Enstatite

Hand Specimen identification: Association, luster, color, and two cleavages at about 90° to each other identify pyroxene, but distinguishing enstatite from other pyroxenes can be problematic. Exact composition cannot be determined without additional analytical data. It may also be confused with amphibole, but the latter has two cleavages at about 60° to each other.

Chemical Composition : Mg2Si2O6

Hardness : 5 to 6

Specific Gravity3.2 to 3.5

Cleavage/Fracture : two perfect prismatic{210}/uneven

Luster/Transparency : vitreous, pearly/translucent

Color : gray, green

Streak : white, gray

Optical Properties : Enstatite is nearly colorless in thin section. If Fe has replaced some of the Mg, it may exhibit slight to pronounced pink to green pleochroism. An 86° cleavagengle, parallel extinction in prismatic section, and relatively low birefringence distinguish it from clinopyroxene. Biaxial 1+ 2, a = 1.657, b = 1.659,g = 1.665, d = 0.008, 2V = 54°.

Crystallography : Orthorhombic, a = 18.22, b = 8.81, c = 5.21, Z = 4; space group P21/b21/c21/a; point group 2/m2/m2/m.

Habit : Enstatite is usually massive, blocky, fibrous, or lamellar. Individual crystals may be prismatic or acicular.

Structure and composition : Enstatite is one of two principal orthopyroxene end members; the other is ferrosilite, Fe2Si2O6. Enstatite and other pyroxenes contain chains of zigzagging SiO4 tetrahedra running parallel to the c-axis. Each tetrahedron shares two oxygens with neighbors in its chain and has one unshared oxygen at an apex pointing perpendicular to the c-axis. Pairs of chains face each other; Mg is located in four adjacent octahedral sites between unsharedapices of tetrahedra. Complete solid solution exists between enstatite and ferrosilite. Except at high temperature, only limited solid solution exists with the clinopyroxene end members diopside, CaMgSi2O6, and hedenbergite, CaFeSi2O6. Mn, Cr, Al, and Ti may also be present in small amounts.

Occurrence and associations : Enstatite is common in mafic igneous rocks, including gabbro, basalt, and norite, commonly associating with plagioclase and clinopyroxene. It is also found in some high-grade metamorphic rocks and is considered diagnostic for the granulite facies.

Varieties : Bronzite 1Mg 7 7 Fe2 and hypersthene 1Mg 7 Fe2 are varietal names given to Mg-Fe orthopyroxene.

Related Minerals : Enstatite is isostructural or isotypical with other pyroxenes. It is closely related to ferrosilite, Fe Si O , and1 2 2 2 6 donpeacorite, Mn,Mg 2Si2O6. Most natural orthopyroxenes are Mg-Fe solid solutions with Mg dominant.

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