Canadian Rockhound: Tancoite

Tancoite
By Richard Gunter

Tancoite, HLiNa₂Al(PO₄)₂(OH), is an extremely rare mineral, named after its only occurrence, the Tanco Mine, Bernic Lake, Manitoba. Here, tancoite occurs as 1 - 1.5 cm, platy, vitreous crystals associated with powdery, white coloured aluminum phosphate which fluoresces bright green, euhedral apatite and colourless cleavable lithiophosphate. Tancoite had been previously found only on material from the mine dump, from which it was described in 1981.

The first in-situ discovery of tancoite took place underground on November 24, 1993. The tancoite crystals on this sample were noted by microscopic examination. It has not been confirmed by X-ray diffraction. The majority of the find, a piece of altered spodumene-quartz, 60 x 30 cm, with three 10 - 15 cm masses of lithiophosphate went to the Manitoba Museum of Man and Nature. This was the first reported in-situ occurrence of lithiophosphate. Some of the best cesium analcime found from the Tanco Mine was directly associated with the lithiophosphate. The occurrence, since mined out, was on a production bench on the southwest corner of 210 Floor, adjacent to 19SK pillar. The occurrence was very small, just the three lithiophosphate samples described plus one cesium analcime. Peter Vanstone checked the rubble pile after the blast and discovered a second major lithiophosphate sample, that went to the National Museum [in Ottawa]. Peter informed Dr. Cerny of the lithiophosphate find and Dr. Cerny was there the next day.

The fluorescent mineral has not been documented before but a powdery white aluminum phosphate was noted as an unknown mineral, but not described during the original description of tancoite. The associated minerals from this locality are virtually identical to that of the original tancoite sample.

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The preceding article was first published in the April 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.