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Plagioclase

Triclinic

(Na,Ca)Al1-2Si3-2O8

Plagioclase feldspars are important Na- and Ca-bearing aluminosilicates that occur in most igneous rocks, as well as in many metamorphic and sedimentary rocks. Plagioclases are fundamental constituents of the Earth’s crust and also of the anorthosite crust of the Moon (the highlands). The name plagioclase derives from the Greek πλάγιος (plágios, “oblique”) + κλᾰ́σῐς (klásis, “fracture”), in reference to its two cleavage systems.

Structure and chemistry
Plagioclase feldspars are framework silicates with a structure similar to that of alkali feldspars. They show a network of (Si,Al)O4 tetrahedrons linked to one another (sharing oxygens) in all directions, distorted by large cation sites that may contain Na+, Ca2+, and some K+. Likewise alkali feldspars, plagioclase may contain traces of Fe2+, Fe3+, Ba, Ti, Mg, Sr, and Mn. The plagioclase series represents a solid-solution between albite (Na[AlSi3O8]) and anorthite (Ca[Al2Si2O8]). The structure of albite, representing also the sodic end-member of the alkali feldspars, is triclinic. Anorthite is also triclinic, but its structure is more ordered compared to albite, with atoms of Si and Al alternating with strict regularity. The solid-solution between albite and anorthite is possible, since Na+ and Ca2+ have similar ionic radius (Na+ = 0.97 Å; Ca2+ = 1.00 Å). The different charge of these ions is compensated by the contemporaneous substitution of Si4+ by Al3+ (i.e. NaSi → CaAl). The names of plagioclase are arbitrary and indicate a specific compositional range. Considering the anorthite end-member (An), the following names for specific plagioclase compositions can be assigned:

albite
: An0-10
oligoclase: An10-30
andesine: An30-50
labradorite: An50-70
bytownite: An70-90
anorthite: An90-100

zoned plagioclase

Zoned plagioclase phenocryst. Etna volcano, Italy. CPL image. (Field of view = 7mm). Photo © Alessandro da Mommio (alexstrekeisen.it).

Properties
Habit: prismatic, tabular
Hardness: 6 – 6.5
Cleavage: {001}, {010} perfect (two planes intersecting at closely 90°)
Twinning: Carlsbad ({hko}, usually {01o} composition plane – [001] twin axis), Baveno ({021}/{0-21} composition plane – [021]/[0-21] twin axis), Manebach ({001} composition plane – [001] twin axis) simple twinning; Albite ({010} composition plane – [010] twin axis), Pericline ({0kl} composition plane – [010] twin axis) polysynthetic twinning
Color: colorless or white, sometimes yellow, green, dark grey, and iridescent
Luster: vitreous
Streak: white
Alteration: sericite, saussurite
In thin section…
α(α^a albite to andesine; α^b labradorite to anorthite): 1.528 (albite) – 1.575 (anorthite)
β(β^c albite to labradorite; β^a bytownite to anorthite): 1.532-1.533 (albite) – 1.583 (anorthite)
γ(γ^b albite to labradorite; γ^c bytownite to anorthite): 1.534-1.539 (albite) – 1.588 (anorthite)
*angle of extinction on (010) relative to trace of (010): – 20° (albite) – 55-60° (anorthite)
2V: 50°(-) high albite; 77° (+) low albite; 78°(-) anorthite
Color: colorless
Birefringence (δ): 0.007-0.013 (first-order greys)
Relief: low, higher or lower than quartz
Optic sign: + or –
[Mindat]

feldspar ternary plot

Classification diagram of feldspars. Plagioclase feldspars constitute a solid-solution between albite and anorthite. Compositional intervals on the series have specific names.

The miscibility of albite and anorthite is complete at high-temperatures (i.e. high-albite – high-plagioclase – body-centered anorthite; T ~ 1000 – 1400 °C). At lower temperatures, plagioclases should unmix (e.g. similar to perthites in alkali feldspars). However, unmixing is energetically not favored and plagioclases develop submicroscopic albite and anorthite lamellae (note: invisible at the optical microscope). The presence of such lamellae causes optical phenomena, like iridescent colors in some plagioclases (i.e. labradorite). Three types of unmixing occur at different plagioclase compositions: peristerite (albite-oligoclase), Bøggild (andesine-labradorite), and Huttenlocher (labradorite-bytownite). As said above, the plagioclase series can contain some orthoclase (Or; KAlSi3O8) in solid-solution, normally below the 5%. The miscibility with Or is favored at higher temperatures and in Na-rich plagioclase. At lower temperatures, orthoclase can unmix from plagioclase forming antiperthites (irregular lamellae of K-feldspar within plagioclase).

Twinning
Plagioclase feldspars obey to the same twinning laws observed in alkali feldspars. Their characteristic is to show repeated, polysynthetic twinning on a microscopic scale, following the Albite law (i.e. a 180° rotation between two twins along an axis perpendicular to the intermediate, b-axis) and, less commonly, the Pericline law (i.e. a 180° rotation between two twins along the intermediate, b-axis; perpendicular to Albite twins). Polysynthetic twinning in plagioclase forms during the transition from high-temperature to low-temperature structures and may be absent at all in plagioclase (i.e. nearly pure albite) that grow in low-grade metamorphic rocks. Plagioclase feldspars commonly show also simple, Carlsbad twinning (i.e. a 180 ° rotation along the long axis or c-axis). Baveno (approximate 45° rotation around the c-axis) and Manebach twinning (180° rotation around the short axis or a-axis) occur but are less common.

feldspar twinning

Some common twinning laws in feldspars. Modified after Deer et al. (2013).

plagioclase

Albite twinning is very rarely visible in the field. This (huge) plagioclase crystal from a pegmatite shows long, straight, and parallel white ‘bands’ of less than a millimeter, which are Albite twins. Oligoclase (An20) sample from a pegmatitic granitoid from Evje, Norway. Length of sample: 12 cm. Photo © Siim Sepp (sandatlas.org).

iridescence in plagioclase

The presence of submicroscopic unmixing causes iridescent colors in some plagioclase. This is an example of iridescence (in the specific case also called labradorescence) in labradorite. Photo © Prokofiev (wikimedia.org).

Field features
Feldspars have tabular to prismatic habit, with rectangular to square-like 2D sections. This habit is commonly well-visible in volcanic and plutonic igneous rocks, where they tend to be euhedral or subhedral. Feldspars have two prismatic cleavage systems intersecting at 90°. For this reason, they are vitreous on broken surfaces (i.e. cleavage planes reflect light very well). All feldspars have hardness around 6 on the Mohs scale (harder than metal but weaker than quartz). In igneous rocks, feldspars commonly constitute the light-colored part of a rock, together with quartz (easily identifiable because it is transparent and show conchoidal fracture). Plagioclase can be distinguished in the field from alkali feldspar based on color: plagioclase tends to be white to green, whereas alkali feldspar is white to pale red and reddish (warning: it can also be green sometimes). Care is especially needed when only white feldspar grains are visible: in this case alkali feldspar tends to be pale pink, while plagioclase is more milk white. Otherwise, it can be useful trying to recognize Albite twinning (which appears as tiny, straight bands) with a hand lens. Carlsbad twinning is commonly visible in plagioclase as a surface separating two twins with different orientation (i.e. reflecting light at different angles).

andesite with plagioclase glomerocrysts

Andesite with glomerocrysts (aggregates) of white, prismatic plagioclase crystals in a fine-grained volcanic groundmass. Field of view: 5 cm. Photo © Richard Droker.

plagioclase phenocrysts

Porphyric basalt with white plagioclase phenocrysts. The rock is 8 cm in length. Isle of Mull, Scotland. Photo © Siim Sepp.

granite

When they occur together, alkali feldspars tend to be pink to red, while plagioclase tends to be white. Note, however, that both may be white or show different colors (e.g. green)! In this granite you can also see grey, transparent quartz and black biotite. Width of view 21 cm. Photo © Siim Sepp.

Plagioclase in thin section
Plagioclase is colorless and show very low relief at PPL. At CPL, plagioclase shows first-order greys interference colors and the characteristic polysynthetic twinning, appearing as sharp dark grey/light grey parallel twins. Plagioclase has two perfect cleavage systems ({001}, {010}) that intersect at nearly 90° on prismatic sections. Plagioclase commonly alters to sericite (i.e. fine-grained mixtures of white micas) and – hydrothermally – to saussurite: fine-grained intergrowths of zoisite, albite, calcite, sericite, and zeolites.
Polysynthetic twinning is the main feature that allows the easy recognition of plagioclase in many igneous, metamorphic, and sedimentary rocks. However, polysynthetic twinning may be not visible in fine-grained grains, which may be confused with quartz. In this case, plagioclase may be distinguished from quartz because it shows cleavage planes and, in general, appears more ‘dirty’ than quartz, as it alters more easily. The relief of Na-rich plagioclase is lower than quartz, but calcic plagioclase show higher relief. In low-grade metamorphic rocks, albite may not show polysynthetic twinning at all and show only Carlsbad twinning, appearing very similar to alkali feldspars. In this case, the different alteration (sericite in albite, kaolinite in alkali feldspar) and habit (albite is commonly elongated parallel to the intermediate axis, b) may be diagnostic. Pure albite also lacks perthites.
Another mineral that may be confused with plagioclase is cordierite, characterized by first-order greys interference colors and polysynthetic twinning. However, cordierite shows also cyclic twinning, is orthorhombic (commonly optically negative) and alters to yellowish pinite.
The optical properties of plagioclase vary slightly in the solid-solution: the refractive indices increase continuously from albite to anorthite, as well as the angle between the intermediate optic axis, β, and the long axis (c-axis). The type of plagioclase can be determined by looking at the angle of extinction of plagioclase twins on a section perpendicular to (010), i.e. the plane of Carlsbad twinning.

plagioclase phenocrysts

Large plagioclase phenocrysts surrounded by a fine-grained plagioclase-pyroxene-opaque minerals groundmass. Scale: 46×27 mm. PPL image. Photo © Daniele.51 (wikipedia.it).

andesiteandesite

Above: Plagioclase is transparent at PPL and the characteristic Albite twinning is visible as sharp first-order grey bands at CPL. Note also the prismatic habit. The brownish crystals with high interference colors are hornblende. The rock is an andesite. Field of view: 7 mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

plagioclase grains

Plagioclase grains with typical first-order greys, prismatic habit, and Albite twinning. Field of view: 7 mm. CPL Image. Photo © Alessandro Da Mommio (alexstrekeisen.it).

Albite
Nearly pure albite occurs commonly in low-grade metamorphic rocks and hydrothermally-altered rocks. Contrarily to plagioclase, such low-grade albite grains may appear very similar to a other alkali feldspars, as they may not show evident Albite twinning. It is important to check the presence of albite vs alkali feldspar when investigating these rocks.

Above: In this schist, there are both quartz and albite. Both are transparent at PPL and grey at CPL, but quartz is more limpid and albite is slightly more ‘dirty’ (e.g. the albite grain on the extreme right). Albite is also altered to sericite (yellowish dot-like masses) on its rims. Finally, albite has lower relief than quartz (e.g. the quartz inclusions it contains here). The other minerals visible are white mica, chlorite, and tourmaline (high interference colors). Width: about 1 mm. Albite schist from the Posada Valley, Sardinia (Italy).

Above: The same image with the minerals present labelled.

albite

Euhedral albite grain in schist. Even if it lacks any twinning, note the stocky habit and the first-order greys that are ‘dirtier’ than quartz. Note also the presence of a vertical set of cleavage planes. Schist from the Posada Valley, Sardinia (Italy). CPL Image. Width: 1 mm.

albite

Albite in low-grade schist with evident Carlsbad twinning. Albite schist from the Posada Valley, Sardinia (Italy). CPL Image. Width: about 1 mm.

Occurrence
Plagioclase feldspars occur in nearly all igneous rocks, from granites to gabbros and their volcanic counterparts. In general, plagioclase feldspars are more sodic in acid igneous rocks and tend to be calcic in mafic ones. Acid rocks commonly show Na-rich plagioclases (albite to oligoclase), frequently associated with alkali feldspars and quartz. Intermediate compositions are characterized by the presence of andesine, which is diagnostic of dioritic/andesitic rocks compared to mafic rocks, like gabbros and basalts that contain labradorite and even bytownite. Ultramafic rocks, like cumulates and peridotites, may also contain a few percent of calcic plagioclase. Plagioclase may occur also in silica-undersaturated rocks, in association with alkali feldspar and feldspathoids.
In metamorphic rocks, the composition of plagioclase becomes more calcic with increasing metamorphic grade (hence temperature). Low-grade rocks, including metapelites and metamafic rocks, commonly show albite as part of the metamorphic assemblage. Albite also forms from hydrothermal alteration, for instance during ocean-floor metamorphism. At higher-grade (amphibolite- to granulite-facies), metamorphic rocks typically display plagioclase with oligoclase and – rarely – andesine composition. More calcic compositions occur in high-grade metamorphic Ca-rich rocks like impure marbles, calcschists and calc-silicate rocks, occurring in contact aureoles and high-grade metamorphic terranes. The presence of plagioclase feldspar is restricted to regional metamorphic conditions. At blueschist- and eclogite-facies, indeed, plagioclase is replaced by other sodic and calcic minerals, such as epidote, glaucophane, lawsonite, omphacite, garnet, and paragonite.
In sedimentary rocks, plagioclase may occur as detrital grains. Ca-rich plagioclase alters more easily and, therefore, it is easier to find Na-rich plagioclase as detrital grains. Pure albite may also form in sedimentary to diagenetic environment as an authigenic mineral, replacing detrital material or as percolating in cavities as cement.

References and Further Reading
Bruni, P. (1976). Plagioclase determination through measurement of the extinction angles in sections normal to (010) and (001).
Gay, P. (1956). The structures of the plagioclase felspars: VI. Natural intermediate plagioclases1. Mineralogical magazine and journal of the Mineralogical Society31(232), 21-40.
Liu, M., & Yund, R. A. (1992). NaSi-CaAl interdiffusion in plagioclase. American Mineralogist77(3-4), 275-283.
Maruyama, S., Liou, J. G., & Suzuki, K. (1982). The peristerite gap in low-grade metamorphic rocks. Contributions to Mineralogy and Petrology81(4), 268-276.
Ribbe, P. H. (Ed.). (2018). Feldspar mineralogy (Vol. 2). Walter de Gruyter GmbH & Co KG.
Smith, J. V., & Brown, W. L. (1988). Intimate Feldspar Intergrowths. In Feldspar Minerals (pp. 555-625). Springer, Berlin, Heidelberg.

        

See also
Alexstrekeisen.it – Plagioclase (plutonic rocks)
Alexstrekeisen.it – Plagioclase (volcanic rocks)
Sandatlas.org – Feldspars

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