What is Granite?

Jim Williams
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Thrust up over millennium, veins of granite are solidified liquid magma. Derived from the Latin root granum - a grain - the word granite refers to the coarse-grained structure of this igneous, crystalline rock. Magma, flowing up but not reaching the surface, flows horizontally. Granite must cool over millions of years to the form the crystals, that are a more strong and hard than man-made, compressed minerals; and, to allow the dissolved gases to escape.

When granite has properly cooled, there are few voids - porosity, where gas was trapped - and it is very hard as a result of the crystals. The Mohs Hardness Scale rates diamonds as 10 and granite is 7. Granite is a combination of three essential minerals - feldspar, mica and quartz. These minerals, in addition to a variety of other minerals, combine to form unique colors, textures and structural characteristic.

Feldspar is comprised of varying combinations of aluminum, calcium, sodium or potassium. Feldspar is the "white" in granite, comprising more than 50% of the earth's mantle. Each of the 28 known classifications of mica has its own name. However, two are the most prevalent - muscovite (light colored) and biotite (black). The small black granules in granite are biotite mica. When magma cools, large silica crystals are formed, called quartz. The quartz content in magma varies.

However, any stone formed from magma must contain at least 20% quartz to be considered a granite. The more quartz, the harder the granite, and the more difficult it is to quarry. The hardness variations in granite come from the quartz content. The porosity, or lack of it, of different granites comes from the rate of cooling. All granite deposits will have cooled more quickly at the edge than in the center.

Consequently, the first granite quarried from a new cite is less hard and less desirable for building and construction and is thrown out. While the quartz crystals are the same in each granite, they can be of different sizes and there can be more or less of them. Colored granites result from various impurities and are lighter or darker, depending upon the type of feldspar present. Granite, in addition to other crystalline rocks, comprises the essence of our continental masses; being the most common intrusive rock exposed at the earth's surface.

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