Crystallochemical effects of heat treatment on Fe-dominant tourmalines from Dolní Bory (Czech Republic) and Vlachovo (Slovakia)

P. Bačík, D. Ozdín, M. Miglierini, P. Kardošová, Martin Pentrak, J. Haloda

Research output: Contribution to journalArticlepeer-review

Abstract

Heat treatment was performed on selected Fe-dominant tourmalines to establish the nature of any change in optical properties. Two tourmaline samples from Dolní Bory, Czech Republic (TDB) and Vlachovo, Slovakia (TVL) were heated at 450, 700 and 900°C at 0.1 mPa and ambient oxidation conditions for 8 h. EMPA study shows that tourmaline from Vlachovo has schorlitic composition and tourmaline from Dolní Bory is alkali-depleted schorl to foitite. Although the black colour remained unchanged after heating at 450°C, it changed to brown at 700°C and reddish brown at 900°C. No significant changes of chemical composition were observed during heating. X-ray diffraction, infrared and Mössbauer study showed negligible oxidation of tourmaline heated at 450°C, but a significant change in iron valency state and deprotonization at 700°C. The oxidation of Fe is the main cause of tourmaline colour change, and the substitution vector for oxidation of Fe is Fe3+OFe-12+(OH)-1. The predicted deprotonization of OH was confirmed by infrared spectroscopy, which documented a decrease in OH groups in both samples, mainly at the V site. The oxidation of Fe is mostly significant in the Y site as documented on the compression of the Y-site octahedra and subsequent decrease in the a lattice parameter. This feature is consistent with lattice dimensions in the transition from schorl and foitite dimensions to those consistent with fluor-buergerite. The Z-site octahedra did not compressed and were not affected by heating-induced Fe oxidation, which indicates only negligible content of ZFe2+ in original samples. After heating at 900°C, the tourmaline structure collapsed likely due to the thermally induced weakening of bonds in Y and Z octahedra, which results in amorphization of tourmaline. Subsequently, breakdown products including Fe-oxides and mullite replaced alkali-depleted amorphized tourmaline.

Original languageEnglish (US)
Pages (from-to)599-611
Number of pages13
JournalPhysics and Chemistry of Minerals
Volume38
Issue number8
DOIs
StatePublished - Sep 2011
Externally publishedYes

Keywords

  • Disintegration of structure
  • Electron backscatter diffraction
  • Electron microprobe
  • Fe oxidation
  • Heating
  • Infrared spectroscopy
  • Mössbauer spectroscopy
  • Powder X-ray diffraction
  • Tourmaline

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Materials Science(all)

Fingerprint Dive into the research topics of 'Crystallochemical effects of heat treatment on Fe-dominant tourmalines from Dolní Bory (Czech Republic) and Vlachovo (Slovakia)'. Together they form a unique fingerprint.

Cite this