Granitic pegmatite

from the Bancroft region of the Grenville province in S.E. Ontario, Canada.

Pegmatite [64 kb]

"Rock of the Month # 47, posted for May 2005" --- Sample GCW 2143. A granitic pegmatite composed largely of pink and greenish-white feldspars and vitreous grey smoky quartz. The pink alkali feldspar displays perthitic exsolution of plagioclase within a host of microcline.

Pegmatites are amongst the last products to crystallize from a cooling magma. These typically coarse-grained intrusive rocks are generally encountered as metre-scale dykes and sills, irregular masses and pockets, and in some cases as segregations in the cupola (roof) of a magma chamber. The most-frequently encountered pegmatite bodies are of granitic composition, but coarse rocks with cm- to dm-scale crystals may also form from more basic (e.g., gabbroic) magma. In all cases, pegmatites concentrate volatiles (such as hydrogen [OH, H2O], boron, and the halogens chlorine and fluorine) and elements which are "incompatible" with common rock-forming minerals. Pegmatites may thus be enriched in rare or unusual elements of possible economic value. These may be metals such as tin and tungsten, tantalum and niobium, as well as alkali metals such as lithium, rubidium and caesium. Some of these elements form minerals of actual or potential value as semi-precious stones and gemstones, including tourmaline (with essential boron) and beryl (rich in beryllium). Many rare and/or complex mineral species occur, including oxides of iron, titanium, rare-earth elements and other cations. Precambrian cratons, including those of Canada, are particularly well-endowed with concentrations ("swarms") of pegmatites of many chemical and mineralogical classes (see selected references for many pegmatite articles).

The figured pegmatite is from the old Cairns mine in Monteagle township, Hastings county, one of many small, abandoned feldspar mines in the Proterozoic Grenville province in southeastern Ontario (Sabina, 1986, p.32; Udd, 1999, pp.41-42). The mine, a small open pit, was an insignificant feldspar producer in the early 1920s. It is situated in the Hybla valley area of Monteagle township, northeast of Bancroft, an area particularly noted for feldspar pegmatites (Hewitt, 1967, 1969; Sabina, 1986).

This striking granite pegmatite is composed largely of beautiful, pale pink, perthitic microcline plus subordinate pale greenish-white plagioclase feldspar and grey smoky quartz. The alkali feldspar may attain 10-12 cm in maximum dimension, the quartz and plagioclase are generally 3 cm or less. The pale plagioclase has a tabular habit and exhibits fine parallel striation on cleaved faces, an indicator of the albite twin law. The alkali feldspar in contrast is simple-twinned. Small blebs and larger crystals of quartz are hosted by both feldspars (arrays of quartz blebs in the microcline are a coarse example of the texture known as graphic granite. Deep rusty spots surround small pyrite crystals. Other minerals reported here include hornblende, magnetite, titanite (sphene) and rarer phases such as bastnaesite (a rare-earth carbonate) and betafite (a complex oxide containing uranium, calcium, titanium, niobium and tantalum: thus a good example of the chemical complexity common in many pegmatites).


ANDERSON,AJ, SIMMONS,WB and GROAT,LA (editors) (1998) Granitic Pegmatites: The Cerny-Foord Volume. Can.Mineral. 36 part 2, 249-682.

CERNY,P (1991) Rare-element granitic pegmatites. Part II: Regional to global environments and petrogenesis. Geoscience Canada 18, 68-81.

HEWITT,DF (1967) Pegmatite Mineral Resources of Ontario. Ontario Department of Mines Industrial Mineral Report 21, 83pp.

HEWITT,DF (1969) Geology and Scenery, Peterborough, Bancroft and Madoc Area. Ontario Department of Mines Geological Guide Book 3, 114pp.

LONDON,D (1996) Granitic pegmatites. Trans.Roy.Soc.Edinburgh: Earth Sciences 87, 305-319.

MARTIN,RF and CERNY,P (editors) (1992) Granitic Pegmatites. Can.Mineral. 30 part 3, 497-954.

SABINA,AP (1986) Rocks and Minerals for the Collector: Bancroft - Parry Sound area and Southern Ontario. Geological Survey of Canada Misc.Rep. 39, 182pp.

UDD,JE (1999) The Mines of Ottawa: A Guide to the Mineral Deposits of Southeastern Ontario and Southwestern Quebec. CJ Multi-Media Inc., 89 Ridgefield Crescent, Nepean, Ontario, 1st edition, 375pp.

Graham Wilson, 13 June 2005

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