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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

Yves Moelo, Emmanuel Fritsch, Eric Gloaguen and Olivier Rouer
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 (March 2020) 55 (1): 83-95

Abstract

Several generations of chamosite, including a red variety, occur in the Ordovician hydrothermalized oolitic ironstone from Saint-Aubin-des-Chateaux (Armorican Massif, France). Their chemical re-examination indicates a low Mg content (0.925<Fe/(Fe+Mg)<0.954), but a significant variation in (super IV) Al. Minor vanadium is present at up to 1.1 wt.% oxide. Variations in (super IV) Al, 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 (Fe (sub 5-x) Al (sub 1+x) )(Si (sub 3-x) Al (sub 1+x) )O (sub 10) (OH) (sub 8) , with 0.2<x<0.8 (0.2: equilibrium with quartz; 0.8: SiO (sub 2) deficit). The red chamosite (IIb polytype) has a mean composition of (Fe (sub 3.87) Mg (sub 0.23) Mn (sub 0.01[]0.07) Al (sub 1.74) V (sub 0.07) )(Si (sub 2.33) Al (sub 1.67) )O (sub 10) (OH) (sub 8) . This chamosite is strongly pleochroic, from pale yellow (E(001)) to deep orange red (E perpendicular to (001)) Visible-near-infrared absorbance spectra show a specific absorption band centred at approximately 550 nm for E perpendicular to (001), due to a proposed new variety of Fe/V intervalence charge-transfer mechanism in the octahedral sheet, possibly Fe (super 3+) -V.4+-->V (super 3+) . 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 f (sub O2) related to oxidizing hydrothermal solutions.


ISSN: 0009-8558
EISSN: 1471-8030
Coden: CLMIAF
Serial Title: Clay Minerals
Serial Volume: 55
Serial Issue: 1
Title: 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
Affiliation: Universite de Nantes, Institut des Materiaux Jean Rouxel, Nantes, France
Pages: 83-95
Published: 202003
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 71
Accession Number: 2020-056628
Categories: Geochemistry of rocks, soils, and sedimentsSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N46°30'00" - N49°45'00", W04°49'60" - E00°00'00"
Secondary Affiliation: BRGM, FRA, FranceUniversite de Lorraine, FRA, France
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202016
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