Andalusite is an aluminum-containing silicate gem mineral. It is homogeneous with kyanite and sillimanite. The name “andalusite” comes from the place name where the andalusite mineral was first found in Andalusia, Spain. Since then, it has been found in Sri Lanka and Brazil that andalusite often contains elements such as iron, manganese, potassium, sodium and carbon. It belongs to the trapezi crystal system, similar to tetragonal prism, and has clear columnar cleavage.
The typical subspecies of andalusite are Chiastolite, which is rhombic columnar crystal. The carbonaceous and clayey materials attached to the near square or quadrangular cross-section in the horizontal section are arranged in a directional manner, forming a black cross-shaped dark band.
On the vertical section, it presents a black stripe in the same direction, which is also known as Christianite, across stone, etc.
Basic Property of Andalusite
Chemical composition
The chemical formula of Andalusite is Al2SiO5, which can contain trace elements such as V, Mn, Ti, Fe, etc. AL3+is often replaced by Fe3+and Mn3+.
Crystallographic characteristics
It is a trapezine crystal system, columnar, almost square cross-section;
It is usually a rhombic prism, rhombic pyramid and parallel double-sided combination form, with the dense longitudinal grain on the crystal surface;
The aggregate of andalusite is often granular and radial. The petals of chrysanthemum stone are also composed of andalusite (some are celestite).
Optical properties
1) Color: green, brown-green, brown-yellow, pink, brown-red, etc., colorless and purple are rare; Some green andalusite from Brazil is called Viridine because of their manganese content.
2) Gloss and transparency: glass luster, usually transparent to opaque.
3) Optical characteristics and polychromatic: orthorhombic system, biaxial crystal, negative optical property. The green and reddish brown andalusite have strong tricolor, which is usually visible to the naked eye. Generally, brown green/brown orange/brown red, and yellow-green/green/reddish brown can also appear; green/olive green/carnation (Brazil); blue/light blue/colorless (Belgium), etc.
4)Refractive index, birefringence and dispersion: the refractive index is 1.634~1.643 (± 0.005), but the refractive index of green andalusite can reach 1.660~1.690 due to manganese content;
The birefringence is 0.007 to 0.013, and the birefringence of Viridine is 0.029. Very low dispersion.
5)Absorption spectrum: green, reddish brown andalusite shows the absorption spectrum of iron, with absorption peaks at 436 and 445 nm;
The dark green Viridine has absorption bands at 555.3 and 550.5 nm in the yellow to green region, 547.5 nm in the green region, 518 and 595 nm in the blue to green region, and 455 nm in the purple region.
6)Fluorescence: Fluorescent under long wave ultraviolet fluorescent lamp, green to light yellow-green fluorescence under short wave ultraviolet fluorescent lamp.
The andalusite produced in Massachusetts shows brown fluorescence under the short wave ultraviolet fluorescent lamp;
The brown-green andalusite produced in Brazil shows dark green or yellowish green fluorescence under the short wave ultraviolet fluorescent lamp.
Mechanical properties
There is a group of medium cleavage parallel to {110}, {100} is incomplete, and two directions intersect at nearly right angles; Shell or uneven fracture; The density is 3.17 (± 0.04) g/cm; The Mohs hardness is 7~7.5.
Zoom in
Andalusite can contain various crystal inclusions: short columnar apatite, needle-shaped rutile, muscovite, quartz, etc.
Black carbonaceous inclusions can be seen in the crisscross distribution of the Chiastolite. There are also growth structures such as ribbons, cleavage and double crystal lines are also common.
Variant types
According to the appearance, morphology and structure of andalusite, it can be divided into the following varieties:
1) Chiastolite.
It is produced as columnar crystal, translucent and grayish brown. Some andalusite captures fine carbonaceous (graphite) and clay mineral particles in the growth process, forming black cross inclusions on the cross-section.
Black cross lines can be seen on the vertical section, which is called “chiastolite.”, also called “cross-stone.”
Chiastolite is generally translucent flesh red. Because there are many inclusions, the hardness of Chiastolite is reduced to 5 to 5.5, and the relative density is 3.10 to 3.20. Massachusetts, Russia, Siberia and South Australia are famous producing areas of achiastolite and the Xixia of Henan Province in China also produces better chiastolite crystals.
2) Chrysanthemum stone
The andalusite aggregate sometimes grows radially and looks like chrysanthemum, so it is called chrysanthemum stone. There are two types of chrysanthemum stones in China On the market.
One is Beijing Xishan andalusite chrysanthemum stone. The rock base is black carbonaceous slate, in which gray white radial andalusite aggregates are densely distributed;
Another chrysanthemum stone is mainly produced in Hunan, is composed of petals consisting of lapis lazuli or calcite , which are distributed radially and symmetrically to form white flowers, and is produced in limestone.
3) Manganese-containing andalusite chrysanthemum stone
Dark green andalusite with manganese absorption spectrum, produced in Brazil.
Identification of andalusite
Optimization and identification
The color of andalusite can be changed or improved by heat treatment. The green andalusite is pink after heating, and the color is stable.
The difference between andalusite and similar gemstones
Gems similar to andalusite in appearance include tourmaline, topaz, Alexandrite, Danburite, apatite, etc.
Especially, andalusite has a similar refractive index to tourmaline, topaz, and apatite; we can distinguish them according to andalusite has a special strong trichromatic, axial, refractive index, birefringence, relative density and characteristic inclusions.
The difference between tourmaline and andalusite:
Andalusite has tricolor, while the tourmaline can be seen in double shadow.
Quality evaluation of andalusite
Andalusite crystal can be evaluated from four aspects: color, clarity, cutting and size.
Generally, andalusite crystals are small, with large block size, bright color, good purity and no impurities are rare. Good cutting ratio and polishing can maximize the luster and brightness of andalusite.
Chrysanthemum stone requires beautiful petals and clear boundaries. The whiter the chrysanthemum, the higher the quality; The base is free of cracks, fine and dark.
It is a high-quality gem if Chiastolite meets clear black cross-boundary, uniform distribution, bright color, and few cracks. It is generally cut and ground into arc or bead shape to show its special inclusion shape.
Occurrence and origin of Andalusite
Andalusite is a typical aluminum-rich mineral. Gem grade andalusite comes from placer ore.
Brazil is an important source of gem-grade andalusite. The main producing areas are Sri Lanka, Madagascar, Myanmar, France, Spain and the United States, Xinjiang, Henan and Beijing in China.
World Treasure Collection
Smithsonian Museum, Washington, USA: brown andalusite, weighing 28.3 ct, produced in Brazil; Green and brown, weighing 13.5ct, produced in Brazil. Royal Ontario Museum, Toronto, Canada: 12.44ct, produced in Brazil.
Andalusite value
The andalusite produced in Spain, Austria, Brazil, Sri Lanka and Australia has a gem-grade hardness, color and transparency. However, perhaps the brightness of the color is low, so it can never be a famous gem.
The andalusite has a strong and unique pleochroism. Therefore, when the crystal is rotated, it will show three colors: red, yellow and green; different places of origin, and different colors flash out.
When the “black cross pattern” appears on the square section of the andalusite crystal, it is called “black cross gem .”The black cross-stone is a precious gem many Christians regard as mascots. It is called Chiastolite in mineralogy.
There are few large andalusite particles on the market, usually smaller than one carat. Andalusite without Impurities is rare and usually are some needle-like inclusions.
The largest andalusite without Impurities is about 4 carats on the market, with a price of $100 per carat, which is a gem collected by experts.
The output of andalusite is small, which is not available in general jewelry stores. If you want to collect these rare gems, you should go to the famous gem market.
Conclusion:
Andalusite is named after the region where it was first discovered, Andalusia Province, Spain. The most remarkable feature of andalusite is that its pleochroism is so strong that you can simultaneously see the color from yellow-green to orange-brown.
At the same time, the pleochroism characteristics of yellow, green and red colors are desirable. It is exactly the unique humanistic feelings of Andalrucia-passion.
Andalusite’s pleochroism and its affordable price make it known as “Poor Man’s Alexandrite” in ancient times.
The polychromatic of andalusite presents several different colors under the same light source, which is different from that of Alexandrite because it can display different colors by switching different light sources.
Andalusite also has a variety called Chiastolite, also known as “Cross Stone,” which has a dark cross pattern in its structure because of its carbon content in its crystal appearance. Such gemstones were cut into flat sheets to show the cross pattern and once became very popular religious protectors.
In addition to Spain, other places of origin are Brazil and Sri Lanka. Chiastolite is mainly produced in Siberia, Australia, the United States, Zimbabwe, etc.