PLAGIOCLASE


General Formula: NaAlSi₃O₈-CaAl₂Si₂O₈	Sample: PT-10C	System: Monoclinic
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Ophitic Texture Colourless plagioclase grains, note cleavage to right of label, surrounded by a large oikocryst of clinopyroxene Field of View = 4 cm, plane light	Ophitic Texture Albite, pericline and Carlsbad twinned plagioclase grains enclosed within a second order green clinopyroxene. Field of View = 4 cm, crossed polars	The above image presents a series of block diagrams showing the variation in indicatrix axes and crystallographic axes position with composition.

Optical Properties


Colour Pleochroism	colourless non-pleochroic	Form	occurs as both euhedral and anhedral grains; crystals are tabular parallel to (010), i.e. lath shaped, and elongated parallel to the c or a axis. Chemical zoning is common and is expressed as a variation in the extinction angle from one zone to another.
Relief RI	low positive or negative, dependant on composition n_alpha = 1.527 - 1.577 n_beta = 1.531 - 1.585 n_gamma = 1.534 - 1.590	Cleavage	{001} perfect cleavage, {010} good cleavage. The cleavages intersect at 93° to 94°.
Birefringence Interference Colours	0.007-0.013 first order grey or white	Twinning	polysynthetic twinning is characteristic. Common twin laws are: albite (010) composition plane, polysynthetic, found in all compositions pericline: (h01) composition plane, polysynthetic, found in intermediate to calcic compositions Carlsbad: (010) composition plane, penetration, found in intermediate to calcic compositions
Interference Figure Optic Sign 2V	biaxial positive or negative varies systematically with composition	Optic Orientation	Optic orientation varies in a regular manner with composition. Except by chance none of the indicatrix axes coincides with any of the crystallographic axes.
Composition	plagioclase shows continuous solid solution from albite to anorthite with NaSi replacing CaAl.	Alteration	commonly partially altered to sericite, clay or zeolites. Plagioclase also may alter to saussurite, a fine grained aggregrate of epidote group minerals, albite, sericite and other minerals.
Occurrence	widespread mineral found in nearly all igneous rocks, in many metamorphic rocks and some sediments. The composition of igneous plagioclase will reflect the composition of the melt from which the mineral formed.	Distinguishing Features	low relief, colourless, biaxial figure, polysynthetic twinning.

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