CONNECTIONS AMONG EXPERIMENTALLY DETERMINED PHASE EQUILIBRIA, THE THERMODYNAMIC PROPERTIES OF MINERALS, AND NEWTONIAN BEHAVIOR

A principal interest of this laboratory over the years has been the thermodynamic mixing properties of mineral series. Our main contribution to this work has been the determination of enthalpies of mixing via solution calorimetric measurements at 50 °C under isoperibolic conditions in HF or HCl acid solutions. Among the mineral and glass series studied are alkali feldspar series of various Al-Si ordering states, nepheline-kalsilite series with a range of Al:Si ratios, Rb- and Cs-leucite-analcime analogue series, muscovite-paragonite micas, hydrated alkali aluminosilicate glasses, anhydrous Qz-Or-Ab glasses, sodium silicate glasses, and fluor-chlorapatite crystalline solutions.

Despite our interest in enthalpy, the principal quantities that determine homogeneous phase equlibria in mineral series are the Gibbs free energies of mixing. Although these cannot be measured directly, they can be known indirectly through experimentally reversed phase equilibria. Combination of the latter with directly-measured enthalpies of mixing leads to knowledge of entropies of mixing for the various series. The entropies, in turn, can be divided into vibrational and configurational components, the former determinable through low-temperature heat capacity measurements, the latter from crystallographic site occupancies measured as a function of temperature. Alternatively, directly-measured enthalpies and entropies of mixing can be combined with Gibbs free energies of mixing determined from phase equilibria to infer either the existence or nonexistence of short-range order in minerals. Overall, then, carefully determined reversed phase equilibria are critical to a thermodynamic and structural understanding of minerals. It goes without saying that Robert Newton has been a leader in experimental petrology, having made numerous and valuable contributions to the understanding of phase equilibria, and in turn to the thermodynamic qualities of minerals. This will be demonstrated by the elevated-pressure solvus reversals of Goldsmith and Newton for the feldspar system (1974, An experimental determination of the feldspar solvus, in The Feldspars, editors MacKenzie and Zussman, Manchester University Press, pp. 337-359), with the addition of data from other workers.