Editing Geochemistry (section)

===Mineral constitution===
Except in [[Acidic rock|acid]] or siliceous igneous rocks containing greater than 66% of [[Silicon dioxide|silica]], known as [[felsic]] rocks, quartz is not abundant in igneous rocks. In basic rocks (containing 20% of silica or less) it is rare for them to contain as much silicon, these are referred to as [[mafic]] rocks. If [[magnesium]] and [[iron]] are above average while silica is low, [[olivine]] may be expected; where silica is present in greater quantity over ferromagnesian minerals, such as [[augite]], [[hornblende]], [[enstatite]] or [[biotite]], occur rather than olivine. Unless [[potash]] is high and silica relatively low, [[leucite]] will not be present, for leucite does not occur with free quartz. [[Nepheline]], likewise, is usually found in rocks with much soda and comparatively little silica. With high [[Alkali metal|alkalis]], soda-bearing [[pyroxenes]] and [[amphiboles]] may be present. The lower the percentage of silica and alkali's, the greater is the prevalence of [[plagioclase|plagioclase feldspar]] as contracted with soda or potash feldspar.<ref name=EB1911/>

Earth's crust is composed of 90% silicate minerals and their abundance in the Earth is as follows: [[plagioclase feldspar]] (39%), [[alkali feldspar]] (12%), quartz (12%), [[pyroxene]] (11%), [[amphiboles]] (5%), [[micas]] (5%), [[clay minerals]] (5%); the remaining silicate minerals make up another 3% of Earth's crust. Only 8% of the Earth is composed of non-silicate minerals such as [[Carbonate minerals|carbonates]], [[Oxide minerals|oxides]], and [[Sulfide minerals|sulfides]].<ref>According to [http://www.sandatlas.org/minerals/] {{Webarchive|url=https://web.archive.org/web/20140428104415/http://www.sandatlas.org/minerals/|date=2014-04-28}}, which cites this: Klein, C., Hurlbut, C. S. (1993) ''Manual of Mineralogy'', 21st Edition. John Wiley & Sons.</ref>

The other determining factor, namely the physical conditions attending consolidation, plays, on the whole, a smaller part, yet is by no means negligible. Certain minerals are practically confined to deep-seated intrusive rocks, e.g., microcline, muscovite, diallage. Leucite is very rare in plutonic masses; many minerals have special peculiarities in microscopic character according to whether they crystallized in-depth or near the surface, e.g., hypersthene, orthoclase, quartz. There are some curious instances of rocks having the same chemical composition, but consisting of entirely different minerals, e.g., the hornblendite of Gran, in Norway, which contains only hornblende, has the same composition as some of the [[camptonite]]s of the same locality that contain feldspar and hornblende of a different variety. In this connection, we may repeat what has been said above about the corrosion of porphyritic minerals in igneous rocks. In rhyolites and trachytes, early crystals of hornblende and biotite may be found in great numbers partially converted into augite and magnetite. Hornblende and biotite were stable under the pressures and other conditions below the surface, but unstable at higher levels. In the ground-mass of these rocks, augite is almost universally present. But the plutonic representatives of the same magma, granite, and syenite contain biotite and hornblende far more commonly than augite.<ref name=EB1911/>