Sodic Pyroxene

Occurrence and Compositon
Sodic pyroxenes have general formula (Na,Ca)(Mg,Fe3+,Al)Si2O6. The principle end members/varieties are: acmite (also called aegirine, NaFe3+Si2O6), aegirine-augite (solid solutions between NaFe3+Si2O6 and Ca(Mg,Fe)Si2O6), jadeite (NaAlSi2O6), and omphacite (a sodic clinopyroxene in whch the amount of Al exceeds the amount of Fe3+). Acmite and aegirine-augite are found in a variety of alkalic igneous rocks. Jadeite and omphacite are found in high-pressure metamorphic rocks. Jadeite is typically in bluschists. Omphacite is typically restricted to eclogites.

Identifying a mineral as a pyroxene is generally not difficult, but distinguishing amongst the different pyroxenes can be challenging. Color and pleochroism help distinguish acmite and aegirine-augite from other pyroxenes. Low birefringence and (often) anomalous blue interference colors distinguish jadeite from most other minerals. Omphacite generally has a nondescript light green color, and low-order nterference colors.

Important properties
 ·Color - Acmite and aegirine-augite are brown, yellowish brown or various shades of green. The color is darkest when Fe3+ content is high. Green varieties are pleochroic; color zoning is common. Omphacite is light green and may show faint pleochroism; jadeite is generally colorless but may be very pale green or yellow.
 ·Relief - Acmite and aegirine-augite have high relief; jadeite and omphacite have moderate to relatively low relief.
 ·Interference colors - Acmite may exhibit third order interference colors; aegirine-augite has slightly lower birefringence. When strongly colored, the color of these minerals may partially or completely mask interference colors. Omphacite has considerably lower birefringence (maximum colors are upper first or second order). Jadeite shows only first order or anomalous interference colors.
 ·Appearance, cleavage, extinction - Some views of acmite, aegirine-augite, omphacite or jadeite may reveal four- or eight-sided cross sections with two cleavages that are nearly perpendicular. Short prisms or plates showing one cleavage are more typical. Additionally, omphacite and jadeite both sometimes exist as granular masses, and jadeite may form fibrous aggregates.
 ·Twinning and exsolution - Twinning, simple or lamellar, may be present.
 ·Interference figure - 2V and optic sign vary with compositon.

Similar minerals
 ·Dark colored amphiboles are similar to dark colored Na-pyroxenes in some ways; light colored amphiboles are occasionally confused with omphacite or jadeite. Amphiboles, however, have the characteristic 60o and 120o cleavage angles and are length slow.
 ·Epidote may have similar color and interference colors as Na-pyroxene but has only one good cleavage.
 ·Acmite and aegirine-augite are distinguishable from other pyroxenes because: they have deeper colors and stronger pleochroism; their extinction angle is small (0-20o); they are length fast; their index of refraction and birefringence are larger.
 ·Omphacite may be hard to distinguish from other (calcic) pyroxenes but has larger 2V (55-85o) and generally low order interference colors.
 ·Jadeite and omphacite are similar in some ways, but jadeite has lower index of refraction, low birefringence or anomalous interference colors, small extinction angle, and is generally colorless.
 ·Zoisite may show anomalous blue interference colors similar to jadeite but has parallel extinction and higher relief than jadeite..

Aegirine in an Ijolite from the McClure Mountains, Colorado

The photos above show a large twinned pyroxene (aegirine) crystal with the classic 8-sided ("stop sign") shape and high relief compared to the light white-green surrounding material. The twin divides the pyroxene into two domains with different optical orientations, easily seen in the XP view. The aegririne is fractured, but with a little imagination it is possible to see two cleavages that intersect at near 90o. All the other high-relief green (PP) material is also aegirine. Minor brown (PP) biotite is present as inclusions. The low-relief, nearly clear (PP), material is mostly nepheline but there are also a few small grains of K-feldspar. The nepheline and K-feldspar are hard to distinguish in the PP view. Nepheline has very low birefringence, and so appears almost isotropic in the XP view.

The field of view is about 3.5 mm.

Glaucophane and Epidote with Jadeite (Blueschist)

This section contains abundant blue glaucophane. It has high relief, and many of the grains show the classic diamond-shaped amphibole cross section and cleavage. Smaller grains of high relief epidote are scattered about the thin section. They have slightly higher relief than the glaucophane (visible in PP light) but get lost in the XP view. The nearly clear (PP) matrix material that encloses the glaucophane and epidote is jadeite. It displays low-order inteference colors, anomalous in some grains, in the XP view.

This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm.

Garnet, Omphacite and Titanite in an Eclogite

These photos show a large euhedral garnet (opaque in the lower left) in a sea of mostly omphacite (a high pressure sodic pyroxene), titanite, and a few flakes of white mica. The garnet is slightly altered to chlorite along its edges; note the anomalous interference colors in the XP view. The omphacite is low-relief, pale green (PP) with interference colors that range up to first order red (XP). In a few places, where higher-order colors can be seen, the omphacite is being replaced by hornblende. The titanite crystals are small, have very high relief (PP), and appear as irregular elongate grains, some of which have acute terminations. Several flakes of white mica are present -- they are the clearest grains visible in the PP view.

This specimen is from Sonoma County, California. The field of view is about 1.5 mm across.