Editing Plagioclase (section)

== Plagioclase series members ==
The composition of a plagioclase feldspar is typically denoted by its overall fraction of [[anorthite]] (%An) or [[albite]] (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages.{{sfn|Sinkankas|1964|p=450}}{{sfn|Nesse|2000|p=209}}

{| class="wikitable" style="text-align: center;"
|+ Plagioclase minerals and their compositions
|-
! scope="col" | Name
! scope="col" | %  CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub><br />
! scope="col" | % NaAlSi<sub>3</sub>O<sub>8</sub>
! scope="col" | Image
|-
| [[Anorthite]]   || 90–100 || 10–0   || [[File:Anorthite-rare08-38b.jpg|120px]]
|-
| [[Bytownite]]   || 70–90  || 30–10  || [[File:Anorthite (Variety Bytownite)-464165.jpg|120px]]
|-
| [[Labradorite]] || 50–70  || 50–30  || [[File:Labradorite Labrador MNHN Minéralogie.jpg|120px]]
|-
| [[Andesine]]    || 30–50  || 70–50  || [[File:01722 Andesine.jpg|120px]]
|-
| [[Oligoclase]]  || 10–30  || 90–70  || [[File:Oligoclase-4jg47b.jpg|120px]]
|-
| [[Albite]]      || 0–10   || 100–90 || [[File:Albite - Minas Gerais, Brazil.jpg|120px]]
|}

The distinction between these minerals cannot easily be made [[Field work|in the field]]. The composition can be roughly determined by specific gravity, but accurate measurement requires chemical or optical tests.{{sfn|Klein|Hurlbut|1993|p=542}} The composition in a crushed [[grain mount]] can be obtained by the Tsuboi method, which yields an accurate measurement of the minimum [[refractive index]] that in turn gives an accurate composition. In [[thin section]], the composition can be determined by either the Michel Lévy or Carlsbad-albite methods. The former relies on accurate measure of minimum index of refraction, while the latter relies on measuring the ''[[Extinction (optical mineralogy)|extinction angle]]'' under a  [[Polarization (waves)|polarizing]] [[microscope]]. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab).{{sfn|Nesse|2000|p=217-219}}

=== Endmembers ===

*[[Anorthite]] was named by [[Gustav Rose]] in 1823 from Greek [[wikt:ἀν-|{{transliteration|grc|an-}}]] ('not') + [[wikt:ὀρθός|{{transliteration|grc|orthós}}]] ('straight'), literally 'oblique', referring to its triclinic crystallization.<ref>{{oed|anorthite}}</ref> Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some [[Orogeny|orogenic]] [[calc-alkaline]] suites.<ref name="Deer_etal_1966">{{cite book | title=An Introduction to the Rock Forming Minerals | publisher=Longman |author=Deer, W.A. |author2=Howie, R.A. |author3=Zussman, J. | year=1966 | location=London | page=336 | isbn=0-582-44210-9}}</ref>
*[[Albite]] is named from the Latin {{lang|la|[[wikt:albus|albus]]}}, in reference to its unusually pure white color. The name was first applied by [[Johan Gottlieb Gahn]] and [[Jöns Jacob Berzelius]] in 1815.<ref>{{oed|albite}}</ref> It is a relatively common and important rock-making mineral associated with the more silica-rich rock types, in [[hydrothermal]] veins, with [[greenschist facies]] metamorphic rocks,{{sfn|Jackson|1997|loc=albite}} and in [[pegmatite]] [[dike (geology)|dikes]], often as the variety ''cleavelandite'' and associated with rarer minerals like [[tourmaline]] and [[beryl]].{{sfn|Klein|Hurlbut|1993|p=568}}

=== Intermediate members ===
The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member (albite 2.62) increases 0.02 per 10% increase in anorthite (2.75).

*[[Bytownite]], named after the former name for [[Ottawa]], Ontario, Canada—Bytown—<ref>{{oed|bytownite}}</ref> is a rare mineral occasionally found in more [[basic rock]]s.{{sfn|Klein|Hurlbut|1993|p=543}}

[[File:labradorite detail.jpg|thumb|130px|Labradorite displaying typical iridescent effect termed labradorescence. (unknown scale)]]
*[[Labradorite]] is the characteristic feldspar of the more basic rock types such as gabbro or basalt.{{sfn|Klein|Hurlbut|1993|p=543}} Labradorite frequently shows an [[iridescence|iridescent]] display of colors due to light refracting within the lamellae of the crystal.{{sfn|Nesse|2000|p=213}} It is named after [[Labrador]], where it is a constituent of the [[Intrusive rock|intrusive]] igneous rock [[anorthosite]] which is composed almost entirely of plagioclase.{{sfn|Klein|Hurlbut|1993|p=543}} A variety of labradorite known as [[spectrolite]] is found in [[Finland]].<ref name="Gems">Michael O'Donoghue, ''Gems'', Butterworth-Heinemann, 6th ed., 2006, pp. 238-267, {{ISBN|0-7506-5856-8}}</ref><ref name="Schumann">Walter Schumann, ''Gemstones of the World,'' Sterling, 3rd ed., 2007, pp. 52 – 53, 182 {{ISBN|1-4027-4016-6}}</ref>
*[[Andesine]] is a characteristic mineral of rocks such as diorite which contain a moderate amount of [[silica]] and related [[volcanics]] such as [[andesite]].{{sfn|Klein|Hurlbut|1993|p=543}}
*[[Oligoclase]] is common in [[granite]] and [[monzonite]].{{sfn|Klein|Hurlbut|1993|p=543}} The name ''oligoclase'' is derived from the Greek [[wikt:ὀλίγος|{{transliteration|grc|olígos}}]] ('small, slight') + [[wikt:κλάσις|{{transliteration|grc|klásis}}]] ('fracture'), in reference to the fact that its cleavage angle differs significantly from 90°. The term was first used by Breithaupt in 1826.<ref>{{oed|oligoclase}}</ref> [[Sunstone]] is mainly oligoclase (sometimes albite) with flakes of [[hematite]].{{sfn|Klein|Hurlbut|1993|p=543}}