Synthetic amphiboles and triple-chain silicates in the system Na2O-MgO-SiO2-H2O: phase characterization, compositional relations and excess H

摘要

The presence of structural OH in amphiboles in excess of the usual two OH per formula has been debated for over 40 years (Gier et al., 1964; Leake et al., 1968). However, the reality of the excess-OH phenomenon is still an open question, because accurate water analyses of amphiboles are rarely available. In this study, we review the data available on the chemically simple synthetic system Na₂-MgO-SiO₂-H₂O (NMSH) and present new results from NMR, infrared spectroscopy, and X-ray-diffraction that allow re-interpretation of previous studies of NMSH amphiboles along the pseudobinary join between the two end-member compositions Na₂Mg₆Si₈O₂₂(OH)₂ and Na₃Mg₅Si₈O₂₁(OH)₃.