Abstract

Experiments with tourmaline-group minerals began with synthesis, and later, synthesis with the aim of defining pressure–temperature limits on the stability fields of end-member compositions. As a result, experiments have confirmed what has been evident from natural occurrences, that the common tourmaline-group minerals (solid solutions among schorl, dravite, olenite, foitite and, to a lesser extent, uvite) can be stable from the conditions of diagenesis through the granulite facies. However, the stability field of elbaite, the most common Li-rich species, is not known with any certainty, and most attempts at its synthesis have been unsuccessful. No tourmaline-group mineral has yet been synthesized at atmospheric pressures. A second wave of experimentation focused on the chemical attributes of environments that determine the composition and stability of tourmaline. Most notably, the stability (saturation) of tourmaline has been related to the boron content of melts and fluids, to aluminosity of melt and alkalinity of fluid, to the activities of Y-site cations, oxidation state, and activities of H2O and F in fluids and melts. As a result, the chemical stability of common tourmaline compositions can now be explained in terms of the activity or concentration products of the constituent chemical components for each structural site. Experimental studies of isotopic exchange between tourmaline and aqueous fluid have focused mostly on Δ11B tourmaline–vapor. There is one experimental study of ΔD tourmaline–vapor, but Δ18O tourmaline–vapor or melt has not yet been measured. The absence of an experimental calibration Δ18O tourmaline–vapor or tourmaline–melt is a major limitation on the utilization of tourmaline for geothermometric purposes.

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