Heat capacity of lazulite, MgAl2(PO4)(2)(OH)(2), from 35 to 298 K and a (S-V) value for P2O5 to estimate phosphate entropy

Brunet, F., Morineau, D. and Schmid-Beurmann, P. (2004) Heat capacity of lazulite, MgAl2(PO4)(2)(OH)(2), from 35 to 298 K and a (S-V) value for P2O5 to estimate phosphate entropy Mineralogical Magazine, 68 (1). pp. 123-134. DOI 10.1180/0026461046810175.

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Abstract

The heat capacity of synthetic lazulite, MgAl2(PO4)(2)(OH)(2), has been Measured between 35 and 298 K by means of low-temperature adiabatic calorimetry. A lazulite third-law entropy value of 204(3) J K-1 mol(-1) was retrieved after extrapolation of the lazulite heat capacity by a Cubic temperature function, down to absolute temperature. Identical measurements on alpha-Al2O3 yielded S-298(0) = 51.2 J mol(-1) K-1 instead of the recommended 50.94 J mol(-1) K-1 value. This new lazulite third-law entropy along with other phosphate entropy data either derived front low-temperature adiabatic calorimetry or experimental phase equilibria under pressure and temperature, were computed to retrieve a (S-V)P2O5 value of 57.4(2.9) to predict phosphate entropies from oxide summation. The predicted third-law entropy for trolleite, Al-4(PO4)(3)(OH)(3), amounts to 285.4(8.5) J mol(-1) K-1 similar to13% higher than the previously published value. We propose a new formation enthalpy from the elements of -6504.2 U mol(-1) and a new third-law entropy of 281.7 J mol(-1) K-1 for trolleite, compatible with the predicted entropy from oxide summation as well as with the published brackets oil the trolleite = berlinite + corundum - water reaction.

Document Type: Article
Keywords: heat capacity lazulite phosphate entropy trolleite thermodynamic properties phase-relations stability minerals system oxide
Research affiliation: Kiel University
DOI etc.: 10.1180/0026461046810175
ISSN: 0026-461X
Date Deposited: 24 Jan 2012 05:50
Last Modified: 08 Oct 2012 10:32
URI: http://eprints.uni-kiel.de/id/eprint/16110

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