Locality: Tanco Mine, Bernic Lake, Lac-du-Bonnet area, Manitoba, Canada, about 180 km east-northeast of Winnipeg.
The discovery crystal was a more than one meter long pink crystal frozen in the pegmatite. The analysis of rossmanite was obtained some time after the crystal was sampled. When the mine was revisited, the original crystal was mined out, so only a few tiny pieces were ever recovered. Unfortunately, no photograph exists of this crystal. It is formed in the late stages of crystallization of tourmaline in the deposit. This was the first reported occurrence of rossmanite in a petalite-subtype pegmatite. Other crystals have subsequently been found from this locality that are rossmanite.
Rossmanite from the Tanco Pegmatite.
image courtesy of Prof. Robert Linnen
The compositions of individual analyses in the rossmanite field are indicated by the following diagram:
Reference: J.B. Selway, P. Cerný, F.C. Hawthorne and M. Novák The Tanco pegmatite at Bernic Lake, Manitoba. XIV. Internal tourmaline. Canadian Mineralogist 877-891.