CLINOPYROXENE

General Formula:
(Ca,Mg,Fe,Al)2(Si,Al)206
Sample: AUPI 33 System:
Monoclinic
Clinopyroxene Phenocrysts
Euhedral, 8 sided, clinopyroxene phenocryst, exhibiting two cleavages, in a fine grained matrix of plagioclase microlites, clinopyroxene and olivine.
Field of view 2.7 mm, plane light
Clinopyroxene Phenocrysts
Note the black areas in the right grain due to plucking of the sample during preparation of the thin section.
Field of view 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
usually colourless, gray, pale green or pale brown, darker colours associated with Fe-rich varieties
Titanaugite is more distinctly coloured from brown/pink to violet.
Form crystals form stubby prisms elongate along the c-axis, basal sections are 4 or 8 sided and show two cleavages at ~90°
Relief
RI
high positive
nalpha = 1.664-1.745
nbeta = 1.672-1.753
ngamma = 1.694-1.771
Cleavage typical pyroxene cleavages parallel to {110}, which intersect at ~90°
Birefringence
Interference Colours
0.018-0.034
lower to middle second order
Twinning simple and lamellar twins and composition planes, which in combination may form a herringbone pattern
Interference Figure
Optic Sign
2V
Biaxial
positive
25-70°
Optic Orientation sections parallel to (100) show parallel extinction
sections parallel to (010) show maximum birefringence, a single cleavage and Z^c of 35 to 48°
Composition All clinopyroxenes crystallize with the same structure and there is complete solid solution among all species and they cannot be distinguished reliably based on their optical properties Alteration commonly alter to uralite (a fg, light coloured amphibole), or may alter to serpentine, chlorite, biotite, carbonates and /or other silicates
Occurrence common in mafic igneous rocks, alkali-rich varieties may be found in more silicic rocks, often associated with olivine, orthopyroxene, and plagioclase Distinguishing Features inclined extinction, higher birefringence, 2nd order interference colours, low 2V, optically positive in comparison with opx


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