Abstract
Abstract. 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.
Highlights
Glauconites are green micas significant in sedimentary environments
As the chemical composition of a given glauconite is dependent on its maturation degree (Odin and Matter, 1981; Baldermann et al, 2013), approaching that of celadonite with residence time, this latter mineral has been traditionally contemplated as a possible endmember for glauconites (e.g. Parron and Amouric, 1990)
The study of glauconite and celadonite coexisting at the nanometre scale has allowed us to acquire a deeper insight about their crystal-chemical relationships
Summary
Glauconites are green micas significant in sedimentary environments. Their restricted genetic conditions give them the status of key minerals for palaeoenvironmental interpretations; they usually form in shallow-marine settings (water depths < 500 m) in semi-confined microenvironments with partially reducing conditions in the sediment pore water, and their complete maturation needs long residence time with low sedimentation rate (Odin and Matter, 1981; Odin et al, 1988; Odin and Fullagar, 1988; Amorosi, 1995). The equivalent non-interlayer-deficient mica, having all the crystal-chemical characteristics described in the previous paragraph, would be celadonite, according to the IMA classification. Celadonite typically has a different origin, linked to the ocean floor hydrothermal al-
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