LOCAL ENVIRONMENT AND VALENCE STATE OF IRON IN MICROINCLUSIONS IN FIBROUS DIAMONDS: X-RAY ABSORPTION AND MГ–SSBAUER DATA
A.A. Shiryaev a , Ya.V. Zubavichus b , A.A. Veligzhanin b , and C. McCammon c
a A.V. Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333, Russia Present address: A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991, Russia
b Russian Research Center "Kurchatov Institute", pl. Akademika Kurchatova 1, Moscow, 123182, Russia
c Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
Keywords: XAS, Mössbauer, fibrous diamond, isotopes
Pages: 1262-1266
Abstract
Iron valence state and local environment in a set of fibrous diamonds from Brazilian and Congolese placers were investigated using X-ray absorption and Mossbauer spectroscopies. It is shown that the diamonds could be divided into two main groups differing in the type of dominant Fe-bearing inclusions. In the first group Fe is mostly trivalent and is present in octahedral coordination; diamonds from the second group contain a mixture of Fe2+ and Fe3+, most likely, with Fe2+ in dodecahedral coordination. A few other diamonds contain iron in a more reduced state: The presence of metallic Fe and Fe3O4 is inferred from XAS measurements. Spatially resolved XANES and Mossbauer measurements on polished diamond plates show that in some cases the Fe valence state may change considerably between the core and rim, whereas in other cases Fe speciation and valence remain constant. It is shown that Fe valence does not correlate with water and/or carbonate content or ratio, suggesting that iron is a minor element in the growth medium of fibrous diamonds and plays a passive role. This study suggests that, when present, evolution of the C isotopic composition with diamond growth is largely due to changes in chemistry of the growth medium and not due to variations of f O 2
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