Granite is one of the most common and recognizable intrusive igneous rocks.
It is usually coarse-grained and is primarily composed of feldspar, hornblende, and quartz.
[ Also Read about Weathering – mechanical, chemical ]
Granite – group, mineralogy, test, locale
Group: Igneous; intrusive
Mineralogy: Quartz, feldspar, hornblende, biotite, and magnetite
Key test(s): Often pinkish; exfoliation
Likely locale(s): Mountainous terrain
Granite is sometimes a salt-and-pepper rock; some granites have significant white feldspar, while others have a lot more dark hornblende. Granite may occur as minor intrusions, as larger plutons, or as huge batholiths taking up tens of thousands of square miles.
One field clue is the tendency of granite to weather by exfoliation, where thin sheets peel off like the skin of an onion.
Granite isn’t highly collectible, but artisans cut granite into tabletops and counters or use it in buildings and monuments because granite takes a high polish. Probably the most interesting aspects of granite are its association with economic ore deposits, often found along the margins of granite intrusions, and the presence of pegmatites.
Granite mountains and batholiths are common across the United States – from Maine to Georgia; across the Midwest, such as at Mount Rushmore; across the West; and all the way to Alaska.
[ Related Study Weathering – geology ]