Synthetic amphiboles and triple-chain silicates in the system Na_2O-MgO-SiO_2-H_2O: 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 Na_2O-MgO-SiO_2-H_2O (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 Na_2Mg_6Si_8O_(22)(OH)_2 and Na_3Mg_5Si_8O_(21)(OH)_3. We show that there is extensive solid solution involving excess H at 650—750℃, but also document the presence of a wide miscibility gap below 600℃. This miscibility gap is defined by amphiboles very close to the end-member composition Na_3Mg_5Si_8O_(21)(OH)_3 coexisting with amphiboles with compositions near the 'normal' Na_2Mg_6Si_8O_(22)(OH)_2end member. We also report the characterization of triple-chain silicates (TCS) in the NMSH system and their phase relations with NMSH amphiboles. The upper thermal stability field of the key TCS Na_2Mg_4Si_6O_(16)(OH)_2 relative to its decomposition to two NMSH amphiboles with a combined equivalent composition has been determined and a pronounced backbend of the transformation boundary documented. Phase relations observed in synthesis experiments suggest that at 550—650℃ all TCSs have compositions close to Na_2Mg_4Si_6O_(16)(OH)_2. Infrared spectroscopy indicates that the TCS synthesized on this composition, studied in detail here, vary from end-member Na_2Mg_4Si_6O_(16)(OH)_2 to binary solid solutions with less than approx 6 mol.% clinojimthompsonite component. No clear spectroscopic evidence for a 'Drits' component NaMg_4Si_6O_(15)(OH)_3 (Drits et al., 1975) has been found. Analysis of H_2O by vacuum extraction and Karl-Fischer titration indicates large excesses of H_2O in all the TCSs studied here that clearly exceed the amounts expected from (OH) groups alone. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) indicate that this excess H_2O is structural. We propose that the excess H_2O is likely to be molecular H_2O located in the A-site channels. The observed backbend of the triple-chain decomposition curve is in agreement with a reaction involving dehydration and loss of this molecular H_2O. However, the absolute amount of analysed molecular H_2O exceeds that expected from the change in Clapeyron slope alone. While demonstrating the reality of excess OH in amphiboles, the evidence presented in this paper also points to interesting avenues for future research on both amphiboles and TCSs, such as understanding the dynamics and enhanced crystal chemistry of excess OH and molecular H_2O in pyriboles.