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Earth's mantle dynamics
Article By:
Kerschhofer, Ljuba Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany.
Last reviewed:1999
DOI:https://doi.org/10.1036/1097-8542.YB990655
It is generally accepted that the Earth's upper mantle consists mainly of olivine, an orthorhombic silicate with the composition (Mg1.8,Fe0.2)SiO4, together with some pyroxene and garnet. The natural occurrence of two high-pressure forms (polymorphs) of olivine—orthorhombic wadsleyite, and cubic ringwoodite (with a spinel structure)—was predicted from high-pressure experiments and was later confirmed by meteorite investigations. The names olivine, wadsleyite, and ringwoodite refer only to naturally occurring compositions [(Mg,Fe)2SiO4]. Chemical analogs with the same crystallographic structures but different compositions, such as Mg2GeO4, are more generally called α phase, β phase, and γ phase, respectively. Some authors also use the terms spinel for γ phase, and β-spinel or modified spinel for β phase.
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