Canadian Rockhound: Chromite

Chromite
By Richard Gunter

Chromite, FeCr₂O₄, is a member of the spinel group of cubic oxide minerals. As such, it has a large amount of substitutions for both iron and chromium in its structure. Manitoba chromite contains abundant aluminum and magnesium in its structure. Chromite is the only ore of the metal chromium, which is used as a major raw material in stainless steel. The large majority of the chromite mined in the world comes from South Africa. North America has few deposits of chromite.

The chromite crystals occur as oxide layers in the Bird River Sill in the southeast part of the province. These oxide layers are exposed to the surface. The Bird River Sill is one of North America's largest and richest chromite deposits.

The chromite occurs as 1 - 2 mm black octahedral, euhedral to subhedral crystals. Most of the oxide layers contain only intergrown crystals, not individual ones. Occasional 2 - 3 mm cm gray-green lenses of prehnite (?) occur within the chromite layers. These prehnite lenses contain coarser grained, isolated octahedrons of chromite.

The most accessible occurrence of chromite in the Bird River Sill is the old "Chrome" claims now owned by Gossan Resources Ltd. An overgrown bush road leads from the highway to the chromite location. The culvert over Peterson Creek may be submerged and may require hip waders to cross. Permission to collect at this site should be obtained from Gossan Resources in Winnipeg, Manitoba.

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The preceding article was first published in the May 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.