HR: 1340h
AN: V43B-1580    [Abstracts]
TI: Entropy And Phase Equilibria Of KAlSi₃O₈ Hollandite
AU: * Yong, W
EM: wenjuny@umich.edu
AF: University of Michigan, Dept. of Geol. Sciences 2534 C.C. Little Building 1100 N. University, Ann Arbor, MI 48109-1005 United States
AU: Dachs, E
EM: Edgar.Dachs@sbg.ac.at
AF: University of Salzburg, Inst. Mineralogie Hellbrunnerstr. 34, Salzburg, A-5020 Austria
AU: Withers, A C
EM: withe012@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics , Minneapolis, MN 55455 United States
AU: Essene, E J
EM: essene@umich.edu
AF: University of Michigan, Dept. of Geol. Sciences 2534 C.C. Little Building 1100 N. University, Ann Arbor, MI 48109-1005 United States
AB: Based on new low-T Cp data, the standard entropy of KAlSi₃O₈ hollandite is calculated to be 166.2±0.2 J mol^-1 K^-1, including an 18.7 J mol^-1 K^-1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. Phase relations in the system K₂O-Al₂O₃-SiO₂ are calculated using PerplE_x and confirm a substantial stability field for kyanite-stishovite/coesite-K₂Si₄O₉ wadeite intervening between KAlSi₃O₈ hollandite and sanidine. The calculated phase equilibria are in general agreement with experimental studies. The entropy and enthalpy of formation for KAlSi₃O₈H₂O K-cymrite are slightly modified to better fit the Holland & Powell (1998) data base and the previous experimental runs. Thermodynamic calculations are undertaken on the reaction of K-cymrite to KAlSi₃O₈ hollandite + H₂O, which is located at 8.3-10.0 GPa for the temperature range 800-1600 K, well inside the stability field of stishovite. The decomposition reaction of muscovite to KAlSi₃O₈ hollandite + Al₂O₃ + H₂O is placed at 10.0-10.6 GPa for the temperature range 900-1500 K, in reasonable agreement with some but not all experiments on this reaction. These reactions strongly support that KAlSi₃O₈ hollandite is an important K-bearing mineral down to depths of 400-660 km in the transition zone of the Earth's mantle.
DE: 3630 Experimental mineralogy and petrology
DE: 3939 Physical thermodynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005