HP and HT Polymorphism of AlPO4 and its Solubility in SiO2 -Stishovite : Implication for Phosphorus Geochemistry

The understanding of phosphorus behavior in the deep Earth requires to evaluate the ability of minor phosphorus to be incorporated in the octahedral site of high-pressure mineral structures. For that purpose, we studied the solubility of AlPO₄ in SiO₂-stishovite, the extend of which will indicate the phosphorus affinity for octahedral oxygen coordination. The choice of those compounds was also motivated by their importance in material sciences (e.g. pressure induced amorphization properties). Synthetic AlPO₄ (berlinite) as well as a 1 to 2 molar mixture of AlPO₄ (berlinite) : SiO₄ (quartz) where encapsulated separately in a Pt container and held together at 1600° C, 18 GPa (4.5 h) in the 2000-ton split-sphere apparatus at GRC (Matsuyama, Japan). In the second capsule, micrometer-size stishovite crystals could be analyzed using the electron microprobe and revealed around 0.5 wt.% P₂O₅ (balanced by Al₂O₃). Berlinite, in the first capsule, totally reacted at HP and HT into a form which could not be quenched. This result was the incentive for a characterization of AlPO₄ polymorphism using in-situ HP-HT x-ray diffraction (Spring 8, BL04B1). Upon compression in the MgO-based assembly, the diffraction pattern of AlPO₄ berlinite showed indication of partial amorphization in keeping with the non-hydrostatic nature of the pressure cell at room temperature. Upon the first heating stage, crystallization of another AlPO₄ polymorph had occurred at 600° C, 6.9 GPa (gold scale) in a first experiment and 900° C, 17.3 GPa in a second one. The corresponding diffraction pattern could be indexed in the InPO₄ -structure (Cmcm) where Al is octahedrally coordinated whereas P is fourfold. No other transformation was identified up to 21 GPa, 1600° C. Although, no rutile-type structure was observed for AlPO₄ up to 21 GPa, phosphorus, which is likely to be stored in garnet down to the transition zone, is expected to be further incorporated into silicate structures to greater depths.