PYROXENE
The pyroxenes are a group of common and
important Ca-, Mg-, and Fe-bearing silicate minerals. There are dozens of
specific minerals in the pyroxene group. These include augite, diopside,
hedenbergite, pigeonite, enstatite, bronzite, hypersthene, ferrohypersthene,
eulite, and ferrosilite. Augite, diopside, hedenbergite, and pigeonite
all contain some calcium - they are known as the clinopyroxenes.
Enstatite, bronzite, hypersthene, ferrohypersthene, eulite, and ferrosilite are
pyroxenes that lack calcium (or have very little calcium, <2 wt.%) -
these are the orthopyroxenes. Another group is the sodium
pyroxenes, a special variety that includes aegirine/acmite, omphacite, and
jadeite. Examples from all three groups are shown below.
Pyroxene is most frequently encountered in mafic and
ultramafic igneous rocks (e.g., basalts, gabbros, peridotites).
The garden-variety clinopyroxene is augite
((Ca,Na)(Mg,Fe,Al)(Si,Al)2O6), a "garbage-can"
mineral with a nonmetallic luster, a very dark green to black color, a
whitish-gray streak, a moderate hardness (H = 5 to 6), and two planes of
cleavage meeting at near-right angles (87° & 93°).
Augite pyroxene (3.7 cm across)
Diopside
is another clinopyroxene - it has the formula CaMgSi2O6 -
calcium magnesium silicate. When pure, it is white, but this is
rare. It's usually pale green to green; it gets greener with increasing
iron content. Its physical properties are otherwise similar to augite.
Diopside pyroxene (field of view 5.2 cm across)
The orthopyroxenes range from pure magnesium silicate
(enstatite - MgSiO3) to pure iron silicate (ferrosilite - FeSiO3).
The Mg-rich half of the orthopyroxene spectrum is much more common than the
Fe-rich half. The pure Fe-pyroxene end of the spectrum is quite rare.
Here is bronzite, one of the magnesium-rich
orthopyroxenes. It's a magnesium iron silicate ((Mg,Fe)SiO3)
with only 10-30% iron content. Bronzite is typically greenish-black to
brownish-black in color, is moderately hard (H = 5.5 to 6), and has one good
cleavage plane plus a couple of parting planes. The most famous aspect of
bronzite pyroxene is its metallic, bronze- or copper-like luster in bright
light.
Bronzite pyroxene (field of view ~6.4 cm across) - very large cleavage
fragment (entire specimen is 22.2 cm across) from a much larger single crystal.
Geology:
Troctolite-Anorthosite I zone, Lower Banded Series, Stillwater Complex, late
Neoarchean, 2.7 billion years.
Locality:
40W4800 stope of the Stillwater Mine, Beartooth Mountains, southern Montana,
USA.
Here's one of the sodium pyroxenes. This
is omphacite, and it has the same general chemistry as augite
((Ca,Na)(Mg,Fe,Al)(Si,Al)2O6), except that omphacite is
Na-rich. Omphacite is a rare pyroxene, as it only forms by very high
grade metamorphism at mantle depths. Very few rocks & minerals formed
at these depths reach the surface again in an unaltered state. But, there
are some surface outcrops of the metamorphic rock eclogite, which always contains
a significant omphacite component.
Omphacite has a nonmetallic luster, a bright green
color, is moderately hard (H = 5 to 6), and will display one or two cleavage
planes. It is always associated with other very high-pressure and very
high-temperature minerals, such as pyrope garnet.
Omphacite pyroxene (field of view ~1.6 cm across) in an eclogite from
Norway.
Here's another sodium pyroxene - this is jadeite,
the key component in the rarer of the two major categories of jade.
Jadeite occurs in some metamorphosed mantle peridotite bodies.
Jadeite pyroxene (~6.2 cm across along the base) from the Jurassic of
Burma. A rock consisting principally of jadeite pyroxene is called jadeitite.
(More info. on Burmese jades)