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-Mössbauer 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 Me ²⁺ site. The ⁵⁷Fe-Mössbauer 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 Me ²⁺ and the Al³⁺ positions and vacancies on the Me ²⁺ site. The ⁵⁷Fe-Mössbauer 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 x _Fe  = 12(1)%