granite

Granite is typically a medium to coarse grained felsic, intrusive igneous rock (plutonic) that is usually pink to dark gray, sometimes black, depending on its chemistry and mineralogy. Granites are the commonest basement rocks of the continental crust, many dating from the Precambrian. In some granites, individual crystals are larger than the groundmass (porphyrys). Granites primarily comprises orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, with minor accessory minerals such as magnetite, garnets, zircon and apatite. Rarely, a pyroxene is present. Very rarely, iron-rich olivine, fayalite, occurs. Rapakivi ("crumbly rock") hornblende-biotite granites are named for their texture of plagioclase (oligoclase) rims mantled around large rounded crystals of orthoclase. Granites are classified according to the QAPF diagram for granitoids and phaneritic foidolites (plutonic rocks) that compares the percentages of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar. As a plutonic rock, granite is often exposed in weathered tors, dykes and as massive batholiths.

links: images: hand-specimens: light gray granite, Salinian, 2; pink-gray granite, Llano Uplift, Texas; pink-gray; pink-tan granite; pink granite, Pikes Peak, Colorado; orbicular granite formed by coronas of reaction rims of oriented hornblende formed around xenolithic inclusions, Granite Harbour, Antarctica; close-up: El Capitan Granite contains quartz (gray glassy looking grains), orthoclase feldspar (white grains), and biotite mica (black grains), Yosemite granite; a light gray granite; pink-gray, Gran Violet (Brazil); blue gray, Lavanda Blue (Brazil); pale gray-tan, Giallo Veneziano (Brazil); gray granite, Idaho batholith; Pikes Peak granite; Rapakivi granite; orbicular granite, Caldera, Chile; very dark gray, Azul Noce (Spain); formations: Big Rock, Yosemite, sheeted granite, Tioga Road, Yosemite ; granite; granite boulders; pink granite on Great Head Trail; schist over granite; gorge in pink granite; granite cascade; glacial scratches and grooves on exposed Milbank granite; granite island, Aus; webpages: Rock Gallery Drexel ; Mineral Gallery Drexel ;

image of Rapakivi granite courtesy of Siim Sepp