SYNCHROTRON-RADIATION STUDY ON THE HIGH-PRESSURE AND HIGH-TEMPERATUREPHASE-RELATIONS OF KALSI3O8

In situ X-ray diffraction study on KAlSi3O8 has been performed using t he cubic type high pressure apparatus, MAX90, combined with synchrotro n radiation. We determined the phase relations of sanidine, the wadeit e-type K2Si4O9 + kyanite (Al2SiO5) + coesite (SiO2) assemblage, and ho llandite-type KAlSi3O8, including melting temperatures of potassic pha ses, up to 11 GPa. Our data on subsolidus phase boundaries are close t o the recent data of Yagi and Akaogi (1991). Melting relations of sani dine are consistent with the low pressure data of Lindsley (1966). The breakdown of sanidine into three phases reduces melting temperature, and wadeite-type K2Si4O9 melts first around 1500 degrees C in three ph ase coexisting region. Melting point of hollandite-type KAlSi3O8 is be tween 1700 degrees C and 1800 degrees C at 11 GPa. If these potassic p hases host potassium in the earth's mantle, the true mantle solidus te mperature will be much lower than the reported dry solidus temperature of peridotite.