Editing Feldspar (section)

==Compositions==
[[File:Feldspar group.svg|thumb|upright=1.25|Compositional phase diagram of the different minerals that constitute the feldspar solid solution]]
The feldspar group of minerals consists of [[Silicate minerals|tectosilicate]]s, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network. Compositions of major elements in common feldspars can be expressed in terms of three [[Endmember (mineralogy)|endmembers]]:
* [[orthoclase]] endmember [[Potassium|K]][[Aluminium|Al]][[silicon|Si]]{{sub|3}}[[Oxygen|O]]{{sub|8}}<ref name="feldspar">Feldspar. [http://www.ima-na.org/?page=what_is_feldspar What is Feldspar?] Industrial Minerals Association. Retrieved on July 18, 2007.</ref>
* [[albite]] endmember [[Sodium|Na]]AlSi{{sub|3}}O{{sub|8}}<ref name = "feldspar"/>
* [[anorthite]] endmember [[Calcium|Ca]]Al{{sub|2}}Si{{sub|2}}O{{sub|8}}<ref name = "feldspar"/>

[[Solid solution]]s between orthoclase and albite are called alkali feldspar<!-- phrase links here -->.<ref name = "feldspar"/> Solid solutions between albite and anorthite are called [[plagioclase]],<ref name="feldspar" /> or, more properly, plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, [[immiscibility]] occurs at temperatures common in the crust of the Earth. Albite is considered both a plagioclase and an alkali feldspar.
<!--
[[Sanidine]] ([[monoclinic]]),<ref name="sanidine">[http://www.galleries.com/minerals/silicate/sanidine/sanidine.htm "Sanidine Feldspar"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> [[orthoclase]] (monoclinic),<ref name="orthoclase">[http://www.galleries.com/minerals/silicate/orthocla/orthocla.htm "The Mineral Orthoclase"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> and [[microcline]] ([[triclinic]])<ref name="microcline">[http://www.galleries.com/minerals/silicate/microcli/microcli.htm "Microcline Feldspar"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> refer to [[polymorphism (materials science)|polymorphs]] of K-feldspar.
-->

The ratio of alkali feldspar to plagioclase feldspar, together with the proportion of [[quartz]], is the basis for the [[QAPF classification]] of igneous rock.<ref name="iugs-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-1999">{{Cite journal|date=1999|title=Rock Classification Scheme – Vol 1 – Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf |archive-date=2022-10-09 |url-status=live|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref><ref name="philpotts-ague-2009-139-143">{{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=9780521880060 |edition=2nd |pages=139–143}}</ref> Calcium-rich plagioclase is the first feldspar to crystallize from cooling magma, then the plagioclase becomes increasingly sodium-rich as crystallization continues. This defines the continuous [[Bowen's reaction series]]. K-feldspar is the final feldspar to crystallize from the [[magma]].<ref>{{Cite book|title=The Evolution of the Igneous Rocks|last=Bowen|first=N.L.|publisher=Dover|year=1956|location=Canada|pages=60–62}}</ref><ref>{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut |first2=Cornelius S. Jr. |title=Manual of mineralogy : (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=047157452X |page=559 |edition=21st}}</ref>

===Alkali feldspars===
Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first of these includes:
* [[orthoclase]] ([[monoclinic]])<ref name="orthoclase">[http://www.galleries.com/minerals/silicate/orthocla/orthocla.htm "The Mineral Orthoclase"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> {{chem2|KAlSi3O8}}
* [[sanidine]] ([[Monoclinic crystal system|monoclinic]])<ref name="sanidine">[http://www.galleries.com/minerals/silicate/sanidine/sanidine.htm "Sanidine Feldspar"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> {{chem2|(K,Na)AlSi3O8}}
* [[microcline]] ([[triclinic]])<ref name="microcline">[http://www.galleries.com/minerals/silicate/microcli/microcli.htm "Microcline Feldspar"]. Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008.</ref> {{chem2|KAlSi3O8}}
* [[anorthoclase]] ([[Triclinic crystal system|triclinic]]) {{chem2|(Na,K)AlSi3O8}}

Potassium and sodium feldspars are not perfectly [[Miscibility|miscible]] in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.{{sfn|Klein|Hurlbut|1993||pages=532-536}} Sanidine is stable at the highest temperatures, and microcline at the lowest.<ref name="orthoclase" /><ref name="sanidine" /> [[Perthite]] is a typical texture in alkali feldspar, due to [[exsolution]] of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many [[granite]]s can be seen with the naked eye.<ref>Ralph, Jolyon and Chou, Ida. "Perthite". [http://www.mindat.org/min-6683.html Perthite Profile on mindat.org]. Retrieved on February 8, 2008.</ref> Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

===Ammonium feldspar===
[[Buddingtonite]] is an ammonium feldspar with the chemical formula: NH<sub>4</sub>AlSi<sub>3</sub>O<sub>8</sub>.<ref>{{cite web | url=https://www.mindat.org/min-796.html | title=Buddingtonite }}</ref> It is a mineral associated with hydrothermal alteration of the primary feldspar minerals.

===Barium feldspars===
Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. Barium feldspars are sometimes classified as a separate group of feldspars,<ref name="DeerHowieZussman2001"/> and sometimes they are classified as a sub-group of alkali feldspars.<ref name="MindatGroup">{{cite web | url=https://www.mindat.org/min-1624.html | title=Feldspar Group | publisher=mindat.org | accessdate=4 July 2021}}</ref>

The barium feldspars are monoclinic and include the following:
* [[celsian]] {{chem2|BaAl2Si2O8}}<ref>[http://www.mindat.org/min-32214.html Celsian–orthoclase series on Mindat.org].</ref>
* [[hyalophane]] {{chem2|(K,Ba)(Al,Si)4O8}}<ref>[http://www.mindat.org/min-8731.html Celsian–hyalophane series on Mindat.org].</ref>

===Plagioclase feldspars===
The plagioclase feldspars are [[triclinic]]. The plagioclase series follows (with percent [[anorthite]] in parentheses):
* [[albite]] (0 to 10) {{chem2|NaAlSi3O8}}
* [[oligoclase]] (10 to 30) {{chem2|(Na,Ca)(Al,Si)AlSi2O8}}
* [[andesine]] (30 to 50) {{chem|NaAlSi|3|O|8|&nbsp;– CaAl|2|Si|2|O|8|}}
* [[labradorite]] (50 to 70) {{chem2|(Ca,Na)Al(Al,Si)Si2O8}}
* [[bytownite]] (70 to 90) {{chem2|(NaSi,CaAl)AlSi2O8}}
* [[anorthite]] (90 to 100) {{chem2|CaAl2Si2O8}}

Intermediate compositions of exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in plagioclase solid solutions are more complex than those in alkali feldspars. The play of colours visible in some feldspar of [[labradorite]] composition is due to very fine-grained exsolution [[Lamellar structure|lamellae]] known as Bøggild intergrowth. The [[specific gravity]] in the plagioclase series increases from [[albite]] (2.62) to anorthite (2.72–2.75).