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Nucleation and growth of iron oxides in olivines, (Mg,Fe)2SiO4

Published online by Cambridge University Press:  05 July 2018

P. E. Champness
P. E. Champness*
Affiliation:
Department of Geology, University of Manchester
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Summary

Iron-rich olivines have been oxidized in air in the laboratory and the mechanism of their breakdown has been elucidated using X-ray diffraction and electron microscopy. Low-temperature oxidation (500–800 °C) produces well-oriented hematite- and magnetite-like precipitates together with amorphous silica. The reaction is a cellular one in which thin needles of oxide about 50–100 Å apart grow into the matrix separated by regions of amorphous silica. Nucleation of spherical colonies of the iron oxide and silica occurs on dislocations.

Although the hematite or magnetite always shows the same topotactic relationship with the matrix, the direction in which the needle-like precipitates grow is determined by the orientation of the nucleating dislocation. The small size and highly distorted nature of these precipitates accounts for the diffuseness of their X-ray reflections.

Oxidation at 1000 °C produces undistorted equiaxed grains of the oxides about 0·2 μm in size. They are surrounded by silica, which produces a disordered electron diffraction pattern. As the temperature is raised, the silica achieves more structural order and the oxide grains increase in size.

Type
Research Article
Information
Mineralogical Magazine , Volume 37 , Issue 291 , September 1970 , pp. 790 - 800
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1970

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