.

Andalusite Mineral Facts:

Chemical Formula: Al₂Si0₅  
Aluminium silicate, a basic orthosilicate. Sillimanite and kyanite both have the same empirical chemical composition but are  different minerals.

Colors: Red, reddish brown, olive-green.
The mineral, when fresh, is greenish or reddish and transparent. Usually, however, it is normally altered, and is opaque, or, at most, translucent, and gray, pink or violet. Some specimens posses marked pleochroism with colors being an olive-green parallel to a, oil-green parallel to b and dark red parallel to c.

Hardness: 7.5

Density: 3.16 to 3.20

Cleavage: Its cleavage is good parallel to (110).

Crystallography: Orthorhombic
Usually in coarse, nearly square prisms. Closely related crystallographically to topaz.

Luster:. Vitreous luster; Transparent to opaque.

Optics: (Refractive Index):  a=1.6326, b=1.6390, y=1.6440.

  Above: Andalusite

.
Composition, Structure and Associated Minerals:
Andalusite is found principally in clay slates and schists that have been metamorphosed by contact with igneous masses, and
to a less extent in some gneisses. Andalusite forms under low pressures and moderate to high temperatures. Because Kyanite and sillimanite form at different temperatures, the three different morphs can be used to determine the range of heat and pressure the rock was exposed to. It is often impure and commonly at -least partly altered. Some specimens contain as inclusions large quantities of a dark gray or black material, which may be carbonaceous, arranged in such a way as to give a cross-like figure in cross-sections of crystals. Because of the shape of the figure exhibited by these crystals, this variety was early called chiastolite, and was valued as a sacred charm.
Andalusite alters readily to kaolin, muscovite, and sillimanite.
 

Andalusite, Variety Chiastolite

Identification and Diagnostics
Before the blowpipe the mineral gradually changes to sillimanite and is infusible. When moistened with cobalt nitrate and roasted it becomes blue (test for the presence of aluminum). It is insoluble in acids. The mineral is distinguished by its nearly square cross-section, its
hardness, its infusibility, and the reaction for Al, and by its manner of occurrence in schists and metamorphosed slates.

Occurrence, Localities and Origins:
 Notable localities are in Andalusia, Spain; Braunsdorf, Saxony; at Gefrees, in the Fichtelgebirge; the Tyrol; water-worn clear, gem quality pebbles come from the state of Minas Gerais, in Brazil. Chiastolite is found in Morihan, Brittany and Bimbowrie, South Australia. In the United States at Standish, Maine; Westford, Lancaster and Sterling, Massachusetts; Litchfield and Washington, Connecticut; Delaware County, Pennsylvania. The chiastolite variery occurs at Lancaster and Sterling, Mass..  and Massachusetts.

Use: When clear and transparent may serve as a gem stone. The stone can be cut so that it appears to be different colors when viewed in different directions because of its strong dichroic character. It is an unusual collector's gemstone and not often seen.

Return to the Mineral Collectors Information Page

Nevada Outback Gems