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Amphiboles from the Kola Superdeep Borehole: Fe³⁺ contents from crystal-chemical analysis and Mössbauer spectroscopy

Y. Uvarova¹, E. Sokolova^1,3, F. C. Hawthorne^1,*, C. A. McCammon², V. I. Kazansky^3, and K. V. Lobanov³

¹ Department of Geological Sciences, University of Manitoba Winnipeg, Manitoba, Canada R3T 2N2
² Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
³ Institute of the Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetny 35, 119017 Moscow, Russia

^* E-mail: frank_hawthorne{at}umanitoba.ca

The crystal structures of a suite of amphiboles from the Kola Superdeep Borehole, Russia, have been refined to R values of ~3% using single-crystal Mo-K X-ray diffraction data. The same crystals used in the collection of the intensity data were subsequently analysed by electron microprobe (EMP) and milliprobe Mössbauer spectroscopy. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. The Fe³⁺/(Fe²⁺+Fe³⁺) values were derived (1) by least-squares refinement of the Mössbauer spectra, and (2) SREF (Structure REFinement) by careful analysis of the mean bond lengths at the M(2) site using two possible models for the behaviour of Ti⁴⁺: (i) Ti⁴⁺ at M(2); and (ii) Ti⁴⁺ at M(1). The agreement between the SREF and Mössbauer/EMP values for Fe³⁺ is very close for Ti⁴⁺ assigned to M(2). This result indicates that the calculation of Fe³⁺ contents in amphiboles from refined site populations and < M(2)–O> distances are accurate. This paper presents the first confirmation of this result.

KEYWORDS: amphibole, crystal-structure refinement, electron microprobe analysis, Mössbauer spectroscopy, Kola Superdeep Borehole