Orthopyroxene



Occurrence
Orthopyroxene is common in mafic igneous rocks, ingcluding gabbro, basalt and norite, commonly associated with plagioclase and clinopyroxene. It is also found in some high grade metamorphic rocks. The term "hypersthene" is often used as a synonym for orthopyroxene.

Composition
Orthopyroxenes have general formula (Mg,Fe,Ca)(Mg,Fe,Al)(Si,Al)2O6, but natural compositions are dominated by two major end member components: enstatite, Mg2Si2O6, and ferrosilite, Fe2Si2O6.

Identification
Orthopyroxene is in many ways similar to clinopyroxene. The keys to identifying orthopyroxene are its relief, usually pale green (sometimes pleochroic to pink) color, low-order interference colors, and near 90o cleavage seen in some views.

Important properties
 ·Color - Commonly light green, but some orthopyroxene shows marked pleochroism to pink.
 ·Relief is moderate to fairly high.
 ·Interference colors - Interference colors usually range up to first order yellow.
 ·Appearance, cleavage, extinction -Well developed end sections may appear as blocky four- or eight-sided crystals; more typically orthopyroxene forms stubby prisms, rectangular lathes, or anhedral crystals and masses. Basal sections show cleavage angle near 90o. Longitudinal sections show one cleavage. Extinction is parallel to the cleavage.
 ·Twinning and exsolution - Orthopyroxenes may show fine twinning or exsolution, but the two are sometimes difficult to distinguish.
 ·Interference figure - 2V and optical sign vary with composition.

Similar minerals
 ·Clinopyroxene has higher birefringence and non-parallel extinction.
 ·Andalusite is very similar but does not occur in the same kinds of rocks.


Orthopyroxene (hypersthene) with Cordierite and Biotite

These four views show a spectacular rock from near Kazabazua, Quebec. It contains primarily cordierite and orthopyroxene with lesser amounts of biotite. The cordierite is twinned (and in XP looks a lot like plagioclase) and contains pleochoric halos around zircon inclusions. The halos appear as "burn" marks in PP light. The orthopyroxene is blocky and fractured, shows high relief (PP), and has first order interference colors (XP). It is pleochroic, with color ranging from light pink to light green. Biotite is also pleochroic and here exists as flakes in various shades of brown (PP) and has upper 2nd order interference colors (XP).

The field of view is 2.5 mm.


Orthopyroxene in a Spinel Peridotite Xenolith from Kilbourne Hole, New Mexico

These photos show mostly orthopyroxene (light clear to green, PP; gray interference color, XP). Several small grains of clinopyroxene (2nd order yellow to blue interference colors, XP) are also also present -- mostly along the left side of the photos. Note the near 90o cleavage angle in the orthopyroxene.

The field of view is 4.5 mm.


Orthopyroxene in an Olivine Websterite Xenolith from the Cima Volanic Field, California

These photos show a large grain of orthopyroxene surrounded by clinopyroxene (some of which is near extinction in the XP view). In the XP view, the orthopyroxene shows only first-order interference colors -- the clinopyroxene ranges up to upper second order orange to pink (near the top center of the photo). Only one well developed cleavage is visible in this orthpyroxene grain.


Orthopyroxene in a Xenolith from the Cima Volcanic Field, California

This photos is of a large orthopyroxene grain in a xenolith from a basalt in the Cima Volcanic Field, California. The fractured orthopyroxene grain is clear (PP) and shows first-order yellow to gray interference colors (XP). The finer grained material is mostly a mix of plagioclase -- blocks and lathes (with twinning just faintly visible in the XP view) and brown volcanic glass. The field of view is 4.5 mm.