Tourmaline Structure

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

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Tourmaline is a complex mineral with general formula XY₃Z₆(Si₆O₁₈)(BO₃)₃Q₃R, where:

• X = a large cation like Na, Ca or K. May also be a vacancy.
• Y is normally Fe or Mg but can also include Li,  Mn, Zn, Al, Cr, V, Fe⁺³, Ti, or a vacancy
• Z is normally Al but can also be Mg or a large charge cation like Fe⁺³, Cr, or V
• Si can be replaced by other small cations like Al or B
• B can be replaced by vacancy
• Q and R are anions: Q is O or OH
• R is O, OH or F

Common black tourmaline or schorl has a nominal formula NaFe²⁺₃Al₆Si₆O₁₈(BO₃)₃(OH)₄ and has a complex structure as befits a mineral with such a grab bag of elements. However, it is not as horrendous as most textbook illustrations make it out to be. It contains layers with four main constituents:

1. Trigonal six-membered silica rings. Unlike beryl, which in some systems is not a cyclosilicate because the silica rings are linked by beryllium and aluminum tetrahedra, tourmaline is a true cyclosilicate. However, the rings are not symmetrical hexagons.
2. Clusters of tetrahedra. There are three central iron or magnesium tetrahedra each joined to pairs of aluminum octahedra. These clusters are linked by silica rings. The octahedra comprise components Y, Z, M and Q
3. BO₃ triangles. These span gaps between aluminum tetrahedra.
4. Large cations like sodium or calcium (X). These line up with the centers of the silica rings and are in ten-fold coordination with oxygen or hydroxyl.

Although many sites display the atomic structure of tourmaline, they generally display skeletal models showing atomic bonds, something virtually useless for ionic materials. One very good and useful representation is athttp://staff.aist.go.jp/nomura-k/common/STRUCIMAGES/Tourmaline.gif. However, it suffers from two defects. First, it doesn't represent different layers of the structure very clearly, and second, it persists in representing the silica rings as hexagonal rather than trigonal. Nevertheless it is vastly superior to any of the other representations I have found.

Tourmaline Layer, "Top" View

This view is called "top" because it is the view presented in most texts. We are actually looking at the bases of the silica tetrahedra (yellow), with the hidden edges shown in gray. Ferromagnesian (Y) octahedra are orange, aluminum (Z) octahedra are green, BO₃ triangles are pink, large cations (X) are purple and hydroxyl ions are blue.

The view below shows two layers of the structure. The lower layer is shown in pale colors and the upper layer is in bright colors. We can see that silica rings have a trio of ferromagnesian octahedra below and a trio of BO₃ triangles above. The large cations are coordinated to the central oxygen in the ferromagnesian octahedra cluster, to six oxygens in the silica ring, and to three oxygens at the tips of the overlying BO₃ triangles.

We can see that there actually are three distinct structural elements:

1. Six-membered silica rings enclosing large cations
2. Clusters of ferromagnesian and aluminum octahedra
3. Open rings with BO₃ triangles pointing into the center

The full structure consists of layers alternating in ABCABC fashion, with octahedral clusters overlain by silica rings and those in turn overlain by BO₃ triangles.

Tourmaline Layer, "Bottom" View

By all logic, this should be the top view, except that most texts show the base view of the silica tetrahedra.

Side View

Here we have a side view looking perpendicular to the threefold symmetry axis. The large X cations are in purple, the Y ferromagnesian octahedra are orange, the Z aluminum octahedra are green, silica rings are yellow and the BO₃ triangles, which are seen edge on, are represented by the three pink spheres connected by a bar. The end spheres represent O and the central one represents B.

We can see from the plan views that the symmetry is threefold with mirror planes. The side view shows there is no symmetry perpendicular to the threefold axes. The point group is therefore 3m. We can also see that the different structural modules climb stepwise along diagonal rows. If we connect the four corner X anions of a unit cell in the side view (shown in red at lower left), we see that there are two other X anions along one diagonal. That's the hallmark of a rhombohedral lattice, so the space group is R3m.

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Created 22 November, 2009, Last Update

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