Last but not least: high-temperature behavior of drugmanite and comparative crystal chemistry of the gadolinite supergroup minerals under extreme conditions

Gadolinite supergroup minerals (GSM) are common in nature and widely known as important rare earth elements (REE) carriers, which makes them prominent objects for study both in geology and material science. Their behavior under extreme conditions (high pressure and/or high temperature) could shed light on geochemical cycles of rare lithophile elements (e.g., REE, B, Be) and show the ways of transformation of crystal structures in a series of solid solutions. In this paper, we present new data on drugmanite, Pb2Fe3+(PO4)(PO3(OH))(OH)2, and compare the obtained results with other GSM. Drugmanite is the least stable member of the supergroup under high temperature conditions, which is a result of the low density of the octahedral layer in its crystal structure. The beryllium silicate members of GSM were found to be the most stable in high-temperature conditions, whereas beryllium phosphate members are most stable at high pressures. The mechanisms of mineral deformation under extreme conditions are primarily determined by the composition of the tetrahedral layer and then by the size and charge of the large cations.