CHAPTER VII.
GARNET, ZIRCON, RUTILE, AND OCTAHEDRITE.
GARNET.
The
name garnet is applied not to any single mineral, but to a well-marked
little group, comprising several species and varieties, differing in
color and chemical composition, but very closely related physically.
They all crystallize in the isometric system, and are all constructed
on the same type chemically, though varying considerably in their
components. They are silicates of lime, magnesium, iron, or manganese,
with more or less of alumina, ferric iron or chromium. According to the
presence and the proportions of these substances, the species and
varieties are determined. Several members of the garnet group are
found in North Carolina, some of the commoner kinds in large
quantities, so that they have been mined for use as an abrasive and
some of choicer quality that yield beautiful gems.
Of
the latter are to be noted the following: Almandine or precious garnet,
the iron-alumina variety; pyrope or Bohemian garnet, the
magnesia-alumina variety; rhodolite, a peculiar and beautiful garnet
intermediate between these two; and spessartite, or manganese-alumina
garnet. This last is rare and the only North Carolina occurrence of it
is reported by Dr. J. H. Pratt, in beautiful flattened plate-like
crystals in mica, near Bakersville, some large enough to cut gems of a
carat or more.1 Very elegant crystals of large size have
been found at Amelia Court House, Virginia, in an albite pegmatite.
This variety is not red, but of a peculiar rich brown or fulvous tint
(PI. XIII).
Almandite is
the most frequent variety, and the one that has been mined for garnet
paper and other abrasive purposes, including a so-called "emery," for
which tons of it have been crushed. The color is red, of many shades,
varying to brownish and purplish reds. The peculiar play of color
observed in some of the North Carolina garnets is usually due to
inclusions. In Burke, Caldwell, and Catawba counties are found large
dodecahedral and trapezohedral ahnandite crystals coated externally
with a brown crust of limonite, the result of superficial alteration,
but
