ENSTATITE

Enstatite – (Mg)2[Si2O6] – is an inosilicate of the Pyroxene Group, more specifically of the orthopyroxenes, the orthorhombic pyroxenes. It is a rarer mineral than clinopyroxenes (monoclinic pyroxenes such as augite, pigeonite, diopside, etc.), but an important rock former. It has no economic importance.

Enstatite is one of the extremes of the Enstatite – Ferrosilite Series. The ideal composition of ferrosilite is (Fe)2[Si2O6]. A former classification named the terms of the series as enstatite (En 100-90), bronzite (En 90-70), hypersthene (En 70-50), iron-hypersthene (En 50-30), eulite (En 30-10) and ferrosilite (En 10-0). These intermediate terms were discredited in 1988 as mineral species (see Morimoto et al., 1988), but in some cases they continue to be used by tradition. When the composition of orthopyroxene is accurately determined, its denomination is given by the relative proportions of enstatite and ferrosilite; for example, En30Fs70.

Enstatite is a polymorph of akimotoite, bridgmanite, clinoenstatite and of another yet unnamed mineral. It may contain, as impurities, Ca, Na, Fe, Ti, K, Al, Mn, Co, Ni and Cr. As varieties, the terms bronzite (with Fe), calcium bronzite, vanadiniferous bronzite and chromiferous enstatite (green) are still used.

1. Characteristics

Crystal system: Orthorhombic bipiramidal.          

Color: White, yellowish green, brown, greenish white or grey, olive green.     

Habit: Prismatic, lamellar, can be fibrous or massive.       

Cleavage:  {210} good.      

Tenacity: Brittle.        

Twinning:  Rare, simple or lamellar.    

Fracture: Irregular.       

Mohs Hardness: 5 – 6

Parting: On {100} and {010}.         

Streak: White to gray.         

Lustre: Vitreous to pearly.          

Diaphaneity: Transparent.           

Density (g/cm³):  3.2 – 3.9

     

2. Geology and Deposits

Enstatite is very common in plutonic rocks, especially in ultramafic and basaltic rocks. In pyroxenites and peridotites, enstatite and bronzite predominate (bronzitite, lherzolite), more rarely hypersthene (harzburgites). Bronzite and hypersthene are common in gabbros and norites.

In diorites and in “dry” granites (charnokites) only hypersthene is common. There is a tendency for Fe-rich members in SiO2-rich igneous rocks.

In volcanic rocks, orthopyroxenes occur as phenocrysts, especially in andesitic to dacitic rocks – usually hypersthene. In dark andesites, Fe-rich bronzite occurs. Hypersthene is a characteristic phase of granulite facies in metamorphic rocks.

Orthopyroxenes also occur in skarnites and meteorites.

 

3. Mineral Associations

It is associated with hematite, olivine, phlogopite, ilmenite, zircon, pseudobrookite, tridymite, diopside, sanidine, spinel, garnet (pyrope), fluor-apatite and fluor-edenite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.650 – 1.668     nβ: 1.652 – 1.673     nγ: 1.659 – 1.679

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless when with Mg alone. If with iron, it can be pale green (bronzite), or pale green to pale reddish brown, showing weak pleochroism:

X,Y = yellowish to reddish,

Z = gray-green.

Clinopyroxene exsolution blades are common on (100).   

Relief: Moderate to high.           

Cleavage: {210} excellent. In the prismatic sections there is only one cleavage. In the basal sections there are two cleavages intersecting at angles of 87º and 93º, typical of pyroxenes. Also partitions on {100} and {010}.           

Habits: In volcanic rocks it is columnar, elongated along the c axis: short prismatic with a square or octagonal basal section. In plutonic and metamorphic rocks it occurs as subhedral to anhedral granular crystals. Qeliphytic margins (edges of reaction of the grain with the surrounding rock) may occur. Parallel inclusions (Schiller textures) may occur.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.008 to 0.015, 1st order colors: gray to yellow and 1st order orange. Colors increase with increasing Fe content.           

Extinction: Tend to be paralell.           

Elongation sign: ES(+)            

Twins:  Not very common, by {100}, simple and lamellar. They are not diagnostic.        

Zoning: Zonation is common in volcanic rocks, with Mg-rich cores.

CONVERGENT LIGHT

Character:  Enstatite (B(+), Bronzite B(+) or B(-), Hyperstene (B-) 

2V angle: increases with increasing Fe content. Enstatite: 54-83º, Bronzite: 97-63º, Hypersthene: 63-45º        

Alterations: very tough mineral. Alters to serpentine in ultramafic rocks (harzburgites) and in volcanic rocks, usually as pseudomorphs with oriented growth (bastite; bastitization in harzburgites: serpentine (lizardite) cleavages parallel [100] of orthopyroxene). Mesh textured chrysotile can also be formed, replacing orthopyroxene. Weathering forms carbonates, iron hydroxides or hematite.          

May be confused with: diagnostic are the two cleavages at almost 90º in the basal sections, the low birefringence and the moderate relief.

Clinopyroxenes have oblique extinction.

Epidote Group minerals show stronger and more anomalous interference colors.

Andalusite has SE(-) and lower refractive indices.

Kyanite has oblique extinction and perfect cleavage.

Melilite and scapolite are uniaxial.         

 

Enstatite 01: In PPL, large enstatite grain showing the first pleochroism color. The second color is shown in the next picture.
Enstatite 01: Also in PPL, the same enstatite grain of the previous image, now rotated 90 degrees and showing the second pleochroism color. High relief and cleavage are diagnostic.
Enstatite 01: The same grain of the two previous images, now in XPL, showing colors of the end of the first order.
Enstatite 02: Granulite in PPL with opaques (black), quartz and feldspars (colorless + low relief), clinopyroxenes and orthopyroxenes (both with the same type of cleavage, the same granular shape and the same high relief). But orthopyroxenes show pleochroism in weak pink tones, as shown by the crystal in the middle of the image. See next image.
Enstatite 02: The same section as of the previous image, also in PPL. Several grains of enstatite, especially the one in the middle of the image, show a pinkish color, notably different from the one of the previous image. Clinopyroxenes are not pleochroic.
Enstatite 02: The same section as of the two previous images, now in XPL. Enstatite show colors up to the end of the first order.
Enstatite 03: In PPL, in charnokite, grain of enstatite-Fe (formerly called “hypersthene”) with its weak but sharp pleochroism, between yellowish and green. Images are untreated (no photoshop) to enhance these pleochroism colors. This is the first color, the next image show the second pleochroism color.
Enstatite 03: In PPL, the same enstatite grain of the previous image, rotated 90 degrees to show the second color of pleochroism.
Enstatite 03: The same Fe-enstatite grain (formerly called “hypersthene”) from the above images, now in XPL, showing its typical low, 1st order interference colors. Extinction parallels cleavage.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of enstatite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with orthopyroxenes, like magnetite, ilmenite, pyrite, chalcopyrite and others.

Sample preparation: the polishing of orthopyroxenes follows the polishing of the associated feldspars and presents no problems.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, similar to feldspar and quartz, lighter than micas such as biotite and muscovite.       

Pleochroism: No.      

Reflectivity: Very low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: It is not possible to perceive anisotropy due to the internal reflections.        

Internal reflections:  Generalized darker or lighter, depending on grain thickness and orientation of the cleavage in relation to the plane of the polished section: cleavage parallel to the section plane (dark colors) or cleavage oblique to the section plane (light colors). They can be very light and luminous yellow to bronze to practically black (like augite, for example). Multicolored reflections are common. With a little care, one can find the typical bronze reflections of the orthopyroxene that was called “bronzite”.     

May be confused with: other transparent minerals of yellowish colors. Considering paragenesis, the possibilities of deception are greatly reduced.       

General Characteristics: 

Cleavage causes alignments of polishing pits in larger crystals.

Twins may be visible.   

Enstatite in PPL showing polishing pits aligned according to cleavage in one direction.
Enstatite in PPL showing polishing pits aligned according to the two cleavage directions that are typical for pyroxene basal sections.
Enstatite 01: Enstatite in PPL showing its dark gray color. There are no diagnostic features.
Enstatite 01: The same section as of the previous image, now in XPL. Enstatite is showing internal reflections in various shades of greenish yellow and bronze to very dark, almost black (left in the image). Quantity, intensity and tonality of internal reflections depend on the thickness of the mineral grain at that point, the existence and orientation of fractures and cleavages.
In XPL, enstatite (“bronzite”) showing twins. It is not such a common feature, but it is very easy to see.
In XPL, enstatite with multicolored reflections, whose occurrence depends on the orientation of the cleavages in relation to the plane of the polished section.