- (Na,K) Aluminosilicate Hollandites: Structures, Crystal Chemistry, and High-pressure Behaviour (2007)
- Aluminosilicates with the composition (Na,K)AlSi3O8 and the dense hollandite-type structure, in which all Si and Al are in six-fold coordination, are considered as a possible repository of potassium and sodium in the Earth´s mantle. The aim of this research is to explore the phase relation of the K-Na system at different temperatures and pressures, and to determine the physical-chemical properties and high-pressure behaviour of silicate hollandite-type structures containing K and Na in different concentrations. The (Na,K)AlSi3O8 hollandite solid solution has been synthesised using multi-anvil apparatus in the pressure range between 13 and 26 GPa and temperatures between 1500 and 2200 °C, using (Na0-0.6, K1-0.4)AlSi3O8 glasses, NaAlSi3O8 glass, and Na0.75K0.05Ca0.1AlSi3O8 glass as starting materials. The solubility of Na component into the KAlSi3O8 hollandite end-member increases with increasing pressure and temperature. Homogeneous assemblages with a pure hollandite phase (and maximum 1-2% of stishovite) were synthesized at temperature of 1700 °C and different pressures with up to 50% of NaAlSi3O8 component. No pure NaAlSi3O8 hollandite end-member was succsessfully synthesized. Considering the difference in heat dissipation between the shock events in meteorites and the multi-anvil presses, it appears likely that NaAlSi3O8 hollandite forms as a result of local high pressure and high temperature conditions and really fast quenching under non-equilibrium conditions. All synthesized hollandite samples have tetragonal I4/m symmetry at ambient conditions. The unit-cell volume and lattice parameters of the (Na,K)AlSi3O8 hollandite decreases linearly with increasing Na content. The a cell parameter decreases more rapidly than the c cell parameter, suggesting that changing the cation size in the tunnels of the hollandite structure affects more the a axis than the c axis. Structural refinements of single-crystal data collected for KAlSi3O8 and K0.8Na0.2AlSi3O8 hollandites are consistent with Si and Al disorder among the octahedral sites. The major difference between the KAlSi3O8 hollandite end-member and the K0.8Na0.2AlSi3O8 sample is the presence in the latter of a split site away from the 4th-fold axis. This position, occupied by ~ 75% of the total Na content, is closer to the framework walls and has a very distorted coordination polyhedron with only 5 Na1-O bond distances between 2.4 and 2.6 Angström whereas all other Na1-O bond distances are larger than 3 Angström. The high pressure behaviour of hollandite samples with compositions of KAlSi3O8, K0.8Na0.2AlSi3O8, K0.6Na0.4AlSi3O8, and K0.5Na0.5AlSi3O8 have been studied using diamond anvil cells and different pressure transmitting media, by means of X-ray powder diffraction and Raman spectroscopy. High temperature behaviour of K0.5Na0.5AlSi3O8 hollandite at high pressures has also been explored by means of X-ray powder diffraction. At high pressures, all tetragonal hollandite samples transform to a monoclinic (hollandite II) structure with space group I2/m. The transition pressure decreases with increasing Na component. Na substitution, thus, stabilizes the monoclinic phase, likely because the framework walls are more distorted than in the tetragonal phase and therefore more apt to accommodate the smaller Na atom. Second order Birch- Murnaghan equations of state were calculated for the tetragonal and monoclinic phases. If only experiments using He as pressure transmitting medium are compared, it appears that Na has little effect on the bulk modulus value of the tetragonal aluminosilicate hollandite, but increases the axial anisotropy. Monoclinic hollandites are more compressible, and are stable up to the highest pressures reached during the experiments, suggesting that they may be possible host minerals for Na and K in transition zone and even down to the Earth´s lower mantle. The lattice strains associated with the tetragonal I4/m to monoclinic I2/m transition have been determined. The phase transition is proper ferroelastic with negligible volume strain. The symmetry breaking strains e1-e2=a-b/a0 and e6=a/a0 x cos gamma are proportional to the order parameter Q associated with the transition and their squared values vary linearly with pressure indicating that the transition is second-order in character. The variation with pressure of the symmetry breaking strains is similar in K0.8Na0.2AlSi3O8 and KAlSi3O8 hollandites, suggesting that Na substitution mainly affects the transition pressure but not the transition mechanism. Results from the high pressure experiments show that the tetragonal to monoclinic phase transition is very sensitive to deviatoric stresses present during the experiments due to the different pressure transmitting media. These results might also give an indication of the possible effects arising from stresses on the mineral transitions in the Earth´s mantle.