Experimental Study of some Potassium and Sodium Bearing Aluminosilicates at High Temperatures and High Pressures

Abstract: This dissertation presents the results of a high-temperature and high-pressure experimental study of some abundant aluminositicates, which are the main repositories for the two geochemically important elements, sodium and potassium, in the Earth's crust. Phase transitions of end member alkali feldspars and jadeite clinopyroxene are studied at high and ultrahigh pressures corresponding to the conditions between the transition zone and the deep Earth's lower mantle. Thermal expansion, dehydroxylation, and high-temperature structural behaviour of a hydrous phlogopite up to 1100 °C at 1 bar is also presented in this thesis. High-pressure and high-temperature experiments were carried out in externally- and internally-heated Mao-Bell type diamond anvil cells (DAC). Samples in DAC were heated externally at temperatures between 650 and 800 °C, using a resistive graphite heating system. For high-temperatures between 1200 and 2300 °C, a Nd: YAG laser beam was employed. Recovered samples from high pressures and high temperatures were studied by powder X-ray diffraction technique and synchrotron radiation. High- temperature measurements at 1 bar were performed by using a resistive platinum wire acting as a heater and sample holder under the X-ray radiation. The results of DAC experiments show that NaAlSiO4 calcium ferrite-type structure is the only high-pressure modification of albite and jadeite stable at pressures above 23 GPa up to at least 75 GPa in a wide range of temperatures. It is also found that KA1Si3O8 hollandite-type structure, the high-pressure form of orthoclase above 10 GPa, is a stable mineral at pressures up to at least 95 GPa. The new data, therefore, suggest that calcium ferrite- and hollandite-type structures might be the host for sodium and potassium, respectively, in a deep primitive chondritic lower mantle.