Tourmaline
Most
people consider tourmaline to be a single mineral. But in fact it is a
group named for several different, but closely related minerals.
Members of the Tourmaline Group are favorites among mineral
collectors. Their rich and varied colors can captivate the eye. Even
the black opaque tourmalines can shine nicely and produce sharp
crystal forms. Tourmalines are cut as precious gems, carved into
figurines, cut as cabochons, sliced into cross-sections and natural
specimens are enthusiastically added to many a rock hound's
collection.
There are many unique properties of tourmalines. First, they are
piezoelectric which means that when a crystal is heated or compressed
(or vibrated) a different electrical charge will form at opposite ends
of the crystal (an electrical potential). Conversely if an electrical
potential is applied to the crystal, it will vibrate. Secondly they
are pleochroic which means that the crystal will look darker in color
when viewed down the long axis of the crystal than when viewed from
the side. This property goes beyond the idea that the crystal is just
thicker in that direction. Even equally dimensioned crystals will
demonstrate this trait. This property can be used as an advantage by
gem cutters who may wish to enhance a crystal's pale color or weaken a
strongly colored crystal.
The four most common and well known tourmalines are distinguished by
their color and transparencies. Elbaite is the gemstone tourmaline and
comes in many varied and beautiful colors. It is transparent to
translucent and is highly prized as minerals specimens and as
gemstones. Elbaite is easily the most colorful of all the gemstones.
The iron rich schorl is the most abundant tourmaline and is black and
opaque. It is a common accessory mineral in igneous and metamorphic
rocks and can form nice crystals. Although too opaque to be used as a
gemstone, schorl is used as an ornamental stone when found as
inclusions in quartz, a stone is called "tourmalinated quartz".
Usually when someone refers to tourmaline they are referring to either
elbaite or schorl.
The two other more common tourmalines; dravite and uvite are much
less common than elbaite or schorl, but they are getting noticed for
their beautiful specimens. Some of dravite's crystals are nicely
formed, translucent brown and they can reach a rather large size.
Uvite is a green translucent to opaque tourmaline that is growing in
popularity and is being cut as a gemstone.
The Tourmaline Group has a general formula of AX3Y6(BO3)3
Si6O18(O, OH, F)4. The A can be
either calcium or sodium. The X can be either aluminum, iron, lithium
or magnesium. The Y is usually aluminum, but can also be chromium or
iron. Some potassium can be in the A position, some manganese can be
in the X position and some vanadium can be found in the Y position,
but these elements are usually not represented in the formulas of the
tourmaline members.
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