| General Formula: (Mg,Fe)2Si2O6 |
Sample: W-3 | System: Orthorhombic |
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| Orthopyroxene The similar appearence in plane light of orthopyroxene and clinopyroxene are highlighted in this image. Note that both grains lack any clear colour, although coloured and pleochroic varieties are common. Field of view = 4 mm, plane light |
Orthopyroxene The low interference colours characteristic of orthopyroxene compared to clinopyroxene are evident in this image. Field of view = 4 mm, crossed polars |
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes. |
Optical Properties
| Colour Pleochroism |
pale coloured in thin section with subtle pinkish to greenish pleochroism |
Form | euhedral crystals are usually stubby prisms, basal sections are 4 or 8 sided, with two primatic cleavages at 90° longitudinal sections are rectangular, exhibit one cleavage and parallel extinction |
| Relief RI |
moderate to high nalpha = 1.649-1.768 nbeta = 1.653-1.770 ngamma = 1.657-1.788 Generally increasing with increasing Fe content |
Cleavage | two good cleavages parallel to the {210} prism faces that intersect at 88° |
| Birefringence Interference Colours |
0.007-0.020 usually first order yellow or lower |
Twinning | rare |
| Interference Figure Optic Sign 2V |
Biaxial positive or negative 2VZ = 50-132° |
Optic Orientation | X=b, Y=a, Z=c optic plane parallel to (100) elongate fragments exhibit parallel extinction and are length slow |
| Composition | the effects of the substituion of Fe for Mg in orthopyroxene on the optical properties is evident in examining Figure 13.4 in Nesse. | Alteration | alters to serpentine, talc or fine grained amphibole |
| Occurrence | Mg-rich opx is common in mafic intrusive rocks (gabbro, norite, etc.). Fe-rich opx is found in more siliceous igneous rocks (diorite, syenite etc.). opx is common in high grade regional metamorphic rocks |
Distinguishing Features | distinguished from cpx by: 1) lower birefringence, 2) parallel extinction, 3) pale colour, weak pleochroism, 4) most common opx is optically negative, and 5) high 2V angle. |
