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The Chemical Formula of Tourmaline
Tourmaline is not a single mineral, but a group of isomorphous
minerals with an identical crystal lattice.
Each member of the Tourmaline group has its own chemical formula,
due to a slight difference in elemental distribution. The formula
of the Tourmaline group is very complex, since it contains many
elements and many combinations of elements. It is one of the most
complicated chemical formula in the entire mineral kingdom.
A generic formula for the entire Tourmaline group is:
X1
Y3
Al6 B3 Si6 (OH)4
X = Na
and/or Ca
Y = Mg, Li,
Al, and/or Fe2+
This creates the formula:
(Na,Ca) (Mg,Li,Al,Fe2+)3 Al6 B3 Si6
(OH)4
In some rare varieties, the Al
may be replaced by other elements. For example, in Uvite, the Al is partially replaced
by Mg, and in Chromdravite,
it is fully replaced by Cr.
This expands the formula to:
(Na,Ca) (Mg,Li,Al,Fe2+)3 (Al,Mg,Cr)6 B3 Si6
(OH)4
To add to the complexity, the rare member Buergerite
contains three O
atoms and one F
atom in place of the OH
radical. A Buergerite molecule also contains an Fe atom that is in a +3 oxidation state.
This expands the formula to:
(Na,Ca) (Mg,Li,Al,Fe2+,Fe3+)3 (Al,Mg,Cr)6 B3 Si6
(OH,O,F)4
Note that there are a few additional forms of Tourmaline that
further alter the formula. Those forms are extremely rare and not
applicable for this guide.
The formula for Tourmaline can be written in several ways, but it
all boils down to the same thing.
The formula of individual members of the Tourmaline group are:
Schorl
Na Fe2+3 Al6 B3 Si6 (OH)4
Dravite
Na Mg3 Al6
B3 Si6
(OH)4
Elbaite
Na (Li,Al)3 Al6 B3 Si6 (OH)4
Liddicoatite
Ca (Li,Al)3 Al6 B3 Si6 (OH)4
Uvite
Ca (Mg,Fe2+)3 Al5
Mg B3
Si6 (OH)4)
(Note that the one Al
atom is replaced by a Mg atom)
Chromdravite
Na Mg3 Cr6 B3 Si6 (OH)4
(Note that the Al atoms are replaced by Cr
atoms)
Buergerite
Na Fe3+3 Al6
B3 Si6
O3 F
(Note that the OH
radical is replaced by
three O atoms and one F atom. This leaves a space open for one more
positive charge, which is compensated by three Fe+3 atoms (as opposed to Fe with a +2 charge)
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