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

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 degrees C and P = 0.3 GPa under hydrothermal conditions and controlled oxygen fugacities of the Ni/NiO-buffer. X-ray powder diffractio n and Fe-57-Mossbauer studies show that under these conditions a complete s olid-solution series is formed which is characterized by the substitution o f Mg2+ and Fe2+ on the octahedral Me2+ site. The (57)-Mossbauer spectra whi ch reveal the presence of both ferrous and ferric iron and the compositiona l data were interpreted in terms of a defect model with a distribution of t he ferric ions over both the Me2+ and the Al3+ positions and vacancies on t he Me2+ site, The Fe-57-Mossbauer parameters of the synthetic compounds cor respond to those of natural lazulites except for the total absortption rati o of the Ferric iron A(Fe3+)/(A(Fe3+) + A(Fe2+)), which is significantly hi gher in natural lazulites of the same composition. The total absorption rat io of the ferric iron increases from 4% in purl scorzalite to 15% in a Mg-r ich solid-solution with chi(Fe) = 12(1)%.