Leucite: melting temperature

The ternary compound, potash feldspar, has an incongruent melting point at 1150 + 20°C. In all compositions that lie in this field, considerable difficulty is experienced in obtaining potash feldspar crystals. Even after several months of crystallization at temperatures about 50 to 75°C below liquidus temperature, Schairer and Bowen [75] observed only a few percent of very small feldspar laths in the glasses. For those extremely viscous melts that also lie on or very near the leucite-silica line, Schairer and Bowen never crystallized feldspar. 

A basanite lava from a cinder cone in the Itcha Mountains of British Columbia, Canada contains small phenocrysts of olivine in a groundmass of olivine, augite, plagioclase, alkali feldspar, nepheline, leucite and Fe-Ti oxides. Analytical data on the rock and enclosed minerals are given below. Combining these data with the equations in the Tables and text, estimate the pressures and temperatures at which a melt of the composition of the rock could be in equilibrium with source materials having the following thermodynamic characteristics ... 

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