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Disordering of Fe²⁺ over octahedrally coordinated sites of tourmaline

¹ Department of Mineralogy, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden
² Dipartimento di Scienze della Terra, Università di Roma "La Sapienza," Piazzale Aldo.Moro 5, 00185 Roma, Italy

Correspondence: ^* E-mail: ferdinando.bosi{at}uniroma1.it

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 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, reexamined 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.

Key Words: Chemical analysis • tourmaline • crystal structure • Mössbauer spectroscopy • order-disorder • XRD data

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				V. J. van Hinsberg and J. C. Schumacher The geothermobarometric potential of tourmaline, based on experimental and natural data American Mineralogist, May 1, 2009; 94(5-6): 761 - 770. [Abstract] [Full Text] [PDF]