Abstract
Experiments at 1 to 2 kbar total pressure, 600-850 °C aimed at exchanging fluorine and hydroxyl ions in tremolite under controlled conditions of HF and H2O fugacities have only been partially successful. The fluorine buffer technique of Munoz and Eugster was used in the gas system O-H-F with the solid assemblages anorthite + fluorite + sillimanite + quartz (higher HF/H2O) and wollastonite + fluorite + quartz (lower HF/H2O). Melting of the first buffer at a temperature greater than 700°C at 2 kbar limits its utility.
Fluorine-hydroxyl exchange is much slower in tremolite compared to reported rates for biotites. The slow rate of exchange may be due to the structural differences between sheet and chain silicates, lack of A-site occupancy in this amphibole, and grain size effects. Tremolite also extracts less fluorine from a coexisting vapor than does phlogopite.
A breakdown loop involving the production of talc marks the lower thermal stability of intermediate F/OH tremolite. This loop was found at 700°C, 1 kbar for intermediate (F,OH) tremolite, and could exist at a higher temperature for pure fluortremolite.
