An atoll-shaped garnet usually consists of a complete or almost complete garnet rim enclosing a core that may contain any combination of mica, feldspar, quartz, and iron ore. In some cases double rims occur, i.e., a smaller rim within and separate from the outer rim, and it is also possible to have garnet “islands” within the rims (Fig. 1). Although the term “atoll garnet” is descriptive, it is conventional to exclude similar garnet shapes occurring as coronas or reaction rims, or skeletal garnet growths in quartz-rich matrices. However, the growth of a single garnet around grain boundaries may closely resemble an atoll garnet.
Photomicrograph of an atoll-shaped garnet from Ardara, Co. Donegal, Eire; the rim has been breached in several places but the euhedral outline has remained, particularly where in contact with quartz or a quartz-rich matrix; the original garnet has been replaced by biotite plus a little quartz, with the core remaining as a garnet “island”; PPL, ⋉35.
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Bibliography
Atherton, M. P. and W. M. Edmunds, 1966, An electron microprobe study of some zoned garnets from metamorphic rocks, Earth Planet. Sci. Lett. 1, 185–193.
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© 1989 Van Nostrand Reinhold
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Smellie, J.A.T. (1989). Atoll garnet . In: Petrology. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_20
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