Disordering of Fe2+ over octahedrally coordinated sites of tourmaline

The partitioning of iron among octahedrally coordinated sites in tourmaline, and its stereochemical consequences, were investigated in a Fe-rich dravite in a skam rock from Utö, Sweden. A multi-analytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mössbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and Fe2+ over the Y and Z sites, and ordering of Fe3+ at the Y site. Two Fe-rich tourmalines from the literature, re-examined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Utö tourmaline with Fe2+ disordered over the octahedral sites. This is best explained by disordered Fe2+ distributions that minimize the strain state of the Y-O bonds and provide a shielding effect reducing Y-Z repulsion. This is consistent with predictions from bond-valence theory and Pauling's rules. An indication of Z-site occupancy by Fe2+ in tourmaline may be signaled by a significant correlation between <Z-O> and the c lattice parameter (r2 = 0.96). The c value for a very Fe2+-rich tourmaline and an ideal end-member schorl, with Fe2+ and Al ordered at Y and Z (respectively), yielded <Z-O> values larger than 1.907 Å (the likely bond length for < ZAl-O>). These large <Z-O> lengths indicate that Fe2+ occurs at the Z site. The hypothesis of a dragging effect from <Y-O> to explain lengthening of < ZAl-O> is not supported by experimental evidence.