Sudoite from diagenetic to very low-grade metaclastites of the Betic Cordillera was studied by X-ray diffraction and transmission/analytical electron microscopy. Sudoite formed directly from dickite, the assemblage dickite + sudoite + illite being replaced at increasing metamorphic grade by the assemblage pyrophyllite + sudoite + illite. Sudoite ranges in composition from Mg-rich to Fe-rich chemistries. In addition, a wide variety of mixed-layered structures (illite-sudoite, pyrophyllite-sudoite, and dickite-sudoite) was also identified. Mg-rich sudoite shows a mean chemical composition of (Al2.91Fe2+0.25Mg1.80)(Si3.10Al0.90)O10(OH)8, and a IIb ordered structure with b = 9.055 Å. Intermediate Fe-Mg sudoite exhibits a very variable composition, the Fe-rich phases having a mean composition of (Al2.09Fe3+0.61Fe2+0.87Mg1.44)(Si3.31Al0.69O10(OH)8. These are disordered polytypes with b values ranging from 9.070 to 9.101 Å. Fe occurs in both octahedral sheets, according to two types of substitutions: Fe3+ for Al in the dioctahedral sheet and Fe2+ for Mg in the trioctahedral sheet. Sudoite with such a composition has not been described previously.

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