Canadian Rockhound: The No. 22 Dyke

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The No. 22 Dyke
By Richard Gunter

Some localities in Manitoba warrant individual descriptions rather than being mentioned as part of a mineral description. The No. 22 pegmatite dyke that Alan Anderson described as part of his Master's thesis at the University of Manitoba is one of these important localities. It contains a number of very rare minerals, not all of them described, plus some interesting variations on common minerals.

The No. 22 Dyke occurs on the south shore of a small island about 2 km north of the town of Cross Lake. Depending on the water level in Cross Lake, the dyke is exposed for a width of 2 to 12 metres. The lake is quite shallow between the town and the dyke, so the best time to view the dyke (when water levels are low) is also the hardest time to reach it.

The dyke has an unusual chemical composition, which accounts for some of the rare minerals. It is very rich in manganese, but has very little iron. Most of the minerals in it have some of the highest manganese contents known anywhere.

The No. 22 Dyke is the only known locality for the rare phosphate mineral bobfergusonite, named after the University of Manitoba's long time mineralogy professor. The formula is Na₂Mn²⁺₅Fe³⁺Al(PO₄)₆ and it occurs as olive-green to brown, subhedral crystals 1-3 cm long. The crystals have a prominent schiller effect. Bobfergusonite is currently being re-investigated to determine its exact chemistry and unit cell size, as it is extremely difficult to tell it apart from other related phosphate minerals such as the partially described "elephantite" (informally named for its huge unit cell size).

There are several other phosphates that occur with the bobfergusonite as complex, pod-shaped aggregates. These pods were still intact as of 1994 and are difficult to sample due to the smooth surface of the outcrop, but generally contain a dark blue, manganese rich apatite in addition to the other rare minerals.

Along with the phosphates, there is an interesting texture of tourmaline and muscovite at this site. The tourmaline occurs as flattened blades with a medium brown internal reflection and often occurs within the muscovite sheets. They have not been analyzed as yet, so the variety is unknown. Muscovite occurs as thick books up to 30 cm thick.

The No. 22 Dyke is a very unusual mineral locality. Its remoteness and difficult accessibility will keep it unexploited until it can be properly investigated.

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The preceding article was first published in the September 1997 issue of the Mineral Society of Manitoba newsletter. Reprinted in the Canadian Rockhound with permission from the author.