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(Fe,Mg)2SiO4 occurs in the polymorphic forms of olivine (orthorhombic), wadsleyite (orthorhombic) and ringwoodite (cubic). These are some of the most common minerals in the inner solar system. Olivine occurs in the mantles of the Earth, the Moon and Mars, in meteorites, in plutonic and volcanic igneous rocks as well as in some metamorphic rocks. The high pressure polymorphs wadsleyite and ringwoodite have been reported from shocked meteorites. Diffusion coefficients of different elements in these phases have been widely used in several areas of the Earth, planetary and materials-sciences (see below for examples). The relative simplicity of structure and chemistry combined with its wide range of stability (in P-T space) uninterrupted by phase transitions also makes olivine one of the easier materials to study. As a result, olivine has served as a model material for the study of defects and diffusion in silicates. In this chapter we will focus on diffusion data in olivine, wadsleyite and ringwoodite belonging to the Fe-Mg solid solution series, (Fe,Mg)2SiO4.
Structure of olivine and types of diffusion coefficients
Olivine, with the general formula A2SiO4, is an orthosilicate made up of isolated SiO4 tetrahedra linked to each other by two different kinds of octahedral polyhedra. The octahedral polyhedra are described as M1 and M2 and are usually occupied by divalent cations such as Fe2+, Mg2+, Ni2+, Mn2+ and Ca2+. One aspect of the structure that is relevant for discussion of diffusion data is that the M1 polyhedra form a continuous chain along the  direction (space group: Pbnm) whereas the M2 polyhedra are not interlinked with each other in any direction in the structure. In this chapter reference to the  direction or the c-axis will always be made with reference to …