Magnetite Exsolution in Almandine Garnet

A. J. Brearley and P. E. Champness
Department of Geology, The University, Manchester, M13 9PL

Abstract: Three almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe2+ show that there are no significant departures from stoichiometry due to the presence of Fe3+ in any of the garnets studied. However, in the transmission electron microscope (TEM) all the garnets were found to contain myriad spherical, iron-rich particles up to 400 Å in diameter. Microdiffraction techniques have revealed that the particles are a cubic spinel phase, consistent with magnetite. There is no crystallographic relationship between the host garnet and the particles, a rare situation for exsolution processes. The presence of such particles is interpreted in terms of the exsolution of magnetite from almandine garnet during cooling. This can apparently occur at temperatures below 55°C. The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe3+ content of the host garnet.

Keywords: magnetite • almandine • garnet • exsolution • transmission electron microscopy

Mineralogical Magazine; December 1986 v. 50; no. 358; p. 621-633; DOI: 10.1180/minmag.1986.050.358.07
© 1986, The Mineralogical Society
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