Abstract
Olivine of approximate composition Mg1.8Fe0.2SiO4 is a major constituent of the Earth's upper mantle. Structural changes, which occur in olivine as a function of increasing pressure and which involve the formation of the β-phase and spinel structure, γ-phase polymorphs, have therefore aroused considerable interest1–4. The mechanisms of these transformations are of particular importance because of their effect on the properties of the mantle in the zone in which they occur. Several mechanisms have been proposed4–7, but remain controversial in the absence of sufficient observational data on intergrowths of reactant and product phases. Although these phases have been produced experimentally2, direct observations of such inter-growths have generally been made only on natural material from shock-produced veins in chondritic meteorites5,6,8. Here we report observations made on the natural, high-density polymorphs of (Mg,Fe)2SiO4 found in the Peace River meteorite, which reveal that the mechanism for the spinel to β-phase transformation involves the topotactic replacement of the spinel polymorph by single, β-phase grains.
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Price, G., Putnis, A. & Smith, D. A spinel to β-phase transformation mechanism in (Mg,Fe)2SiO4. Nature 296, 729–731 (1982). https://doi.org/10.1038/296729a0
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DOI: https://doi.org/10.1038/296729a0
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