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 IV Al. Minor vanadium is present at up to 1.1 wt.% oxide. Variations in 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 5-x Al 1+x)(Si 3-x Al 1+x)O 10 (OH) 8 , with 0.2 < x < 0.8 (0.2: equilibrium with quartz; 0.8: SiO 2 deficit). The red chamosite (IIb polytype) has a mean composition of (Fe 3.87 Mg 0.23 Mn 0.01 □ 0.07 Al 1.74 V 0.07)(Si 2.33 Al 1.67)O 10 (OH) 8. This chamosite is strongly pleochroic, from pale yellow (E || (001)) to deep orange red (E ⊥ (001)). Visible-near-infrared absorbance spectra show a specific absorption band centred at ∼550 nm for E ⊥ (001), due to a proposed new variety of Fe/V intervalence charge-transfer mechanism in the octahedral sheet, possibly Fe 2+-V 4+ → Fe 3+-V 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 O 2 related to oxidizing hydrothermal solutions.