Editing Peridotite (section)

==Composition==
[[File:Olivine-Dunit.jpg|thumb|Typical peridotite sample ([[dunite]], left) and large olivine crystal (right)]]
Mantle peridotite is highly enriched in magnesium, with a typical magnesium number{{explain|date=January 2025}} of 89.<ref>{{cite journal |last1=Palme |first1=H. |last2=O'Neill |first2=H.St.C. |title=Cosmochemical Estimates of Mantle Composition |journal=Treatise on Geochemistry |date=2007 |pages=1–38 |doi=10.1016/B0-08-043751-6/02177-0|isbn=9780080437514 }}</ref> In other words{{whose|date=January 2025}}, of the total content of iron plus magnesium, 89 [[mol%]] is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite.

Olivine is the essential mineral found in all peridotites. It is an iron-magnesium [[orthosilicate]] with the variable formula {{chem2|(Mg,Fe)2SiO4}}. The magnesium-rich olivine of peridotites is typically olive-green in color.<ref name=Nesse2000>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |pages=306–310}}</ref>

Pyroxenes are chain silicates having the variable formula {{chem2|(Ca,Na,Fe^{II},Mg)(Cr,Al,Fe^{III},Mg,Mn,Ti,V)Si2O6}} comprising a large group of different minerals. These are divided into orthopyroxenes (with an [[orthorhombic]] crystal structure) and clinopyroxenes (with a [[monoclinic]] crystal structure).{{sfn|Nesse|2000|pp=261–74}} This distinction is important in the classification of pyroxene peridotites<ref name="bgs"/>{{sfn|Philpotts|Ague|2009|p=142}} since clinopyroxene melts more easily than orthopyroxene or olivine. The most common orthopyroxene is [[enstatite]], {{chem2|Mg2Si2O6}}, in which iron substitutes for some of the magnesium. The most important clinopyroxene is [[diopside]], {{chem2|CaMgSi2O6}}, again with some substitution of iron for magnesium ([[hedenbergite]], {{chem2|FeCaSi2O6}}).{{sfn|Nesse|2000|pp=261–74}} Ultramafic rock in which the fraction of pyroxenes exceeds 60% are classified as [[pyroxenite]]s rather than peridotites. Pyroxenes are typically dark in color.{{sfn|Nesse|2000|pp=261–74}}

Hornblende is an [[amphibole]], a group of minerals resembling pyroxenes but with a double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from [[tschermakite]] ({{chem2|Ca2(Mg,Fe)3Al2Si6Al2O22(OH)2}}) to [[pargasite]] ({{chem2|NaCa2(Mg,Fe)4AlSi6Al2O22(OH)2}}) with many other variations in composition.{{sfn|Nesse|2000|pp=277–289}} It is present in peridotite mostly as a consequence of alteration by hydrous fluids.<ref name=BlatterCarmichael1998/><ref name=MatusiakMatekEtal2017/>

Although peridotites are classified by their content of olivine, pyroxenes, and hornblende, a number of other mineral families are characteristically present in peridotites and may make up a significant fraction of their composition. For example, chromite is sometimes present in amounts of up to 50%. (A chromite composition above 50% reclassifies the rock as a ''peridotitic chromitite''.) Other common accessory minerals include [[spinel]], [[garnet]], [[biotite]], or [[magnetite]]. A peridotite containing significant amounts of one of these minerals may have its classification refined accordingly; for example, if a lhertzolite contains up to 5% spinel, it is a ''spinel-bearing lhertzolite'', while for amounts up to 50%, it would be classified as a ''spinel lhertzolite''.{{sfn|Philpotts|Ague|2009|p=138}} The accessory minerals can be useful for estimating the depth of formation of the peridotite. For example, the aluminium in lhertzolite is present as [[plagioclase]] at depths shallower than about {{convert|20|km||sp=us}}, while it is present as spinel between 20&nbsp;km and {{convert|60|km||sp=us}} and as garnet below 60&nbsp;km.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=145}}</ref>