Olivine to spinel transformation in Mg₂SiO₄ via faulted structures

Abstract

The mechanism of the olivine to spinel transformation has been studied in relatively few systems experimentally, most of the previous research being directed to the determination of the phase boundaries in pressure–temperature (P, T) space¹ or crystal structure modelling of the low- and high-pressure phases^2–6. Two of the more recent studies on magnesium germanate⁷ (Mg₂GeO₄) and nickel silicate⁸ (Ni₂SiO₄) revealed that there was no special orientation relationship between the olivine (α) and the spinel (γ). Furthermore, no evidence by way of stacking faults or twins was found in the transforming olivine to support the proposal that the transformation was martensitic-like^4–6. Consequently, it was concluded that in these systems the transformation had occurred by nucleation and growth processes^7,8. In contrast, the α and γ phases have been observed as tabular intergrowths in the partially transformed iron end-member, fayalite (Fe₂SiO₄)⁹, with the interphase boundaries being (100)_Ol or (111)_Sp. This microstructure and the orientation relationship are compatible with the martensitic transformation. We report here the results of transmission electron microscopy of the α → γ transformation in the pure end-member olivine, forsterite (Mg₂SiO₄). Not only is there a special orientation relationship between the two phases during the transformation, that is, (100)_Ol is parallel to (111)_Sp and [001]_Ol, parallel to [1̄10]_Sp, but some of the residual olivine grains have an extremely high density of stacking faults in (100)_Ol as well as a high dislocation density.