Magnetite exsolution in almandine garnet

Abstract

A B S T R A C T. Three almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe 2 + show that there are no significant departures from stoichiometry due to the presence of Fe 3 § 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/~ 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 550 ~ The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe a + content of the host garnet.KEYWORDS: magnetite, almandine, garnet, exsolution, transmission electron microscopy.THE exsolution of oxide minerals is a common phenomenon in many silicate phases. Several examples of such exsolution have been successfully studied by transmission electron microscopy (TEM). Various authors have investigated exsolved oxide phases such as ilmenite and magnetite in olivines and pyroxenes, e.g. Moseley (1981Moseley ( , 1984, and recently Mongkoltip and Ashworth (1983) have discussed the exsolution of rutile and ilmenite from hornblende. In all the above examples the presence of a secondary exsolved, opaque phase had been initially observed by conventional light microscopy. The further use of TEM enabled detailed crystallographic, morphological and, in some cases, chemical information to be obtained in order to deduce a possible mechanism for the exsolution process. In contrast to the examples above, the present paper is concerned with the exsolution of magnetite particles from almandine garnet on a scale which is well beyond the resolution of the light microscope. Sample descriptionsThree samples have been used for this study. All are from pelitic metamorphic rocks with differing ~) Copyright the Mineralogical Society metamorphic histories. Sample localities and petrography are presented below.Sample RM5020 is a pelitic gneiss from the Moine inlier on the Ross of Mull, Isle of Mull, Scotland and has been studied in detail by Brearley (1984). The rock contains the assemblage garnet + biotite + muscovite + plagioclase + quartz. Garnet is abundant, occurring as small crystals less than 500/~m in diameter. The core regions are optically homogeneous ( fig. la) and are separated from the outer inclusion-free rim of the garnet by a zone full of inclusions, dominantly quartz with subordinate opaques. These distinct zones are probably attributable to periods of growth at different rates during the metamorphic evolution of the area. The calculated conditions of formation for t...

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A natural example of the disequilibrium breakdown of biotite at high temperature: TEM observations and comparison with experimental kinetic data

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The surface texture and morphology of magnetite from the Izok Lake volcanogenic massive sulfide deposit and local glacial sediments, Nunavut, Canada: Application to mineral exploration

“…During regional metamorphism, an increase in oxygen fugacity of the fluid phase causes the breakdown of Fe-bearing minerals such as biotite, and subsequent development of iron formations (Nesbitt, 1986). Brearely and Champness (1986) studied three metamorphic almandine garnets with numerous magnetite inclusions. They concluded that magnetite can exsolve from almandine garnet during cooling.…”

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