Heat capacity and phase equilibria of hollandite polymorph of KAlSi3O8

The low-temperature heat capacity (C-p) of KAlSi3O8 with a hollandite structure was measured over the range of 5-303 K with a physical properties measurement system. The standard entropy of KAlSi3O8 hollandite is 166.2 +/- 0.2 J mol(-1) K-1, including an 18.7 J mol(-1) K-1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K2Si4O9 with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K2O-Al2O3-SiO2. The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K2O-Al2O3-SiO2 confirm a substantial stability field for kyanite-stishovite/coesite-Si-wadeite intervening between KAlSi3O8 hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al2SiO5) = corundum ( Al2O3) + stishovite ( SiO2), which is located at 13-14 GPa for 1,100-1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlS(i)3O(8)center dot H2O) were modified to better fit global best fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi3O8 hollandite + H2O, which is located at 8.3-10.0 GPa for the temperature range 800-1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi3O8 hollandite + corundum + H2O is placed at 10.0-10.6 GPa for the temperature range 900-1,500 K, in reasonable agreement with some but not all experiments on this reaction.