9706326 Evans This project will extend the PIs work on the crystallographic and thermodynamic properties of the aluminum-free "quadrilateral" amphiboles into the Al-bearing orthorhombic gedrite series and the monoclinic hornblende series, both of which occur widely in crystalline rocks. X-ray single crystal structure refinements will be done on suites of natural samples encompassing a range of Mg/Fe ratios and Al-contents, prior to and after equilibration at 700o-1000oC and high pressure. Site scattering and bond length data will be incorporated with compositional data to provide a description of octahedral and tetrahedral site order-disorder as a function of temperature. The Gibbs energy of these complex amphiboles is written to explicitly include the measured configurational entropy, as well as end-member, reciprocal and reciprocal-ordering terms. Experimental, thermochemical and natural phase equilibrium data provide additional constraints. These data allow calculation of phase diagrams for magmatic and subsolidus systems involving amphiboles and other minerals for which there are previously developed consistent thermodynamic models.
