felsic

Felsic (feldspar-silica) refers to silicate minerals, magmas, and rocks relatively enriched in the lighter elements such as silica, oxygen, aluminium, sodium, and potassium.

The sial is the upper layer of Earth's crust – the continental crust – and is rich in aluminum/silica minerals (granitic layer). Because felsic-sial rocks are lower in density than mafic-sima rocks, the continental crust 'floats' atop the deeper, denser sima crust. The Conrad discontinuity, arbitrarily set at 2800kg/m^3 marks the base of the sial where it grades into the basalts of the sima.

Felsic minerals are usually light in color, with specific gravities less than 3 (2700 - 2800 kg/m^3). Common felsic minerals include quartz, muscovite, hornblende, orthoclase, and the sodium rich plagioclase feldspars. Felsic rocks contain >75% felsic minerals. Granite is the most common felsic rock. Rocks with greater than 90% felsic minerals are also termed leucocratic, meaning 'light-coloured'.

The term acid rock, although sometimes used as a synonym for felsic rocks, refers to volcanic rocks with high silica content (greater than 63% SiO2 by weight) such as rhyolite.

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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.

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.