Editing Geochemistry (section)

==Earth's crust==
{{See also|Abundance of elements in Earth's crust}}
The more common rock constituents are nearly all [[oxide]]s; [[chloride]]s, [[sulfide]]s and [[fluoride]]s are the only important exceptions to this and their total amount in any rock is usually much less than 1%. By 1911, [[Frank Wigglesworth Clarke|F. W. Clarke]] had calculated that a little more than 47% of the Earth's crust consists of [[oxygen]]. It occurs principally in combination as oxides, of which the chief are [[silica]], [[alumina]], [[iron oxide]]s, and various [[carbonate]]s ([[calcium carbonate]], [[magnesium carbonate]],  [[sodium carbonate]], and [[potassium carbonate]]). The silica functions principally as an acid, forming silicates, and all the commonest minerals of [[igneous rock]]s are of this nature. From a computation based on 1672 analyses of numerous kinds of rocks Clarke arrived at the following as the average percentage composition of the Earth's crust: SiO<sub>2</sub>=59.71, Al<sub>2</sub>O<sub>3</sub>=15.41, Fe<sub>2</sub>O<sub>3</sub>=2.63, FeO=3.52, MgO=4.36, CaO=4.90, Na<sub>2</sub>O=3.55, K<sub>2</sub>O=2.80, H<sub>2</sub>O=1.52, TiO<sub>2</sub>=0.60, P<sub>2</sub>O<sub>5</sub>=0.22, (total 99.22%). All the other constituents occur only in very small quantities, usually much less than 1%.<ref name=EB1911/>

These oxides combine in a haphazard way. For example, [[potash]] (potassium carbonate) and soda ([[sodium carbonate]]) combine to produce [[feldspar]]s. In some cases, they may take other forms, such as [[nepheline]], [[leucite]], and [[muscovite]], but in the great majority of instances they are found as feldspar. [[Phosphoric acid]] with [[Lime (material)|lime]] (calcium carbonate) forms [[apatite]]. [[Titanium dioxide]] with [[ferrous oxide]] gives rise to [[ilmenite]]. Part of the lime forms lime feldspar. Magnesium carbonate and iron oxides with silica crystallize as [[olivine]] or [[enstatite]], or with alumina and lime form the complex ferromagnesian silicates of which the [[pyroxene]]s, [[amphibole]]s, and [[biotite]]s are the chief. Any excess of silica above what is required to neutralize the [[Base (chemistry)|base]]s will separate out as [[quartz]]; excess of alumina crystallizes as [[corundum]]. These must be regarded only as general tendencies. It is possible, by rock analysis, to say approximately what minerals the rock contains, but there are numerous exceptions to any rule.<ref name=EB1911/>

===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/>

===Felsic, intermediate and mafic igneous rocks===
Those rocks that contain the most silica, and on crystallizing yield free quartz, form a group generally designated the "felsic" rocks. Those again that contain the least silica and most magnesia and iron, so that quartz is absent while [[olivine]] is usually abundant, form the "mafic" group. The "intermediate" rocks include those characterized by the general absence of both quartz and olivine. An important subdivision of these contains a very high percentage of alkalis, especially soda, and consequently has minerals such as [[nepheline]] and [[leucite]] not common in other rocks. It is often separated from the others as the "alkali" or "soda" rocks, and there is a corresponding series of mafic rocks. Lastly, a small sub-group rich in olivine and without feldspar has been called the "ultramafic" rocks. They have very low percentages of silica but much iron and magnesia.

Except these last, practically all rocks contain felspars or feldspathoid minerals. In the acid rocks, the common feldspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the mafic rocks labradorite, anorthite, and bytownite prevail, being rich in lime and poor in silica, potash, and soda. Augite is the most common ferromagnesian in mafic rocks, but biotite and hornblende are on the whole more frequent in felsic rocks.<ref name=EB1911/>

{| class="wikitable"
!rowspan=2 | Most Common Minerals
! Felsic
!colspan=2 | Intermediate
! Mafic
! Ultramafic
|-
! Quartz <br /> Orthoclase (and Oligoclase), Mica, Hornblende, Augite
! Little or no Quartz: <br /> Orthoclase hornblende, Augite, Biotite
! Little or no Quartz: <br /> Plagioclase Hornblende, Augite, Biotite
! No Quartz <br /> Plagioclase Augite, Olivine
! No Felspar <br /> Augite, Hornblende, Olivine
|-
| Plutonic or Abyssal type
| [[Granite]] || [[Syenite]] || [[Diorite]] || [[Gabbro]] || [[Peridotite]]
|-
| Intrusive or [[Hypabyssal]] type
| [[Quartz-porphyry]] || [[Orthoclase-porphyry]] || Porphyrite || [[Dolerite]] || [[Picrite]]
|-
| Lavas or Effusive type
| [[Rhyolite]], [[Obsidian]] || [[Trachyte]] || [[Andesite]] || [[Basalt]] || [[Komatiite]]
|}

Rocks that contain leucite or nepheline, either partly or wholly replacing felspar, are not included in this table. They are essentially of intermediate or of mafic character. We might in consequence regard them as varieties of syenite, diorite, gabbro, etc., in which feldspathoid minerals occur, and indeed there are many transitions between syenites of ordinary type and nepheline — or leucite — syenite, and between gabbro or dolerite and theralite or essexite. But, as many minerals develop in these "alkali" rocks that are uncommon elsewhere, it is convenient in a purely formal classification like that outlined here to treat the whole assemblage as a distinct series.<ref name=EB1911/>

{| class="wikitable"
|+Nepheline and Leucite-bearing Rocks
|-
! Most Common Minerals
! Alkali Feldspar, Nepheline or Leucite, Augite, Hornblend, Biotite
! Soda Lime Feldspar, Nepheline or Leucite, Augite, Hornblende (Olivine)
! Nepheline or Leucite, Augite, Hornblende, Olivine
|-
| Plutonic type
| Nepheline-syenite, Leucite-syenite, Nepheline-porphyry
| Essexite and Theralite
| Ijolite and Missourite
|-
| Effusive type or Lavas
| Phonolite, Leucitophyre
| Tephrite and Basanite
| Nepheline-basalt, Leucite-basalt
|}

This classification is based essentially on the mineralogical constitution of the igneous rocks. Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial, but it has grown up with the growth of the science and is still adopted as the basis on which more minute subdivisions are erected. The subdivisions are by no means of equal value. The syenites, for example, and the peridotites, are far less important than the granites, diorites, and gabbros. Moreover, the effusive andesites do not always correspond to the plutonic diorites but partly also to the gabbros. As the different kinds of rock, regarded as [[Aggregate (geology)|aggregate]]s of minerals, pass gradually into one another, transitional types are very common and are often so important as to receive special names. The quartz-syenites and nordmarkites may be interposed between granite and syenite, the tonalites and adamellites between granite and diorite, the monzonites between syenite and diorite, norites and hyperites between diorite and gabbro, and so on.<ref name=EB1911>{{EB1911|wstitle=Petrology |inline=1|volume=21|pages=328–333 |first=John Smith |last=Flett}}</ref>