"Rock of the Month # 91, posted for January 2009" ---

Corundum (Al₂O₃) is a simple oxide, an hexagonal mineral whose crystal habit varies from tapering prisms to stubby barrel- or plate-like forms. Brittle yet tough, and very hard, it occurs in igneous rocks, such as nepheline syenites and pegmatites, and in metasedimentary schists and marbles. The photomicrographs (Figs. 1-2) clearly show the high polishing relief of this exceedingly hard mineral.

The photomicrograph of 2055 (Fig. 1, above left) displays coarse corundum invaded by a scapolite-filled fracture, in a matrix of alkali feldspar. Besides microcline and plagioclase, the section displays the corundum (some crystals appear to be zoned), biotite mica, and accessory phases including scapolite, calcite, magnetite, hercynite, pyrite and (?) zircon.

Sample 2059 (Fig. 2, above right) is a coarse corundum syenite from Burgess corundum mines, southwest of Combermere in Carlow township. This sample is from the Logan Cut, and it is dominated by alkali and plagioclase feldspar, with abundant biotite and corundum, plus muscovite sheaths around the corundum, and accessory magnetite, zircon and hematite. Similar material is also seen at the best-known and overall most prolific site in the district, the Craigmont corundum mine in northwest Raglan township, on the steep flanks of Craigmont Hill, above and west of Conroys Marsh and the York River, south of Combermere.

Fig. 3. Massive corundum syenite from the Lily Robertson (Cooney) occurrence, east of Bancroft.

Corundum is very well-documented, particularly for its gemstone forms (sapphire and ruby, together with the emerald form of beryl, are the "big three" species in the coloured stone trade, subject to steady demand). MINLIB, by early 2021, contained some 949 articles relevant to corundum, of which 113 (almost 12% or 1 in 8) relate to the Grenville province. The corundum occurrences in southeast Ontario occur near Sharbot Lake, the Blue Mountain mine, and the Bancroft belt of alkaline igneous rocks, from which the featured samples were collected. A century ago, the area was the world's biggest producer of abrasive corundum, which was mined in ledges on hillsides and open pit mines, from syenites which often contained 10 percent or more of the tough mineral. A point of historical note is that Willet Green Miller, first provincial geologist of Ontario, wrote an early article on using glacial boulder trains in mineral exploration, citing the distinctive, resistant corundum-bearing boulders from the Bancroft area.

Selected references

Miller,WG (1899) Notes on the corundum-bearing rocks of eastern Ontario, Canada. American Geologist 24, 276-282.

Kerr,DG (1906) Corundum in Ontario. Can.Min.J. 27, 152-157.

Barlow,AE (1915) Corundum, its Occurrence, Distribution, Exploitation, and Uses. GSC Memoir 57, 377pp.

Moyd,L (1950) Structure of the corundum deposit at Craigmont, Renfrew county, Ontario. Proc.GAC 2, 51-56.

Hewitt,DF, Peach,PA and Moyd,L (1953) Petrology of the nepheline and corundum rocks, Bancroft area, eastern Ontario. GSA / GAC Guide Book, Field Trip No.1, Toronto, 28pp. plus 3 maps.

Dreimanis,A (1958) Tracing ore boulders as a prospecting method in Canada. CIM Bull. 51, 49-56.

Fouts,C (1998) Bancroft & District Mineral Collecting Guidebook. Bancroft & District Chamber of Commerce, 50pp.