Lazulite stability relations in the system Al2O3-AlPO4-Mg3(PO4)2-H2O
Cemič, Lado; Schmid-Beurmann, Peter
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 921 - 930
published: Aug 1, 1995
manuscript accepted: Mar 24, 1995
manuscript received: Mar 29, 1994
ArtNo. ESP147050704018, Price: 29.00 €
Abstract Phase relations in the system Al2O3-AlPO4-Mg3(PO4)2-H2O were studied experimentally between 0.01 and 0.31 GPa at temperatures between 487 and 704 °C. Two univariant reactions, which define the upper thermal stability of pure lazulite and of lazulite in the presence of farringtonite and corundum, were determined by bracketing experiments. The reactions are: lazulite ⇔ MgAlPO5 + berlinite + H2O (1) lazulite + farringtonite + corundum ⇔ 4 MgAlPO5 + H2O (2) The mean standard enthalpies and standard entropies of reactions (1) and (2) were calculated from the P-T location of the univariant curves as ΔH0R, 1, = 154(4) kJ, ΔSOR, 1 = 232(5) J/K, ΔHOR,2 - 91(9) kJ, ΔSOR, 2= 172(11)J/K. A combination of the extracted thermodynamic data with the tabulated standard enthalpies of formation and third-law entropies of corundum, berlinite, farringtonite and H2O yield ΔHOLaz = - 4532(7) kJ/mol, ΔHOf,MgAlPO5 = - 2405(9) kJ/mol, and SOLaz= 139(7) J/K-mol, SOMgAlPO5 = 90(9) J/K-mol as the standard enthalpies of formation and third-law entropies of synthetic lazulite and MgAlPO5, respectively. Extrapolation of the experimentally determined univariant equilibria to higher pressures and temperatures predicts an invariant point at 0.36 GPa and 710°C, where lazulite, corundum, MgAlPO5, berlinite, farringtonite and H2O coexist.