Original paper

Polymorphism and solid solution in the system SiO2-AlPO4(-H2O): a review and new synthesis experiments up to 3.5 GPa and 1573 K

Hensel, Nicole; Franz, Gerhard; Riedl, Martin; Gottschalk, Matthias; Wunder, Bernd; Galbert, Francois; Nissen, Jörg


The systems SiO2 and AlPO4 show considerable crystal chemical similarity as shown by an extensive literature review, but the amount of the berlinite substitution 2 [4]Si4+ = [4]Al3+ + [4]P5+ is still controversial. We present new results from hydrothermal synthesis experiments aimed to look for possible solid solution in the stability fields of the different polymorphs α-quartz, β-quartz, tridymite, cristobalite and coesite with the corresponding AlPO4-structure types. Runs were carried out in the pressure range up to 3.5 GPa, at temperatures between 650 °C and 1300 °C. For quartz-berlinite and for coesite the amount of solid solution is small, in the order of one mol%. In addition, no coesite-modification of AlPO4 could be found. Instead, trolleite (in the AlPO4-H2O system) and berlinite (dry conditions) are the stable Al-phosphates at high pressure. The stability of berlinite strongly depends on the H3PO4-content of the fluid. For the low-pressure - high-temperature polymorphs no unequivocal results for possible solid solution could be found due to the problem of metastable growth and persistence of these phases.