Minerals In Thin Section


Formula	NaAlSi₃O₈ - CaAl₂Si₂O₈

Optic class & sign	Biaxial positive or negative

Optical orientation	a, b, c at highly variable angles to X, Y, Z, depending on composition

Optical plane	Variable orientation, depending on composition

Relief	Low negative to low positive

Refractive indices	n_x = 1.527 -1.577

	n_y = 1.531 -1.585

	n_z = 1.534 -1.590

	n increases with increasing anorthite component

Birefringence (max.)	0.007 - 0.013

	Δn decreases slightly from albite to andesine, then increases again to anorthite

Optic Angle	2V_x = to be clarified -

	2V_z = 90 - 75°

Sign of elongation	Length-slow, l (+) and length-fast (l -), composition-dependent

Interference figure	2V is generally high for low-T plagioclase, but may decrease on the Na-side to 50° for high-T plagioclase. Diffuse isogyre images on a white background. Tightly spaced twinning renders it difficult to obtain meaningful interference figures.

Colour / pleochroism	Colourless. May be clouded by alteration minerals

Zoning


Form	Habit	Granular, tabular, elongate (parallel to c or a). Crystals in volcanics tend to have a high aspect ratio.

	Surface	Anhedral to subhedral in plutonic rocks and metamorphic rocks; subhedral to euhedral in volcanics; euhedral if authigenic.

Cleavage		{001} perfect, {010} distinct, intersecting at 94 resp. 86°

Twinning		Large variety of possible twin laws, with multiple, lamellar twinning on (010) being most common (“albite law”). Recognition of albite twins: if twin plane is parallel to cross-hairs, the interference colour of both twin sets is uniform. Simple or, less commonly, multiple twins on (010) showing non-uniform interference colours in the above crystal orientation are Carlsbad twins. The twin axis of albite twins is orthogonal to (010), but parallel to (010) in Carlsbad twins (oriented [001]). A second set of multiple twinning may also be common about 90° to the albite twins, with a twin axis [010]. These are pericline twins.

Extinction		Inclined. Extinction angles vary strongly with composition. Of particular interest is the angle between X’ and the trace of (010) in sections orthogonal to [100], varying over a range of about 70° between Ab and An (see below). This angle can be used to quickly determine plagioclase composition under the microscope. The commonly used methods are the Michel-Lévy and Carlsbad-albite methods which are described in optical mineralogy books. Compositional zoning, as reflected in changing extinction angles, is common, particularly in plagioclase from intermediate igneous rocks. Zoning can be simple core-to-rim continuous (normal: An-rich to Ab-rich, or reverse: Ab-rich to An-rich), or oscillatory.


Reaction textures	Plagioclase coronas around garnet in mafic rocks, commonly interpreted as a decompression effect.

Alteration / decomposition	Sericite, calcite, clinozoisite, zoisite, secondary albite, kaolinite, montmorillonite, zeolites, prehnite. Sodic varieties are generally more stable than calcic ones, which can lead to selective alteration in compositionally zoned plagioclase.


Occurence	Ign	Felsic to mafic igneous rocks, plutonic and volcanic, ultramafic cumulates, pegmatites. Common range is albite/oligoclase/andesine in felsic rocks to labradorite/bytownite in mafic rocks.

	Met	Very-low grade to high-grade felsic, mafic and pelitic rocks. Essentially pure albite up to greenschist facies, then increasing An component at higher temperatures, up to andesine. An-rich Plg in calcsilicate rocks and impure marble. Plg is not stable at high pressures (eclogite facies, mantle conditions). Product of Na-metasomatism.

	Sed	Detrital in sediments and sedimentary rocks, particularly in immature clastic deposits; authigenic in diagenetic environmentsDetrital in sediments and sedimentary rocks, particularly in immature clastic deposits; authigenic in diagenetic environments

	Hyd	Veins

	Other	Stony meteorites


Distinctive properties	Low n and Δn, characteristic twinning, alteration. The expectation of multiple twins in plagioclase can be misleading. Twinning tends to be less common in plagioclase of metamorphic rocks, and may be completely absent. Authigenic albite is commonly untwinned. Note: lamellar twinning in cordierite resembles albite twinning, but occurs in three cross-cutting sets (cyclic twins); twin lamellae are less regular in shape, with a tendency to wedge out or terminate abruptly within a crystal, and/or show steps along twin planes.

Additional comments	K contents of plagioclase do not influence optical properties significantly. Unmixing of high-temperature plagioclase during cooling can produce blebs of K-feldpar in plagioclase, called ‘anti-perthite’. In the absence of twinning, quantitative estimates of plagioclase versus quartz under the microscope may be difficult to make; looking for signs of alteration along grain margins is a sensible approach, even where plagioclase is mostly pristine. Routinely checking for interference figures in randomly oriented grains is clearly impractical. Otherwise, element mapping methods need to be applied.

Plagioclase