Intergrowths of augite and magnetite (type A symplectites) have been examined in eight olivines ranging in composition from Fo86 to fayalite. For Fo86 the Mg/Fe ratio in the exsolved pyroxene is the same as that of the olivine matrix. In more fayalitic compositions the pyroxene is enriched in Mg relative to the olivine matrix.

For olivines of plutonic origin two structural orientations between clinopyroxene and host have been observed. They are related by a 60° rotation about [100]o]. Magnetite forms only one orientation relationship with olivine, but because of its high symmetry, magnetite–augite lattice orientations are always the same. These symplectites are plate-like lying on (100)oi, with a tendency to be elongated along 〈010〉01 or 〈013〉o1; directions that are pseudosymmetrically related. For both pyroxene orientations the common habit plane is (100)ol║(100)px║(111)mt. There is evidence to suggest that magnetite can start to form before pyroxene. It is possible that both pyroxene and magnetite grow into the olivine by the propagation of ledges along their interface with the host phase.

It is suggested that the formation of these intergrowths is caused by the presence of ferric iron in the olivine. On cooling the olivine structure eventually contracts to a critical size below which the electrostatic repulsion between Fe3+ and Si4+ becomes too great for stability. This initiates a reaction which can be summarized by the equation:

where X = Ca, Mg, Fe. The pyroxene phase acts as a sink for elements not compatible with the olivine structure, such as A1 and Ca. Augite is formed as long as there is sufficient Ca present and the temperature is high enough for it to diffuse to the reaction sites. Exhaustion of Ca or a drop in temperature can lead to the spinel–two pyroxene assemblages reported in some earlier studies.

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