Crystal chemistry and OH defect concentrations in spodumene from different granitic pegmatites

摘要

Thirty spodumene samples of distinct paragenetic types (primary magmatic, secondary after petalite and hydrothermal) from variety of granitic pegmatites were characterized by electron microprobe, polarized FTIR spectroscopy and Mössbauer spectroscopy. The FTIR spectra of OH (weak sharp pleochroic bands at 3,425, 3,410, 3,395 cm−1 and in the 3,500–3,470 spectral region) are strongly polarized with maximum absorption parallel to nγ. The majority of OH dipoles are presumably generated by a partial replacement of O2 oxygen atoms with an orientation pointing above the Li vacancy site. The separation of the bands probably resulted from a replacement of the coordinating Al by Fe and Si by Al. Homogeneous spodumene mostly close to its ideal formula LiAlSi2O6 shows Fe (0.00–0.10 apfu as Fe3+; Fe3+ Fe2+) and Na (0.00–0.04 apfu) as the only minor cations and Fe3+Al−1 substitution up to 10 mol% of the LiFe3+Si2O6 component. Hydrogen concentrations (from 0.1 up to <5 ppm H2O by weight) vary as a function of genetic type with the highest amounts in high-temperature magmatic spodumene. Differences among particular genetic types of spodumene are related to maximum solubility of OH in spodumene structure at given P–T conditions and at actual chemical composition of spodumene. OH defect concentrations in spodumene follow a trend, LT/LP pyroxenes containing lower hydrogen contents compared to HT/HP ones. The hydrogen contents in particular genetic types of spodumene and their decrease with decreasing T and P are consistent with petrologic models of the pegmatite (sub)types formations.