Tourmaline (7 x 5 x 4 cm)

A = Ca, Na, K, or is vacant (large cations);

D = Al, Fe²⁺, Fe³⁺, Li, Mg²⁺, Mn²⁺ (intermediate to small cations – in valence balancing combinations when the A site is vacant);

G = Al, Cr³⁺, Fe³⁺, V³⁺ (small cations);

T = Si (and sometimes minor Al, B³⁺);

X = O and/or OH;

Z = F, O and/or OH.