ORTHOPYROXENE

General Formula:
(Mg,Fe)2Si2O6
Sample: W-3 System:
Orthorhombic
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.


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