File:Tourmaline 001 single.png

English: This diagram depicts a single unit of a tourmaline crystal seen along the c axis. The trigonal symmetry is evident. White spheres are oxygen ions; the light blue sphere is an X cation; small black spheres are T (usually silicon) ions; red are red spheres are the Y cations; small dark blue spheres are the B cations; green spheres are the Z cation; and yellow spheres are the V and W anions. The unit consists of a single large X cation bound to a six-member T ring , which in turn binds to a layer of Y, Z, V, and W cations; which in turn binds to three triangular B ions at the bottom. These units are stacked vertically into columns; three columns are packed into a bundle with vertical offsets of 0, 1/3, and 2/3 the unit vertical length; and bundles are packed together to form the final crystal structure. This unit is not actually the unit cell, which contains a slice of a bundle of three columns to capture the complete structure.