Abstract
Bernal1 and Jeffreys2 first suggested that olivine might transform to a denser spinel structure under the high pressures existing deep in the Earth's mantle. Such a transformation would be of considerable geophysical significance. This suggestion had been stimulated by Goldschmidt's3 observation that the compound Mg2GeO4 displayed olivine and spinel polymorphs. Support for the Jeffreys–Bernal hypothesis afterwards emerged when I found that the olivines Fe2SiO4 and Ni2SiO4 inverted directly to spinel structures at high pressure accompanied by large increases in density4,5. Further studies6–8 of common olivine, Mg2SiO4, also revealed a strong probability that this compound would invert to a spinel structure at high pressure.
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