Abstract

Tourmalines with Al3+, Mg2+, Fe3+, Fe2+, Mn2+, V3+, and Cr3+ cation disorder at the two octahedrally coordinated sites (Y, Z) have recently been described. Their complete structural and compositional description enabled a detailed statistical study of Mg2+, Al3+, and Fe2+ disorder reactions. The sizes of the a and c unit-cell parameters are positively correlated with <Y–O> and <Z–O> bond lengths, respectively, and this enables inference of octahedral distortion. While total octahedral Al3+ and Mg2+ show weak negative correlations with a consistent with their relatively smaller ionic radii, Fe2+ has a weak positive correlation with a because of its larger ionic radius. Negative Al3+ and positive Mg2+ correlations with c show that increased Mg2+ content, rather than Al3+, is related to increased c and ZO6 octahedral dilation. Fe2+ negatively correlates with c, and this suggests that none or only a negligible Fe2+ proportion can be incorporated at the Z site. All Y-site bond lengths except Y–O2 increase with the occupancy of Fe2+ at the Y site and decrease with the occupancy of Mg at the Y site. Al3+ has no significant effect except for Y–O2 bond-length contraction. In contrast, Al3+ negatively correlates with all bonds involving the Z site, and Mg2+ has the opposite effect. Fe2+ shows negative correlation with Z–O bond lengths, which also suggests that Fe2+ is not present at the Z site in any significant amount. Consequently, although Al-Mg disorder may be very common in Mg-bearing tourmalines but should never be over 2 apfu Mg at the Z site, disorder involving ZFe2+ seems unlikely. Complementary bond-topology graphs revealed that ZR2+ induces increased bond valence and decreased Y–O6 bond length, with consequent Y–O2 bond-valence decrease and bond-length increase. This corresponds with variations in bond lengths dependent on site occupancy, especially since YR2+ requires the presence of R3+ at neighboring Y sites. A topological study of selected structural arrangements reveals that YR2+ZR3+ZR3+ and YR3+ZR3+ZR2+ arrangements with WOH can exist together in bond-valence stability. In contrast, bond-valence requirements involving WO have more degrees of freedom. Consequently, although these bond-valence requirements may produce structures with Al-Mg disorder, the disorder most likely occurs in arrangements with WOH.

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