Lazulite stability relations in the system Al (sub 2) O (sub 3) -AlPO (sub 4) -Mg (sub 3) (PO (sub 4) ) (sub 2) -H (sub 2) O

Phase relations in this system were studied at 0.01 and 0.31 GPa, at 487-704 degrees 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: lazulite = MgAlPO (sub 4) + berlinite + H (sub 2) O and lazulite + farringtonite + corundum = 4 MgAlPO (sub 5) + H (sub 2) O. The mean standard enthalpies and entropies were calculated. A combination of these results with the standard enthalpies of formation and third-law entropies of corundum, berlinite, farringtonite and H (sub 2) O yield Delta H (super 0 ) (sub roman f,laz ) = -4532 kJ/mol, Delta H (super 0 ) (sub roman f,MgAlPO sub 5 ) = -2405 kJ/mol, and S (super 0 ) (sub roman laz ) = 139 J/K.mol, S (super 0 ) (sub roman MgAlPO sub 5 ) = 90 J/K.mol. as the standard enthalpies of formation and third-law entropies of synthetic lazulite and MgAlPO (sub 5) , respectively. Extrapolation of the experimentally determined univariant equilibria to higher P and T predicts an invariant point at 0.36 GPa and 710 degrees C, where lazulite, corundum, MgAlPO (sub 5) , berlinite, farringtonite and H (sub 2) O coexist.