Crystal chemical properties of synthetic lazulite-scorzalite solid-solution series

Abstract

Members of the lazulite–scorzalite (MgAl2- (PO4)2(OH)2-FeAl2(PO4)2(OH)2) solid-solution series were synthesized in compositional steps of 12.5 mol% at T = 485 °C and P = 0.3 GPa under hydrothermal conditions and controlled oxygen fugacities of the Ni/NiO-buffer. X-ray powder diffraction and 57Fe-Mossbauer studies show that under these conditions a complete solid-solution series is formed which is characterized by the substitution of Mg2+ and Fe2+ on the octahedral Me2+ site. The 57Fe-Mossbauer spectra which reveal the presence of both ferrous and ferric iron and the compositional data were interpreted in terms of a defect model with a distribution of the ferric ions over both the Me2+ and the Al3+ positions and vacancies on the Me2+ site. The 57Fe-Mossbauer parameters of the synthetic compounds correspond to those of natural lazulites except for the total absorption ratio of the ferric iron A(Fe3+)/(A(Fe3+)+A(Fe2+)), which is significantly higher in natural lazulites of the same composition. The total absorption ratio of the ferric iron increases from 4% in pure scorzalite to 15% in a Mg-rich solid-solution with xFe = 12(1)%