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The Defining Features of Granite
Granite is an example of an igneous rock, meaning rock that forms when molten rock, or magma, cools near the surface of the earth. More specifically, granite is referred to as an intrusive igneous rock. The term intrusive refers to the fact that rocks that have cooled into solid form within or under the crust of the earth, as opposed to magma which was pushed to the surface or exploded into the atmosphere by volcanic activity, known as extrusive igneous rock. Intrusive igneous rock is also sometimes referred to as plutonic rock, in a reference to Pluto, god of the underworld. Because intrusive igneous rock cools at a slower rate beneath the earth's surface than extrusive rocks, crystallization occurs more fully, creating the large crystals that give granite its characteristic mottled appearance.
Key Components of Granite
The other defining feature of granite is a high concentration of silicon dioxide (SiO2). According to the U.S. Geological Survey, by definition granite must contain at least 65 percent silica (a form of silicon dioxide). The key silica elements comprising granite are quartz, feldspar and mica.
Components in Granite
A variety of intrusive igneous rocks containing these elements are generally referred to as "granitite" and may sometimes be used interchangeably with true granite. Granite, however, is specifically defined by the proportions in which its various elements occur.
Quartz
The dominant element is quartz, which makes up 20 to 60 percent of granite. Quartz is made of silicon dioxide that forms clear, glassy crystals shaped like six-sided prisms.
Feldspar
Granite includes two types of feldspar in varying proportions. Potassium feldspar has prism-shaped crystals, typically pink or white in color. The other type is plagioclase feldspar, which has similarly shaped crystals that are more grayish or white in color, and may have a glassy luster. Granite usually contains more potassium feldspar than plagioclase feldspar, but without at least a small amount of the latter, the rock is technically a different type, called alkili granite.
Mica
The final defining component of granite is mica. The term mica refers to a group of silicate materials comprised of aluminum, potassium, magnesium, iron and water, which form flat, flaky crystals. The most common micas in granite are biotite and muscovite. Biotite appears as black or dark brown flakes within the granite, while muscovite is light-colored or clear. Many types of granite also include hornblende, which forms dark green, needle-like crystals.
Characteristics and Uses
Granite's naturally enduring qualities have lead to its association with strength and permanence, which may be why it is a popular choice for gravestones and memorial markers. It's other characteristics, including a wide range of colors and patterns formed by its varying mineral crystals, have lead to its popularity both as a constructive material, as in paving stones, and in polished slabs used for facing stones and counter tops.