Omphacite is a member of the clinopyroxene group of

silicate minerals Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...

with formula: ( Ca, Na)( Mg, Fe²⁺, Al) Si₂ O₆. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in

eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

, which is the high-pressure

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

. Omphacite is the

solid solution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

of Fe-bearing diopside and

jadeite Jadeite is a pyroxene mineral with composition Na Al Si2 O6. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, but is most often found in shades ...

. It crystallizes in the

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

with prismatic, typically twinned forms, though usually anhedral. Its

space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

can be P2/n or C2/c depending on the thermal history. It exhibits the typical near 90° pyroxene

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

. It is brittle with

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

of 3.29 to 3.39 and a

Mohs hardness The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fair ...

of 5 to 6.

Formation and occurrence

Omphacite is the dominated phase in the subducted

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

in the Earth's upper mantle. The Mid-Ocean Ridge Basalt, which makes up oceanic crust, goes through ultrahigh-pressure metamorphic process and transforms to

at depth ~60 km in the

subduction zones Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second pla ...

. The major mineral components of eclogite include omphacite,

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

and high-pressure silica phases (

coesite Coesite () is a form (polymorphism (materials science), polymorph) of silicon dioxide (silicon, Sioxide, O2) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first ...

and

stishovite Stishovite is an extremely hard, dense tetragonal form ( polymorph) of silicon dioxide. It is very rare on the Earth's surface; however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle. Stishovite w ...

). As depth increases, the omphacite in eclogite gradually transforms to majoritic garnet. Omphacite is stable up to 500 km depth in the Earth's interior. Considering the cold geotherm of subducted slabs, omphacite can be stable even in deeper mantle. It also occurs in blueschist facies and ultrahigh-pressure

metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

. It is also found in eclogite xenoliths from

kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...

as well as in crustal rocks metamorphosed at high pressures. Associated minerals in eclogites except the major minerals include

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure Polymorphism (materials science), polymorph of andalusite and sillimanite, and the presence of kyani ...

, phengite, and lawsonite. Minerals such as glaucophane, lawsonite, titanite, and

epidote Epidote is a calcium aluminium iron sorosilicate mineral. Description Well developed crystals of epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in ha ...

occur with omphacite in

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and similar rocks at relatively low temperatures () but very high overburden pressure, pressure corresponding to a depth of . The b ...

facies metamorphic rocks. The name "jade", usually referring to rocks made of

, is sometimes also applied to rocks consisting entirely of omphacite.

Chemical composition

Omphacite is the solid solution of Fe-bearing diopside (CaMgSi₂O₆) and jadeite (NaAlSi₂O₆). Depending on how much the coupled substitution of (Na, Al)-(Mg-Fe, Ca) happens, the

chemical composition A chemical composition specifies the identity, arrangement, and ratio of the chemical elements making up a compound by way of chemical and atomic bonds. Chemical formulas can be used to describe the relative amounts of elements present in a com ...

of omphacite varies continuously from pure diopside to pure jadeite. Due to the relatively small radius of (Na, Al) atoms, the

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector In mathematics, a unit vector i ...

volume linearly decreases as jadeite component increases. In addition, the coupled substitution also stiffens the crystals. The bulk and shear modulus linearly increases as jadeite component increases.

Space group

Although omphacite is the solid solution of diopside and

, its

may be different with them. The space group of diopside and jadeite is C2/c. However, omphacite can show both P2/n and C2/c space group. At low temperature, the partial coupled substitution of (Na, Al)-(Mg-Fe, Ca) in omphacite orders the atoms in the unit cell and makes omphacite shows a relatively low symmetry space group P2/n. As temperature increases, the movements of the atoms increase and finally the coupled substitution will not influence the order of the structure. When temperature reaches ~700–750 °C, the structure of omphacite becomes totally disordered and the space group will transform to C2/c. Natural omphacite may show C2/c structure even at room temperature if the omphacite crystal went through fast temperature decreasing. Although the atomic positions in the two space groups have a subtle difference, it does not clearly change the physical properties of omphacite. The absolute unit cell volumes are a little different for the two different space group, the compressibility and

thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

does not show obviously different within experimental uncertainties.

Etymology and history

It was first described in 1815 in the Münchberg Metamorphic complex, Franconia,

Bavaria Bavaria, officially the Free State of Bavaria, is a States of Germany, state in the southeast of Germany. With an area of , it is the list of German states by area, largest German state by land area, comprising approximately 1/5 of the total l ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

. The name ''omphacite'' derives from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''omphax'' or ''unripe grape'' for the typical green color.

References

{{commonscat, Omphacite Inosilicates Calcium minerals Sodium minerals Magnesium minerals Iron(II) minerals Aluminium minerals Monoclinic minerals Minerals in space group 13 Minerals in space group 15