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Mineralogical Magazine; February 2004; v. 68; no. 1; p. 123-134; DOI: 10.1180/0026461046810175
© 2004 Mineralogical Society of Great Britain and Ireland
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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

F. Brunet1,*, D. Morineau2 and P. Schmid-Beurmann3

1 Laboratoire de Géologie, CNRS-UMR 8538, Ecole normale supérieure, 24 rue Lhomond, F-75005 Paris, France
2 Laboratoire de Chimie Physique, UMR 8000, Université Paris-sud, Bat. 349, F-91405 Orsay, France
3 Institut für Geowissenschaften, Universität Kiel, Olshausenstr. 40, D-24118 Kiel, Germany

* E-mail: brunet{at}geologie.ens.fr

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 (S0298) 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 S0298 = 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 from 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, Al4(PO4)3(OH)3, amounts to 285.4(8.5) J mol–1 K–1 ~13% higher than the previously published value. We propose a new formation enthalpy from the elements of –6504.2 kJ 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 on the trolleite = berlinite + corundum + water reaction.

KEYWORDS: heat capacity, lazulite, phosphate, entropy, trolleite




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P. Schmid-Beurmann, F. Brunet, V. Kahlenberg, and E. Dachs
Polymorphism and thermochemistry of MgAlPO4O, a product of lazulite breakdown at high temperature
European Journal of Mineralogy, March 1, 2007; 19(2): 159 - 172.
[Abstract] [Full Text] [PDF]


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