The crystal structure of B-free kornerupine: Conditions favoring the incorporation of variable amounts of B through [4]B Å [4]Si substitution in kornerupine

B-free kornerupine from the Limpopo Belt, southern Africa, a = 16.117(4), b = 13.728(3), c = 6.749(2) Å, space grotp Cmcm, Z = 4, has been refined to R = 0.034 and R_w = 0.028 for 1550 observed reflections. The formula for one eightfold, five octahedral, and three tetrahedral sites based on the structure refinement (F from the chemical analysis), (☐_0.71-Mg_0.16Fe_0.13)(Mg_1.64Fe_0.20☐_0.16)(Mg)(Al_1.64Mg_0.36)(Al)₄(Si)₂(Al_1.12Si_0.88)(Si_0.68Al_0.32)O_20.55(OH)_1.2-F_0.25, is in good agreement with wet chemical analyses for H₂O and electron microprobe analyses of the other constituents. The (OH + F) content of 1.5 per formula unit determined by wet chemical analysis exceeds the theoretical rnaximum of 0.67(OH) predicted by P. B. Moore and colleagues for kornerupine with 33% occupancy of the eightfold X site. Protons are bound to only one O atom, and thus two-thirds of these O sites must be bound to two protons. Comparison with three kornerupine samples having B contents of 0.41–0.69 B atoms per formula unit indicates coupled substitution of B for Si and Al in T3 and Si for Al in T2, and it implies that T3 in kornerupine with low B contents (possibly 0–0.5 atoms) contains Al as well as Si and B. We suggest that the following factors enable kornerupine to incorporate variable amounts of B: (1) flexibility of the structure in the vicinity of the triad of T2 and T3 tetrahedra, allowing the structure to adjust to the size differences between B, Si, and Al, (2) a low T3-O7-T2 angle, which leads to a minimum-energy situation for a B-O-Si bridge, and (3) compositional disorder on T2 and T3, so that B-O-Al bridges are avoided.