GRANITE
Granite
is an very common intrusive igneous rock. Garden-variety granites are
composed of quartz, potassium feldspar (K-feldspar), sodic plagioclase
feldspar, hornblende amphibole, and mica. Granites have a felsic
chemistry. Felsic igneous rocks are generally light-colored, have >65%
silica (“silica” = SiO2 chemistry) (felsic has also been defined as
>70% silica), are rich in potassium (K) & sodium (Na), and are dominated
by the minerals quartz and K-feldspar.
Most granites have a phaneritic texture
(coarsely crystalline; all crystals between 1 mm and 1 cm in size), while other
granites are porphyritic (a mix of large & small crystals) or pegmatitic
(almost exclusively composed of very large crystals - almost all >1 cm in
size).
Granite
Some granites have such a low ferromagnesian mineral
content that they appear to consist only of quartz and potassium feldspar.
Such granites are called alaskites.
Alaskite
(pegmatitic alaskite) (7.8 cm across at its widest) from the Mount Evans
Batholith having pinkish K-feldspar and whitish-gray quartz. This 1.44
billion-year-old rock comes from near the summit of Mt. Evans, Clear Creek
County, WSW of Denver, central Colorado, USA.
Collected by Dan Leavell.