ANDALUSITE, a mineral with the same chemical composition as cyanite and sillimanite, being a basic aluminium silicate, Al2SiO5. As in sillimanite, its crystalline form is referable to the orthorhombic system. Crystals of andalusite have the form of almost square prisms, the prism-angle being 89 deg. 12'; they are terminated by a basal plane and sometimes by small dome-faces. As a rule the crystals are roughly developed and rude columnar masses are common, these being frequently altered partially to kaolin or mica. Such crystals, opaque, and of a greyish or brownish colour, occur abundantly in the mica-schist of the Lisens Alp near Innsbruck in Tirol, while the first noted of the many localities of the mineral is in Andalusia, from which place the mineral derives its name. The unaltered mineral is found as transparent pebbles with topaz in the gem-gravels of the Minas Novas district, in Minas Geraes, Brazil. These pebbles are usually green but sometimes reddish-brown in colour, and are remarkable for their very strong dichroism, the same pebble appearing green or reddish-brown according to the direction in which it is viewed. Such specimens make very effective gem-stones, the degree of hardness of the mineral (H.= 7 1/2) being quite sufficient for this purpose. Its specific gravity is 3.18; it is unattacked by acids and is infusible before the blowpipe.
Andalusite is typically a mineral of metamorphic origin, occurring most frequently in altered clay-slates and crystalline schists, near the junction of these with masses of intrusive igneous rocks such as granite. It has been recognized also, however, as a primary constituent of granite itself.
A curious variety of andalusite known as chiastolite is specially characteristic of clay-slates near a contact with granite. The elongated prismatic crystals enclose symmetrically arranged wedges of carbonaceous material, and in cross-section show a black cross on a greyish ground. Cross-sections of such crystals are polished and worn as amulets or charms. Crystals of a size suitable for this purpose are found in Brittany and the Pyrenees, while still larger specimens have been found recently in South Australia. The name chiastolite is derived from the Greek chiastos, crossed or marked with the latter ch: cross-stone and macle are earlier names, the latter having been given on account of the resemblance the cross-section of the stone bears to the heraldic macula or mascle. (L. J. S.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)