CATION ORDERING AT OCTAHEDRAL SITES IN SCHORL-DRAVITE SERIES TOURMALINES

Abstract

Tourmalines with Al 3+ , Mg 2+ , Fe 3+ , Fe 2+ , Mn 2+ , V 3+ , and Cr 3+ 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 Mg 2+ , Al 3+ , and Fe 2+ 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 Al 3+ and Mg 2+ show weak negative correlations with a consistent with their relatively smaller ionic radii, Fe 2+ has a weak positive correlation with a because of its larger ionic radius. Negative Al 3+ and positive Mg 2+ correlations with c show that increased Mg 2+ content, rather than Al 3+ , is related to increased c and Z O 6 octahedral dilation. Fe 2+ negatively correlates with c , and this suggests that none or only a negligible Fe 2+ proportion can be incorporated at the Z site. All Y -site bond lengths except Y –O2 increase with the occupancy of Fe 2+ at the Y site and decrease with the occupancy of Mg at the Y site. Al 3+ has no significant effect except for Y –O2 bond-length contraction. In contrast, Al 3+ negatively correlates with all bonds involving the Z site, and Mg 2+ has the opposite effect. Fe 2+ shows negative correlation with Z –O bond lengths, which also suggests that Fe 2+ 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 Z Fe 2+ seems unlikely. Complementary bond-topology graphs revealed that Z R 2+ 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 Y R 2+ requires the presence of R 3+ at neighboring Y sites. A topological study of selected structural arrangements reveals that Y R 2+ – Z R 3+ – Z R 3+ and Y R 3+ – Z R 3+ – Z R 2+ arrangements with W OH can exist together in bond-valence stability. In contrast, bond-valence requirements involving W O 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 W OH.