The univariant reaction Al4(PO)3(OH)3(trolleite)3 AlPO4(berlinite)+12Al2O3+3/2H2O has been investigated at pressures between PH2O = 10.0 and 23.0 kbar using a piston–cylinder apparatus, and below PH2O = 5.0 kbar using standard cold-seal bombs. From these experimental results the third-law entropy and the Gibbs free energy of formation of trolleite were calculated to be 64.7±6 eu and −1452.55±3.5 kcal/mole, respectively. Based on experimental data including reversal reactions demonstrated in this study plus previous experimental results in the system Al2O3–AlPO4–H2O, a revised phase diagram for this system is presented wherein four univariant curves define an invariant point at 4.70 kbar and 542°C.

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