The phase MgAlPO4O is a thermal decomposition product of the metamorphic phosphate mineral lazulite, MgAl2(PO4) (OH) ; its polymorphism and thermochemistry were investigated. The room-temperature β″-polymorph of MgAlPO4O was found to undergo an isosymmetric and reversible phase transition towards a non-quenchable α′-polymorph at 758 K (485 °C). The two structures are found to be closely related to each other. They are characterised by the presence of sheets of five-fold coordinated Mg and layers containing zweier single chains of Al-tetrahedra as well as additional PO4 groups providing a connection between the chains. Due to the specific linkage of the chains and the phosphate groups, a single tetrahedral layer consists of both twofold and fourfold connected tetrahedra in the ratio Q4:Q2 = 1:1. The β″- α′ transition is accompanied by a transformation enthalpy of ΔHβ″– α′ = 0.65 kJ/mol (DSC measurement), a volume reduction of −2.3 % and it is characterised by switching of bonds between Mg2+ and the coordinating O1 ions. The corresponding negative Clapeyron slope of the transition implies that the β″-polymorph is stable in a restricted low-P (< 0.5 GPa) and low-T (< 758 K) field as confirmed by in situ X-ray diffraction experiments (MAX80, Hasylab). A third-law entropy value (S2980) of 110.7(1) J/mol.K was obtained for β″-MgAlPO4O by heat-pulse calorimetry using the Physical Properties Measurement System (PPMS) produced by Quantum Design®. The reaction 3 MgAlPO4O ⇔ farringtonite + berlinite + corundum was bracketed up to 1373 K and 1 GPa in an internally heated pressure vessel. The combination of the experimental brackets with the PPMS entropy value allows us to derive a standard enthalpy of formation (ΔH0f,298) of −2394(5) kJ/mol for β″-MgAlPO4O.