Silicate hollandites: structures, crystal chemistry and geochemical implication
Final Report Abstract
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 was 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 major results of this study are the following: a) The solubility of Na into the KAlSi3O8 hollandite end-member increases with increases pressure and temperature. However, the maximum solubility found is 50% of NaAlSi3O8 component. No pure NaAlSi3O8 hollandite end-member was successfully synthesised. b) All synthesised hollandite samples have tetragonal /4/m symmetry at ambient conditions. The unit-cell volume of the (Na,K)AlSi308 hollandite decreases linearly with increasing Na content. Structural refinements of single-crystal data collected for KAlSi3O8 and Ko.8Nao.2AlSi3O8 hollandites are consistent with Si and Al disorder among the octahedral sites. The Na cations occupy partially a split site away from the 4th-fold axis and closer to the framework walls. c) At high pressures, all tetragonal hollandite samples transform to a monoclinic (hollandite II) structure with space group /2/m. The transition pressure decreases with increasing Na component. Na substitution, thus, stabilises 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 atoms. d) 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, if continental cmst is subducted to those depths. e) The lattice strains associated with the tetragonal /4/m to monoclinic Film transition have been determined. The variation with pressure of the symmetry breaking strains is similar in Ko.8Nao.2AlSi3O8 and KAlSi3O8 hollandites, suggesting that Na substitution mainly affects the transition pressure but not the transition mechanism. The major surprise was to observe that the deviatoric stresses present in soft media normally used in high-pressure experiments (i.e. Ar and LiF) are not negligible and substantially influence the high-pressure measurements. These results give, thus, an indication of the possible effects arising from stresses on the mineral transitions in the Earth's mantle.
Publications
- (2007) Equation of state and thermal expansivity of LiF and NaF. High Pressure Research, 27, 483 - 489
J. Liu, L. Dubrovinsky, T. Boffa Ballaran, W. Crichton
