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The thermodynamic model of the two-feldspar thermometer has been revised. From recent enthalpy and volume measurements in the (Na,Ca)-and (K,Ca)-feldspar binaries, new interaction parameters have been derived and previous ones have been updated. Entropy parameters have been fi tted to the phase equilibrium data of Seck (1971) and Elkins and Grove (1990). The two data sets could be suitably combined into one. Ideal Ab, Or, and An activities have been expressed in terms of both the molecular mixing and Al-avoidance models.Two-feldspar pairs from high-grade metamorphic rocks that cooled slowly under dry conditions suffer from a distinct type of retrograde resetting. Whereas the original An content in both the plagioclase and the alkali feldspar is preserved because the intercrystalline Ca + Al ↔ (Na,K) + Si diffusion is sluggish, Na and K may be freely exchanged between phases. Mathematical reversal of the Na-K exchange at constant An yields the temperature at which the two feldspars originally coexisted. The shifts in Ab and Or contents obtained from the reversal refl ect the relative plagioclase/alkali feldspar proportions observed in thin sections. Good agreement between calculated and measured ratios was found for feldspar pairs from Sri Lankan granulites. This observation represents a successful test of the reliability of the calculated Ab-Or shifts.In contrast to dry metamorphic rocks, similar application of chemical constraints is not indicated in the case of volcanic rocks. Then the two-feldspar thermometer delivers three, usually incongruent temperatures: T(Ab), T(Or), and T(An). From the abundance of temperatures, Fuhrman and Lindsley (1988) suggested adjusting compositions within assumed chemical uncertainties (e.g., ±2 mol%) so that congruent temperatures could be obtained. However, the result is not unique. Depending on minute variations in the starting compositions, the temperatures may vary by several tens of degrees. In addition, temperatures vary to a similar extent depending on the type of search algorithm. Therefore, we advise users to completely abandon this practice. Instead, a statistical procedure is suggested: Two-feldspar compositions are randomly generated according to Gaussian distributions with their means at the observed compositions and standard errors chosen according to the quality of the chemical analysis. This procedure returns normally distributed temperatures [T(Ab), T(Or), T(An)] together with means and standard deviations. From the overlap of the three Gaussian curves the question of equilibrium or non-equilibrium crystallization of feldspar pairs may be addressed.

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Plagioclase undergoes complex exsolution and ordering and phase transition processes during their evolution in nature, and this has hindered attempts to define simple trends relating the major peaks of their Raman spectra with composition. Here, the peak position and linewidth of major Raman features have been calibrated for a set of 20 plagioclases, spanning from albite to anorthite in composition, with symmetry and ordering states that were already well characterized. Point group symmetry is the most important factor determining the Raman peak behaviour with composition, though C true1¯, I true1¯, and P true1¯ plagioclases show different trends for the position of the main peak νa at ~500 cm−1. Using a simplifying approach, which merges the effect of Al–Si ordering and incommensurate modulations, a method has been developed to estimate the plagioclase composition from calibration of a few determinative Raman peaks. This makes use of the wavenumber difference Δab between the most intense peaks νa and νb around 500 cm−1, the linewidth Гa of the strongest νa peak, and the wavenumber difference Δcb between νc and νb peaks, where νc is a Raman feature at ~560–580 cm−1. The calibration was completed from data sets composed of spectra from metamorphic to pegmatitic plagioclase. The results were then tested against a further data set, mostly made by volcanic plagioclase. In most samples, the difference between electron micro probe analysis (EMPA) and Raman compositions is less than 5%. Higher residuals (beyond 10%) are observed for intermediate plagioclase, suggesting that some differences in Δab exist between volcanic and metamorphic plagioclase of intermediate compositions. The Raman compositional results for a plagioclase from Marsili submarine volcano agree with composition and zoning found from the analysis by laser ablation.

Highlights► A method for low-temperature heat capacity measurements on powders is presented. ► The method is based on combined PPMS and DSC measurements. ► It allows entropy determinations on powders with an accuracy of better than 1%. ► The sample is not mixed with Apiezon N grease during measurement. ► This is useful for samples that are only available in tiny amounts.