Variations, paleoenvironment implications, and agronomic considerations of glauconite expandable layers: A case study from Egypt

Abstract

As a contribution to many works conducted on glauconites, the current study represents an interdisciplinary approach combining geoscience and agronomy of these K-mineral resources, depending on the variations of expandable layers %. Through this context, two condensed sections of the Up. Cretaceous and Up. Eocene glauconites, located in the Western Desert of Egypt, were undergone field, petrographic, mineralogical, and geochemical investigations. The agronomic considerations were studied by simulating the organic acid-solubility of glauconite, using 2 % oxalic acid, in cases of its direct application as a slow-release K-fertilizer. Geologically, all of the studied glauconites were deposited authigenically by the neoformation of Fe-smectite precursor. The latter is thought to be more ferruginous in case of the Up. Cretaceous glauconite (nascent species, 3–4.1 wt% K₂O) than the Up. Eocene counterpart (slightly evolved-evolved species, 5.05–6.71 wt% K₂O). Depending on K₂O contents and ethylene glycol solvation of glauconite grains, expandable layers were calculated and averaged at 40.53 % and 24.03 % for the Up. Cretaceous and Up. Eocene glauconites, respectively. Also, expandable layers showed a gradual increment from base to top of the studied sections where shoaling upward occurs. Depending on field observations, mineralogical, and geochemical data, along with literature, Fe²⁺ availability and sea level changes are found to be the main controlling factors of glauconite expandable layers. Agronomically, oxalic acid-based solubility tests showed that K⁺ ions aren't completely tightly fixed into the interstratified structure of the highly expandable glauconite, causing faster solubility rates than the less expandable species. According to the concept of slow-release fertilizers, the more evolved, less expandable glauconite deposits are preferred for the long-lasting K supply required by plants and keeping the soil fertility of plant macro-nutrients. On the other side, the smectite-rich glauconite can be exploited as a soil conditioner for enhancing its cation exchange capacity.