Canadian Rockhound: Sodalite, a Rare Silicate Mineral

Sodalite is considered relatively rare since there are only three large deposits known in the world. One of these deposits is located only two and a half miles east of the town of Bancroft in Hastings County, Ontario. Two other locations are Ice River in British Columbia and Litchfield in the State of Maine. In Ontario, the mineral has been observed along a belt of syenitic rocks in the townships of Glamorgan, Faraday, Dungannon, Monteagle, Raglan, Brudnell and as far as Clear Lake, usually in ill-defined, irregular masses and patches.

Frank D. Adams first discovered the deposit near Bancroft about 1892 while investigating the geology of the Haliburton-Hastings area for the Geological Survey of Canada. In 1893 a rough specimen was shown in the Ontario Provincial exhibit of the World's Columbian Exposition in Chicago. The Geological Survey of Canada also displayed cut and polished specimens at the Exposition.

The royal blue colour of this sodalite piece is clearly visible. Sodalite is commonly associated with natrolite, which forms veins and masses in the sodalite. Photo courtesy of Dirk Schmid.

Sodalite gemstone
Sodalite cuts very nice gemstones. Gemstones related to the mineral nepheline are of special interest to Canadians, since they are relatively exclusive to this country and sodalite is the best known.

Carving
Bird carving. This carving is one of many attractive uses for the massive form of sodalite. Photo by Marilyn Fraser.

Sodalite is a mineral of alkaline igneous and plutonic rocks associated with feldspathoids and occurring in massive form in nepheline syenite gneiss and pegmatite. The chemical formula is Na₂Al₃Si₃O₂Cl, and hence the mineral is a sodium aluminum silicate with chlorine. The colour ranges from royal blue to light blue and white and it is commonly associated with natrolite, which forms veins and masses in the sodalite. Natrolite ranges in colour from orange-red, to pink, to white. It also occurs as transparent acicular crystals and as pearly-white tabular or prismatic crystals in small cavities.

In certain types of silica-poor igneous rocks, the feldspars are replaced by a special group of minerals, the feldspathoids. Feldspathoids are nepheline and sodalite. They are not common. The finer-grained varieties are found in dykes and sills.

Sodalite is found in regions where volcanic action was responsible for nepheline rocks being invaded from below by sodium chloride. This is what gives the mineral its deep blue colour.

The blue colour is very typical of this mineral and can be confused with lazulite and lazurite. On heating, sodalite loses colour and fuses to white glass with the strong yellow flame of sodium. The fused mineral will fluoresce blue in shortwave ultraviolet light. Hackmanite is brilliantly fluorescent without heating.

This mineral has a glassy luster with a Mohs hardness of 5.5-6. The specific gravity is 2.2-2.3. Its fracture is conchoidal to uneven with a poor dodecahedral cleavage.

In 1905, when the Prince and Princess of Wales visited Canada, they were presented with a piece of sodalite. The couple had such admiration for the beautiful azure blue stone with its red and white colouring that arrangements were made to ship some of it to England. Thus the Princess Sodalite Quarry was named. The name as since been changed to the Princess Sodalite Mine.

1906 saw the first commercial operation of the deposit, when Mr. Thomas Morrison employed six men to extract 200 cu. ft. (130 tons) of sodalite for use as a decorative stone. This production, valued at $6,000 was shipped to London for use in Marlborough House, for the residence of Sir Ernest Cassell, Park Lane, Hyde Park.

Over the years this mine has been operated by a number of owners:

1893-1905: Dr. Allen, England
1905-1906: Thomas Morrison
1906-1946: English Crown
1946-1954: Mr. Bowers
1953-1955: Dept. of Mines
1955-1960: Bill Morrison
1960-1970: Carl Bosiack
1970-1996: Paul Rasmussen
1996-Today: Andy Christie

Other deposits in the area are at Cancrinite Hill, Davis Hill, Davis Quarry, Egan Chute, Goulding-Keene Quarry, Lily Robertson and the Morrison Quarry.

At the Goulding-Keene Quarry the deposit is about 80 ft. long by 60 ft. wide with a 40 ft. face. It is located in the side hill on the west bank of the York River. Grey nepheline and white plagioclase include deposits of blue massive sodalite. At Egan Chute sodalite occurs in irregular masses in gneiss.

Map

Sodalite cuts very nice gemstones. Gemstones related to the mineral nepheline are of special interest to Canadians, since they are relatively exclusive to this country and sodalite is the best known.

References:

Bell, P., and D. Wright. Macmillan Field Guides, Rocks & Minerals. Macmillan Publishing Company. 1985.
Dana, E.S. Minerals and How to Study Them. 3rd ed. John Wiley & Sons Inc. 1965.
Sabina, Ann P. Geological Survey of Canada Miscellaneous Report 39 - Rocks and Minerals for the Collector. 1986.
Ince, Bill. Collecting Minerals. McClelland & Stewart Limited. 1977.
Metz, R. Gems and Minerals in Color. Hippocrene Books, Inc. 1973.
Ontario Geological Survey Memorandum #6. 1911.
Pough, F. H. Field Guide to Rocks & Minerals. The Riverside Press. 1960.

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The preceding article was first published in the Summer 1989 issue of Cab & Crystal (Vol. 1, No. 2). Updated December 2000. Reprinted in the Canadian Rockhound with permission from the author.