andalusite

n.
A mineral, aluminum silicate (Al₂SiO₅), usually found in orthorhombic crystals of various colors.

[After ANDALUSIA.]

A nesosilicate mineral, composition Al₂SiO₅, crystallizing in the orthorhombic system. It occurs commonly in large, nearly square prismatic crystals. There is poor prismatic cleavage; the luster is vitreous and the color red, reddish-brown, olive-green, or bluish. Transparent crystals may show strong dichroism, appearing red in one direction and green in another in transmitted light. The specific gravity is 3.1–3.2; hardness is 7.5 on Mohs scale, but may be less on the surface because of alteration. See also Silicate minerals.

Andalusite was first described in Andalusia, Spain, and was named after this locality. It is found abundantly in the White Mountains near Laws, California, where for many years it was mined for manufacture of spark plugs and other highly refractive porcelain. Chiastolite, in crystals largely altered to mica, is found in Lancaster and Sterling, Massachusetts. Water-worn pebbles of gem quality are found at Minas Gerais, Brazil.

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Al
Orthorhombic -- Rhombic bipyramidal

Environment

Metamorphic rocks and in contact-metamorphic zones near granite intrusions; grossly different in different parageneses.

Crystal description

Coarse, embedded crystal masses, usually dull-surfaced with blunt box-like terminations, and generally slightly altered on the surface so any original luster is lost. A variety known as chiastolite forms rounded fossil or cigar shapes, bulges wrapped in dark schist. In cross section, each rod displays a pattern of light and dark areas, apparently created as the growing crystal thrust dark foreign particles into definite areas. The pattern of checkerboard squares changes in successive slices through the length of the crystal. Smaller gemmy, waterworn andalusite crystals have been found in Brazil and Sri Lanka (Ceylon), but their parent is unsure; no such crystals have been collected from a matrix.

Physical properties

Gray, pink, brown, white, and emerald green. Luster glassy; hardness 7Ɖ; specific gravity 3.1-3.2; fracture conchoidal; cleavage fair to good prismatic. Transparent to translucent. Uncommon green andalusite, known as viridine, is strongly dichroic in yellow and green; gemmy white varieties are very dichroic in green and brown.

Composition

Aluminum silicate (63.2% Al ₂ O ₃ , 36.8% SiO ₂ ).

Tests

Infusible and insoluble, but powder is slightly colored blue by strong firing after it is moistened with cobalt nitrate (aluminum test).

Distinguishing characteristics

The variety chiastolite is easily recognized. The altered, mica-wrapped cigar shape and dull surface with the square cross section (dominant prisms are virtually at right angles to each other) are very distinctive. The bluish cobalt nitrate coloration test is more easily obtained with kyanite and sillimanite powders than with andalusite.

Occurrence

Like the next two aluminum silicates, andalusite can be used in spark plugs and other porcelains requiring high heat resistance. The clear material makes an interesting gem because of its two-color effect. Chiastolite is found in Fresno, Kern, and Mariposa counties, California, and in boulders in farmers' stone walls near Lancaster, Massachusetts. Gemmy pinkish andalusite grains from shattered small crystals are found in the mica schist of Mt. Washington, New Hampshire. Great gray to white masses were mined at White Mountain, Laws, Inyo Co., California. Good opaque crystals have come from Standish, Maine, and Delaware Co., Pennsylvania. The viridine Brazilian variety is said to be manganiferous.

Gemmy andalusite, in somewhat waterworn pebbles and crystals, comes from Minas Gerais in Brazil and from Sri Lanka (Ceylon). As a gemstone, it is distinctive because of the two colors it shows in the sides and ends of the cut stone: grayish green sides and reddish brown ends in cushion-cut stones, their best shape.

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