Sillimanite is named after Professor B. Silliman of Connecticut, USA. The other word Bucholzite (ByxOJIbIIHT), is also a fibrous subspecies of Sillimanite, named after the German chemist C.F. Bucholz.
Sillimanite, andalusite and kyanite are homogeneous and polymorphic varieties with the same composition.
It is mainly used to manufacture high alumina refractory materials and acid-resistant materials.
Those with bright colors can be used as raw materials for gemstones. Single crystals can be grinded into faceted gemstones.
Basic Properties of Sillimanite
a. Mineral name
Sillimanite belongs to the family of Sillimanite.
b. Chemical composition
Al2 [SiO4] O often contains a small amount of Fe, Ti, Ca, Mg and other trace elements.
c. Crystal system and crystallization
Orthorhombic crystal system, the crystal is in the form of column or fiber extending parallel to the Z-axis, with no crystal plane at both ends, and the cross-section is nearly square rhombic or rectangular. The cylinder surface has stripes. The aggregates are radial or fibrous.
d. Optical properties
1. Color
The common colors of Sillimanite are white to gray, brown, green, and occasionally purple-blue to gray-blue.
2. Gloss and transparency
Glass luster, some varieties have silk luster, translucent to transparent.
3. Optical property
The Sillimanite is biaxial crystal positive light, and the fibrous aggregate shows aggregate characteristics under the orthogonal polarized light.
The light film is generally colorless, sometimes with light brown and light blue tones. Np=1.657 ～ 1.660, Nm=1.658 ～1.661,Ng=1.677~1.682,Ng-Np=0.020～0.022.
The highest interference color is Grade II blue. It is parallel extinction. Positive ductility. Two Axis (+), 2V=21 °~30 °.
4. Refractive index and birefringence
The refractive index is 1.659 ～ 1.680 (+0.004, － 0.006); The birefringence is 0.015~0.021. The refractive index is 1.66, which can be as low as 1.64.
e. Pleochroism
The pleochroism of white to grayish white Sillimanite is not obvious; Blue Sillimanite can have strong pleochroism, and the pleochroism colors are colorless, light yellow and blue.
f. Fluorescence
blue Sillimanite Under ultraviolet light can have weak red fluorescence, and other color varieties show fluorescence.
g. Absorption spectrum characteristics
There can be 410nm, 441nm and 462nm weak absorption lines in the visible light spectrum.
h. Change:
easily altered into mica and clay minerals
Mechanical properties
- Cleavage: A group of complete cleavage {010} can be seen in the Sillimanite.
- Hardness: The Mohs hardness is 6~7.5.
- Density: 3.25 (+0.02, – 0.11) g/cm, translucent gemstones can be as low as 3.20g/cm.
Microscopic characteristics
Rutile, spinel, biotite and other inclusions can be seen in crystal.
A group of fibrous inclusions arranged in parallel can be seen in Sillimanite cat’s eye, and the sillimanite cat’s eye in Sri Lanka is caused by the arrangement of fibrous hypersthene and some rutile needles. Sillimanite can also show cat’s eye effect by a fibrous aggregate.
Special optical effects
Common cat eye effect
Type of Sillimanite
Sillimanite cat’s eye
It is a common gem with cat’s eye effect in the market, mainly from India and Sri Lanka. It is composed of fibrous aggregates of Sillimanite, often grayish green, containing black inclusions. The inclusions are magnetite and hematite.
The light band of cat’s eye is usually wide and fuzzy, and the effect is poor; However, occasionally, there are beautiful yellow-green and cat’s eyes with very clear light bands, which are indistinguishable from Chrysoberyl cat’s eyes. But the sillimanite cat’s eye has a low refractive index and low relative density than that of Chrysoberyl cat’s eyes.
Fibrolite
It refers to Sillimanite in fibrous or massive aggregate.
Synthetic Sillimanite
Under the high pressure of about 1 million kPa and the high temperature of 900 ℃, the Sillimanite can be synthesized by adding fluorine to kaolinite.
Identification of Sillimanite and Similar Gemstones
a. Identification of Sillimanite and Similar Gemstones
The appearance of grayish brown Sillimanite is similar to that of smoke crystal, which can be distinguished by its high refractive index and density.
The fibrous inclusion of the Sillimanite is usually an obvious identification feature, which can be confused with the Prehnite, but can be distinguished by the refractive index and density.
b. Differentiation of Sillimanite Cat’s Eye from Similar Gems
Sillimanite cat’s eyes are common, usually grayish green, brown, gray-white, translucent to opaque, rarely transparent; and the fibrous structures or fibrous inclusions can be seen under magnification.
Point measurement of refractive index 1.66. Four bright and four dark or collective polarized lights can be displayed under the polarizer.
1. Tourmaline cat’s eye
Tourmaline cat’s eye is usually blue, green and pink in color. Its transparency is higher than that of Sillimanite cat’s eye, and coarse tubular inclusions can be seen with the naked eye.
Point measurement of refractive index 1.64. It often shows four bright and four dark under the polarizer.
2. Apatite cat’s eye
Apatite cat’s eyes are usually yellowish green and brownish green, with thin tubular inclusions visible when magnified.
Point measurement of refractive index 1.63 or 1.64. The visible characteristic spectrum is a 580nm double line. It often shows four bright and four dark under the polarizer.
3. Actinolite cat’s eye
The actinolite cat’s eyes are usually green and yellowish green, without obvious inclusions when magnified.
Under the light, it shows a “milky white honey yellow” color (also called “milk honey” phenomenon).
Point measurement of refractive index 1.62 or 1.63. the Visible characteristic spectrum is a 505nm absorption line. Collective polarization is often displayed under the polarizer.
Occurrence and origin of Sillimanite
Sillimanite is a typical metamorphic mineral, which is widely distributed. It is commonly found in the contact zone between igneous rocks (especially granite) and aluminum-rich rocks, as well as in the areas where schist and gneiss are developed.
The Sillimanite is very stable during the weathering process, so it is common in alluvial placer, eluvium and broken bases.
The green, blue and violet Sillimanite mainly come from Mogu in Sri Lanka and Myanmar, and the gray-green Sillimanite cat’s eyes are produced in Sri Lanka. Colorless and light blue Sillimanite is produced in Kenya.
World Treasure Collection
- Smithsonian Museum in Washington, USA: black Sillimanitecat’s eye, 5.9ct, produced in South Carolina, USA.
- Natural History Museum, London, England: fibrous, 35ct. Geological Museum, London, England: fibrous, 17ct.
Sillimanite in China
Sillimanite jade produced in Liaoning mainly refers to the gemstones produced in Huanren and Dandong, commonly known as blue and white jade.
The main components of Sillimanite jade are Sillimanite mineral aggregates and mica, kaolin and other impurities. It is different from the common gem-grade sillimanite cat’s eye (white, gray, brown translucent to transparent single crystal) in the markets.
It is more common in the local market of Huanren County, the main producing area.
In addition to the raw stone of Sillimanite, it is commonly used to make bracelets, carvings and other finished products.
Characteristics of Sillimanite jade
a. Basic properties
1. Mineral composition and microscopic characteristics
Huanren Sillimanitejade jade is composed mainly of Sillimanite mineral aggregate, mica, kaolin, and other impurity minerals.
The content of Sillimaniteis generally more than 96%, containing a small amount of biotite and kaolinite.
Sillimanite mineral is a columnar fibrous crystalloblastic structure, which is mainly fibrous and columnar, with locally hairy and uneven particle size.
It is locally fine, with mineral particle size of less than 0.1mm, and locally coarse, with a maximum mineral particle size of about 0.5mm.
Due to the uneven of crystal size and structure, the transparency of Sillimanite jade is also uneven. The fine part of mineral particles has good transparency, while the coarse part has poor transparency.
Under the single polarizing microscope, it shows light brown, partial brown-yellow, and the overall pleochroism is not obvious.
Some brown yellow minerals have obvious pleochroism, ranging from light yellow to brown yellow.
The interference color is first-grade yellow to first-grade dark gray under orthogonal polarization and parallel extinction, and the second-grade bright interference color of biotite can be seen locally.
Sillimanite jade produced in Dandong area is mainly composed of Sillimanite and a small amount of mica, limonite, carbonaceous, epidote (locally distributed), actinolite (locally distributed) and other minerals.
The biotite and Sericite are in the form of fine scales, the carbon is mostly in the form of small points, epidote is in the form of granules, and actinolite is in the form of columns, and the aggregate is in the form of radiation and fibers.
From the mirror, except epidote and actinolite, the main minerals in the rock-Sillimanite, biotite, Sericite and carbonaceous are all arranged in a certain direction. Sometimes they are gathered into thin strips and extend in the same direction, and some are twisted into small wrinkles, forming the so-called phyllitic structure.
Color:
Huanren Sillimanite jade is mostly gray white and gray black, and some samples have black, brown and yellow mottled distribution characteristics.
The color is generally uneven, with black spots and white flocculent inclusions, and the distribution is uneven, with no direction arrangement.
Sometimes with green lumps, Fibrous and radial structures can also be seen locally in some samples; The mineral particles are thick, columnar and flaky.
Sillimanite jade produced in Dandong is white, light yellowish-green, dark-green, light-grayish yellow, light grayish white, etc. The texture in the stone will be mixed with its variegated color.
b. Quality and identification
Sillimanite jade produced in Huanren County, Liaoning Province, is mainly composed of the aggregate of Sillimanite minerals.
Its appearance is obviously different from that of Nephrite and Xiuyan nephrite. It can be identified by observing its color, luster, structure, mineral particle size, and moisture; it can also be accurately identified by density tests, spectroscopy tests, and other methods.
1)Compared with amphibole jade, the Quality of the Sillimanite jade produced by Huanren is obviously inferior.
2)Generally, the most common Sillimanite jade structure is blanket -like fiber interwoven structure, and the particle size of the main mineral of the jade is generally less than 0.01 mm, which means it should be a cryptocrystalline structure.
The mineral grain size of the main part of Huanren Sillimanite jade is 0.05-0.50 mm, which is obviously too coarse and uneven.
Generally speaking, the thicker the mineral particles are, the more disorderly they are arranged, the greater the spacing between particles is, and the more internal reflective interfaces are.
When light travels between different particles, the refraction and scattering are enhanced, reducing the light transmission capacity and transparency.
3)The Smoothness of Sillimanite jade is poor, and the grain feeling is very obvious; Fibrous and radial structures can also be seen locally in some samples;
The mineral particles are thick, columnar and flaky; Some samples have yellowish brown shells, and the mineral composition and structure of the shells are consistent with the internal jade.
4)A large number of coarse grain Sillimanite, mica minerals and local unequal grain structure in Huanren Sillimanite jade results in poor fineness and uneven color.
At the same time, the mottled distribution of biotite minerals forms black spots, which also reduces the Quality of the jade.
Fe+content in jade is related to black. The higher the Fe+ content, the darker the color. The Quality of dark color is obviously inferior to that of light color.
However, the black spots of Sillimanite jade are of great significance for identifying the origin.
In the process of processing, we can use their color differences to make some beautiful handicrafts, which can be regarded as an exclusive feature.
5) Dandong’s Sillimanite jade is delicate and smooth, especially with rich colors. The light gray or white jade tendons are slightly convex, while the concave part is black ink dots. It gives a blue and white feeling as a whole.
Conclusion:
Sillimanite is a kind of aluminum-containing silicate mineral, which is isomorphic and allomorphic with andalusite and kyanite.
It belongs to the Orthorhombic crystal system; the crystals are needle-shaped, mostly fibrous or radial aggregates and massive aggregates.
The color is blue, blue-green, brown-green, yellow, black, gray-white, white and other colors;
Sillimanite is a mineral formed during thermal metamorphism at a higher temperature than andalusite. It is mainly found in the high-temperature contact metamorphic belt between argillaceous rocks and extrusive rocks or granites. It often coexists with iolite and garnet.
Most Gem grade crystals are taken from placers. Among them, green, blue and violet gemstones are mainly produced in Sri Lanka and Myanmar;
Light blue and colorless gemstones are mainly produced in Kenya; Grey green cat’s eye Sillimanite is mainly produced in Sri Lanka and Madras, India;
There are felt blocks and fibrous blocks in Idaho, USA. This slightly transparent grayish-white to slightly brownish-green block stones are very similar to low-quality jadeite, so they are often used as jadeite.
In terms of physical property, except for the relative density is lower, they are very similar to jadeite, which is difficult to distinguish between them;
However, it can be distinguished by spectroscopic spectrum, and jadeite jade often has 437nm absorption, while Sillimanite has weak absorption lines of 410nm and 462nm.