Thermally induced cation redistribution in fluor-elbaite and Fe-bearing tourmalines
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An Fe-rich fluor-elbaite was thermally treated in air and hydrogen atmosphere up to 800 °C to study potential changes in Fe- and Al-ordering over the octahedrally coordinated Y and Z sites. Overall, the experimental data (structural refinement, electron and ion microprobe, Mössbauer, infrared and optical absorption spectroscopy) show that thermal treatment of fluor-elbaite results in an increase of Fe contents at the Z site balanced by an increase of Al at the Y site. On the basis of this and previous experimental studies on Fe–Mg–Al-bearing tourmalines, it can be stated that the intersite Fe–Mg–Al exchange rates are significant at temperatures around 700–800 °C. Thermal treatment results in an increase of ca. 0.30 Fe atoms per formula unit at the Z site compensated by a similar increase of (Mg + Al) at the Y site, following the exchange reaction YFe + Z(Mg + Al) → ZFe + Y(Mg + Al). Since the tourmaline nomenclature is based on the occupancy of ions at each structural site, the intersite Fe–Mg–Al ordering may determine the tourmaline species. This means that effectively the name associated with a given composition may be a function of the sample thermal history.
KeywordsTourmaline Fluor-elbaite Crystal structure refinement Infrared spectroscopy Mössbauer spectroscopy Optical absorption spectroscopy Thermal treatment Cation redistribution
Funding by Sapienza University of Rome (Prog. Università 2017 to F.B.) and the Swedish Research Council (H.S.) is gratefully acknowledged. E. Tillmanns and D.J. Henry are thanked for their constructive comments.
- Bosi F, Naitza S, Skogby H, Secchi F, Conte AM, Cuccuru S, Hålenius U, De La Rosa N, Kristiansson P, Nilsson EJC, Ros L, Andreozzi GB (2018a) Late magmatic controls on the origin of schorlitic and foititic tourmalines from late-Variscan peraluminous granites of the Arbus pluton (SW Sardinia, Italy): crystal-chemical study and petrological constraints. Lithos 308–309:395–411CrossRefGoogle Scholar
- Burns PC, MacDonald DJ, Hawthorne FC (1994) The crystal chemistry of manganese-bearing elbaite. Can Mineral 32:31–41Google Scholar
- Deloule E, Chaussidon M, Allé P (1992) Instrumental limitations for isotope ratios measurements with a Cameca IMS 3f ion microprobe: the example of H, B, S, Sr. Chem Geol 101:187–192Google Scholar
- Ertl A, Kolitsch U, Dyar MD, Hughes JM, Rossman GR, Pieczka A, Henry DJ, Pezzotta F, Prowatke S, Lengauer CL, Körner W, Brandstatter F, Francis CA, Prem M, Tillmans E (2012a) Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: evidence from Fe2+- and Mn2+-rich tourmaline. Am Mineral 97:1402–1416CrossRefGoogle Scholar
- Federico M, Andreozzi GB, Lucchesi S, Graziani G, César-Mendes J (1998) Crystal chemistry of tourmalines. I. Chemistry, compositional variations and coupled substitutions in the pegmatite dikes of the Cruzeiro mine, Minas Gerais, Brazil. Can Mineral 36:415–431Google Scholar
- Henry DJ, Novák M, Hawthorne FC, Ertl A, Dutrow B, Uher P, Pezzotta F (2013) Erratum Am Mineral 98:524Google Scholar
- Libowitzky E (1999) Correlation of O-H stretching frequencies and O–H⋯O hydrogen bond lengths in minerals. Monatsh Chemie 130:1047–1059Google Scholar
- Lussier A, Ball NA, Hawthorne FC, Henry DJ, Shimizu R, Ogasawara Y, Ota T (2016) Maruyamaite, K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O, a potassium-dominant tourmaline from the ultrahigh-pressure Kokchetav massif, northern Kazakhstan: description and crystal structure. Am Mineral 101:355–361CrossRefGoogle Scholar
- Sheldrick GM (2013) SHELXL2013. University of Göttingen, GermanyGoogle Scholar