The Guide to Rocks and Minerals

In focus: Andalusite

Chemical Composition: Al2SiO5
Group: Silicate
Colour: Dark green, Grey, Brown, Red, Green
Streak: white
Hardness: 6.5-7.5
Density: 3.1-3.2
Luster: Vitreous
Cleavage: Distinct in one direction

Andalusite is an anhydrous aluminosilicate mineral (Al₂SiO₅). It was first found in Andalusia in Spain, which is why it has its name, though it is actually a relatively common mineral widely distributed in many countries including France, Austia, Germany, USA South Africa, Australia and Brazil. Andalusite shares its chemical structure with two other minerals, namely kyanite and sillimanite. These three minerals are called polymorphs, because although their chemical structure is the same, their crystal structures are different. A unique variety of andalusite is called "chiastolite". This contains black or brown clay and/or carbonaceous material inclusions in the crystal. These inclusions are arranged in regular symmetrical shapes usually in the form of a cross or X.

Andalusite is found predominantly in sedimentary rocks where it is associated with kyanite, sillimanite, cordierite, corundum, garnet, tourmaline and mica. Chiastolite is common in some metamorphic rocks. Water-worn pebbles of gem quality are found at Minas Gerais, Brazil.

After calcining (a thermal treatment process in which ores and other solid materials undergo thermal decomposition, phase transition, or removal of a volatile fraction) at high temperatures Andalusite is converted into mullite (3Al₂O₃.2SiO₂) and silica. These are refractory materials (stable at high temperature) and are therefore used in the production of furnaces, boilers, ladles, and kilns, in metallurgy, and among others, in the glass, chemical, and cement industries. One metric ton of mineral concentrate following calcining yields about 0.88 t of mullite.

Mineral uses

as gemstones especially from the water-polished pebbles found in Brazil
manufacture of highly refractive porcelain
used in producing furnaces, boilers, ladles, and kilns
collector's items

Andalusite comes in a number of colours, namely: gray, pink, brown, white, and emerald green. Green andalusite, known as viridine, is rare. Viridine is strongly dichroic in yellow and green. The white variety of andalusite is also strongly dichroic in green and brown. Minerals which are dichroic display two colours when light passes through them; one by transmitted light and the second colour by reflected light.

Identification

Andalusite is infusible and insoluble, but the powder is slightly coloured blue when fire is applied to it after it has been moistened with cobalt nitrate (the aluminum test).

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