ANDALUSITE, a mineral with the same chemical compo sition as kyanite and sillimanite, being aluminium silicate, 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° As a rule the crystals are roughly developed and rude columnar masses are common, these being fre quently partially altered to kaolin or mica. Such crystals, opaque and of a greyish or brownish colour, occur abun dantly in the mica-schist of the Lisens Alp near Innsbruck in Tirol, and in An dalusia, from which place it 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 effec tive gemstones, the degree of hardness of the mineral (H. = 7 ) being quite sufficient for this purpose.
A curious variety of andalusite known as chiastolite is spe cially characteristic of clay-slates near a contact with granite. The elongated prismatic crystals enclose symmetrically arranged wedges of carbonaceous material, and in cross-section (see fig.) 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.