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Experimental cation redistribution in the tourmaline lucchesiite, CaFe2+3Al6(Si6O18)(BO3)3(OH)3O
Sapienza Università di Roma, Italy.
Swedish Museum of Natural History, Department of Geology.
Swedish Museum of Natural History, Department of Geology.
Associazione Micromineralogica Italiana,Torino, Italy.
2018 (English)In: Physics and chemistry of minerals, ISSN 0342-1791, E-ISSN 1432-2021, Vol. 45, no 7, p. 621-632Article in journal (Refereed) Published
Abstract [en]

Natural Mg-rich lucchesiite was thermally treated in air and hydrogen atmosphere up to 800 °C to study potential changes in Fe-, Mg- and Al ordering over the octahedrally coordinated Y-  and Z -sites, and to explore possible applications to intracrystalline geothermometry based on tourmaline. Overall, the experimental data (structural refinement, Mössbauer, infrared and optical absorption spectroscopy) show that thermal treatment of lucchesiite results in an increase of Fetot contents at Z balanced by an increase of Mg and Al at Y . This process is accompanied by a significant deprotonation of the O3 anion site. The Fe order–disorder reaction depends more on temperature, than on redox conditions. During heat treatment in H2 ,reduction of Fe3+ to Fe2+ was not observed despite strongly reducing conditions, indicating that the fO2  conditions do not exclusively control the Fe oxidation state at the present experimental conditions. On the basis of this and previous studies, the intersite order–disorder process induced by thermal treatment indicates that Fe redistribution is an important factor for Fe–Mg–Al-exchange and is significant at temperatures around 800 °C. As a result, Fe–Mg–Al intersite order–disorder is sensitive to temperature variations, whereas geothermometers based solely on Mg–Al order–disorder appear insensitive and involve large uncertainties. The presented findings are important for interpretation of the post-crystallization history of both tourmaline and tourmaline host rocks, and indicate that successful tourmaline geothermometers may be developed by thermal calibration of the Fe-Mg–Al order–disorder reaction.

Place, publisher, year, edition, pages
2018. Vol. 45, no 7, p. 621-632
National Category
Geochemistry
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-2819DOI: 10.1007/s00269-018-0947-0OAI: oai:DiVA.org:nrm-2819DiVA, id: diva2:1233113
Available from: 2018-07-16 Created: 2018-07-16 Last updated: 2018-07-16Bibliographically approved

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