This
mineral has the same chemical composition as cyanite, it being a simple
aluminum silicate. It differs, however, from that mineral in color,
specific gravity, system of crystallization and various other
properties, so that there is little danger of mistaking the two.
Andalusite
occurs generally as an opaque mineral, commonly in argillaceous and
mica schists. The transparent pieces cut for gems are obtained almost
entirely from the province of Minas Geraes, Brazil, where they occur in
the beds of streams, together with topaz. These transparent pebbles
have a pale green color in one direction and in another are brownish
red. This difference of color is due to the pleochroism of the mineral,
which is strong, and the directions of which should be borne in mind in
cutting. It is when looked at in the direction of the vertical axis
that the reddish color of andalusite is apparent, while at right angles
to this the green color appears.
Andalusite
crystallizes in the orthorhombic system, the crystals usually taking
the form of nearly square prisms. It has a marked prismatic cleavage,
which does not, however, interfere with the cutting of it to any
extent. Its luster is vitreous. In hardness it is somewhat superior to
quartz, the degree of hardness being 7.5. The specific gravity is
3.16-3.20. Like cyanite, andalusite is infusible before the blowpipe,
and is not attacked by acids.
In
addition to the use of the transparent forms of andalusite, mention
should be made of the fact that sections of the opaque crystals are
sometimes
worn, being prized on account of the cross-like markings which they
contain. These result from the shape taken by inclusions of
carbonaceous
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