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ANDALUSITE

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Originally appearing in Volume V01, Page 955 of the 1911 Encyclopedia Britannica.
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ANDALUSITE , a See also:

mineral with the same chemical See also:composition as See also:cyanite and See also:sillimanite, being a basic See also:aluminium silicate, Al2SiO5. As in sillimanite, its crystalline See also:form is referable to the orthorhombic See also:system. Crystals of andalusite have the form of almost square prisms, the See also:prism-See also:angle being 89° 12'; they are terminated by a basal See also:plane and some-times by small See also:dome-faces. As a See also:rule the crystals are roughly See also:developed and See also:rude columnar masses are See also:common, these being frequently altered partially to See also:kaolin or See also:mica. Such crystals, opaque, and of a greyish or brownish See also:colour, occur abundantly in the mica-schist of the Lisens See also:Alp near See also:Innsbruck in See also:Tirol, while the first noted of the many localities of the mineral is in See also:Andalusia, from which See also:place the mineral derives its name. The unaltered mineral is found as transparent pebbles with See also:topaz in the See also:gem-gravels of the See also:Minas Novas See also:district, in Minas Geraes, See also:Brazil. These pebbles are usually See also:green but sometimes reddish-See also: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. = 71) being quite sufficient for this purpose. Its specific gravity is 3.18; it is unattacked by acids and is infusible before the See also:blowpipe. Andalusite is typically a mineral of metamorphic origin, occurring most fre- quently in altered See also:clay-slates and crystalline See also:schists, near the junction of these with masses of intrusive igneous rocks such as See also:granite. It has been recognized also, however, as a See also: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 See also:

cross-See also:section show a See also:black cross on a greyish ground. Cross-sections of such crystals are polished and worn as amulets or charms. Crystals of a See also:size suitable for this purpose are found in See also:Brittany and the See also:Pyrenees, while still larger specimens have been found recently in See also:South See also:Australia. The name chiastolite is derived from the See also:Greek xraoros, crossed or marked with the See also:letter x: cross-See also:stone and made are earlier names, the latter having been given on See also:account of the resemblance the cross-section of the stone bears to the heraldic macula or mascle. (L. J.

End of Article: ANDALUSITE

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