Omphacite

Chemical formula: (Ca,Na)(Mg,Fe²⁺,Al)Si₂O₆

A pyroxene group silicate, a key component of the metamorphic rock eclogite, characterized by its distinctive dark green color.

## Characteristics Omphacite is a complex silicate of sodium, calcium, magnesium, iron, and aluminum, belonging to the clinopyroxene group. It is a rock-forming mineral, constituting, along with garnet, the main component of eclogites – metamorphic rocks formed under conditions of high pressure and temperature. It rarely forms well-developed crystals visible to the naked eye. It usually occurs in granular or massive aggregates, intimately intergrown with other minerals, mainly red garnets, creating the characteristic green-red appearance of the rock. ## Physical Properties Its Mohs hardness ranges from 5 to 6. It has a vitreous luster, and on fracture surfaces, it can also be dull or greasy. It is translucent to opaque, especially in thicker fragments. Density varies depending on chemical composition, typically within the range of 3.29-3.39 g/cm³. ## Colors and Varieties Omphacite most often ranges in color from light green, through grass-green, to dark green, almost black. The color depends on the proportion of iron and magnesium in its structure. No named varieties are distinguished, although in gemology, rock material rich in omphacite and jadeite is sometimes referred to by the trade name "omphacite jade." ## History and Name The mineral's name, given in 1815 by Alexis Boubée, comes from the Greek word "omphax" (όμφαξ), meaning "unripe grape," which alludes to its typical green color. It was described based on samples from eclogite outcrops in Weißenstein, Bavaria (Germany), which is its type locality. ## Uses Omphacite is primarily a mineral of scientific and collector's interest. As an indicator of high-pressure metamorphism conditions, it provides geologists with valuable information about processes occurring deep within the Earth's crust and in the upper mantle. Eclogite rock, rich in omphacite, is sometimes polished and used as an ornamental or decorative stone.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: greenish white
Density: 0
Cleavage: Good on {110}
Fracture: Uneven, Conchoidal
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Omphacite's key diagnostic feature is its green color and its co-occurrence with red or pink garnet (most often pyrope or almandine) in eclogite rock. It has a vitreous luster and a white to greenish streak. Unlike many other green minerals, it rarely forms independent, well-developed crystals. ## Distinguishing from Similar Minerals Omphacite can be confused with other green pyroxenes, such as diopside or hedenbergite, from which differentiation without specialized tests (chemical analysis, optical methods) is often impossible. It can also be confused with jadeite, with which it forms a solid solution; however, jadeite is usually lighter and more homogeneous. It differs from epidote by its often grassier shade of green and lack of distinct cleavage in one direction. It differs from olivine by its geological context (olivine is rare in eclogites) and hardness. ## Crystal Forms Well-developed crystals are rare and usually appear as short, prismatic or thick-tabular forms, typical of clinopyroxenes. Most often, it occurs as anhedral grains or compact, granular aggregates within the rock mass.

Geological environment

## Genesis Omphacite is a characteristic mineral of high-pressure metamorphism and moderate to high temperatures (eclogite facies). It forms deep within the Earth's crust or in the upper mantle, as a result of the transformation of basaltic rocks (e.g., oceanic crust) in subduction zones. It forms at pressures exceeding 1 GPa. ## Mineral Associations Its most common and characteristic associated mineral is garnet (mainly pyrope, almandine). It also co-occurs with minerals such as kyanite, rutile, coesite (a high-pressure polymorph of quartz), phengite (a variety of muscovite), glaucophane, zoisite, and diamond (in kimberlitic eclogites). ## Localities Significant deposits and occurrences of omphacite-bearing eclogites are found worldwide. The most important include the type locality Weißenstein in Germany, the Saualpe and Koralpe regions in Austria, the Nordfjord area in Norway (famous garnets-eclogites), the Western Alps (Italy, Switzerland), as well as occurrences in California (USA), Guatemala, Japan, and within the Siberian craton (Russia), where it is associated with diamond deposits.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most valuable specimens are those that show a clear color contrast between intensely green omphacite and red or pink garnet crystals within the eclogite rock. Rare, well-formed omphacite crystals are also sought after, although they are usually small. The purity and intensity of the green color increase the specimen's value. Large, aesthetically composed eclogite slabs with evenly distributed minerals are highly prized. ## Popular Localities The most classic and highly valued specimens of eclogites with omphacite and garnet by collectors come from historical localities in Norway (Nordfjord region) and the Austrian Alps (Saualpe). Specimens from these locations are considered exemplary for this mineral association.

Care and storage

## Cleaning Omphacite specimens are best cleaned with a soft, dry brush to remove dust. If necessary, a damp cloth and distilled water can be used, followed by thorough drying of the specimen. It is generally water-resistant. ## What to Avoid Avoid contact with strong acids, which can damage the mineral's surface. It is not particularly sensitive to sunlight, but prolonged exposure to extreme temperature changes is not advisable. ## Storage Omphacite is relatively hard and durable; however, to prevent scratches, it should be stored in separate boxes or pouches, separated from harder minerals (e.g., quartz, corundum). Specimens in matrix rock (eclogite) are stable and well-suited for display.