The Occurrence of Authigenic Clay Minerals in Alkaline-Saline Lakes, Pantanal Wetland (Nhecolândia Region, Brazil)

Isis Armstrong Dias,Gustavo Barbosa Athayde,Anelize Manuela Bahniuk Rumbeslperger,Leonardo Fadel Cury,Larissa da Rocha Santos,Guilherme Fedalto,Ana Paula Soares,Camila de Vasconcelos Müller Athayde,Bruno Guimarães Titon

doi:10.3390/min10080718

Abstract

Mg clay minerals are usually associated with carbonates in alkaline-saline environments, precipitated from solution and/or transformation from other minerals. The aim of this research is to identify the mineralogy and geochemistry of clay minerals in different alkaline lakes in the Nhecolândia region, the southernmost region of the Pantanal wetland (Brazil). Sediment samples were analyzed by X-ray diffraction, X-ray fluorescence, scanning electron microscopy and transmission electron microscopy. Water samples were analyzed, determining their main cations and anions, in order to understand their relationship with the clays. The analyses allowed classifying the water bodies as saline, oligosaline and freshwater lakes. The sediments are composed mainly of quartz and a fine-clay fraction, dominated by illite, kaolinite and smectite. The XRD results showed illite and smectite mixed-layered in the saline lakes at Barranco Alto farm, whereas at Nhumirim farm, trioctahedral smectite was only observed in one lake. The smectite minerals were normally identified coupled with calcite at the top of the sequences, associated with exopolymeric substances (EPS) in the lakes, suggesting that these minerals are precipitating due to the physical-chemical and biological conditions of the water bodies.

Highlights

Clay minerals occur in different geological environments under varying climate, geological and chemical conditions
Data, fresh water has less than mg L−1 and we considered in this work, based on the data obtained on the field campaigns, the oligosaline lakes with total dissolved solids (TDS) values between and 1500 mg L−1 and the saline lakes with values higher than 1500 mg L−1 [26]
The clay minerals from the Pantanal lakes are controlled by distinctive processes of transportation, transformation and neoformation

Summary

Introduction

Clay minerals occur in different geological environments under varying climate, geological and chemical conditions. Minerals 2020, 10, 718 diagenesis and authigenic clays that normally take place in soil profiles in alkaline-saline environments under certain concentrations in arid to semi-arid conditions [6,7]. The authigenic clay minerals have frequently been reported as magnesium silicate minerals, e.g., kerolite, trioctahedral smectites, palygorskite and sepiolite [5,6,7,8]. Regarding the smectite minerals, depending on the concentration of the solution and the amount of detrital material available, dioctahedral or trioctahedral minerals are favored [7,8]. Dioctahedral smectites are mainly formed through transformation of detrital material and they are associated with environments rich in Al3+ and Fe3+ , while trioctahedral smectites are formed by precipitation from aqueous solutions, associated with Mg2+ [7,9,10]

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