Synthetic laihunite (☐xFe2−3x2+Fe2x3+SiO4)⁠, an oxidation product of olivine

Laihunite $(☐xFe2−3x2+Fe2x3+SiO4)$ has been produced by heating single crystals of synthetic fayalite (Fe₂SiO₄) in the air at 400, 600, and 700°C. Opaque complexes produced at the surfaces and internal defects of the heated crystals consist of iron oxides, amorphous silica and laihunite. No laihunite has been observed in the crystals heated above 800°C.

Chemical analysis of the heated fayalite by an analytical electron microscope shows that diffusion of Fe²⁺ ions to the surfaces and internal defects took place during the oxidation process, and produced iron oxide-silica complexes. This diffusion resulted in Fe-depletion zones in fayalite surrounding the complexes. Three types of laihunite have been formed in the Fe-depletion zone. Laihunite-2M$(☐0.37Fe0.902+Fe0.733+SiO4)$ overgrew on hematite-silica intergrowths, and laihunite-3M$(☐0.24Fe1.282+Fe0.483+SiO4)$ in the intermediate region between laihunite-2M and fayalite. Planar precipitates of laihunite of the Guinier-Preston zone type, with thickness of about 18Å, have also been found in fayalite around laihunite-3M. The synthetic laihunites within fayalite have larger unit cells than the natural laihunite-3M$(☐0.4Fe0.82+Fe0.83+SiO4)$ due to elastic strain.