HR: 0800h
AN: V41C-1409    [Abstracts]
TI: New Standard State Entropy for Sphene (Titanite)
AU: * Manon, M R
EM: mrmanon@umich.edu
AF: University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109-1063 United States
AU: Dachs, E
EM: Edgar.Dachs@sbg.ac.at
AF: Univ. Salzburg, Inst. Mineralogie, Hellbrunnerstrasse 34, Salzburg, 5020 Austria
AU: Essene, E J
EM: essene@umich.edu
AF: University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109-1063 United States
AB: Several recent papers have questioned the accepted standard state (STP) entropy of sphene (CaTiSiO$_{5}$), which had been considered to be in the range 129-132 J/mol.K (Berman, 1988: 129.3 Robie and Hemingway, 1995: 129.2 J/mol.K; Holland and Powell, 1995: 131.2 J/mol.K.). However, Xirouchakis and Lindsley (1998) recommended a much lower value of 106 J/mol.K for the STP entropy of sphene. Tangeman and Xirouchakis (2001) inferred a value less than 124 or 120 J/mol.K, based on based on enthalpy constraints combined with the tightly reversed reaction sphene+kyanite=rutile+anorthite by Bohlen and Manning (1991). Their recommendations are in conflict with the accepted values for STP entropy for sphene, including values calculated by direct measurement of Cp from 50 to 300 K by King (1954). In order to resolve this discrepancy, we have collected new data on the Cp of sphene between 5 and 300 K. Our measurements were made in the PPMS at Salzburg on a 21.4 g sample of sphene generously furnished by Tangeman and Xirouchakis (2001), the same sample as used in their experiments. The Cp data are slightly lower than those of King (1954) but merge smoothly with data of Tangeman and Xirouchakis (2001) from 330 to 483 K (or whatever) where a transition is recorded in the Cp data as a lambda anomaly. Tangeman and Xirouchakis also obtained data above the transition up to 950K. Integration of the new Cp data yields a STP entropy of 127.3 J/mol.K, lower than the generally accepted value by ca. 2 J/mol.K. A change in the STP entropy of sphene will have an effect on many Ti-bearing reactions which occur within the earth, although the magnitude of this change is not nearly as large as that suggested by Xirouchakis and Lindsley (1998). Above 700 K, the entropy calculated using the new STP entropy with the heat capacity equation of Tangeman and Xirouchakis (2001) is within 1 J/mol.K of the value tabulated in Robie and Hemingway (1995) and of that calculated from Berman (1988). The effect on most phase equilibrium calculations will not be large except for reactions with small $\Delta$S. The use of 127.2 J/mol.K as the standard entropy of sphene is recommended especially in calculations of geobarometers involving that phase.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3939 Physical thermodynamics
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting