This study presents a mineralogical analysis of the effect of hypersaline lake environments rich in organic matter on the mineral transformations in sediments of the eastern part of the Guadix-Baza Basin (Betic Cordillera). Dark sediments with fragments of organic matter are rich in phyllosilicates (K-white mica, illite-smectite mixed layers, –I/S– minor amounts of paragonite, palygorskite, chlorite and kaolinite) and contain quartz, feldspars, and framboidal pyrite. X-ray diffraction patterns of I/S (R0 type, disordered) revealed a continuous between the positions of pure smectite and 30% of illite layers. Chemical composition of I/S mixed layers indicated a significant beidellitic substitution of Si by AlIV, Fe + Mg > 1 atoms per formula unit (a.p.f.u.) and K content up to 0.42 a.p.f.u. These data suggested the presence of a minor and variable amount of illitic layers among the major smectitic ones. Saline waters of the lake and the reducing environment of the sediment favored Mg and Fe2+ uptake, generating octahedral negative charge, which together with the beidellitic substitution would provide clay layer negative charge enough to admit K incorporation in the interlayer, hence the beginning of a diagenetic low temperature illitization.

This work has been financed by research projects PGC2018-094573- B-I00 and TASCUB (RTI2018-100737-B-I00) from the MCIU-AEIFEDER, and research groups RNM-325 and RNM-179 of the Junta de Andalucía. We acknowledge the suggestions and comments from two anonymous reviewers and from the editor (Dr. M.V. Villar). María del Mar Abad is especially recognized for helping with the HRTEM and for being able of providing us the AEM analyses during the COVID-19 lockdown. The “Centro de Instrumentación Científico-Técnica of the Universidad de Jaén” is aknowledged for the use of the PANalytical Empyrean diffractometer and the Merlin Carl Zeiss electron microscope. The use of the PANalytical X'Pert Pro diffractometer of the “Departamento de Mineralogía y Petrología, Universidad de Granada, Spain” is also acknowledge. The access to the HAADF FEI TITAN G2 microscope and the Philips CM20 (STEM) microscope was facilitated by the “Centro de Instrumentación Científica of the Universidad de Granada”. The “Instituto de Recursos Naturales y Agrobiología of Sevilla, CSIC” is acknowledged for provide organic carbon analyses.