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| Titel |
Fe2+-Ti4+ vs. Fe2+-Fe3+ charge-transfer and short-range order in single chains of face-sharing octahedra: ellenbergerite and dumortierite |
| VerfasserIn |
C. Chopin, K. Langer, V. Khomenko |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027155
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| Zusammenfassung |
| In zoned pyrope megacrysts from the Dora-Maira UHP terrane, new, dark-violet colour
varieties of the hexagonal, high-pressure silicate ellenbergerite extend the range of known Fe
contents for this mineral from 0–0.1 to 0–0.4 atom pfu, for Ti contents commonly in the
range 0.2–0.4 pfu. The new varieties show an extremely intense pleochroism, colourless for E
perpendicular to c to deep Prussian blue for E//c, as compared to colourless to lilac or reddish
purple for classical Fe-poor ellenbergerite. These features were the incentive for an
electronic absorption spectroscopic study and a reappraisal of the interpretation of the
charge transfers (CT), colour and ordering schemes in this group and the structurally
related borosilicate dumortierite. Both structures are characterized by the presence
of infinite single chains of face-sharing, partly vacant octahedra along the 6-fold
screw axis and pseudo-hexad axis, respectively, in which the Fe and Ti atoms are
partitioned.
In the spectra of Fe-poor ellenbergerite, the presence of a single Fe2+-Ti4+ CT band near
19000 cm˘1 was taken as evidence for complete short-range ordering of Mg(Fe),
Ti and vacancies in the octahedral single chain [1]. The E//c spectra of Fe-rich
ellenbergerite show the same absorption band near 19000 cm˘1 but consistently
flanked by another CT band near 14000 cm˘1 , the intensity of which increases
with total Fe content. The latter is similar to the 12400 cm˘1 CT band observed
as the single feature in E//c spectra of the isotructural (Ti-free and Fe-bearing)
phosphoellenbergerite, and clearly assigned to Fe2+-Fe3+ CT in the octahedral single chain
[1].
The same colour pattern occurs in the dumortierite group, with red Fe-poor, Ti-rich
crystals showing a single CT band near 20000 cm˘1, blue Ti-poor crystals showing a
single CT band near 16500 cm˘1, and violet Fe- and Ti-rich crystals showing a
combination of the two bands [2]. In the light of the new data, we reinterpret the
dumortierite colour scheme as due to both Fe2+-Fe3+ (16500 cm˘1) and Fe2+-Ti4+
(20000 cm˘1) CT, rather than to Fe2+-Ti4+ CT only with two extreme types of Fe-Ti
dimers [- v - Fe2+ - Ti4+ - v -] and [- M - Fe2+ - Ti4+ - v -], where v stands for
vacancy. We discuss the implications in terms of energy and of short-range ordering of
vacancies in the octahedral single chains of the ellenbergerite and dumortierite groups.
Optical spectroscopy appears as a very sensitive structural probe of minor or trace
elements.
[1] Chopin C. & Langer, K. (1988): Fe2+-Ti4+ charge transfer between face-sharing
octahedra: polarized absorption spectra and crystal chemistry of ellenbergerite. Bull.
Minéral., 111, 17-27.
[2] Platonov, A.N., Langer, K., Chopin, C., Andrut, M., Taran, M.N. (2000): Fe2+-Ti4+
charge-transfer in dumortierite. Eur. J. Mineral., 12, 521-528. |
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