A calorimetric investigation of spessartine: Vibrational and magnetic heat capacity

Dachs, E., Geiger, C. A., Withers, A. C. and Essene, E. J. (2009) A calorimetric investigation of spessartine: Vibrational and magnetic heat capacity Geochimica Et Cosmochimica Acta, 73 (11). pp. 3393-3409. DOI 10.1016/j.gca.2009.03.011.

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The heat capacity (C,,) of two synthetic spessartine samples (Sps) was measured oil 20-30 mg-size samples in the temperature range 2-864 K by relaxation calorimetry (RC) and differential scanning calorinictry (DSC). The polycrystalline spessartine samples were synthesized in two different laboratories at high pressures and temperatures from glass and oxide-mixture starting materials and characterized by X-ray powder diffraction and electron-microprobe analysis. The low-temperature heat capacity data show a prominent lambda transition with a peak at 6.2 K, which is interpreted to be the result of a paramagnetic-antiferromagnetic phase transition. The DSC data around ambient T agree excellently with the RC data and can be represented by the C(p) polynomial for T> 250 K: C(p)(Sps) = 610 - 3060. T(-0.5) - 1.45.10(7).T(-2) + 1.82.10(9).T(-3). Integration of the low temperature C,, data yields a calorimetric standard entropy for the two different samples of S(o) = 334.6 +/- 2.7 J/mol K and 336.0 +/- 2.7 J/mol.K. The preferred standard third-law entropy for spessartine is S(o) = 335.3 +/- 3.8 J/mol.K, which is the mean value from the two separate determinations. The lattice (vibrational) heat capacity of spessartine was calculated using the single-parameter phonon dispersion model of Komada and Westrum. The lattice entropy at 298.15 K is S(ulb)(298.15) = 297.7 J/mol.K, which represents 89% of the calorimetric entropy. The magnetic heat capacity and entropy of spessartme, S(mag) at 298.15 K were also calculated. The S(mag) of the two samples is 38.7 and 37.4 J/mol.K, which is 87% and 83% of the maximum possible magnetic entropy given by 3Rln6 = 44.7 J/mol . K. Published model-dependent lattice-dynamic calculations S(ulb)(298.15) are analyzed and compared to the experimental data. Using the calorimetrically determined S(o) and the C(p) polynomial for spessartine, together with high P-T experimental phase-equilibrium data oil Mn(2+)-Mg partitioning between garnet and olivine, allows calculation of the standard enthalpy of formation of spessartine. This gives Delta H(f,Sps)(o) = -5693.6 +/- 1.4 kJ/rnol, a value nearly 50 kJ more negative than some published values. The Gibbs free energy of spessartine was also calculated and gives Delta G(f,Sps)(o) = -5364.3 kJ/mol at 298.15 K. The new standard entropy and enthalpy of formation values for spessartine lead to revised estimates for the enthalpies of formation of other Mn(2+)-silicates. Resulting Delta H(f)(o) values for Mn-biotite, Mn-chlotire, Mn-cordierite, Mn-staurolite and Mn-chloritoid are 7-34 kJ more negative than their values listed in the thermodynamic database "THERMOCALC". As an example, the new standard entropy and enthalpy of formation for spessartine have been applied to Mn-Fe partitioning between garnet and orthopyroxene from manganiferous iron formations. Excellent agreement between the predicted and observed distribution coefficient was obtained. (C) 2009 Elsevier Ltd. All rights reserved.

Document Type: Article
Keywords: thermodynamic properties solid-solutions lattice-vibrations phase-equilibria 1000 k garnet entropy minerals mn3al2si3o12 parameters
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.1016/j.gca.2009.03.011
ISSN: 0016-7037
Date Deposited: 22 Dec 2011 05:01
Last Modified: 09 Oct 2012 03:55
URI: http://eprints.uni-kiel.de/id/eprint/15765

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