hosted in pegmatitic dikes on the Island of Elba, in Italy, were analyzed for the major and minor elements, including the light elements, by EMPA and SIMS techniques. Back-scattered electron (BSE) images and X-ray maps were used to define the complex textures of these crystals and to highlight the pattern of element distribution in each sample. The majority of the analyzed crystals exhibit a complex pattern of zoning caused by changes in the chemical composition, which follow the main mechanisms of substitution defined for the crystal chemistry of beryl. The following main features are illustrated: (1) the occurrence of crystals with octahedral-site or tetrahedral-site substitutions in different domains of the same crystal, (2) the occurrence of crystals with exclusively tetrahedral or exclusively octahedral substitutions, and (3) the occurrence of crystals or crystal domains having a limited extent of octahedral-site and tetrahedral-site substitutions. However, the analytical data confirm the presence of a gap between the two series, having octahedral-site or tetrahedral-site substitutions. The observed enrichment in Cs and Li is strictly related to the geochemical and petrological features of the LCT host pegmatites. The presence of Ca in some beryl crystals in pegmatites emplaced in carbonate rocks suggests an interaction of the late hydrothermal fluids with country rocks. An enrichment in Fe could also be ascribed to the circulation of fluids having such a contaminant element attributed to the contribution of country rocks or the destabilization of early-crystallized Fe-rich minerals.