Elasticity of Hollandite
Abstract
KAlSi3O8 (K-hollandite) is a high-pressure polymorph of potassium feldspar and has been believed to be the most abundant phase in the continental crustal material that has been subducted to pressure and temperature conditions relevant to the mantle transition zone. They are possible reservoirs for large ion lithophile elements such as K, Na, Pb, Sr and are potential candidates to generate EMII geochemical signatures. The stability of K-hollandite has been experimentally studied. Although there are geochemical signatures for crustal components, how well could we detect such pockets geophysically? Our study aims to address this issue by determining elastic property of hollandite. We are undertaking an indepth analysis of crystal chemistry of hollandite relating structure-property and stability of these phases using first-principle electronic structure simulations. We have explored the elastic properties of K- and Na- hollandite over a wide range of pressures. Results of compression for the K - hollandite phase is well represented by a third order Birch-Murnaghan finite strain expression with KO = 207 GPa, K'= 4.47 and VO= 233.12 Å 3. The zero pressure volume is 1.8 % smaller whereas bulk modulus is 13 % larger than experimental values. This is expected since the comparison is not direct, our results are static (0 K) as opposed to experimental data at 300 K. Corresponding values for Na and Ca- hollandite phase are KO= 196.83 GPa, K'= 3.54, VO= 225.85 Å 3 and KO= 197.92 GPa, K'= 3.31, VO= 232 Å 3. The bluk modulus of Na and Ca endmember are similar owing to similarity of the cation size, where as K being a larger cation renders a much larger volume and stiffer structure. Hollandite has a tetragonal symmetry (I4/m) at ambient conditions and undergoes phase transition at 22 GPa to a lower symmetry monoclinic phase (I2/m). For K-hollandite we are exploring this phase transitions and a finite strain fit for the high pressure phase shows KO= 192 GPa, K'= 4.28 and VO= 234.37 Å 3. At 0 K, the phase transition, based on crossover of enthalpy of tetragonal and monoclinic structure occurs at ~36 GPa as opposed to ~20 GPa at ambient (300 K). We will determine full elastic tensor for high symmetry (I4/m) hollandite and develop velocity models based on crustal components to compare with local seismic models and infer about the chemistry in the interior of the earth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMDI41A1750S
- Keywords:
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- 1042 Mineral and crystal chemistry (3620);
- 3621 Mantle processes (1038);
- 3900 MINERAL PHYSICS;
- 3909 Elasticity and anelasticity;
- 3924 High-pressure behavior