Heat Capacity of Hollandite-Type KAlSi3O8 and Related Phase Equilibria

The behavior of potassium in the deep mantle is of great significance from geophysical and geochemical points of view. Several experimental studies of phase transitions of stoichiometric KAlSi3O8 have been undertaken (Ringwood et al., 1967; Liu, 1978; Kinomura et al., 1975; Yagi et al., 1994; Urakawa et al., 1994; Fasshauer et al., 1998; Akaogi et al., 2004). Hollandite-type KAlSi3O8 is suggested to be a possible reservoir of potassium in the earth¡_s lower mantle. Tutti et al. (2001) showed in experiments that hollandite-type KAlSi3O8 was a stable phase under the pressure of at least 2200 km deep in the lower mantle and suggested that it is important host for potassium in that region. However, because of the lack of low-temperature heat capacity data of hollandite-type KAlSi3O8, the S¡a(298) is not well determined, hence the phase equilibria related to hollandite-type KAlSi3O8 are still not well constrained in terms of slopes of the reactions. In this study, we first synthesized hollandite-type KAlSi3O8 from sanidine glass as starting material at ca. 10 GPa and 1273K using a multi-anvil device at the Univ. of Minnesota. The hollandite structure was confirmed by XRD analysis. The heat capacity was measured in the range of 5-303 K using a PPMS at Salzburg. The low-temperature heat capacity data merge smoothly with data measured on hollandite-type KAlSi3O8 between 160 and 700 K by Akaogi et al. (2004). The entropy of hollandite-type KAlSi3O8 is calculated as 147.6 J/mol.K at 298.15 K. In combination with the existing thermodynamic and experimental data, the phase boundary between hollandite-type KAlSi3O8 and wadeite, kyanite, and coesite is more accurately fixed.