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Abstract

The ambient-temperature compressibility and room-pressure thermal expansion of two Mg3(PO4)2 polymorphs (farringtonite=Mg3(PO4)2-I, with 5- and 6-fold coordinated Mg, and chopinite=“Mgsarcopside”=[6]Mg3(PO4)2-II), three Mg2PO4OH polymorphs (althausite, hydroxylwagnerite and ɛ-
Mg2PO4OH, all with [5]Mg and [6]Mg) and phosphoellenbergerite ([6]Mg) were measured on synthetic powders
using a synchrotron-based multi-anvil apparatus to 5.5 GPa and a laboratory high-temperature diffractometer,
with whole-pattern fitting procedures. Bulk moduli range from 64.5 GPa for althausite to 88.4 GPa for hydroxylwagnerite, the high-pressure Mg2PO4OH polymorph. Chopinite, based on an olivine structure with ordered
octahedral vacancies (K0=81.6 GPa), and phosphoellenbergerite, composed of chains of face-sharing octahedra
(K0=86.4 GPa), are distinctly more compressible than their homeotypical silicate (127 and 133 GPa, respectively). The compressibility anisotropy is the highest for chopinite and the lowest for phosphoellenbergerite.
First-order parameters of quadratic thermal expansions range from v1=2.19x10-5K-1 for ɛ-Mg2PO4OH to
v1=3.58x10-5K-1 for althausite. Phosphates have higher thermal-expansion coefficients than the homeotypical silicates. Thermal anisotropy is the highest for farringtonite and the lowest for hydroxylwagnerite and chopinite. These results set the stage for a thermodynamic handling of phase-equilibrium data obtained up to
3 GPa and 1000°C in the MgO–P2O5–H2O and MgO–Al2O3–P2O5–H2O systems.