Disordering of Fe2+ over octahedrally coordinated sites of tourmaline

Abstract

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 2+ over the Y and Z sites, and ordering of Fe 3+ at the Y site. Two Fe-rich tourmalines from the literature, reexamined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Uto tourmaline with Fe 2+ disordered over the octahedral sites. This is best explained by disordered Fe 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 2+ in tourmaline may be signaled by a significant correlation between Z -O> and the c lattice parameter ( r 2 = 0.96). The c value for a very Fe 2+ -rich tourmaline and an ideal end-member schorl, with Fe 2+ and Al ordered at Y and Z (respectively), yielded Z -O> values larger than 1.907 A (the likely bond length for Z Al-O>). These large Z -O> lengths indicate that Fe 2+ occurs at the Z site. The hypothesis of a dragging effect from Y -O> to explain lengthening of Z Al-O> is not supported by experimental evidence.