LAZULITE STABILITY RELATIONS IN THE SYSTEM AL2O3-ALPO4-MG-3(PO4)(2)-H2O

Phase relations in the system Al2O3-AlPO4-Mg-3(PO4)(2)-H2O were studie d experimentally between 0.01 and 0.31 GPa at temperatures between 487 and 704 degrees C. Two univariant reactions, which define the upper t hermal stability of pure lazulite and of lazulite in the presence of f arringtonite and corundum, were determined by bracketing experiments. The reactions are: lazulite double left right arrow MgAlPO5 + berlinit e + H2O (1) lazulite + farringtonite + corundum double left right arro w 4 MgAlPO5 + H2O (2) The mean standard enthalpies and standard entrop ies of reactions (1) and (2) were calculated from the P-T location of the univariant curves as Delta H-R,1(0) = 154(4) kJ, Delta S-R,1(0) = 232(5) J/K, Delta H-R,2(0) = 91(9) kJ, Delta S-R,2(0) = 172(11)J/K. A combination of the extracted thermodynamic data with the tabulated sta ndard enthalpies of formation and third-law entropies of corundum, ber linite, farringtonite and H2O yield Delta H-f,Laz(0) = -4532(7) kJ/mol , Delta H-f,MgAlPO5(0) = - 2405(9) kJ/mol, and S-Laz(0) = 139(7) J/K . mol, S-MgAlPO5(0) = 90(9) J/K . mol as the standard enthalpies of for mation and third-law entropies of synthetic lazulite and MgAlPO5, resp ectively. Extrapolation of the experimentally determined univariant eq uilibria to higher pressures and temperatures predicts an invariant po int at 0.36 GPa and 710 degrees C, where lazulite, corundum, MgAlPO5, berlinite, farringtonite and H2O coexist.