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The Mineral omphacite

Omphacite is a rare member of the pyroxene group. It is intermediary in chemical composition between Augite and Jadeite. It is one of the primary minerals that forms eclogite deposits, which is a dense, course grained rock formed at extremely high pressures. It is occasionally associated with Diamond in Diamond deposits.

Chemical Formula

(Ca,Na)(Mg,Fe²⁺,Fe³⁺,Al)Si₂O₆

Color

Light to dark green

Crystal System

Monoclinic

Properties

Streak
White to light green

Hardness
5 - 6

Transparency
Translucent to opaque. Rarely transparent.

Specific Gravity
3.3 - 3.4

Luster
Vitreous, dull

Cleavage
1,2 - prismatic at cleavage angles of 87º and 93º (Characteristic of minerals in the pyroxene group).

Fracture
Uneven

Tenacity
Brittle

Crystal Habits

As prismatic or equant crystals with a square cross-section. Crystals are often twinned. Also grainy, massive, and encrusting.

Additional Information

Composition
A silicate of calcium, sodium, magnesium, iron, and aluminum

In Group
Silicates; Inosilicates; Pyroxene Group

Striking Features
Crystal forms, cleavage habits, and environment.

Environment
A major constituent of eclogite deposits; also in other ultra-high pressure metamorphic rocks.

Rock Type
Metamorphic

Noteworthy Localities

First described in Stammbach, Bavaria, Germany. Also found in Found in Val d'Osta (Ivrea), and Bric Vert, Italy; Hof, Bayreuth, Germany; Andersdorf, Koralpe Mountains, Austria; Sogn og Fjordane, Norway; and Oyashirazu beach, Itoigawa, Japan. In the U.S, it is found at Reeds Station in the Tiburon Peninsula, Marin County, California; and in Healdsburg, Sonoma Co., California.

Common Mineral Associations

Quartz, Biotite, Muscovite, Glaucophane, Almandine, Pyrope, Kyanite, Coesite

Distingushing Similar Minerals

May be difficult to distinguish from other pyroxene minerals such as Augite, Enstatite, and Hedenbergite.