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 Rw = 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-Mg0.16Fe0.13)(Mg1.64Fe0.200.16)(Mg)(Al1.64Mg0.36)(Al)4(Si)2(Al1.12Si0.88)(Si0.68Al0.32)O20.55(OH)1.2-F0.25, 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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.