PLAGIOCLASE

General Formula:
NaAlSi3O8-CaAl2Si2O8
Sample: PT-10C System:
Monoclinic
Click here
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 nalpha = 1.527 - 1.577
nbeta = 1.531 - 1.585
ngamma = 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:
  1. albite (010) composition plane, polysynthetic, found in all compositions
  2. pericline: (h01) composition plane, polysynthetic, found in intermediate to calcic compositions
  3. 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.


Back to: