| General Formula: (Ca,Mg,Fe,Al)2(Si,Al)206 |
Sample: AUPI 33 | System: Monoclinic |
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| 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 |
