Canadian Rockhound: Cordierite

Cordierite occurs in Saskatchewan and in Manitoba. The picture shown here is a faceted, gem quality example of cordierite, though not from these provinces. Cordierite can also be found in the Northwest Territories. Picture Copyright ©1996 theImage.com.

Cordierite
By Richard Gunter, Winnipeg, MB
In the last issue of the Canadian Rockhound, Doug Miller of Saskatoon described the occurrance of cordierite in Saskatchewan. Cordierite in Manitoba will be described in two columns to provide a more detailed summary of the occurrences first described in the November 1995 issue of the Mineral Society of Manitoba newsletter.
Part I.
Cordierite, Mg₂Al₄Si₅O₁₈, is a relatively common mineral in rocks that have been metamorphosed to the proper temperature and pressure. However, most of it is quite nondescript and partially altered. Cordierite of interest to collectors occurs in two distinct parageneses (i.e. its manner of formation):

In feldspathic neosomes within regionally metamorphosed gneisses. (A neosome is the part of a gneiss that has been melted, remobilized and recrystallized into new mineral assemblages, typically feldspar, quartz and biotite).

In metamorphosed hydrothermal alteration zones, often related to ore forming processes. These include the alumina rich zones found at the copper-zinc mines around Snow Lake and the iron-magnesium rich zones occurring at Star Lake, Sherridon and Rat Lake near Leaf Rapids.

This column will concentrate on the first of these, the regionally metamorphosed gneisses. Cordierite is fairly common in these gneisses in Manitoba. Collector quality crystals can occur where the gneisses have been recrystallized into a coarse grain size and there has been little of the late-stage alteration which can convert cordierite into a mass of fine-grained mica known informally as "pinite".
The most accessible location where these conditions occur is a roadstone quarry north of Highway 391 where it crosses the Footprint River, north of the community of Nelson House. Crushed rock from the excavation of the quarry contains abundant euhedral to subhedral cordierite crystals up to 3 cm in length. There has been some surface alteration of the crystals but most are clear and only moderately fractured. The quarry is on the north side of the highway on the west side of the river. One can park on the rivers edge and walk a short distance uphill to the quarry.
Part II: Alteration Zones

Copyright ©1997 Richard Gunter

This article may not be copied, distributed or reprinted in any form without the author's permission. To contact the author, please use the e-mail address provided. If you are unable to contact the author, please contact the Canadian Rockhound. Authorized reprints must acknowledge the author, original source and the Canadian Rockhound, and include the website URL address of the Canadian Rockhound.
The preceding article was first published in the April, 1997 issue of the Mineral Society of Manitoba Newsletter. Reprinted in the Canadian Rockhound with permission from the author and editor.
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Document Number: CR9701207

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