The structure of an aluminous, alkali‐deficient schorl has been refined (R = 0.054 for 3084 intensity data) and its data integrated with those of 12 other refined tourmaline structures to provide a better understanding of the structural accommodation of diverse substitutions in the tourmaline group. Alkali‐deficient schorl, (Na0.550.45)(Fe1.762+Al0.89Mg0.33Ti0.02)Al6(BO3)3(Si5.86Al0.14)O18.48((OH)3.38F0.14), is rhombohedral with a = 15.963(3) and c = 7.148(2) Å its space group is R3m.

The 3a alkali site in the alkali-deficient schorl with an occupancy of (Na0.550.45) is the largest yet observed in tourmaline. Substitution of cations smaller than Na, notably Ca, affects the configuration of the six‐membered tetrahedral ring by decreasing its degree of “crimping” and by increasing its ditrigonality and the distortion of its tetrahedra. The only observed effect of Al → Si substitution is an increase in the size of the tetrahedra. R2+ → R3+ substitution in the 9b octahedral site in response to the dehydroxylation‐type substitution, (OH)- + R2+ = O2- + R3+, and alkali defect‐type substitution, R+ + R2+ = 3a□ + R3+, results in increased “puckering” (inward rotation of the tetrahedra of the six-membered ring) and in a marked compression of the smaller 18c octahedra with which the larger 9b octahedra share edges. The volume of the unit cell is highly correlated (r = 0.98) to the weighted‐mean size, [9b-O + 4(18c-O)]/5, of the 9b and 18c octahedra, which explains why lattice parameters can be successfully used to estimate octahedral-site occupancies. The chemistries of natural as well as synthetic tourmalines in the systems Na20‐Al203‐Si02‐B2O3‐H2O and MgO‐Al2O3‐SiO2‐B2O3‐H2O are also rationalized on the basis of the observed structural trends.

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