Mineralogy and Genesis of Smectites in an Alkaline-Saline Environment of Pantanal Wetland, Brazil

Abstract

Smectite formation in alkaline-saline environments has been attributed to direct precipitation from solution and/or transformation from precursor minerals, but these mechanisms are not universally agreed upon in the literature. The objective of this work was to investigate the mineralogy of smectites in the soils surrounding a representative alkaline-saline lake of Nhecolândia, a sub-region of the Pantanal wetland, and then to identify the mechanisms of their formation. Soils were sampled along a toposequence and analyzed by XRD, TEM-EDS, and ICP-MS. Water was collected along a transect involving the studied toposequence and equilibrium diagrams were calculated using the databases PHREEQC and AQUA. The fine clay fraction is dominated by smectite, mica, and kaolinite. Smectites are concentrated at two places in the toposequence: an upper zone, which includes the soil horizons rarely reached by the lake level variation; and a lower zone, which includes the surface horizon within the area of seasonal lake level variation. Within the upper zone, the smectite is dioctahedral, rich in Al and Fe, and is classified as ferribeidellite. This phase is interstratified with mica and vermiculite and has an Fe content similar to that of the identified mica. These characteristics suggest that the ferribeidellite originates from transformation of micas and that vermiculite is an intermediate phase in this transformation. Within the lower zone, smectites are dominantly trioctahedral, Mg-rich, and are saponitic and stevensitic minerals. In addition, samples enriched in these minerals have much lower REE contents than other soil samples. Water chemistry shows a geochemical control of Mg and saturation with respect to Mg-smectites in the more saline waters. REE contents, water chemistry, and the presence of Mg-smectite where maximum evaporation is expected suggest that saponitic and stevensitic minerals originate by chemical precipitation from the water column of the alkaline-saline lake.

Biogeochemical diversity and hot moments of GHG emissions from shallow alkaline lakes in the Pantanal of Nhecolândia, Brazil

“…In this context, the Fe-mica (Furquim et al, 2010a;Barbiero et al, 2016), whereas alkalinity is involved in the formation of other clay minerals and calcite. The cations Ca 2+ and Mg 2+ are controlled by the formation of Mg-calcite with about 5 % Mg (Barbiero et al, 2008) and Mg-smectites of stevensite and saponite types (Furquim et al, 2008;2010b). Since the waters evolve in a sodicalkaline pathway, the salinity range is associated with a wide range in pH values, which affects both surface water (5.5 < pH 5 < 10.5) and lake sediments (5.7 < pH < 9.9).…”

Soil mineral genesis and distribution in a saline lake landscape of the Pantanal Wetland, Brazil

“…2B) are summarized below and more fully discussed by Furquim et al (2008). The soil consists of a pale brown (10YR 6/3), single grain, sand surface material (horizon 1), which is laterally replaced toward the lake by a dark gray (10YR 4/1), mostly prismatic, loamy sand, surface material, rich in decomposed algae (horizon 2).…”

“…1B) was carried out by our research team, the results of which are beginning to be disseminated (e.g. Furquim et al, 2008). In spite of the extensive number of saline lakes in the Nhecolândia, our studies represent a pioneering effort in understanding the silt and clay mineralogy in these landscapes.…”

Sediment geochemistry, mineral precipitation and clay neoformation on the Mkuze River floodplain, South Africa

“…Less commonly considered are solutes such as Si, Ca and Mg which have the potential to accumulate in mineral forms. Research conducted in the Okavango Delta, Botswana (McCarthy and Metcalfe, 1990;McCarthy et al, 1993), and the Pantanal, Brazil (Barbiero et al, 2002;Furquim et al, 2008), has shown that large wetland systems can accumulate significant quantities of solutes. Mass balance calculations indicate that chemical accumulation is the dominant aggradational process occurring in the Okavango basin, where it is responsible for shaping the structure and function of this ecosystem (McCarthy and Metcalfe, 1990).…”