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Andalusite



andalusite


Crystal system Orthorhombic
Transparency Transparent to opaque
Luster Vitreous
Fracture Uneven
Cleavage Distinct Prismatic
Specific Gravity 3.17 ± .04
Hardness 7 - 7.5
Optical Character Biaxial - ; Double Refractive
Refractive index 1.634-1.643
Birefringence 0.008 - 0.013
Fluorescence Strong, Brownish-red & green
Pleochroism Green to yellowish green
Chemical Formula Al2SiO5
Comments Powder dissolves slowly in HCL, somewhat faster in H2SO4

Andalusite is on the borderline between what could be called a jewelry stone and the type of material that is more associated with hobbyists and serious collectors than with jewelry. Although the mineral was named for the Andalusian area of Spain, where it was first found, it was not until gem qualities were discovered in Ceylon, and particularly Brazil, that it first became noted as a gemstone. Since it has the necessary hardness, color, transparency and, to some, the requisite beauty, about the only reason why andalusite could not become a gemstone of some prominence is its fairly low color intensities.

Andalusite varies in color from slightly brownish green to brownish red. It is very strongly dichroic, the two colors being slightly brownish green and reddish-brown or brownish-red. Sometimes it is almost a pure green, with little of the other dichroic color. The dichroic colors are somewhat reminiscent of two of the three colors of alexandrite; however, andalusite does not exhibit the daylight to incandescent light color change of alexandrite. Andalusite is very rarely brown, pink or violet. One variety, chiastolite, or macle (ki-ASS-tow-lite; MACK-el) is a curious opaque stone that has a crossed dark band in its center on a white, gray, reddish or light brown background. This cross, which varies in size and shape from one end of the crystal to the other, is caused by a concentration of carbonaceous impurities. Chiastolite is derived from a Greek word meaning "arranged diagonally"; macle comes to us from the Latin word "macula", meaning "blemish" or "spot". It is often sold as a curio stone or a good luck piece. As might be expected, this unusual stone was regarded as having mystical and religious significance by early peoples. If worn so as to touch the skin, it was said to staunch the flow of blood. It was also thought to increase the secretion of milk. If chiastolite were worn suspended from the neck, it was thought to cure almost any kind of fever, and the divine symbol it bore served to drive away evil spirits from the vicinity of the wearer.

Andalusite is usually found as a contact mineral in clay slates that have been metamorphosed from shale as are sult of granitic intrusions. It is often associated with sillimanite, iolite, garnet, corundum and tourmaline. Sources include Minas Gerais, Brazil; the gem gravels of Ceylon; Andalusia, Spain; and in the United States in California, New Hampshire, Maine and Pennsylvania. Massachusetts and Siberia are noted localities for chiastolite. Impure andalusite has been mined for use in making the porcelain of spark plugs.

The chemical formula for this gem mineral is Al2SiO5,an aluminum silicate. Together with kyanite and sillimanite (fibrolite), andalusite illustrates trimorphism, for all three minerals correspond to the same formula. It crystallizes in the orthorhombic system, but the crystals are almost square in cross-section and have basal faces; they resemble tetragonal prisms more than orthorhombic pinacoids. The hardness is 7 to 7 1/2, the toughness is fair to good, and the cleavage is distinct (prismatic). The fracture is uneven (subconchoidal), the specific gravity is within a plus or minus .04 of 3.17, and the streak is white. Inclusions may be irregular, dense clouds of minute particles consisting of curved, elongated liquid inclusions or tiny fissures. Narrow striations parallel to the basal plane of the crystal or hair like hexagonal crystals oriented parallel to the C axis may also be noted.

Andalusite ranges from transparent to opaque, fracture surfaces have a dull to vitreous luster, and polished surfaces are vitreous; the refractive indices vary from approximately 1.628 - 1.640 for the low index to 1.641 - 1.647 for the high index, and the birefringence varies from .008 to .013 (highest when the indices are lowest). The optic character is biaxial negative. Pleochroism is strong (the red color mentioned above is visible in a direction parallel to the prism edge, and at right angles to this the strong color is green), the dispersion is fairly weak, and no phenomenon is exhibited. It is not attacked by acids and is infusible in the flame of the jeweler's torch or the blowpipe.

The two stones most likely to resemble andalusite are tourmaline and alexandrite. Alexandrite and its imitations can be distinguished readily by their higher refractive indices and specific gravities. The R.I. and S.G. of tourmaline are in the same general range as those of andalusite, but it is separated easily from andalusite by its considerably higher birefringence and the fact that one index is constant at 1.644, in contrast to the variability of both indices in andalusite.

Transparent material usually faceted in the step or brilliant style. Since cleavage is not troublesome and it is not heat sensitive to any phase of the fashioning process, only the usual precautions are required. Because of the strong pleochroism of this mineral, however, careful orientation is necessary. In the crystals possessing a red and green pleochroic color, if both colors are desired in the finished stone, the table should be oriented parallel to the red color; in this way, the red color will be visible through the center of the stone and the green will appear around the edges. If the stone is oriented so that the table is at right angles to the red color, its color will be predominantly red, and the green color will be visible only when observation is made through the girdle. Linde A powder on a tin lap produces an excellent polish. Suggested brilliant-cut facet angles are 43° for the crown and 39° for the pavilion. Chiastolite is always cut en cabochon. Because of the low hardness of the carbonaceous material forming the cross, it undercuts easily; therefore, it is usually wet sanded with a very fine sanding cloth and then polished on a leather lap with Linde A powder as the polishing agent. In order to obtain the dark cross, the rough must be oriented so that the base of the cabochon is at right angles to the length of the crystal.