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Moelo, Y., Fritsch, E., Gloaguen, E. & Rouer, O. (2020) Polygenic chamosite from a hydrothermalized oolitic ironstone (Saint-Aubin-des-Chateaux, Armorican Massif, France): crystal chemistry, visible-near-infrared spectroscopy (red variety) and geochemical significance. Clay Minerals, 55 83–95. 
Added by: Richard Baschera (2020-07-20 13:14:31)   Last edited by: Richard Baschera (2020-09-30 16:17:36)
Type de référence: Article
DOI: 10.1180/clm.2020.13
Numéro d'identification (ISBN etc.): 0009-8558, 1471-8030
Clé BibTeX: Moelo2020
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Catégories: IMN, MIOPS
Mots-clés: Armorican Massif, chamosite, crystal chemistry, France, Vanadium, visible-NIR spectroscopy
Créateurs: Fritsch, Gloaguen, Moelo, Rouer
Collection: Clay Minerals
Consultations : 2/589
Indice de consultation : 9%
Indice de popularité : 2.25%
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Several generations of chamosite, including a red variety, occur in the Ordovician hydrothermalized oolitic ironstone from Saint-Aubin-des-Châteaux (Armorican Massif, France). Their chemical re-examination indicates a low Mg content (0.925 {<} Fe/(Fe + Mg) {<} 0.954), but a significant variation in IVAl. Minor vanadium is present at up to 1.1 wt.% oxide. Variations in IVAl, the vanadium content and the colour of chamosite are related to the hydrothermal reworking of the ironstone. Taking into account other published data, the ideal composition of chamosite is (Fe5–xAl1+x)(Si3–xAl1+x)O10(OH)8, with 0.2 {<} x {<} 0.8 (0.2: equilibrium with quartz; 0.8: SiO2 deficit). The red chamosite (IIb polytype) has a mean composition of (Fe3.87Mg0.23Mn0.01□0.07Al1.74V0.07)(Si2.33Al1.67)O10(OH)8. This chamosite is strongly pleochroic, from pale yellow (E {textbar}{textbar} (001)) to deep orange red (E ⊥ (001)). Visible–near-infrared absorbance spectra show a specific absorption band centred at {textasciitilde}550 nm for E ⊥ (001), due to a proposed new variety of Fe/V intervalence charge-transfer mechanism in the octahedral sheet, possibly Fe2+ – V4+ → Fe3+ – V3+. While the formation of green chamosite varieties is controlled by reducing conditions due to the presence of organic matter as a buffer, that of red chamosite would indicate locally a weak increase of fO2 related to oxidizing hydrothermal solutions.
Publisher: Cambridge University Press
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