Mindat gives the following general formula for Tourmaline:
It then provides the following:
"Notes on the general formula:
" - Additional G and D site cations are reported substituting in minor amounts for the principal cations of these sites.
" - The Y and Z sites are usually combined in the simplified formula for the Group as Z4.
"The structure for the group is one in which SiO4 tetrahedra are linked into six-member rings having a hexagonal pattern and are stacked up with intervening distorted triangular BO3 groups, linked by D site cations; the SiO4 tetrahedra are linked vertically by G site cations and Y site anions, while the columns are linked horizontally by both D site cations and Y site anions. The A site cations and the Z site anions occupy the channels down the centre of the columns (or the A site may be vacant.)"
Note that the general formula does not include any of the anionic Y sites refered to in the notes. In the list of ions associated with each site, only the X and Z sites are anionic sites. Probably "Y" should be changed to "X" in the Notes.