GRANITE
Granite
is
a common and widely occurring type of intrusive, felsic, igneous rock. Granites
usually have a medium- to coarse-grained texture. Occasionally some individual crystals are larger than
the groundmass, in
which case the texture is known asporphyritic. A
granitic rock with a porphyritic texture is sometimes known as a porphyry. Granites
can be pink to gray in color, depending on their chemistry and mineralogy. By definition, granite
is an igneous rock with at least 20% quartz by volume. Granite differs from granodioritein that
at least 35% of the feldspar in granite is alkali
feldspar as opposed to plagioclase; it is
the alkali feldspar that gives many granites a distinctive pink color. Outcrops of granite tend to form tors and rounded massifs. Granites
sometimes occur in circular depressions
surrounded by a range of hills, formed by the metamorphic
aureole or hornfels. Granite
is usually found in the continental
plates of the Earth's crust.
Granite
is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use
as a construction stone. The word granite comes from the Latin granum, a grain, in
reference to the coarse-grained structure of such a crystalline rock.
Granite
has been extensively used as a dimension
stone and
as flooring tiles in public and commercial buildings and monuments. Aberdeen in
Scotland, which is constructed principally from local granite, is known as "The Granite City". Because of its
abundance, granite was commonly used to build foundations for homes in New England.
The Granite Railway,
America's first railroad, was built to haul granite from the quarries inQuincy,
Massachusetts, to the Neponset
River in
the 1820s. With increasing amounts of acid
rain in
parts of the world, granite has begun to supplant marble as
a monument material, since it is much more durable. Polished granite is also a popular choice
for kitchen countertops due
to its high durability and aesthetic qualities. In building and for countertops, the term "granite" is often
applied to all igneous rocks with large crystals, and not specifically to those with a granitic
composition.
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