Calorimetric study on high-pressure transitions in KAlSi3O8

KAlSi3O8 sanidine dissociates into a mixture of K2Si4O9 wadeite, Al2SiO5 kyanite and SiO2 coesite, which further recombine into KAlSi3O8 hollandite with increasing pressure. Enthalpies of KAlSi3O8 sanidine and hollandite, K2Si4O9 wadeite and Al2SiO5 kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 +/- 7.4 kJ mol(-1) for sanidine --> wadeite + kyanite + coesite and 99.3 +/- 3.6 kJ mol(-1) for wadeite + kyanite + coesite --> hollandite. The isobaric heat capacity of KAlSi3O8 hollandite was measured at 160-700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi3O8 hollandite above 298 K as C-p=3.896 x 10(2)-1.823 x 10(3)T(-0.5)-1.293 x 10(7)T(-2)+1.631 x 10(9)T(-3) (C-p in J mol(-1) K-1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite --> hollandite intersects with the coesite-stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase-equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO4 kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi3O8 determined in this study put some constraints on the stability range of KAlSi3O8 hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.