Granite, coarse- or medium-grained intrusive igneous rock that
is rich in quartz and feldspar; it is the most common plutonic
rock of the Earth's crust, forming by the cooling of magma (silicate
melt) at depth. Because of its use as paving block and as a building
stone, the quarrying of granite was, at one time, a major industrial
activity. Except for tombstones, however, for which there is a
continuing demand, the present production of granite is geared
to the fluctuating market for curbing in highway construction
and veneer used in the facing of large industrial and commercial
buildings.
Granite may occur in dikes or sills (tabular bodies injected in
fissures and inserted between other rocks), but more characteristically
it forms irregular masses of extremely variable size, ranging
from less than eight kilometers (five miles) in maximum dimension
to larger masses (batholiths) that are often hundreds or thousands
of square kilometers in area.
The principal constituent of granite is feldspar. Both plagioclase
feldspar and alkali feldspar are usually abundant in it, and their
relative abundance has provided the basis for granite classifications.
In most granite, the ratio of the dominant to the subdominant
feldspar is less than two. This includes most granites from the
eastern, central, and southwestern United States, southwestern
England, the Fennoscandian (Baltic Shield) area, western and central
France, Spain, and many other areas. Granites in which plagioclase
greatly exceeds alkali feldspar are common in large regions of
the western United States and are thought to be characteristic
of the great series of batholiths stretching from Alaska and British
Columbia southward through Idaho and California into Mexico. Granites
with a great excess of alkali feldspar over plagioclase are known
from New England; they occur in smaller bodies at numerous sites
in British Tertiary rocks and in the Oslo region of Norway, but
their most extensive development is in northern Nigeria.
Rocks containing less than 20 percent quartz are almost never
named granite, and rocks containing more than 20 percent (by volume)
of dark, or ferromagnesian, minerals are also seldom called granite.
The minor essential minerals of granite may include muscovite,
biotite, amphibole, or pyroxene. Biotite may occur in granite
of any type and is usually present, though sometimes in very small
amounts. The sodic-amphiboles and pyroxenes (riebeckite, arfvedsonite,
aegirine) are characteristic of the alkali granites. If neither
feldspar is in great excess, neither amphibole nor pyroxene is
likely to be an essential constituent; the dark minerals will
then ordinarily be either biotite or muscovite, or both.
Stone has been used by man for construction purposes for many
thousands of years. Geology, which seeks to describe and explain
these materials, is a relatively new science, having developed
largely since the early 1800's. As a consequence the nomenclature
employed by the construction industry is often scientifically
inaccurate.
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Company
Highlights
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