Nano-scale microstructure of Fe (super 3+) -, OH (super -) -bearing crystalline inclusions in experimentally oxidized olivine from a mantle nodule

An olivine grain from peridotite nodule 9206 (Udachnaya kimberlite) was heated in air at 700 degrees C for 9 h. It was investigated by EMPA, by IR and UV spectroscopy and by TEM. The TEM examination reveals hexagon-like inclusions up to several hundred nm in size with a core and rim structure. The AEM data show that the inclusions contain only Mg, Fe and Si as cations. The rims have almost the same composition as the olivine matrix whereas the cores are enriched in iron and depleted in Mg. Electron diffraction data, SAED and CBED, indicate that the rims are composed of a "secondary" olivine while the cores are composed of coexisting feroxyhite FeOOH, bernalite Fe(OH) (sub 3) and beta -cristobalite SiO (sub 2) . The presence of nm-sized inclusions in minerals is expected to influence the interpretation of spectroscopic results if spectroscopy is carried out without knowledge of the microstructure. It is speculated that the complex microstructure of the inclusions is a result of a solid state transformation of a precursor phase, probably a hydrous magnesian silicate, during the experimental heating and oxidation.