Granite

granite.jpg

Granites are coarse-grained rocks with a mottled appearance. They comprise a mixture of glassy quartz, white, pink or red alkali feldspar, minor amounts of dark minerals, and often white, sodic plagioclase. Their low density and resistance to weathering means that they usually form high, rugged terrains. Quarrymen use the term granite for almost any hard rock. Geologists use it either in a restricted sense, or as a blanket term for granitoids of acid or intermediate composition. Their complex and varied origins give rise to a great variety of types. Weather-resistant stones are polished for use as decorative facings on buildings. Granites that weather to pure kaolin provide china clay.

Colour: colourless grains, also mottled in white, pink or red, some grey or dark grains.
Mineralogy: essential quartz, alkali feldspar and plagioclase in variable amounts, usually with hornblende and/or biotite; muscovite may occur.
Classification: acid plutonic igneous rock.
Occurrence: intrusive, most commonly occurring in batholiths.
Texture: coarse to very coarse-grained; usually granular; may be porphyritic with well-shaped phenocrysts (large crystals) of feldspar; sometimes foliated; coarse intergrowths of quartz and feldspars can form a 組raphic� texture.
Structure: commonly contain xenoliths, may contain cavities (druses) into which well-formed crystals project; may be associated with much late-stage mineralization.
Examples: Shap granite, Cornish Granite (below)
cornish.jpg Left: This granite is a biotite granite with a black and white appearance. It contains milky-white orthoclase and white plagioclase. There is grey coloured quartz and approximately 10% biotite. This granite, from the South-West of England, is often used as a decorative building stone because it is hard and crystalline.
Right: This specimen from the Shap area in North-West England is an example of a pink granite. It has an equigranular texture with much pink orthoclase, grey quartz and biotite. shap.jpg