Al-free di-trioctahedral substitution in chlorite and a ferri-sudoite end-member

A compilation of Fe3+-bearing chlorite analyses is used: (1) to investigate the Alfree di-trioctahedral (AFDT) substitution 2Fe(3+) +square= 3(Mg, Fe2+) in chlorite; and (2) to estimate the composition of a ferri-sudoite end-member (Si3Al)[(Fe2+, Mg) 2Fe(2)(3+)square Al] O-10(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 mu XANES 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.