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lines called striae, and the prism may be more or less rounded by the addition of other planes.
If
a doubly terminated crystal be examined carefully, it will be seen that
the planes on the two ends are not alike, either in number or
inclination. On one end there may be three planes, on the other six, or
even twelve. If the planes on one end make a blunt termination, those
on the other may make a sharply pointed one. Such a peculiarity of
crystal form is possessed by but few minerals. Those possessing it are
said to be hemimorphic, i. e., half formed. Some minerals, among which
is tourmaline, which exhibit this peculiarity of form are often also
pyroelectric, i. e., become electric on heating. It was this
development of electricity which caused the stones with which the
Dutch children played, to pick up ashes, etc., when the stones were
warmed by the heat of the sun. Any one can repeat their observation by
gently heating crystals, or even fragments of tourmaline, and applying
them to bits of paper. The electrical attraction will often be found to
be very strong, though it varies with different crystals. The fragments
should not be overheated, the electricity being most strongly developed
between 100° and 200° Fahrenheit.
In
composition tourmaline is a complex silicate, chiefly of aluminum and
boron. Iron, magnesium, the alkalies, and water also enter in varying
amounts into it. In fact, so complicated is its chemical nature that
perhaps no other mineral has been so often analyzed or had its analyses
so much discussed.
Ruskin,
in his " Ethics of the Dust," thus describes the composition of
tourmaline: "A little of everything; there's always flint and clay and
magnesia in it; and the black is iron according to its fancy; and
there's boracic acid, if you know what that is, and if you don't, I
cannot tell you to-day, and it doesn't signify; and there's potash and
soda; and on the whole, the chemistry of it is more like a mediaeval
doctor's prescription than the making of a respectable mineral."
Tourmaline
is both harder and heavier than quartz, its hardness being 7-7.5, and
its specific gravity 2.98-3.20. It is thus sufficiently hard for use as
a gem. It is, however, quite brittle, and even at times friable. Cracks
therefore frequently cut across good crystals, and spoil what would
otherwise make a good gem. It is very common to find tourmalines in the
rocks broken into a number of pieces, and the fragments "mended"
together with quartz or calcite. This has been true of the black
tourmaline shown in the accompanying plate. Scarcely any other mineral
exhibits this change so often as tourmaline, a result due probably to
its brittleness and the character of the rock in which it occurs.
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