Enstatite is a mineral; the magnesium endmember of the
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
silicate mineral series enstatite (MgSiO
3) – ferrosilite (FeSiO
3). The magnesium rich members of the
solid solution series are common rock-forming
minerals found in
igneous and
metamorphic rocks. The intermediate composition, , has historically been known as
hypersthene, although this name has been formally abandoned and replaced by orthopyroxene. When determined
petrographically or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En
80Fs
20).
Polymorphs and varieties

Most natural
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...
s are
orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
(
space group P''bca'') although three
polymorphs are known. The high temperature, low pressure polymorphs are protoenstatite and protoferrosilite (also orthorhombic, space group P''bcn'') while the low temperature forms, clinoenstatite and clinoferrosilite, are
monoclinic (space group P2''
1/c'').
Weathered enstatite with a small amount of iron takes on a submetallic
luster and a bronze-like color. This material is termed
bronzite, although it is more correctly called altered enstatite.
Bronzite and hypersthene were known long before enstatite, which was first described by
G. A. Kenngott in 1855.

An emerald-green variety of enstatite is called chrome-enstatite and is cut as a
gemstone. The green color is caused by traces of
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and h ...
, hence the varietal name. In addition, bronzite is also sometimes used as a gemstone.
Identification
Enstatite and the other orthorhombic pyroxenes are distinguished from those of the monoclinic series by their optical characteristics, such as straight extinction, much weaker
double refraction and stronger
pleochroism.
They also have a prismatic
cleavage that is perfect in two directions at 90 degrees. Enstatite is white, gray, greenish, or brown in color; its
hardness is 5–6 on the
Mohs scale, and its
specific gravity is 3.2–3.3. This prismatic form is used in gemstones, and for academic purposes.
Occurrence
Isolated crystals are rare, but orthopyroxene is an essential constituent of various types of
igneous rocks and
metamorphic rocks. Magnesian orthopyroxene occurs in
plutonic rocks such as
gabbro (norite) and
diorite. It may form small idiomorphic phenocrysts and also groundmass grains in volcanic rocks such as
basalt
Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90% of a ...
,
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...
, and
dacite.
Enstatite, close to En
90Fs
10 in composition, is an essential mineral in typical
peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
and
pyroxenite of the
Earth's mantle.
Xenoliths of peridotite are common in
kimberlite and in some basalt. Measurements of the
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
,
aluminum, and
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and h ...
contents of enstatite in these xenoliths have been crucial in reconstructing the depths from which the xenoliths were plucked by the ascending magmas.
Orthopyroxene is an important constituent of some metamorphic rocks such as
granulite. Orthopyroxene near pure enstatite in composition occurs in some metamorphosed
serpentine
Serpentine may refer to:
Shapes
* Serpentine shape, a shape resembling a serpent
* Serpentine curve, a mathematical curve
* Serpentine, a type of riding figure
Science and nature
* Serpentine subgroup, a group of minerals
* Serpentinite, a ...
s. Large crystals, a foot in length and mostly altered to
steatite, were found in 1874 in the
apatite veins traversing
mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
-
schist and
hornblende-schist at the apatite mine of Kjørstad, near
Brevik in southern
Norway
Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...
.
Enstatite is a common mineral in meteorites.
Crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...
s have been found in stony and iron
meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s, including one that fell at
Breitenbach in the
Ore Mountains,
Bohemia. In some meteorites, together with
olivine it forms the bulk of the material; it can occur in small spherical masses, or
chondrules, with an internal radiated structure.
Enstatite in space
Enstatite is one of the few silicate minerals that have been observed in crystalline form outside the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, particularly around evolved stars and
planetary nebulae such as
NGC 6302. Enstatite is thought to be one of the early stages for the formation of crystalline silicates in space. Many correlations have been noted between the occurrence of the mineral and the structure of the object around which it has been observed.
Enstatite is thought to be a main component of the
E-type asteroids.
[H. U. Keller, et all - E-Type Asteroid (2867) Steins as Imaged by OSIRIS on Board Rosetta - Science 8 January 2010: Vol. 327. no. 5962, pp. 190 - 193 ]
The
Hungaria asteroids are the main examples in the Solar System.
See also
*
*
References
*Deer, W. A., Howie, R. A., and Zussman, J. (1992). ''
An introduction to the rock-forming minerals (2nd ed.)''. Harlow: Longman
{{Commons category
Inosilicates
Magnesium minerals
Iron(II) minerals
Pyroxene group
Orthorhombic minerals
Minerals in space group 61