Editing Plagioclase (section)

== Petrogenesis ==
[[File:Bowen's Reaction Series.png|thumb|upright=1.6|Bowen's reaction series]]
[[File:Qapf diagram plutonic 05.svg|thumb|upright=1.75|QAPF diagram for classification of plutonic rocks]]
Plagioclase is the primary aluminium-bearing mineral in [[mafic]] rocks formed at low pressure.<ref>{{cite book |last=McBirney |first=Anthony R. |title=Igneous Petrology |date=1984 |publisher=Freeman, Cooper, and Company|page=270}}</ref> It is normally the first and most abundant feldspar to crystallize from a cooling [[primitive magma]].{{sfn|McBirney|1984|page=104}} Anorthite has a much higher melting point than albite, and, as a result, calcium-rich plagioclase is the first to crystallize.{{sfn|Klein|Hurlbut|1993|p=543}} The plagioclase becomes more enriched in sodium as the temperature drops, forming [[Bowen's reaction series|Bowen's continuous reaction series]]. However, the composition with which plagioclase crystallizes also depends on the other components of the melt, so it is not by itself a reliable thermometer.{{sfn|McBirney|1984|page=107}}

The [[liquidus]] of plagioclase (the temperature at which the plagioclase first begins to crystallize) is about {{convert|1215|C||sp=us}} for [[olivine basalt]], with a composition of 50.5 wt% silica; {{convert|1255|C||sp=us}} in [[andesite]] with a silica content of 60.7 wt%; and {{convert|1275|C||sp=us}} in [[dacite]] with a silica content of 69.9 wt%. These values are for dry magma. The liquidus is greatly lowered by the addition of water, and much more for plagioclase than for mafic minerals. The [[eutectic]] (minimum melting mixture) for a mixture of anorthite and [[diopside]] shifts from 40 wt% anorthite to 78 wt% anorthite as the water vapor pressure goes from 1 bar to 10 kbar. The presence of water also shifts the composition of the crystallizing plagioclase towards anorthite. The eutectic for this wet mixture drops to about {{convert|1010|C||sp=us}}.{{sfn|McBirney|1984|pages=318-320}}

Crystallizing plagioclase is always richer in anorthite than the melt from which it crystallizes. This ''plagioclase effect'' causes the residual melt to be enriched in sodium and silicon and depleted in aluminium and calcium. However, the simultaneous crystallization of mafic minerals not containing aluminium can partially offset the depletion in aluminium.{{sfn|McBirney|1984|page=396}} In volcanic rock, the crystallized plagioclase incorporates most of the potassium in the melt as a trace element.{{sfn|McBirney|1984|page=104}}

New plagioclase crystals nucleate only with difficulty, and diffusion is very slow within the solid crystals.{{sfn|McBirney|1984|p=107}} As a result, as a magma cools, increasingly sodium-rich plagioclase is usually crystallized onto the rims of existing plagioclase crystals, which retain their more calcium-rich cores. This results in compositional zoning of plagioclase in igneous rocks.{{sfn|Klein|Hurlbut|1993|p=543}} In rare cases, plagioclase shows reverse zoning, with a more calcium-rich rim on a more sodium-rich core. Plagioclase also sometimes shows oscillatory zoning, with the zones fluctuating between sodium-rich and calcium-rich compositions, though this is usually superimposed on an overall normal zoning trend.{{sfn|Nesse|2000|p=215}}

=== Classification of igneous rocks ===
Plagioclase is very important for the classification of crystalline igneous rocks. Generally, the more silica is present in the rock, the fewer the mafic minerals, and the more sodium-rich the plagioclase. Alkali feldspar appears as the silica content becomes high.{{sfn|Klein|Hurlbut|1993|p=543}} Under the [[QAPF classification]], plagioclase is one of the three key minerals, along with quartz and alkali feldspar, used to make the initial classification of the rock type. Low-silica igneous rocks are further divided into [[Diorite|dioritic]] rocks having sodium-rich plagioclase (An<50) and [[gabbro]]ic rocks having calcium-rich plagioclase (An>50). [[Anorthosite]] is an [[intrusive rock]] composed of at least 90% plagioclase.<ref name="lebas-streckeisen-1991">{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen|first2=A. L.|title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–833|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|citeseerx=10.1.1.692.4446|s2cid=28548230}}</ref><ref name="bgs">{{Cite journal|date=1999|title=Rock Classification Scheme – Vol 1 – Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref><ref name=PhilpottsAgue2009>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |edition=2nd |pages=139–143}}</ref>

Albite is an end member of both the alkali and plagioclase series. However, it is included in the alkali feldspar fraction of the rock in the QAPF classification.<ref name="PhilpottsAgue2009" />

=== In metamorphic rocks ===
Plagioclase is also common in metamorphic rock.{{sfn|Nesse|2000|p=219}}{{sfn|Klein|Hurlbut|1993|p=543}} Plagioclase tends to be albite in low-grade metamorphic rock, while oligoclase to andesine are more common in  medium- to high-grade metamorphic rock. Metacarbonate rock sometimes contains fairly pure anorthite.{{sfn|Nesse|2000|pp=219-220}}

=== In sedimentary rocks ===
Feldspar makes up between 10 and 20 percent of the framework grains in typical [[sandstone]]s. Alkali feldspar is usually more abundant than plagioclase in sandstone because Alkali feldspars are more resistant to chemical weathering and more stable, but sandstone derived from volcanic rock contains more plagioclase.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0-13-154728-3 |edition=4th |pages=120–121}}</ref> Plagioclase weathers relatively rapidly to [[clay minerals]] such as [[smectite]].<ref>{{cite book |last1=Leeder |first1=M. R. |title=Sedimentology and sedimentary basins: from turbulence to tectonics |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex, UK |isbn=978-1-4051-7783-2 |edition=2nd |pages=10–11}}</ref>

=== At the Mohorovičić discontinuity ===
The [[Mohorovičić discontinuity]], which defines the boundary between the [[Earth's crust]] and the [[upper mantle]], is thought to be the depth where feldspar disappears from the rock.{{sfn|Philpotts|Ague|2009|p=2}} While plagioclase is the most important aluminium-bearing mineral in the crust, it breaks down at the high pressure of the upper mantle, with the aluminium tending to be incorporated into [[clinopyroxene]] as ''Tschermak's molecule'' ({{chem2|CaAl2SiO6}}) or in [[jadeite]] {{chem2|NaAlSi2O6}}. At still higher pressure, the aluminium is incorporated into [[garnet]].{{sfn|McBirney|1984|page=270}}

=== Exsolution ===

At very high temperatures, plagioclase forms a solid solution with potassium feldspar, but this becomes highly unstable on cooling. The plagioclase separates from the potassium feldspar, a process called ''[[exsolution]]''. The resulting rock, in which fine streaks of plagioclase (''lamellae'') are present in potassium feldspar, is called [[perthite]].{{sfn|Sinkankas|1964|p=450}}

The solid solution between anorthite and albite remains stable to lower temperatures, but ultimately becomes unstable as the rock approaches ambient surface temperatures. The resulting exsolution results in very fine lamellar and other intergrowths, normally detected only by sophisticated means.{{sfn|Klein|Hurlbut|1993|p=542}} However, exsolution in the andesine to labradorite compositional range sometimes produces lamellae with thicknesses comparable to the wavelength of visible light. This acts like a [[diffraction grating]], causing the labradorite to show the beautiful play of colors known as ''[[chatoyance]]''.{{sfn|Nesse|2000|p=213}}