Tourmaline Composition and
(Na1+, Ca2+)(Li1+, Mg2+, Al3+).
(Al3+, Fe3+, Mn3+)6(BO3)3
complexity of tourmaline's chemical formula reflects the very large range of
its possible compositions and the complexity of the mineral structure.
Some chemical elements
are essential to tourmaline's basic structure. However, some parts of
the atomic structure will allow the addition of small amounts of practically
any of the common chemical elements.
3+, 2+, 1+, and -2 in the formula
indicate the oxidation
state or valence
of the ions in
tourmaline. Minerals are held together
by chemical bonds created by the attraction between positively and negatively
charged ions in their structure. Ions that are very similar in
size and valence are the most likely to substitute for each other in any given
part of a mineral's structure. The pressure and temperature
conditions under which the substitution takes place have significant effects
on the stability of the mineral.
HOLDER FOR STRUCTURE DIAGRAM
The formula for
tourmaline doesn't list every single chemical element that may be present in a
given crystal, but is limited only to the most commonly found ones.
Groups of elements enclosed in parentheses and separated by commas can
substitute for one other in the mineral structure. This diagram of
tourmaline's chemical structure shows where some of these substitutions take
place. For example, calcium ions (Ca2+) may substitute for
sodium ions (Na1+) in the "holes" or "channels"
in the center of the rings of linked silica tetrahedra.
The rings of six
linked silica tetrahedra dominate the structure of tourmaline and other
cyclosilicate minerals such as beryl. Each silica tetrahedron
(SiO4)-4 in the ring consists of one positively
charged silicon ion (Si4+) surrounded by four negatively charged
oxygen (O-1) ions. Because each silica tetrahedron shares two
oxygens with adjacent silica tetrahedra, the composition of each ring is (Si6O18)-12.
The hydroxyl radical (OH)- alternates with the Na1+ or
Ca2+ ions along the channels in the rings of silica tetrahedra.
As for the
remainder of the tourmaline structure, the three triangular-shaped (BO3)-3
groups are located approximately equidistant from each other, between the
linked silica tetrahedra in the ring. The relatively small, low valence
cations (Li1+, Mg2+, Al3+) link the Si6O18
rings, the BO3 groups, and the hydroxyl radicals
together. The slightly higher valence cations (Al3+ Fe3+,
Mn3+) link the columns of (Si6O18)-12 rings
Knowing what you
now do about how tourmaline is composed of stacks of rings of silica
tetrahedra interlayered with three (BO3)-3 groups,
should give you some idea why tourmaline crystallizes in the hexagonal crystal
system and typically forms elongated crystals with a roughly triangular
Latest update: 31 July 2003