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Experimental evidence for partial Fe2+ disorder at the Y and Z sites of tourmaline: a combined EMP, SREF, MS, IR and OAS study of schorl

Published online by Cambridge University Press:  02 January 2018

Ferdinando Bosi ,
Giovanni B. Andreozzi ,
Ulf Hålenius  and
Henrik Skogby
Ferdinando Bosi*
Affiliation:
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro, 5, I-00185 Rome, Italy Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden
Giovanni B. Andreozzi
Affiliation:
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro, 5, I-00185 Rome, Italy
Ulf Hålenius
Affiliation:
Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden
Henrik Skogby
Affiliation:
Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden
*
*E-mail: [email protected]
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Abstract

An experimental study of an Al-rich schorl sample from Cruzeiro mine (Minas Gerais, Brazil) was carried out using electron microprobe analysis, structural refinement and Mössbauer, infrared and optical absorption spectroscopy in order to explore the disordering of Fe2+ over the Y and Z sites of the tourmaline structure.

A structural formula was obtained by merging chemical and structural data. The cation distribution at the two non-equivalent octahedrally coordinated sites (Y and Z) was obtained by two different optimization procedures which, minimizing the residuals between observed and calculated data, converged to the formula: X(Na0.65〈0.32Ca0.02K0.01)Σ1.00Y(Fe1.652+Al1.15Fe0.063+Mn0.052+Zn0.05Ti0.044+)Σ3.00Z(Al5.52Fe0.302+Mg0.18)Σ6.00[T(Si5.87Al0.13)Σ6.00O18](BBO3)3V(OH)3W[(OH)0.34F0.28O0.38]Σ1.00.

This result shows a partial disordering of Fe2+ over the Y and Z sites which explains adequately the mean atomic number observed for the Z site (13.5±0.1). Such a disordering is also in line with the shoulder recorded in the Mössbauer spectrum (fitted by a doublet with isomer shift of 1.00 mm/s and quadrupole splitting of 1.38 mm/s) as well as with the asymmetric bands recorded in the optical absorption spectrum at ∼9000 and 14,500 cm–1 (modelled by four Gaussian bands, centred at 7677 and 9418 cm–1, and 13,154 and 14,994 cm–1, respectively).

The high degree of consistency in the results obtained using the different methods suggests that the controversy over Fe2+ order can be ascribed to the failure to detect small amounts of Fe2+ at Z (typically <<10% atoms/site) rather than a steric effect of Fe2+ on the tourmaline structure.

Keywords

tourmalineschorlcrystal-structure refinementelectron microprobeMössbauer spectroscopyoptical absorption spectroscopyinfrared spectroscopy
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 3 , June 2015 , pp. 515 - 528
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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