The name Enstatite comes from Greek, which means confrontation, and is named because of its high melting point.
Enstatite belongs to the pyroxene group, and is mainly produced in basic and ultrabasic igneous rocks rich in magnesium.
It belongs to the Orthorhombic crystal system; the crystal is columnar, usually with different crystal tops. However, most gem-grade crystals are produced in the form of pebbles, mainly from placers.
Enstatite is transparent to slightly transparent, with glass luster and pearl luster on the cleavage surface.
The main colors are reddish brown, brown-green, yellow-green and brown, and a few are gray and colorless.
With the increase of iron content, the crystal color will become dark, even black.
The fibrous inclusions are common in Enstatite, which can make brown gemstones have bronze-like luster after polishing. Cat’s eyes can be formed if there are a lot of directional fibrous inclusions.
India is the main producer of enstatite cat’s eye, and the color is mostly grayish yellow-green. High-quality transparent yellow-green gem-grade crystals are mainly produced in Myanmar and South Africa.
Colorful Enstatite is mainly produced in Canada. Other producing areas include Tanzania, Sri Lanka, China, the United States, etc.
- Chemical composition: (Mg, Fe) 2Si2O6
- Crystal system: orthorhombic crystal system
- Crystallization habit: columnar crystal
- Transparency: transparent to translucent
- Gloss: glass luster
- Refractive index: 1.663~1.673
- Birefringence: 0.008~0.011
- Optical characteristics: biaxial crystal, positive optical
- Polychromism: weak to medium
- Cleavage: two groups of complete cleavage
- Relative density: 3.25
- Mohs hardness: 5~6
- Color: red-brown, green-brown, yellow-green, colorless (rare)
- Main origin: Myanmar, India, South Africa, Tanzania, China, Sri Lanka, the United States, Canada
Basic properties of Enstatite
Chemical composition: the chemical formula of Enstatite is (Mg, Fe) 2Si2O6, and it can contain elements such as Ca and A1. The substitution rate of Fe and Mg can reach 1:1.
When the mass fraction of Fe is less than 5%, it is Enstatite, 5%~13% is bronzite, and more than 13% is hypersthene.
Crystal system and crystallization habit: orthorhombic crystal system, columnar, complete crystal form is rare.
Optical properties
A. Color
Enstatite is characterized by dark reddish brown to brownish green or yellowish green; Gray or colorless varieties are occasionally seen.
Bronzite is usually characterized by bronzing color. When there is fibrous inclusion, it will produce bronzing luster. Hypersthene is usually gray-green.
B. Transparency and luster
Transparent and semi-transparent, glass luster and pearl luster can be seen on the cleavage surface.
C. Phototropism
Enstatite is a biaxial crystal with positive luster.
D. Refractive index and birefringence
The refractive index of Enstatite is 1.663~1.673 (± 0.010). The higher the Fe content in the composition, the greater the refractive index value;
The birefringence is 0.008-0.011.
E. Polychromaticity
Weak to medium, and the pleochroism is brown-yellow, yellow to green, and yellow-green.
F. Luminescence
Fluorescence inertness under ultraviolet light.
G. Absorption spectrum
There is a strong absorption line at 505nm and a weak absorption line at 550nm.
H. Special optical properties
It can show the cat’s eye effect. The Enstatite with cat’s eye effect is generally brown and translucent.
Mechanical properties
A. Cleavage
The two groups of {110} cylinders have complete cleavage, and there are often fractures in the direction parallel to the bottom, with staggered fracture.
B. Hardness: Mohs hardness is 5~6.
C. Density: 3.25 (+0.15, － 0.02) g/cm.
Internal and external microscopic characteristics
It can be seen that gas-liquid inclusions and mineral inclusions. It can form a cat’s eye if there are a large number of directional inclusions.
How to identify Enstatite
General identification characteristics
Enstatite is magnesium silicate, its chemical formula is (Mg, Fe) SiO3, and less than 5% Fe often replaces Mg.
The content of Fe increases or even completely replaces Mg; it becomes Hypersthen [(Mg, Fe) 2Si2O6].
Enstatite belongs to the orthorhombic system. The crystal is usually short columnar, with a hardness of 5.5, and has obvious cylindrical cleavage. The angle of intersection of the two groups of cleavage is nearly 90 °.
The fracture is shell-shaped, with a relative density of 3.25, a refractive index of 1.65~1.68 (iron-rich hypersthene 1.71~1.73), and a birefringence of 0.010 (iron-rich hypersthene 0.016). The dispersion is weak.
Transparent to slightly transparent, glassy luster, positive dimorphism, and negative dimorphism for iron-rich hypersthene.
The colors are green, brown and colorless. For iron-rich varieties are green, green black and brown black.
Brown varieties have obvious pleochroism, and are green and light brown green.
Cat’s eye and starlight effects can be seen in some gem-grade enstatite. There is a strong absorption band at 506nm in the blue-green region under the spectroscope, which can be used as the identification feature.
- Spectroscopic observation: it has a typical spectrum and various colors (including colorless). There is a strong absorption line at 506nm in the blue-green area. In addition to this absorption line, there is a pair of absorption lines in the red area for chrome coloration. 506 nm absorption line is the diagnostic line of Enstatite.
- Refractometer measurement: positive optical property of biaxial crystal, refractive index 1.65~1.68, birefringence 0.010.
- Under the dichroic microscope: polychromatic, strong brown, weak green.
- Magnification observation: due to low hardness and poor surface wear resistance, stepped fracture can be seen at the fracture.
Identification of Enstatite and similar gemstones
Enstatite is easily mixed with chrysoberyl, tourmaline, olivine, Diopside, Kornerupine, etc.
It can be distinguished by refractive index, birefringence and absorption spectrum;
Its low birefringence can be distinguished from olivine and Diopside;
Spectral characteristics and pleochroism can distinguish it from Kornerupine.
1. Difference from olivine
Olivine has a refractive index of 1.65 ～ 1.69 and a relative density of 3.32 ～ 3.37, which is close to Enstatite (refractive index of 1.65 ～ 1.68 and a relative density of 3.20 ～ 3.30).
The main differences are:
1) The birefringence is different. The birefringence of olivine is 0.036, with double shadow; The birefringence of Enstatite is 0.010, with no double shadow.
2) The absorption spectrum is different: there are three typical absorption lines in the blue region for olivine; Enstatite has only one absorption line at 506 nm.
2. Difference from Diopside
The relative density of Diopside is 3.3, and the refractive index is 1.67~1.70, which is similar to that of Enstatite (relative density 3.20~3.30, refractive index 1.65~1.68). The main differences are:
1) The birefringence is different. The refractive index of Diopside is 0.025, and small prism double shadow is visible; the refractive index of Enstatite is 0.010, without double shadow.
2) The absorption spectra are different. Enstatite has an absorption line at 506nm, while Diopside does not.
3. Difference from Kornerupine
The refractive index of Kornerupine is 1.66 ～ 1.68, and the relative density is 3.25 ～ 3.35, which is close to Enstatite (refractive index is 1.65~1.68, relative density is 3.20~3.30).
The main differences are:
1)On refractometer:
enstatite: shadow boundary of high refractive index move obviously;
Kornerupine: βis close, When rotating the gem, the low refractive index shadow boundary moves.
2) Absorption spectrum:
Enstatite has an absorption line at 506nm; Kornerupine has a weak absorption zone in the blue-green area and a strong absorption zone in the purple area.
Origin of Enstatite
Enstatite is mainly produced in basic and ultrabasic rocks, layered intrusive rocks, igneous rocks, metamorphic rocks, and other rocks.
The gem-grade Enstatite Magu in Myanmar and Sri Lanka are mainly produced in the form of gravel.
Fresh green Enstatite is produced in Near Kimberley, South Africa, but the particle size is only 1-2 ct.
Enstatite cat’s eye mainly comes from Sri Lanka.
Brown crystals and starlight enstatite is produced in India. Other producing areas include Tanzania and Arizona in the United States.
Gem grade enstatite collection
- The Enstatite with weighing 12.97ct from Myanmar, is collected by the Royal Ontario Museum in Toronto, Canada;
- A Brown enstatite with weighing 11.0ct from Sri Lanka and a Brown enstatite with weighing 3.9 ct produced in Austria are collected by the Smith Museum, Washington, USA;
Conclusion:
Enstatite often coexists with olivine and spinel. Gem-grade crystals are found in basic igneous rocks and contact metasomatic rocks. These gemstones are produced in Mogu, Myanmar, India, South Africa, Austria and China.