Granite is a common type of felsic intrusive igneous rock that is granular
and phaneritic in texture. Granites can be predominantly white, pink, or
gray in color, depending on their mineralogy. The word “granite” comes
from the Latin granum, a grain, in reference to the
coarse-grained structure of such a holocrystalline rock. Strictly
speaking, granite is an igneous rock with between 20% and 60% quartz by
volume, and at least 35% of the total feldspar consisting of alkali
feldspar, although commonly the term “granite” is used to refer to a
wider range of coarse grained igneous rocks containing quartz and
feldspar. The term “granitic” means granite-like and is applied to granite and a
group of intrusive igneous rocks with similar textures and slight
variations in composition and origin. These rocks mainly consist
of feldspar, quartz, mica, and amphibole minerals, which form an
interlocking, somewhat equigranular matrix of feldspar and quartz with
scattered darker biotite mica and amphibole (often hornblende) peppering
the lighter color minerals. Occasionally some individual crystals
(phenocrysts) are larger than the groundmass, in which case the texture
is known as porphyritic. A granitic rock with a porphyritic texture is
known as a granite porphyry. Granitoid is a general,
descriptive field term for lighter-colored, coarse-grained igneous
rocks. Petrographic examination is required for identification of
specific types of granitoids.[1] The extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive (lacking any internal structures),
hard and tough, and therefore it has gained widespread use throughout
human history as a construction stone. The average density of granite is
between 2.65 and 2.75 g/cm3 (165.4 – 171.7 lb/ft3)[3], its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3–6 • 1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F); it is strongly reduced in the presence of water, down to 650 °C at a few kBar pressure.
Granite has poor primary permeability, but strong secondary permeability.