CRYSTAL-CHEMICAL STUDY OF KAlSi3O8- KFeSi3O8 SOLID SOLUTION SERIES. V.K. Taroev1, V.L. Tauson1, H. Kroll2, J. Gottlicher3, 1Institute of Geochemistry, Irkutsk, 664033, Russia; 2Institute of Experimental Mineralogy, Munster Univ., D-48149, Germany; 3Institute of Technical Mineralogy, Karlsruhe, D-76021, Germany
The monoclinic mixed crystals K(Fe,Al)Si3O8 are synthesized under hydrothermal conditions from pure oxides in KOH solution at temperatures 500 and 600deg.C and pressure ca. 100 MPa. The mixed crystals up to 2 mm were obtained. An extensive formation of solid solition was observed only under sufficiently high oxygen fugacities corresponding to Cu-Cu2O buffer assemblage (up to 60 mole per cent of KFeSi3O8). The unit cell edges vs. mole fraction graphs are close to linearity, their extrapolation to monoclinic end member agrees well with the data available from the literature. Both the Mossbauer specroscopy and X-ray structure refinement studies indicate the absence of Fe2+ species, and preferential occupancy of T1 position by Fe3+ and Al3+ in the proportion of their bulk concentrations. The data obtained are in poor agreement with the well-known EberhardÙs solvus for monoclinic Al-Fe - potassium feldspar (Eberhard, 1965). The descripancy may be explained by the dispersed state of phases in EberhardÙs experiments, and so, the possibility of crystallite size effect (Tauson & Akimov, 1993) or the absence of oxygen fugacity control.
References:
Eberhard, E. (1965) Tchermaks. Min. Petr. Mitt. 10, 400-408. Tauson, V.L. & Akimov, V.V. (1993) Chem. Geol. 109, 113-118.