Chemical composition of the tourmaline group

The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. Its composition varies widely because of isomorphous replacement (solid solution), and its general formula can be written as

XY3Z6(T6O18)(BO3)3V3W,

where:

X = Ca, Na, K, vacancy

Y = Li, Mg, Fe2+, Mn2+, Zn, Al, Cr3+, V3+, Fe3+, Ti4+, vacancy

Z = Mg, Al, Fe3+, Cr3+, V3+

T = Si, Al, B

B = B, vacancy

V = OH, O

W = OH, F, O