Sunday, 30 June 2013

Introduction to Andalusite (Al2sio5)

AndalusiteANDALUSITE (Al2sio5)

Silicate of aluminium.

Crystal system

Orthorhombic

Appearance

It occurs as squat prismatic crystals with poor luster, or aggregates of elongated crystals arranged in "sheaves.' The color varies from light yellowish brown to green-brown, light brownish pink, grayish green or definite green with pronounced pleochroism making it hard to identify the main color. Some crystals have dense dancer inclusions in the center which in cross section, exhibit a cruciform pattern. These stones are known as chiastolite

Physical properties

It has a hardness of 7-7.5 and a density of about 3.13-3.20 9icrn3. The refractive indices are about no 1.632, fly 1.643. It has distinct prismatic cleavage.

Genesis

Andalusite groupAndalusite is a characteristic mineral of areas of contact metamorphism between granites and argillaceous rocks.

Occurrence

It is not a rare mineral. Fine crystals are found in Spain (Andalusia), whence the name. Other sources are Brazil, the United States (California, Maine, Massachusetts), Australia, Sri Lanka, Burma, Austria, and occasionally Italy.

Andalusite

This gem is named after Andalusia, where fine maul occurs.

Appearance

It is a dull yellowish green with faint pinta brown reflections, or it may be yellowish brown-greens even violet brown-green or, rarely, a definite green lie difficulty in describing the color of this gem is because its strong pleochroism, which is heightened by the facets/ Cut stones, to the point of being almost iridescent; bulls colors are never very lively. It has unexceptional, vitro luster. It is regularly given a faceted, mixed oval square or rectangular step cuts are also used, but there are less suited to its modest luster and cannot justice its exceptional pleochroism.

Distinctive features

Its peculiar color range, unusual pleochroism, and modest luster make it quite easy to distinguish from all other gems. Some say it can be conk with alexandrite, but the pleochroism of andalusite is very different from the true color change of the other. It she that when alexandrite is a yellowish green with a fantods change, it may look vaguely similar; but checking thirsty would immediately distinguish between the two stones. The chiastolite variety with its black cross-shape is unmistakable.

Occurrence

The gem variety comes mainly from Brad (state of Minas Gerais), but also Sri Lanka, Burma, the Soviet Union, the United States and, of course. Andalusia

Value

Chiastolite
Quite low, probably because of the rather drab color and poor luster, It is anything but a rare stone, yet it is not plentiful on the market and is mainly in demand with collectors. The most valuable stones are those with the most pronounced greenish to reddish pleochrotsm chiastolite variety has a longer history of gem use thanes transparent type, having been worn as an amulet, but Is value is equally low and similar to that of opaque ornamental materials.

Simulants and synthetics

Not being valuable or wet known, it is not imitated. Nor has it been produced synthetically, at any rate on a commercial scale.

Writer – Curzio Cipriani and Alessandro Borelli

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