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American Mineralogist, 91, 327-332, (2006) [doi: 10.2138/am.2006.1879]

Equation of state and phase transition in KAlSi₃O₈ hollandite at high pressure

T. Ferroir, T. Yagi, T. Onozawa, S. Merkel, N. Miyajima, N. Nishiyama, T. Irifune, T. Kikegawa

The tetragonal hollandite structure (KAlSi₃O₈ hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the high- pressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn₈O₁₆ hollandite. The gamma angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state fit to pressure-volume data obtained for KAlSi₃O₈ hollandite yields a bulk modulus Ko pressure-volume data obtained for KAlSi₃O₈ hollandite yields a bulk modulus K₀ = 201.4(7) GPa with K' = 4.0.

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