HIGH-PRESSURE CRYSTAL-CHEMISTRY OF KALSI3O8 HOLLANDITE

A high-pressure single-crystal XRD study of KAlSi3O8 with the tetragonal hollandite structure has been completed to 4.47 GPa. The a axis is approximately twice as compressible as c, so c/a increases with pressure. This anisotropy is similar to that of the structurally related mineral stishovite, though KAlSi3O8 hollandite is approximately 53% more compressible along both axes. The relative incompressibility of the c axis can be explained by the strong cation to cation repulsive forces across the shared octahedral edge in the double chain; Si(Al)-O bonds perpendicular to c are more compressible than those in other directions. P-V data give an isothermal bulk modulus of 180(3) GPa using a Birch-Murnaghan equation of state with K(T)' = 4.0 and constraint of V0. The polyhedral bulk modulus of the Si(Al)O6 octahedron is 153(9) GPa, the smallest among rutile-related oxides. The KO8 tetragonal prism has a polyhedral bulk modulus of 181(43) GPa, which is unusually large for an alkali cation site. The volume of the K coordination polyhedron is constrained by the rigid tetragonal octahedral framework, so the bulk modulus is expected to be independent of the size and charge of the central cation.