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As
a rock-forming mineral it often occurs as long, slender prisms,
frequently about the size of a darning-needle, and radiating in all
directions. The only mineral for which it is likely to be mistaken in
this form is hornblende. It can be distinguished from this in the
following manner: On fusing the powdered mineral with a mixture of
bisulphate of potash and fluor-spar (best done on a little loop of
platinum wire) tourmaline will color the flame green, while hornblende
will produce no coloration.
The
black opaque crystals of tourmaline often reach a large size, some
being known four feet in length. Both black and brown tourmaline are
usually opaque, and hence have no value as gems. Tourmaline of other
colors, however, is often transparent, and this is of gem value.
The
gem tourmalines are to be found in only a few localities. They occur in
Maine, Connecticut, and California in our own country, and also in
Brazil, Russia, and Ceylon. The crystals are usually in the form of
long, slender prisms, often having the peculiarity of being
differently colored in different portions. Thus a crystal may be green
at one end and red at the other, and in cross-section may show a blue
center, then a colorless zone, then one of red, and then one of green.
Some of the crystals from Paris, Maine, change from white at one
termination to emerald green, then light green, then pink, and finally
are colorless at the other termination. In some crystals again the red
passes to blue, the blue to green, and the green to black.
Exactly
what produces these differences of color is not known. It is known that
black tourmaline has an excess of iron, the red and green an excess of
sodium and lithium, and the yellow and brown an excess of magnesium in
their composition. These same differences of composition characterize
similar colors in portions of the same crystal as well as separate
crystals. Hence the evidence is quite conclusive that the color in some
way depends on the composition. Many transparent tourmalines, while
appearing of a uniform color when viewed in any one direction, exhibit
different colors when viewed in different directions. Thus, one of the
long, slender crystals may appear green when held lengthwise in front
of the eye, but when looked at from the end appears brown. Again, some
crystals appear perfectly transparent when viewed perpendicularly to
the sides of the prism, but when viewed from the end are nearly opaque.
This may be true even when the thickness is less in the latter
direction. Both these properties are due to the arrangement of the
molecules of tourmaline, which is such as to make the power of
absorbing light different in different directions.
The
form of crystals of tourmaline is usually that of a three-sided prism.
The sides of the prism are usually marked by narrow parallel
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