Editing Plagioclase (section)

== Properties ==
Plagioclase is the most common and abundant [[mineral group]] in the [[Earth's crust]]. Part of the [[feldspar]] family of minerals, it is abundant in [[igneous rock|igneous]] and [[metamorphic rock]], and it is also common as a [[detrital]] mineral in [[sedimentary rock]].<ref>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=978-0-19-510691-6 |page=219}}</ref><ref name=KleinHurlbut1993>{{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=0-471-57452-X |edition=21st |page=543}}</ref> It is not a single [[mineral]], but is a [[solid solution]] of two [[endmember|end members]], [[albite]] or sodium feldspar ({{chem2|NaAlSi3O8}}) and [[anorthite]] or calcium feldspar ({{chem2|CaAl2Si2O8}}). These can be present in plagioclase in any proportion from pure anorthite to pure albite.{{sfn|Klein|Hurlbut|1993|p=542}} The composition of plagioclase can thus be written as {{chem2|Na_{1-x}Ca_{x}Al_{1+x}Si_{3-x}O8|}} where ''x'' ranges from 0 for pure albite to 1 for pure anorthite. This solid solution series is known as the plagioclase series.<ref name=Allaby2013>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-965306-5 |edition=Fourth |chapter=plagioclase}}</ref><ref name=Jackson1997>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0-922152-34-9 |edition=Fourth |chapter=plagioclase}}</ref> The composition of a particular sample of plagioclase is customarily expressed as the [[mol%]] of anorthite in the sample. For example, plagioclase that is 40&nbsp;mol% anorthite would be described as An40 plagioclase.<ref name=Sinkankas1964>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0-442-27624-9 |page=450}}</ref>

The ability of albite and anorthite to form solid solutions in any proportions at elevated temperature reflects the ease with which calcium and aluminium can substitute for sodium and silicon in the plagioclase crystal structure. Although a calcium ion has a charge of +2, versus +1 for a sodium ion, the two ions have very nearly the same effective radius. The difference in charge is accommodated by the coupled substitution of aluminium (charge +3) for silicon (charge +4), both of which can occupy tetrahedral sites (surrounded by four oxygen ions). This contrasts with potassium, which has the same charge as sodium, but is a significantly larger ion. As a result of the size and charge difference between potassium and calcium, there is a very wide [[miscibility gap]] between anorthite and [[potassium feldspar]], ({{chem2|KAlSi3O8}}), the third common rock-forming feldspar end member. Potassium feldspar does form a solid solution series with [[albite]], due to the identical charges of sodium and potassium ions, which is known as the [[alkali feldspar]] series. Thus, almost all feldspar found on Earth is either plagioclase or alkali feldspar, with the two series overlapping for pure albite. When a plagioclase composition is described by its anorthite mol% (such as An40 in the previous example) it is assumed that the remainder is albite, with only a minor component of potassium feldspar.{{sfn|Nesse |2000|pp=208-209}}

Plagioclase of any composition shares many basic physical characteristics, while other characteristics vary smoothly with composition.<ref name="Allaby2013" /> The [[Mohs hardness]] of all plagioclase species is 6 to 6.5,{{sfn|Nesse |2000|pp=208-209}} and cleavage is perfect on [001] and good on [010], with the cleavage planes meeting at an angle of 93 to 94 degrees.{{sfn|Nesse|2000|p=216}} It is from this slightly oblique cleavage angle that plagioclase gets its name, [[Ancient Greek]] {{transliteration|grc|plágios}} ({{wikt-lang|grc|πλάγιος}} 'oblique') + {{transliteration|grc|klásis}} ({{wikt-lang|grc|κλάσις}} 'fracture'). The name was introduced by [[August Breithaupt]] in 1847.<ref name="Jackson1997" /> There is also a poor cleavage on [110] rarely seen in hand samples.{{sfn|Nesse|2000|p=216}}

The [[Lustre (mineralogy)|luster]] is vitreous to pearly and the [[diaphaneity]] is transparent to translucent.{{sfn|Klein|Hurlbut|1993|p=542}} The [[Tenacity (mineralogy)|tenacity]] is brittle, and the [[fracture (mineralogy)|fracture]] is uneven or conchoidal, but the fracture is rarely observed due to the strong tendency of the mineral to cleave instead.{{sfn|Sinkankas|1964|p=457}} At low temperature, the crystal structure belongs to the [[triclinic system]], space group ''P{{overline|1}}''{{sfn|Klein|Hurlbut|1993|p=541}}{{sfn|Nesse|2000|p=215}} Well-formed crystals are rare and are most commonly sodic in composition.{{sfn|Sinkankas|1964|pp=456-457}} Well-shaped samples are instead typically cleavage fragments. Well-formed crystals are typically bladed or tabular parallel to [010].{{sfn|Klein|Hurlbut|1993|p=542}}

Plagioclase is usually white to greyish-white in color, with a slight tendency for more calcium-rich samples to be darker.<ref name="Allaby2013" /> Impurities can infrequently tint the mineral greenish, yellowish, or flesh-red.{{sfn|Klein|Hurlbut|1993|p=542}} [[Ferric iron]] (Fe<sup>3+</sup>) gives a pale yellow color in plagioclase feldspar from [[Lake County, Oregon]].<ref>{{Cite web |title=Minerals Colored by Metal Ions |url=http://minerals.gps.caltech.edu/color_causes/Metal_Ion/index.html |access-date=2023-03-01 |website=minerals.gps.caltech.edu}}</ref> The specific gravity increases smoothly with calcium content, from 2.62 for pure albite to 2.76 for pure anorthite, and this can provide a useful estimate of composition if measured accurately.{{sfn|Klein|Hurlbut|1993|p=542}} The index of refraction likewise varies smoothly from 1.53 to 1.58, and, if measured carefully, this also gives a useful composition estimate.{{sfn|Nesse|2000|p=216}}

Plagioclase almost universally shows a characteristic [[polysynthetic twinning]] that produces twinning striations on [010]. These striations allow plagioclase to be distinguished from alkali feldspar. Plagioclase often also displays Carlsbad, Baveno, and Manebach Law twinning.{{sfn|Klein|Hurlbut|1993|p=542}}