Canadian Rockhound: Almandine Garnet

Almandine Garnet
By Richard Gunter

Almandine garnet, Fe₃Al₂(SiO)₄)₃, is a very common mineral, the most common of the garnet group, that forms whenever rocks rich in iron and magnesium are raised to the proper metamorphic pressure and temperature. Examples of rocks that contain garnet when metamorphosed are basalt, shale and altered gabbro.

Generally the garnets are small and not very interesting as collecting crystals. Under unusual conditions the almandine garnets can form large euhedral crystals, that delight mineral collectors. The conditions for almandine garnet growth are a slow cooling of metamorphic temperature, which allows the crystals to grow to large size, and a growth medium, such as chlorite, that allows the crystals to develop faces without interference from the growth of other crystals.

An example of such an occurrence was found in 1985 on the 1250 level of the then active Chisel Mine, near Snow Lake [Manitoba]. The altered rock beneath the copper-zinc ore deposit had originally been altered to an iron-rich clay by the hot waters that deposited the sulphides. As the area was metamorphosed euhedral almandine crystals began to grow in the clay.

Only a few isolated almandine garnet crystals started to grow and the result was a few large garnets rather than many small crystals. The iron-rich clay became a fine-grained chlorite which allowed both the garnet crystals to grow and allow easy removal of the chlorite when the garnet crystals were collected.

This particular location is currently inaccessible with the closure of the Chisel Mine, but there are several locations in the Snow Lake Area where garnet crystals can still be collected.

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The preceding article was first published in the February 1998 issue of The Mineral Vein, the official newsletter of the Mineral Society of Manitoba. Reprinted in the Canadian Rockhound with permission from the editor.