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 small differences in their 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 of all minerals.
A simple generic formula for the Tourmaline group is:
X1Y3Al6(BO3)3Si6O18(OH)4
Where:
X = Na and/or Ca
Y = Mg, Li, Al, and/or Fe2+
This creates the formula:
(Na,Ca)(Mg,Li,Al,Fe2+)3Al6(BO3)3Si6O18(OH)4
In some less common varieties, the Al may be replaced by other elements. For example, in Uvite, the
Al is partially replaced by Mg.
This expands the formula to:
(Na,Ca)(Mg,Li,Al,Fe2+)3(Al,Mg,Cr)6(BO3)3Si6O18(OH)4
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 further expands the formula to:
(Na,Ca)(Mg,Li,Al,Fe2+,Fe3+)3(Al,Mg,Cr)6(BO3)3Si6O18(OH,O,F)4
The chemical formula for Tourmaline can be written several different ways.
Note that there are several additional forms of Tourmaline that further alter the formula.
These forms are extremely rare and not applicable for this guide.
The chemical formula of individual members of the Tourmaline group are:
Schorl:
NaFe2+3Al6(BO3)3Si6O18(OH)4
Dravite:
NaMg3Al6(BO3)3Si6O18(OH)4
Elbaite:
Na(Li,Al)3Al6(BO3)3Si6O18(OH)4
Liddicoatite:
Ca(Li,Al)3Al6(BO3)3Si6O18(OH)4
Uvite:
Ca(Mg,Fe2+)3Al5Mg(BO3)3Si6O18(OH)4
(Note that the one Al atom is replaced by a Mg atom)
Buergerite:
NaFe2+3Al6(BO3)3Si6O18O3F
(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 Fe3+ atoms (as opposed to Fe with a 2+ charge).
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