Granite, is an igneous rock of visible crystalline formation and texture. It is composed of feldspar (usually potash feldspar and oligoclase) and quartz, with a small amount of mica (biotite or muscovite) and minor accessory minerals, such as zircon, apatite, magnetite, ilmenite, and sphene. Granite is usually whitish or gray with a speckled appearance caused by the darker crystals. Potash feldspar imparts a red or flesh color to the rock. Granite crystallizes from magma that cools slowly, deep below the earth’s surface. Exceptionally slow rates of cooling give rise to a very coarse-grained variety called pegmatite. Granite, along with other crystalline rocks, constitutes the foundation of the continental masses, and it is the most common intrusive rock exposed at the earth’s surface.
Although granite has been known as igneous rocks derived from, molten masses or magmas, but there is wide evidence that the origin of some granite may be attributed to regional metamorphism or preexisting rocks, rearrangement and recrystallization taking place without a liquid or molten stage.
The specific gravity of granite ranges from 2.63 to 2.75. Its crushing strength is from 1050 to 14,000 kg per sq cm (15,000 to 20,000 lb per sq in). Granite has greater strength than sandstone, limestone, and marble and is correspondingly more difficult to quarry. It is an important building stone, the best grades being extremely resistant to weathering.
Normally granite is classified in three different groups:
* FINE GRAIN: Fine grain granites are those which the feldspar crystals average about 1/16 – 1/8″ in diameter.
* MEDIUM GRAIN: Medium grain granites are those in which the feldspar crystals average about 1/4″ in diameter.
* COARSE GRAIN: Coarse grain granites are those in which feldspar crystals average 1/2″ , and greater diameter or several centimeters in maximum dimension. Coarse grain granites may have a lower density.
In recent years about 83 percent of the stone used for monuments has been granite, about 17 percent, marble.