Formation of phoxite, (NH4)2Mg2(C2O4) (PO3OH)2•4H2O, from struvite and its occurrence in Petrogale Cave, Western Australia

ABSTRACT The mineral unknown A reported by Bridge from the Petrogale Cave in Western Australia has been shown to be phoxite, a recently described mineral from the Rowley mine in Arizona, USA. This is only the second recorded occurrence of phoxite, and while at the type locality, it occurs with struvite, (NH4)Mg(PO4)·6H2O, at the Petrogale Cave, it occurs with weddellite, Ca(C2O4)·2H2O. Phoxite can be synthesised from struvite or newberyite, MgHPO4·3H2O, by reaction with ammonium oxalate in water or by grinding the solids together. Mixing newberyite or struvite with ammonium oxalate in high humidity enables a clean synthesis of phoxite. A competing reaction is the formation of magnesium oxalate dihydrate (the mineral glushinskite). However, while stable in the solid, washing with water allows phoxite to revert to struvite, as part of a solid liquid equilibrium.