A compilation of Fe3+-bearing chlorite analyses is used: (1) to investigate the Al-free di-trioctahedral (AFDT) substitution 2Fe3+ + □ = 3(Mg,Fe2+) in chlorite; and (2) to estimate the composition of a ferri-sudoite end-member (Si3Al)[(Fe2+,Mg)2Fe3+2□Al]O10(OH)8 within the chlorite solid-solution domain. According to our observations, up to two Fe3+ cations might be allocated in the M2-M3 chlorite sites by the substitution AFDT, which does not involve Al. These unexpected observations were made possible by the development of µXANES techniques allowing in situ measurements of XFe3+ (Fe3+/(Fe2+ + Fe3+)) in heterogeneous chlorite. Although further studies are required to confirm the crystallographic position of Fe3+ and refine its ionic/magnetic behaviour in chlorite, this development creates opportunities for developing new geothermometers.

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