Textural and genetic relationships between glauconite and celadonite at the nanoscale: two different structural-compositional fields

We acknowledge the use of the LMA-INA (Universidad de Zaragoza) for access to dual beam equipment and expertise, the technical and human support provided by CICT of Universidad de Jaén (UJA, MINECO, Junta de Andalucía, FEDER), and the use of the PANalytical X'Pert Pro diffractometer of the Departamento de Mineralogía y Petrología, Universidad de Granada, Spain. María del Mar Abad is especially recognized for helping with the HRTEM work. 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. Thanks are extended to Javier Cuadros and an anonymous reviewer for their critical reviews and very helpful comments and suggestions.Glauconite and celadonite coexist at the nanometre scale in Early Jurassic submarine volcanic rocks of the Betic Cordillera (southern Spain) as a result of microbial activity. Samples from the limit between the two micas, recognizable in scanning electron microscopy, have been extracted using the focussed ion beam technique and studied by high-resolution analytical electron microscopy. Both micas are present as randomly oriented differentiated small crystals in the boundary area. They define clearly distinct compositional fields with gaps affecting to Fe, Mg and K. At the lattice scale, celadonite shows a high degree of order, with homogeneous orientation of the visible lattice parameters being a difference from glauconite, formed by packets no more than 10-layers thick. Smectite layers were also detected alongside glauconite packets, in accordance with X-ray diffractograms which indicate that glauconite is a mica-smectite interstratification being more than 90 % mica layers. The compositional gap indicates that celadonite is not the endmember of the glauconitic series and the two micas represent two different structural tendencies of mica, with glauconite having more distorted octahedral sheets, indicated by systematically higher b parameters than celadonite.MCIU-AEIFEDER PGC2018-094573-B-I00 ,PID2019-104624RB-I00 and RTI2018-093419-B-I00Junta de Andalucia RNM-325, RNM-200 and RNM-179Gobierno de Aragon E18_17REuropean Fun