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 Granite and other Silica Rich Igneous Rocks 
pinkish tan exposures of the Cadillac granite on the top of Mt Cadillac, Mt Desert Island, Maine.

Cadillac Granite,
Acadia National Park, Maine

Granites are intrusive felsic igneous rocks. They cooled slowly enough to have visible crystals. These crystals are mostly quartz and k-feldspars, with a small portion of other minerals, such as muscovite, biotite, or hornblende. Properly, a granite is a felsic rock of which between 10 and 50 percent of the felsic minerals are quartz and between 65 and 90 percent of the feldspars are rich in sodium or potasium.

granite with coarse crystals, unknown locality, new england. George Flagg Collection

New England
(4 cm field of view)

This is a coarse grained (thus slow cooling) granite. - clear large crystals of quartz and orthoclase feldspar (plus in this case, small fraction of muscovite mica).

Slowest cooling times let crystals grow very large - up to several meters in size. These very coarse grained rocks are called pegmatites.

Tarana granite with fine grains, Anzac Memorial, Sidney, Australia

Tarana Granite,
Anzac Memorial, Sydney, Australia
(4 cm field of view)

This is a finer grained, thus more rapid cooling, granite, with quartz and feldspar crystals a couple of millimeters across.

Pumice, flotsam on beach, east coast, Australia

(1 cm field of view)

With much more rapid cooling, such as occurs on eruption at the earth's surface, crystals are no longer visible. The rock, though felsic in composition, is no longer a granite. Felsic magmas that are extruded and rapidly cooled form rhyolites. Very rapidly cooled felsic magmas form volcanic glasses. A lava that has little in the way of dissolved gasses coming out of solution at the time it cools forms a solid volcanic glass, obsidian. A lava that has lots of dissolved gasses coming out of solution (just like a soda fizzes when its container is opened) as it is very rapidly cooled forms a frozen volcanic froth. The pumice figured to the right is light colored, and is formed of crystals too small to see. It is a mass of shards of volcanic glass and is full of holes. Thus we can deduce that it has a felsic composition, and it cooled very rapidly in an eruption of a felsic melt that contained lots of dissolved gasses.

Sources: Press and Siever, 1978
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Written by Paul J. Morris
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Date Created: 2000 Dec 30
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