We members of our Brampton Club should occasionally remind ourselves that we have rock as well as mineral interests. And so, always in the superior judgement of our editors, this space would like to offer some – hopefully – interesting information on this literal foundation of geology – rocks.

Even among elementary school children Granite is the most correctly identified favorite. For that matter it is also for adults. The reason is more or less obvious – it’s usually spotted with large "grains", hence the name.

There is a vast literature and equally vast disagreement about granite, centering mostly on its origin. It certainly is "plutonic" – its large crystals attesting to deep slow cooling in the crust. But does it come from recycled sediments of uncertain source or metamorphic material or has it crystallized from certain kinds of magmas? The dispute rages, but with our better understanding of crustal-plate movement many scientists favor the idea that as the continents drift across ocean floors, the thick deep-sea sediments are scraped off and forced under the continents to great melting depths.

It’s suggested that this now-buoyant liquid (magma) or mush begins to rise as great balloons – remember the once-fashionable lava lamps? – melting and/or displacing overlying country rock as they head toward the surface of the crust. It appears significant that these great bodies of granite – batholiths – are found under mountain ranges. There, hoisted by mountain building (orogenesis) the overlying rocks are often removed, exposing these granite masses to view. For example we have the Nevada, Idaho, Nelson and Coast Range batholiths (deep-rocks) beautifully available for our inspection. Well, sort of. While surface exposures may be hundreds of kilometers wide by thousands long parallel to the Coast Ranges, there is no way of knowing how deeply they extend, their uniformity with depth or the mechanics of their formation. No technology in sight will be of use.

Even the definition of granite has been hugely corrupted, mostly by the building-stone industry where almost any rock with visible crystals may get plugged into the term. Presently true granite is made up something like this "rubbery" analysis: quartz 20 – 60 %; feldspar(s) 30 – 70 % and dark (mafic) minerals as biotite and horneblende in small amounts. Granite forms the cores of all continents because its SG of about 2.7 allows it to "float" on the heavier – SG 3.3 – basalt of the mantle. It’s also lighter in color thanks to quartz and feldspars.

The sloppy definition of granite is largely due to the kinds and quantities of feldspars present. If you can hang in there long enough we’ll try to sort out the feldspar mess another time.

Uses? Well, if we’ve lived the good life, perhaps caring survivors will plant a plain block or a handsomely carved monument over our bones – tombstones. A more conspicuous use is as facing on (bank!) towers and commercial buildings.

Have you ever walked or cycled over old European streets? "Belgian blocks". We may suspect that these were laboriously hacked into shape in prison quarries.

The word means "joined together", first applied in 1822 to describe graphic granite, an uncommon form which looks like some primitive "writing." Now the term includes any coarse-grained igneous rock. We usually mean granite pegmatite but in fact pegmatites may be formed in many kinds of igneous rock, even basalts. So don’t ignore any igneous masses when searching for specimens.

Grains in pegmatites vary widely in size from centimetres to metres. Giants are rare but do exist; a single crystal of spodumene (LiAlSi206) a lithium mineral, in a mine in South Dakota, is 15 metres long. Beryl crystals to 54 tonnes have been found in Brazil. But, aside from crystal sizes, all pegmatites "look igneous."

Simple pegmatites are rather homogeneous, with mostly microcline feldspar and quartz with a little biotite (black) mica and black tourmaline. But complex pegmatite’s – the kind collectors love – have besides various feldspar, goodies to warm the heart – large crystals of muscovite (clear mica), beryl (emerald, aquamarine, morganite, etc.), topaz, coloured tourmaline and spodumene. Much rarer ones too, like lithium, niobium, tantalum, cesium, uranium and the so-called rare earths. Treasure indeed!

Simple pegmatites usually occur as dikes – near-vertical injections cutting country rock. They are rather common in our Shield rocks. But "good" ones occur as pods or irregular bodies within the country rock surrounding large igneous masses, or seemingly with metamorphic bodies.

Origin? Uncertain. Most pegmatites form from late stage liquids "squeezed" out of cooling magmas.

As collectors, most of us settle for "thumb-nails" or micro-sizes of specimens, so lucky indeed is the bushwhacking collector who stumbles onto such a pegmatite where rare giant treasures of museum quality may lurk.