(Na,K) Aluminosilicate Hollandites: Structures, Crystal Chemistry, and High-pressure Behaviour
Year: 2007
Type: dissertation
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 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 Angstrom whereas all other Na1-O bond distances are larger than 3 Angstrom. 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…
Author: Jun Liu
Language: English
Subfield: Geophysics
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2278877322