Disordering of [Fe.sup.2+] 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 skarn rock from Uto, Sweden. A multi-analytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mossbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and [Fe.sup.2+] over the Y and Z sites, and ordering of [Fe.sup.3+] 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 Uto tourmaline with [Fe.sup.2+] disordered over the octahedral sites. This is best explained by disordered [Fe.sup.2+] 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 [Fe.sup.2+] in tourmaline may be signaled by a significant correlation between and the c lattice parameter ([r.sup.2] = 0.96). The c value for a very [Fe.sup.2+]-rich tourmaline and an ideal end-member schorl, with [Fe.sup.2+] and Al ordered at Y and Z (respectively), yielded values larger than 1.907 [Angstrom] (the likely bond length for ). These large lengths indicate that [Fe.sup.2+] occurs at the Z site. The hypothesis of a dragging effect from to explain lengthening of is not supported by experimental evidence. Keywords: Chemical analysis, tourmaline, crystal structure, Mossbauer spectroscopy, order-disorder, XRD data