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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.
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minerals
Article
The Occurrence of Authigenic Clay Minerals
in Alkaline-Saline Lakes, Pantanal Wetland
(Nhecolândia Region, Brazil)
Isis Armstrong Dias 1,* , Leonardo Fadel Cury 1, Bruno Guimarães Titon 1,
Gustavo Barbosa Athayde 2, Guilherme Fedalto 1, Larissa da Rocha Santos 1,
Ana Paula Soares 1, Camila de Vasconcelos Müller Athayde 2and
Anelize Manuela Bahniuk Rumbeslperger 1
1Instituto LAMIR, Universidade Federal do Paraná, Centro Politécnico, Jardim das Américas,
Curitiba 81531-980, PR, Brazil; cury@ufpr.br (L.F.C.); btiton47@gmail.com (B.G.T.);
fedalto.guilherme@gmail.com (G.F.); lrsantos.geo@gmail.com (L.d.R.S.); a_p_soares@yahoo.com (A.P.S.);
anelize.bahniuk@ufpr.br (A.M.B.R.)
2Laboratório de Pesquisas Hidrogeológicas, Universidade Federal do Paraná, Centro Politécnico,
Jardim das Américas, Curitiba 81531-980, PR, Brazil; gustavo.athayde@ufpr.br (G.B.A.);
camilavmuller@ufpr.br (C.d.V.M.A.)
*Correspondence: armstrong.oi.dias@gmail.com
Received: 19 May 2020; Accepted: 13 July 2020; Published: 17 August 2020


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 dierent alkaline lakes in the
Nhecol
â
ndia region, the southernmost region of the Pantanal wetland (Brazil). Sediment samples were
analyzed by X-ray diraction, 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.
Keywords: alkaline lakes; clay minerals; EPS; chemical precipitation
1. Introduction
Clay minerals occur in dierent geological environments under varying climate, geological
and chemical conditions. The weathering environment, rock type, topography and the presence of
organisms and organic matter also play a prominent role in the formation of clay minerals [
1
]. The clays
correspond to nearly 70% of the sedimentary rocks found worldwide and can be designated as detrital,
transformed and neoformed clays [
2
].The detrital clays are derived from erosion of soils and weathered
rocks, being transported and accumulated; transformed clays are secondary clay minerals originated by
reactions undergone at the deposition, burial and diagenesis processes and neoformed clays precipitate
directly from solutions [
1
5
]. The last two types of clay minerals formation are associated with
Minerals 2020,10, 718; doi:10.3390/min10080718 www.mdpi.com/journal/minerals
Minerals 2020,10, 718 2 of 26
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
8
]. The authigenic clay minerals can also
comprise corrensite, dioctahedral smectite minerals, a variety of mixed-layer minerals, as well as illite
formed by illitization processes [
6
]. 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 Al
3+
and Fe
3+
, while trioctahedral smectites are
formed by precipitation from aqueous solutions, associated with Mg2+[7,9,10].
The Nhecol
â
ndia sub-region of the Pantanal wetland is known as a modern alkaline-saline
environment, influenced by seasonal weather conditions, located in western Brazil [
11
,
12
].
The Nhecol
â
ndia region comprises thousands of fresh-water and alkaline-saline lakes, which are
distributed next to each other, but display a wide chemical variability [
7
]. The salinity of the lakes used
to be attributed to an inherited past, but more recently it has been related to a continuous evaporitic
concentration of the freshwater [
13
]. The chemical variability is possibly related to precipitation of
calcite or Mg-calcite and formation of Mg-silicates as the solutions become more saline [13,14].
Several works have already discussed the distribution, classification and geochemistry of the
lakes, as well as the main mineral compositions of the sediments. Researches in alkaline-saline lakes
in the Nhecol
â
ndia studied the mineralogy of smectite minerals, indicating magnesium clay minerals
precipitate under exogenous conditions by organic control [
8
,
14
]. In this context, the aim of this
research is to study the mineralogical and geochemical features of the clay minerals and lakes from
two distinct regions at the Nhecol
â
ndia region, as well as to identify a possible relation to carbonate
precipitation through biologically influenced mineralization.
2. Geological Setting
The Pantanal is the largest tropical wetland of the world and its origin is dated to the Proterozoic
times [
15
]. The Pantanal is currently an active sedimentary basin, one of the four geographic areas of
the Central Andean foreland basin system, corresponding to the most eastern region, also referred
to as back-bulge depozones [
16
]. The third depozone described by the aforementioned authors is
known as forebulge and it is found in eastern Bolivia and northern Paraguay, located west, parallel to
the Pantanal wetland. The subsidence of the Pantanal is related to an elastic response caused by the
extension of the upper crust when the forebulge initiated its development [17].
The floodplains of Pantanal are surrounded by highlands and plateaus, where the Proterozoic and
Cambrian basement rocks are exposed [
15
,
17
]. Besides the magmatic and low-grade metamorphic rocks
of the Cuiab
á
Group basement rocks, the highlands also comprise rocks of the Corumb
á
Group at the west
edge, and Paleozoic and Mesozoic rocks of the Paran
á
Basin, at the east edge of the Pantanal wetland [
12
].
The depocenter of the Pantanal basin reaches approximately 500 m [
17
]. The basin is filled by
siliciclastic sediments (primarily quartzose sands) from dierent source areas, mainly from the adjacent
highlands, such as Paleozoic and Mesozoic sediments from the Paran
á
Basin on the eastern limit
and quaternary sediments from the alluvial fans of Taquari River to the northeast, which consists in
the Quaternary Pantanal Formation [
12
,
18
,
19
]. The Pantanal Formation includes conglomerates and
coarse-grained sandstones in the bottom part, with fining upward sequences, where fine and medium
quartz sands prevail. Locally, the presence of iron oxide cements and reddish sands are observed [
19
].
The Pantanal wetland is inserted in the Upper Paraguay River drainage basin, where the Paraguay
River flows to N–S and cross-cutting the marginal alluvial fans, which flows to E–W [
19
]. Most of
the Pantanal area is dominated by hydrodynamic alluvial fans and the most important of them is the
Taquari River alluvial fan, extending over an area of 54,125 km
2
, with a 250 km diameter and circular
shape; [12,19].
Minerals 2020,10, 718 3 of 26
The southern half of the Taquari alluvial fan is called Nhecol
â
ndia, showing a very low topographic
gradient and corresponds to the oldest depositional lobe of the Taquari alluvial fan [
12
,
20
]. The Nhecol
â
ndia
region is characterized by thousands of lakes which vary widely on the basis of spatial, physical, chemical
and biological features [20], surrounded by elevations of 3–4 m, locally called “cordilheiras.
The physiography of the region, the formation of the lakes, as well as their variabilities and the
sediment features may be related to dierent origins [
21
]. Several works have related the lakes and
the quartzose sands to an eolian origin, once the sands are white fine to medium-grained, and the
cordilheiras would be morphological expressions of ancient dunes [
12
,
21
]. On the other hand, there is a
hypothesis that indicates a fluvial genesis for the lakes, and the textural features of the sediments might
be related to their source area, therefore being an inherited feature [
22
]. The authors have proposed
that the current landscape was formed after the Late Pleistocene glaciation, when the elevation of the
regional phreatic level would have formed the lakes, and the waters would have become progressively
briny. The morphology of the lakes and the sand banks (cordilheiras) could be explained by the shifting
of the river channels, being the result of the cut-omeanders. In all these studies, the evolution of the
Nhecolândia is related to wet and arid phases during the Pleistocene [13].
Currently, the Pantanal experiences annual flooding from September to April and a dry season
from May to August [
19
]. The annual rainfall provides a heterogeneous filling of rivers, lakes and
groundwater, promoting a high chemical variability to the waters. The lakes generally show high
alkalinity values and, some of them, high salinity. The origin of dierent types of water in the lakes
is still under debate: there is the hypothesis that lakes have become progressively brinier [
22
] and
another hypothesis indicates that the lakes derived from the same water source and underwent
dierent degrees of evaporation and, thus, dierent concentrations of ions [
7
,
13
]. Additionally, the last
hypothesis was complemented with studies showing that the evaporation rates are the same on the
region and the distinct degrees of concentration are related to dissimilar insulation degrees, caused by
a sub-superficial continuous horizon of low-porosity materials, as amorphous silica and smectites [
23
].
Several works have been conducted to investigate and classify these lakes according to pH, electric
conductivity (EC) and total dissolved solids (TDS) values [
13
,
18
,
21
,
24
]. These lakes can be distinguished
between hyperalkaline and alkaline lakes, despite showing a huge variance [
18
]. They can also be
classified between freshwater and saline lakes, based on EC values [
21
] and finally, discriminated into
five types according to EC and TDS values: freshwater; water with low to medium salinity; water with
high salinity; water with very high salinity; and hypersaline water [24].
The most recent classification has distinguished the lakes according to biological and geochemical
features [
25
]. The lakes are classified as macrophyte or freshwater lakes, where autotrophic metabolism
predominates above the lake waterline (EC <2000
µ
S/cm and pH <7.5); bacterial or oligosaline lakes,
where heterotrophic metabolism predominates (500 <EC <5000
µ
S/cm and 7.9 <pH <10.5); or saline
lakes, where metabolism is also autotrophic, but predominates below the lake waterline (700 <EC <
65,000 µS/cm and 7.9 <pH <10.5).
3. Materials and Methods
3.1. Study Area and Sampling
The study areas are located on the south and southwestern parts of the Taquari alluvial fan,
at Barranco Alto farm, near to Aquidauana/MS, and Nhumirim (Embrapa) farm, near to Corumb
á
/MS,
respectively (Figure 1). Three field campaigns were conducted in dierent seasons. The first was at
Barranco Alto farm, from 26 to 30 June 2017 at the beginning of the dry season. The second was at
Nhumirim farm, from 20 to 24 August 2018, and the third again at Barranco Alto farm, from 17 to 21
September 2018. The last two campaigns were scheduled to happen at the end of the dry season,
however, despite the low rate of rainfall in the region during the wet season and similar rainfall rate
during the wet season in 2017–2018, the lakes had not dried and the water level was significantly
higher than in 2017.
Minerals 2020,10, 718 4 of 26
In Barranco Alto farm, we studied twelve lakes and the Negro River. In Nhumirim farm, we studied
five lakes and the Paraguay River. In each lake and river, we collected samples of water and sediments
following a sequence extending from the border of the lakes to their central part (P1, P2, P3 and P4),
resulting in two to four collection points. When it was necessary, the sampling points were divided into
bottom, middle and top samples (Figure 2). We collected 160 samples of sediment, but only 95 were
analyzed: 31 from the first campaign, 38 from the second and 26 from the third campaign. The samples
were chosen according to the amount of sand and clay.
The water temperature (WT
C), the pH, the EC (mS cm
1
) and the TDS (mg L
1
) were measured
in situ with a multiparameter equipment. The lakes were classified adapting recent criteria, which take
functional biogeochemistry, pH and EC values to classify them as saline, oligosaline and freshwater
lakes [
25
]. In this work, we dierentiate the lakes based on the names proposed by the authors;
however, we used the TDS values as the main parameter to dierentiate them. According to the TDS
data, fresh water has less than 500 mg L
1
and we considered in this work, based on the data obtained
on the field campaigns, the oligosaline lakes with TDS values between 501 and 1500 mg L
1
and the
saline lakes with values higher than 1500 mg L1[26].
3.2. Water Analyses
The water samples were analyzed for cations and anions in the Laborat
ó
rio de Pesquisa Hidrogeol
ó
gica
(LPH-UFPR), whose methods are described on Table 1. The samples were prior filtered in the field
through 0.45
µ
m surfactant-free cellulose acetate (SFCA) syringe filters [
27
]. In the laboratory,
a fiberglass membrane and then a cellulose ester membrane were used to filter the samples from the
campaign of 2017, whose water had more suspension material than those from 2018.
Table 1. Equipment and methodologies used for water analyses.
Element Equipment Methodology Quantification Limit
Alkalinity
Automatic burette Titration
0.10 mg L1CaCO3
HCO30.10 mg L1HCO3
CO30.10 mg L1CO3
Ca Atomic absorption
Spectrometer Atomic absorption
Spectrometer 0.10 mg L1
Mg
Cl
Spectrophotometer
UV/VIS II (1901100 nm)
REF 918 20 NANOCOLOR®0.20 mg L1Cl
Si REF 936 225 VISOCOLOR®2.00 mg L1SiO2
Fe REF 918 36 NANOCOLOR®0.01 mg L1Fe
SO4Turbidimetric determination 1.00 mg L1SO4
Na Flame photometer Flame photometer 0.10 Na mg L1
K0.10 K mg L1
Minerals 2020,10, 718 5 of 26
Minerals 2020, 10, x FOR PEER REVIEW 5 of 32
Figure 1. Map of the Barranco Alto and Nhumirim farms in the sub-region of Nhecolândia, Pantanal
wetland (Brazil).
Figure 1.
Map of the Barranco Alto and Nhumirim farms in the sub-region of Nhecol
â
ndia, Pantanal
wetland (Brazil).
Minerals 2020,10, 718 6 of 26
Minerals 2020, 10, x FOR PEER REVIEW 6 of 32
Figure 2. (a) Aerial view of Barranco Alto farm; (b) sampling points in the margins (formed by sand or
a crust of sediments, carbonate and iron oxide) and inside the lakes; (c) sampling core with the
sediments split in bottom, middle and top portions; (d) water samples in plastic containers.
3.3. Mineralogical Analyses
The mineralogical analyses were carried out at Instituto LAMIR (Laboratório de Análises de
minerais e rochas) of the Geology Department at Federal University of Parana. The composition of the
sediments was determined by means of X-ray diffraction (XRD) through a PANalytical
diffractometer model Empyrean with an X-celerator detector. Scans from bulk sediments and clay
minerals were run from 2θ angles of 3°to 70°and 3°to 30°, respectively, using a step-size of 0.016° and
count time of 10.16 s per step [27]. A total of 74 bulk sediment samples and 69 clay mineral samples
were analyzed, since some of the bulk sediments were composed solely by sand.
To analyze the mineralogy of the clay minerals, approximately 30 g of the bulk sediment was
sifted in a 350-mesh sieve with distilled water. The solution was firstly centrifuged for 7 min at 800
rpm to decant the coarser fraction. The supernatant fluid was transferred to another plastic tube and
centrifuged again for 30 min at 3000 rpm. The resulting mixture was poured on a glass slide covering
at least 1 cm2 of the glass slide. The samples were air-dried under room temperature and pressure
(STP) to obtain oriented analyses with three different treatments: air-dried sample, ethylene-glycol
vapor saturation for 8–12 h (EG) and heating at 550 °C for 2 h [27].
The clay minerals were identified mostly by their 001 peak after the three treatments cited above.
The dioctahedral and trioctahedral character of smectites were identified by their 060 diffraction
/peaks. Table 2 shows some clay minerals and their respective 001 and 060 peaks. Sepiolite and
palygorskite present 110 reflections at 12.0–12.3 and 10.4–10.5, respectively.
Table 2. Clay minerals and their 001 and 060 peaks [28].
Clay Minerals d(001) d(060) Å
Air Dried Ethylene-Glycol 550 °C
Kaolinite 7.0–7.1 No change Collapse 1.49
Illite 10.1 No change No change 1.50
Glauconite 10.1 No change No change 1.51–1.52
Figure 2.
(
a
) Aerial view of Barranco Alto farm; (
b
) sampling points in the margins (formed by sand or a
crust of sediments, carbonate and iron oxide) and inside the lakes; (
c
) sampling core with the sediments
split in bottom, middle and top portions; (d) water samples in plastic containers.
3.3. Mineralogical Analyses
The mineralogical analyses were carried out at Instituto LAMIR (Laborat
ó
rio de An
á
lises de minerais
e rochas) of the Geology Department at Federal University of Parana. The composition of the sediments
was determined by means of X-ray diraction (XRD) through a PANalytical diractometer model
Empyrean with an X-celerator detector. Scans from bulk sediments and clay minerals were run from 2
θ
angles of 3
to 70
and 3
to 30
, respectively, using a step-size of 0.016
and count time of 10.16 s per
step [
27
]. A total of 74 bulk sediment samples and 69 clay mineral samples were analyzed, since some
of the bulk sediments were composed solely by sand.
To analyze the mineralogy of the clay minerals, approximately 30 g of the bulk sediment was
sifted in a 350-mesh sieve with distilled water. The solution was firstly centrifuged for 7 min at 800 rpm
to decant the coarser fraction. The supernatant fluid was transferred to another plastic tube and
centrifuged again for 30 min at 3000 rpm. The resulting mixture was poured on a glass slide covering
at least 1 cm
2
of the glass slide. The samples were air-dried under room temperature and pressure
(STP) to obtain oriented analyses with three dierent treatments: air-dried sample, ethylene-glycol
vapor saturation for 8–12 h (EG) and heating at 550 Cfor2h[27].
The clay minerals were identified mostly by their 001 peak after the three treatments cited above.
The dioctahedral and trioctahedral character of smectites were identified by their 060 diraction /peaks.
Table 2shows some clay minerals and their respective 001 and 060 peaks. Sepiolite and palygorskite
present 110 reflections at 12.0–12.3 and 10.4–10.5, respectively.
Minerals 2020,10, 718 7 of 26
Table 2. Clay minerals and their 001 and 060 peaks [28].
Clay Minerals d(001) d(060) Å
Air Dried Ethylene-Glycol 550 C
Kaolinite 7.0–7.1 No change Collapse 1.49
Illite 10.1 No change No change 1.50
Glauconite 10.1 No change No change 1.51–1.52
Smectite (dioc) 15.0 16.9 10.0 1.49–1.52
Smectite (trioc) 15.0 16.9 10.0 1.52–1.54
Vermiculite ~14.0 No change No change 1.54
Chlorite ~14.0 No change
Increase the intensity
1.54
There is a wide variation in 060 diraction peaks among dioctahedral and trioctahedral smectites.
In dioctahedral smectites, for instance, montmorillonite and beidellite show d060 at 1.49–1.50 Å,
while nontronite shows d060 at 1.52 Å [
28
]. In trioctahedral smectites, stevensite and saponite present
060 at 1.52 Å and hectorite presents d060 at 1.53 Å.
Mixed-layered clay minerals are very common in natural environments [
2
]. The identification of
mixed-layered minerals is based on the entire diffraction pattern, like breadth, symmetry, intensity and
peak position [
28
]. Illite/smectite (I/S) are the most common mixed-layered clay minerals in sedimentary
rocks and soils. They can be recognized by their altered diffraction pattern under EG solvation treatment
and after heating the sample to 375for 1 h, resulting in a pattern similar to that of illite [28].
Non-clay minerals are very common in sedimentary rocks, such as quartz, feldspar, zeolites,
carbonates, apatite, pyrite, gypsum and others [
28
]. Beyond the carbonates, calcite and dolomite
are normally associated with clay minerals. The d-spacings of the most intense peak of calcite and
dolomite are 3.04
and 2.89
2
θ
, respectively [
28
]. The empirical curve of a calcite-disordered dolomite
solid-solution series was used to identify the amount of mol% MgCO
3
[
29
]. The semi-quantitative
XRD percentages of each sample were obtained through the Rietveld multi-phase standard
analysis, performed in the HighScore Plus PANanytical Software (Version 3.0, Malvern, Amsterdam,
The Netherlands).
A thin section of a crust sample was examined under transmitted light using the Zeiss Imager
A2m Microscope (Carl Zeiss Microscopy, New York, NY, USA) aiming to observe the contact relations
between grains and cements.
Images and chemical composition of specific points in 10 samples were collected using a JEOL
scanning electron microscope (SEM, JEOL Ltd., Peabody, MA, USA) model 6010LA, equipped with
an energy dispersive X-ray spectrometer (EDX, JEOL Ltd., Peabody, MA, USA) EX-94410T1L11 for
elemental analyses. The morphology of illite crystals consists of ribbon-like flakes projections, as well
as filamentous pore-lining and pore-bridging for authigenic illites [
30
]. The kaolinite occurs as
stacks of pseudohexagonal plates or blocks and vermiform crystals. Smectite consists of a webby or
highly-crenulated pore lining and thin ribbon of pore-bridging morphologies [30].
Images of one selected fine-clay fraction sample were obtained by transmission electron microscopy
(TEM) using a JEOL JEM 1200EX-II instrument (JEOL Ltd., Peabody, MA, USA) operated at 120 kV
(medium resolution) at Centro de Microscopia Eletr
ô
nica (CME) in the Federal University of Parana.
Images, qualitative analyses and quantitative chemical analyses of 6 samples collected in 2017 were
performed by scanning tunneling electron microscopy (STEM) with a FEI TITAN G2 instrument
operated at 300 kV (high resolution—HRTEM) at Centro de Instrumentaci
ó
n Cient
í
fica (CIC) in Granada
University (Spain). The chemical composition was determined by analytical electron microscopy
(AEM) in the HRTEM and the data were used to calculate the chemical formulae.
The HRTM analyses usually show illite particles as straight and relatively defect-free lattice fringes,
with continuous and constant 10 Å interlayer spacings, with a mottled contrast, whereas smectite
particles show anastomosing and imperfect 14 Å lattice fringe images [
31
]. AEM is a technique for
quantitative chemical analysis of crystals of clay minerals, where data can be interpreted by phase
diagrams [
2
,
31
33
]. Muscovite, albite, biotite, spessartite, olivine and titanite were used to obtain
Minerals 2020,10, 718 8 of 26
k-factors to the correct energy dispersion X-ray data by the thin-film method [
34
]. Errors for analyzed
elements (two standard deviations) expressed in percentage of the atomic proportions are 6 (Na),
3 (Mg), 2 (Al), 4 (K), 4 (Ca), 5 (Ti), 3 (Mn) and 3 (Fe). Instrumental conditions for spectra acquisition
were 200 s of live time for all elements except for K and Na, for which a time of 30 s was used due to
volatilization problems.
Contents of the major oxides of 92 bulk-sample sediments were measured by means of X-ray
fluorescence (XRF) using a PANalytical spectrometer model AXIOS MAXDY 5297 (Malvern, Amsterdam,
The Netherlands), through quantitative analyses at Instituto LAMIR (Laborat
ó
rio de An
á
lises de minerais
e rochas) of the Geology Department at the Federal University of Parana.
4. Results
4.1. Water Chemistry
Despite the proximity between the studied lakes, they show a high spatial, physical and chemical
variability. In the first field campaign at Barranco Alto farm (2017), the lakes showed differences related to
their physical-chemical parameters. Table 3shows the mean results of temperature, pH, EC and TDS
measured on field and the concentrations of cations and anions in meq L
1
(except SiO
2
, whose value is
in mmol L
1
), measured at the LPH. The two field campaigns of 2018 were scheduled to happen in a drier
season than 2017. However, as mentioned before, the region was still flooded, and the physical-chemical
aspects of the lakes showed lower values than 2017 (in the case of Barranco Alto farm).
In 2017, the northernmost lakes in Barranco Alto farm (Tubar
ã
o, Burro Branco, Coraç
ã
oand Mara
Maravilha) were considered saline lakes. These lakes also exhibited the highest pH (10–11) and EC
(
>3 mS cm1
) values. The Mineira Grande and Sete lakes were considered as oligosaline lakes and the
others (nearest to the Negro River) were classified as freshwater lakes. In 2018, the lakes from Barranco
Alto farm showed a great dierence in comparison with the results of 2017. Mara Maravilha was the
only saline lake, while Sete, Coraç
ã
o, Nova, Burro Branco, Sete C, Jap
ã
oand Jacar
é
were classified as
oligosaline lakes and the others were considered as freshwater lakes.
In Nhumirim farm, the Meio and Oito lakes were considered as oligosaline. Nevertheless, Meio lake
has shown a pH >10.18 and TDS value of 926.00 mg L
1
, while at Oito lake, the pH is 8.60 and TDS
value is 651 mg L1. The other lakes were classified as freshwater.
Considering the average values of pH, the freshwater lakes exhibited values lower than 9,
the oligosaline lakes values near 9.5 and the saline lakes values higher than 10.
It was also possible to distinguish the lakes according to their spatial and morphological features
in 2017. The saline lakes have an average depth of 50 cm with a distinctive white beach devoid of
vegetation around them, which is ringed by carandas palm trees, Gramineae,Bromeliaceae and the forest
towards the “cordilheiras” [
20
]. The freshwater lakes, on the other hand, show depths higher than
1 m, do not have beaches surrounding them, and aquatic vegetation is very common, as well as the
presence of animals, like alligators. In 2018, however, those features were not evident, and all the lakes
showed similar morphological features.
Similarly, the measured elements in the water and listed in Table 3show the same proportion
related to the pH, EC and TDS values, when the data are split in milliequivalent per liter (Figure 3).
Comparing the average values of saline, oligosaline and freshwater lakes in the three field campaigns
(in meq L1), it is possible to recognize the highest values among saline and oligosaline lakes.
Regarding the anion content, bicarbonate shows the highest value in almost all of the campaigns,
except for the saline lakes from Barranco Alto 2017 and freshwater lakes from Nhumirim 2018, where the
carbonate ions were higher. The results from Barranco Alto in 2018, on the other hand, have exhibited
larger amounts of Cl
than CO
32
in all types of lakes. This feature is only observed in 2017 in the
freshwater lakes.
Minerals 2020,10, 718 10 of 26
In all of the lakes, the main cations are Na
+
and K
+
, except for the saline lakes at Barranco Alto
2017 and freshwater lakes at Nhumirim 2018, where the carbonate ions were higher. Both saline and
oligosaline lakes of 2018
0
s campaigns have exhibited similar compositions of silica, Ca
2+
, Mg
2+
and Fe
2+
.
For all the studied parameters, Barranco Alto has shown the highest concentrations, highlighting
Na
+
, K
+
, Cl
, C.E and TDS. Only Ca
2+
and Mg
2+
mean concentrations are higher in the Nhumirim
farm than in the Barranco Alto, where there are lower amounts of Na+and K+.
Minerals 2020, 10, x FOR PEER REVIEW 2 of 32
Figure 3. Distribution of the main elements in the saline, oligosaline and freshwater lakes from the
three sampling campaigns. The values are given in meq L1, except for SiO2, whose values are
normally given in mmol L1.
4.2. Occurrence of Microbial Mats
Exopolymeric substances (EPS) have been observed in both studied sites. In Barranco Alto, there
were EPS in the Sete, Sete B, Mineira Pequena and Tubarão lakes, mainly in 2017, with a green color and
Figure 3.
Distribution of the main elements in the saline, oligosaline and freshwater lakes from the three
sampling campaigns. The values are given in meq L
1
, except for SiO
2
, whose values are normally
given in mmol L1.
Minerals 2020,10, 718 11 of 26
4.2. Occurrence of Microbial Mats
Exopolymeric substances (EPS) have been observed in both studied sites. In Barranco Alto, there
were EPS in the Sete, Sete B, Mineira Pequena and Tubar
ã
olakes, mainly in 2017, with a green color and a
viscous aspect (Figure 4b). In those lakes, the EPS were normally associated with algae, forming a
green viscous algalic net with some bubbles. In Nhumirim, EPS were observed in the Oito and Seis
lakes and showed an orange color, formed just by bubbles (Figure 4a). At some points of some lakes,
the algae were observed with no association with the EPS, and were identified as charophyte (Figure 4c).
A microbial mat was identified above the sediments on the crust of Seis lake, showing green and red
interbedded sheets of 2–3 mm each (Figure 4d).
Minerals 2020, 10, x FOR PEER REVIEW 3 of 32
a viscous aspect (Figure 4b). In those lakes, the EPS were normally associated with algae, forming a
green viscous algalic net with some bubbles. In Nhumirim, EPS were observed in the Oito and Seis
lakes and showed an orange color, formed just by bubbles (Figure 4a). At some points of some lakes,
the algae were observed with no association with the EPS, and were identified as charophyte (Figure
4c). A microbial mat was identified above the sediments on the crust of Seis lake, showing green and
red interbedded sheets of 2–3 mm each (Figure 4d).
Figure 4. (a) Orange bubbles forming the exopolymeric substances (EPS) at Seis Lake, in Nhumirim
2018; (b) green viscous EPS at Sete lake, in Barranco Alto in 2017; (c) the algae type charophyte
observed at Sete B lake in 2017 and (d) The sediment crust from Seis lake, with the microbial mat on
the top the sample.
4.3. Petrographic Characterization
White unconsolidated sands predominate in “cordilheiras” and lakes’ margins at Pantanal, they
are composed mainly of quartz, feldspar and iron minerals, and the grains are fine, homogeneous
and rounded, probably associated with the top sequences of the Pantanal Formation. The “beach”
around the saline and oligosaline lakes is also formed by carbonate. The carbonate was observed as
a thin white crust over the sand. Two thin sections were made of a consolidated rock of Coração lake
and Negro River (Barranco Alto farm) from the 2017 and 2018 campaigns, respectively. Both samples
are composed mostly of quartz. The grains show a mottled aspect, varying from fine to medium
granulometry and sub-rounded morphology with high sphericity.
In 2017, Coração lake exhibited a wide area composed of a concentric crust of interbedded
sandstones, carbonates and iron nodules (Figure 5a). Microscopically, besides the quartz grains, it
was also noticed a very thin light brown material around the grains (Figure 5b), which could be
constituted of clay minerals.
At higher magnification, the fine-grained material shows wispy laminations and it seems to be
coating the quartz grain boundaries and pores (Figure 5c).
The sample collected on the banks of the Negro River could be described as a massive
agglomerate of sandy clay (Figure 5d). In thin sections, there are clays with brown color (black arrow)
and clays with greenish-brown color (red arrow), the latter being the most common in the sample
(Figure 5e). Observing these greenish-brown clays in a higher magnification (red arrows), it is noticed
Figure 4.
(
a
) Orange bubbles forming the exopolymeric substances (EPS) at Seis Lake, in Nhumirim
2018; (
b
) green viscous EPS at Sete lake, in Barranco Alto in 2017; (
c
) the algae type charophyte observed
at Sete B lake in 2017 and (
d
) The sediment crust from Seis lake, with the microbial mat on the top
the sample.
4.3. Petrographic Characterization
White unconsolidated sands predominate in “cordilheiras” and lakes’ margins at Pantanal, they
are composed mainly of quartz, feldspar and iron minerals, and the grains are fine, homogeneous
and rounded, probably associated with the top sequences of the Pantanal Formation. The “beach”
around the saline and oligosaline lakes is also formed by carbonate. The carbonate was observed as a
thin white crust over the sand. Two thin sections were made of a consolidated rock of Coraç
ã
olake
and Negro River (Barranco Alto farm) from the 2017 and 2018 campaigns, respectively. Both samples
are composed mostly of quartz. The grains show a mottled aspect, varying from fine to medium
granulometry and sub-rounded morphology with high sphericity.
In 2017, Coraç
ã
olake exhibited a wide area composed of a concentric crust of interbedded
sandstones, carbonates and iron nodules (Figure 5a). Microscopically, besides the quartz grains, it was
also noticed a very thin light brown material around the grains (Figure 5b), which could be constituted
of clay minerals.
Minerals 2020,10, 718 12 of 26
At higher magnification, the fine-grained material shows wispy laminations and it seems to be
coating the quartz grain boundaries and pores (Figure 5c).
The sample collected on the banks of the Negro River could be described as a massive agglomerate
of sandy clay (Figure 5d). In thin sections, there are clays with brown color (black arrow) and clays
with greenish-brown color (red arrow), the latter being the most common in the sample (Figure 5e).
Observing these greenish-brown clays in a higher magnification (red arrows), it is noticed that they
form a “bridge” (red circle) between the quartz grains (Figure 5f), which widely occurs in the section.
Minerals 2020, 10, x FOR PEER REVIEW 4 of 32
that they form a “bridge” (red circle) between the quartz grains (Figure 5f), which widely occurs in
the section.
Figure 5. (a) Layered crust around Coração lake in 2017; (b) photomicrograph of the crust composed
by quartz grains, vugy pores and brown clay material coatings; (c) detail of the lamination in the
clayey material; (d) the banks in the margins of the Negro River. The rock is described as a
consolidated sandy clay; (e) photomicrograph of the sample, with predominance of quartz grains.
The black and red arrows indicate, respectively, the brown and the greenish-brown clays; (f) detail of
the greenish-brown clays between the quartz grains. They usually develop a “bridge” aspect (red
polygons) between adjacent grains.
4.4. Mineral Characterization
XRD analyses of the bulk samples confirmed the dominance of quartz, mainly in the samples
from the crust and border. The second mineral is potassium feldspar, followed in some samples by
calcite, that is usually present in the saline and oligosaline lakes. Table 4 shows the abundance of
three representative samples from each campaign. This abundance represents a relative proportion
of the phases, measured by using the automatic semi-quantification program.
Table 4. Relative abundance of selected samples from each campaign. Qz—quartz; Kfs—K-feldspar;
Ilt—illite; Kln—kaolinite; Cal—calcite; Sme—smectite; (-) not observed; (*) not measured.
Figure 5.
(
a
) Layered crust around Coraç
ã
olake in 2017; (
b
) photomicrograph of the crust composed by
quartz grains, vugy pores and brown clay material coatings; (
c
) detail of the lamination in the clayey
material; (
d
) the banks in the margins of the Negro River. The rock is described as a consolidated sandy
clay; (
e
) photomicrograph of the sample, with predominance of quartz grains. The black and red
arrows indicate, respectively, the brown and the greenish-brown clays; (
f
) detail of the greenish-brown
clays between the quartz grains. They usually develop a “bridge” aspect (red polygons) between
adjacent grains.
4.4. Mineral Characterization
XRD analyses of the bulk samples confirmed the dominance of quartz, mainly in the samples
from the crust and border. The second mineral is potassium feldspar, followed in some samples by
calcite, that is usually present in the saline and oligosaline lakes. Table 4shows the abundance of three
Minerals 2020,10, 718 13 of 26
representative samples from each campaign. This abundance represents a relative proportion of the
phases, measured by using the automatic semi-quantification program.
Table 4.
Relative abundance of selected samples from each campaign. Qz—quartz; Kfs—K-feldspar;
Ilt—illite; Kln—kaolinite; Cal—calcite; Sme—smectite; (-) not observed; (*) not measured.
Farms Year Lakes Bulk Rock (%) Clay Fraction (%)
Qz Kfs Ilt Kln Cal Ilt Kln Sme Cal
Barranco
Alto
2017
Burro Branco 78 17 - - 5 66 29 - 5
Coração84 12 - - 4 26 71 3 -
Negro River 69 16 - 15 - 25 70 4 -
2018
Sete C 90 10 - - - 70 28 - 2
Mineira 89 11 - - - 41 59 * -
Negro River 85 15 - - - 32 52 16 -
Nhumirim 2018
Meio Lake-P2 92 8 - - - 63 30 5 2
Meio Lake-P3 91 8 - - 1 51 24 25 -
Meio Lake-P4 90 8 - - 2 85.9 8 * 6
In 2017, calcite widely occurred in the sediments of the Mara Maravilha and Burro Branco lakes, as
well as in the crusts of the Coraç
ã
o,Tubar
ã
oand Sete B lakes. In 2018, calcite occurred in the sediments
of the Coraç
ã
o,Burro Branco,Tubar
ã
o,Mara Maravilha and Sete B lakes in Barranco Alto farm and in the
Seis,Oito and Meio lakes in Nhumirim farm. In the clay fraction, calcite only occurred in the middle of
those lakes, often at the top of the sequences.
The clay fraction is dominated by illite and kaolinite and secondly by smectite [
28
]. The clays
were identified mostly by their 001 peaks. Illite and kaolinite were distinguished by their distinctive
peaks at 10.0–10.1 and 7.0–7.1 Å, respectively [
28
]. Smectite was identified by its 001 peak ~15 Å in the
air-dried sample and the shift to 16–18.5 Å after EG treatment, and by the shift to ~10 Å after heating
at 550
C. The dioctahedral and trioctahedral characters were identified by its 060 diraction peak,
between 1.49–1.52 and 1.52–1.54 Å, respectively [28,35,36].
Some samples from Barranco Alto farm did not show the illite 001 peaks ranging from 10 to 12 Å in
air-dried condition, exhibiting a broad reflection, with an inclination toward the small angles (Figure 6).
Most of the samples have not shown a shifting after the EG treatment, however, after heating the
samples at 550
C, the peaks became sharper and more intense, as it is shown in the first row (Burro
Branco and Sete C lakes). The Coraç
ã
oand Mineira lakes show broader peaks in air-dried condition
(second row) and although these peaks are connected, they exhibit two points of maximum intensity at
13 and 14 Å. After heating, these peaks shift to the peak of 10 Å.
Smectites have been observed in the sediments of the Negro River and in (third row) Meio lake.
They were identified by a shift of the 001 peak from ~15 Å under air-dried condition to 17–18 Å under
EG treatment and to ~10 Å after heating the samples at 550
C (Figure 6). In Meio lake, the smectite
was identified at the top of the three collection points inside the lake, normally associated with calcite.
The XRD pattern was accompanied by a high background, which hampered the identification of the
060 peaks. The 060 diraction peak of smectite from the Negro River was not distinguished in 2017,
but in 2018 showed values of 1.542 Å. The smectite of Meio lake showed 060 peaks at 1.521 (P3) and
1.524 Å (P4), but the peak from sample P2 was not distinguished. The other lakes of Nhumirim farm
are composed mainly of illite and kaolinite. On the lakes Seis and Oito, there is also calcite present,
normally at the center of the lakes and in the top of the sequences.
Minerals 2020,10, 718 14 of 26
Minerals 2020, 10, x FOR PEER REVIEW 6 of 32
Figure 6. XRD patterns of the clay fraction of some samples from the Barranco Alto and Nhumirim
farms. All the sediments are from the top of the sequences collected at the center of the lakes. At the
first row of the diffractograms of Barranco Alto (Burro Branco and 7C lakes), there is no peak at 15 ;
in the second row (Coração and Mineira lakes), a broad peak is observed between 10 and 14 and the
diffractograms of Negro River showed two different peaks at 10 and 15 , the latter being more
intense. The numbers above the dashed lines represent the d—spacing values. Sme—smectite, Ilt—
illite, K—kaolinite, Qz—quartz, Cal—calcite.
Smectites have been observed in the sediments of the Negro River and in (third row) Meio lake.
They were identified by a shift of the 001 peak from ~15 under air-dried condition to 17–18 under
EG treatment and to ~10 after heating the samples at 550 °C (Figure 6). In Meio lake, the smectite
was identified at the top of the three collection points inside the lake, normally associated with calcite.
The XRD pattern was accompanied by a high background, which hampered the identification of the
060 peaks. The 060 diffraction peak of smectite from the Negro River was not distinguished in 2017,
but in 2018 showed values of 1.542 . The smectite of Meio lake showed 060 peaks at 1.521 (P3) and
1.524 Å (P4), but the peak from sample P2 was not distinguished. The other lakes of Nhumirim farm
are composed mainly of illite and kaolinite. On the lakes Seis and Oito, there is also calcite present,
normally at the center of the lakes and in the top of the sequences.
4.5. Microstructure of the Clay Minerals
The SEM observations of samples of the Mara Maravilha, Coração and Meio lakes allowed the
investigation of the morphologies and contact relationships between the crystals. The observations
indicated dominance of quartz. The grains display spherical and angular morphologies, with smooth
surfaces. There are few visible calcite crystals displaying angular and irregular morphologies. The
significant amount of fine-grained minerals is observed over and between quartz and calcite grains.
They can be distinguished as small aggregations (Figure 7a) or as a thin sheet coating the other grains
(Figure 7b). Furthermore, whole structures and pieces of diatoms are noticed along the samples.
Figure 6.
XRD patterns of the clay fraction of some samples from the Barranco Alto and Nhumirim
farms. All the sediments are from the top of the sequences collected at the center of the lakes. At the
first row of the diractograms of Barranco Alto (Burro Branco and 7C lakes), there is no peak at 15 Å;
in the second row (Coraç
ã
oand Mineira lakes), a broad peak is observed between 10 and 14 Å and
the diractograms of Negro River showed two dierent peaks at 10 and 15 Å, the latter being more
intense. The numbers above the dashed lines represent the d—spacing values. Sme—smectite, Ilt—illite,
K—kaolinite, Qz—quartz, Cal—calcite.
4.5. Microstructure of the Clay Minerals
The SEM observations of samples of the Mara Maravilha,Coraç
ã
oand Meio lakes allowed the
investigation of the morphologies and contact relationships between the crystals. The observations
indicated dominance of quartz. The grains display spherical and angular morphologies, with
smooth surfaces. There are few visible calcite crystals displaying angular and irregular morphologies.
The significant amount of fine-grained minerals is observed over and between quartz and calcite grains.
They can be distinguished as small aggregations (Figure 7a) or as a thin sheet coating the other grains
(Figure 7b). Furthermore, whole structures and pieces of diatoms are noticed along the samples.
EDX analyses are consistent with XRD and the morphological observations. The fine-grained
minerals are composed of Si, Ca, Fe, Al, K, Mg and Na. The composition of these minerals is indicative
of clay minerals, which do not show a specific morphology, but in some samples, it is possible to
distinguish a filamentous pore-bridging structure (structure 1, Figure 7c) and a crenulate/webby shape
(structure 2, Figure 7c) between the quartz grains.
Observing the morphologies and their chemical composition, it is possible to establish that there
are illite and smectite minerals among the clay minerals.
In Figure 7e, it is possible to observe a thin and elongated structure (indicated as 1) above the
quartz grain at the center of the image. This mineral phase is composed of Si, Al and Mg. The mineral
Minerals 2020,10, 718 15 of 26
type - 2 is composed of Si, Al and K, and the rounded structure indicated in “3” is composed of a
high content of Ca, corresponding to calcite. TEM images of the sample from Coraç
ã
olake show a
large thin euhedral sheet mineral, which overlaps, probably in the basal plane, and a lenticular shape,
representing a longitudinal plane or other kind of clay mineral. Dierent shades among the crystals
were noticed, which might represent a dierent electric absorption pattern (Figure 8a).
The thin euhedral and subhedral sheets are arranged close to lenticular crystals in the same
aggregate and they exhibit dierent shades. Besides these characteristics, the samples have also
shown irregular morphologies, as “cotton-like”, with a filamentous pore lining growing up from
the core of the boundaries of some material (Figure 8b). It is possible to notice crystals with thin
euhedral morphologies and elongated sheets associated with a cotton-like material disposed together
(Figure 8c,d), as well as rounded shapes among a cotton-like mineral, as Figure 8e shows.
Minerals 2020, 10, x FOR PEER REVIEW 8 of 32
Figure 7. Scanning electron photomicrographs. (a) General overview of the sediments. The yellow
arrows indicate the aggregations of clay minerals; (b) thin sheet of clay minerals coating quartz
crystals; (c) two different structures of clays between the grains (structure 1—pore-bridging and
structure 2—webby shape); (d) acicular clay minerals scattered on the surface of other crystals; (e)
elongated Mg-rich clays above quartz (1), associated with clays rich in Al and K (2) and calcite (3).
Qz—quartz, Cal—calcite, Dt—diatom.
Figure 7.
Scanning electron photomicrographs. (
a
) General overview of the sediments. The yellow
arrows indicate the aggregations of clay minerals; (
b
) thin sheet of clay minerals coating quartz crystals;
(
c
) two dierent structures of clays between the grains (structure 1—pore-bridging and structure
2—webby shape); (
d
) acicular clay minerals scattered on the surface of other crystals; (
e
) elongated
Mg-rich clays above quartz (1), associated with clays rich in Al and K (2) and calcite (3). Qz—quartz,
Cal—calcite, Dt—diatom.
Despite the dierent morphologies observed, in the EDX analyses of several points along
the minerals, they showed similar chemical composition. Observing dierent points from similar
morphologies, it was not possible to establish a specific chemical composition for each one of them.
Si and O are dominant, followed by Al (3.0–15.0 wt.%), Fe (2.6–17.4 wt.%), K (0.5–8.0 wt.%) and Mg
(1.1–4.2 wt.%) and some other elements as Ca, Mn and Na. The “cotton-like” morphology mentioned
above seems to be growing up from a barite grain in Figure 8b. The cotton-like shaped clay mineral is
bound with the organic material through pore-lining or pore-bridging projections, which may indicate
Minerals 2020,10, 718 16 of 26
an authigenic clay [
30
]. In Figure 8d, the structure highlighted by the dashed line show a composition
rich in P and Ca, which was not observed in any other sample and might indicate an apatite grain.
In Figure 8e, the crystal surrounded by a dashed line is composed only of Si and Al, which corresponds
to kaolinite.
Minerals 2020, 10, x FOR PEER REVIEW 9 of 32
Figure 8. Transmission electron photomicrographs. (a) Lath-like clay particles (red line) and flakes
(blue line) in the same aggregate; (b) “cotton-like” clays with filamentous pore-lining (red arrows)
bond in the core (yellow dashed line) composed of S and Ba; (c) “cotton-like” clays associated with
elongated clays; (d) different types of morphologies of clay minerals. The yellow dashed line indicates
a portion rich in Ca and P; (e) aggregation of “cotton-like” and pseudohexagonal clays. The circulated
area (yellow dashed line) is composed mainly of Si and Al, comprising kaolinite; (f) detail of the
pseudohexagonal kaolinite. Kln—kaolinite.
4.6. Chemical Composition and Structural Formulae of the Clay Minerals
The XRF analysis of major elements (expressed as oxides) in the bulk sediments confirm the
dominance of SiO2 and Al2O3, followed by Fe2O3, CaO and K2O (Table 5).
The Table 5 presents the average of each lake and it is possible to notice the higher concentrations
of the oxides (except SiO2) in the Barranco Alto lakes in 2017, possibly related to the lower water level,
caused by higher evaporation rates. In 2018, the SiO2 concentration increased from a mean value of
83.79% (2017) to 93.80%, while the other oxides decreased from a mean value of 1.63% in 2017 to
0.63% in 2018. Furthermore, comparing the averages of the two farms in 2018, just Al2O3 has shown
significant differences.
Figure 8.
Transmission electron photomicrographs. (
a
) Lath-like clay particles (red line) and flakes
(blue line) in the same aggregate; (
b
) “cotton-like” clays with filamentous pore-lining (red arrows)
bond in the core (yellow dashed line) composed of S and Ba; (
c
) “cotton-like” clays associated with
elongated clays; (
d
) dierent types of morphologies of clay minerals. The yellow dashed line indicates
a portion rich in Ca and P; (
e
) aggregation of “cotton-like” and pseudohexagonal clays. The circulated
area (yellow dashed line) is composed mainly of Si and Al, comprising kaolinite; (
f
) detail of the
pseudohexagonal kaolinite. Kln—kaolinite.
4.6. Chemical Composition and Structural Formulae of the Clay Minerals
The XRF analysis of major elements (expressed as oxides) in the bulk sediments confirm the
dominance of SiO2and Al2O3, followed by Fe2O3, CaO and K2O (Table 5).
Minerals 2020,10, 718 17 of 26
The Table 5presents the average of each lake and it is possible to notice the higher concentrations
of the oxides (except SiO
2
) in the Barranco Alto lakes in 2017, possibly related to the lower water level,
caused by higher evaporation rates. In 2018, the SiO
2
concentration increased from a mean value
of 83.79% (2017) to 93.80%, while the other oxides decreased from a mean value of 1.63% in 2017 to
0.63% in 2018. Furthermore, comparing the averages of the two farms in 2018, just Al
2
O
3
has shown
significant dierences.
Table 5. Major elements (wt.% of the bulk sediments, measured by XRF).
Farm Year Lake SiO2TiO2Al2O3Fe2O3MnO MgO CaO Na2O K2O P2O5LOI
Barranco
Alto
2017
Tubarão
88.33
0.20 2.18 1.98 0.27 0.63 0.78 0.98 1.73 0.10 2.80
Burro Branco
82.20
0.20 3.36 2.10 0.28 0.60 1.28 2.14 1.84 0.16 5.70
Coração
87.43
0.28 3.33 2.28 0.90 0.50 0.98 0.38 1.28 0.17 2.47
Mara Maravilha
69.17
0.36 5.63 4.04 0.77 2.09 4.01 1.20 3.04 0.12 9.00
Mineira G.
78.40
0.30 3.30 1.50 0.20 100 3.00 0.40 1.40 0.50 9.65
Mineira P.
78.60
0.40 4.60 2.20 0.10 0.50 0.80 0.20 1.20 0.20
11.08
Sete B
66.00
0.30 4.90 5.80 0.40 1.50 5.60 0.20 2.60 0.10
11.90
Sete
94.83
0.13 1.90 0.53 0.00 0.17 0.20 0.13 0.90 0.10 1.05
Boi Preto
96.90
0.10 1.20 0.20 0.00 <0.1 <0.1 <0.1 0.60 0.10 0.76
Jacaré
95.70
0.20 1.40 0.20 0.00 <0.1 0.10 0.10 0.90 0.10 1.21
Japão
97.80
0.10 0.90 0.20 0.00 0.00 <0.1 0.10 0.50 0.10 0.40
Negro River
71.95
0.80
14.80
4.80 0.20 0.85 0.30 0.10 1.65 0.10 4.30
2018
Tubarão
88.99
0.15 1.82 1.94 0.17 0.43 0.61 0.14 1.27 0.04 4.48
Burro Branco
97.10
0.09 0.85 0.38 0.03 0.04 0.36 0.10 0.53 0.05 0.80
Coração
97.25
0.08 0.67 0.17 0.03 0.13 0.54 0.09 0.41 0.02 0.83
Mara Maravilha
89.37
0.21 2.13 1.51 0.25 0.49 1.18 0.38 1.31 0.04 3.42
Mineira G.
83.70
0.25 2.82 1.12 0.09 0.38 0.84 0.18 0.85 0.10 9.98
Sete C
97.22
0.13 1.28 0.34 0.01 0.07 0.07 0.10 0.55 0.01 0.76
Sete B
82.43
0.24 3.11 3.43 0.20 0.67 1.67 0.12 1.61 0.03 6.55
Sete
97.41
0.10 1.04 0.32 0.01 0.07 0.05 0.06 0.50 0.01 0.50
Nova
98.30
0.06 0.53 0.16 0.01 0.05 0.04 0.05 0.31 0.01 0.61
Boi Preto
97.99
0.08 0.66 0.16 0.01
<0.01
0.03 0.03 0.32 0.01 0.76
Jacaré
98.83
0.07 0.51 0.11
<0.01
0.01 0.02 0.04 0.26 0.01 0.24
Japão
98.24
0.09 0.65 0.14 0.01 0.01 0.03 0.05 0.32 0.02 0.56
Negro River
92.54
0.23 3.01 1.00 0.03 0.18 0.09 0.06 0.68 0.02 1.74
Nhumirim 2018
Oito Lake
96.75
0.10 0.72 0.37 0.07 0.13 0.31 0.07 0.47 0.02 0.99
Meio Lake
96.45
0.13 0.81 0.19 0.04 0.20 0.52 0.12 0.49 0.02 1.02
Seis Lake
88.57
0.21 1.14 3.33 0.28 0.52 1.16 0.11 1.58 0.27 3.18
Salitrada
93.39
0.25 1.51 0.38 0.02 0.05 0.10 0.06 0.56 0.03 3.66
SalvinaDoce
95.59
0.12 0.85 0.43 0.04 0.06 0.07 0.05 0.42 0.02 2.35
Regarding CaO, MgO and Na
2
O, in the farthest lakes from the Negro River (Tubar
ã
oand Burro
Branco) in 2017, Na
2
O prevailed over CaO and MgO. In the other saline and oligosaline lakes (Mara
Maravilha and Coraç
ã
o), CaO predominates over MgO and Na
2
O. In the Negro River, the sequence of
predominance is MgO-CaO-Na2O.
In 2018, the dominant sequence CaO-MgO-Na
2
O continued in most of the lakes. However,
some of them showed a preponderance of Na
2
O. Nevertheless, the concentrations of these three oxides
continued the same for Negro River in the two years. The Nhumirim lakes showed the same sequence
of concentrations than Barranco Alto lakes (CaO-MgO-Na
2
O), except for Salitrada lake, where Na
2
O
prevails over MgO. Analyzing each collection point (Table S1 in Supplementary Materials), there is an
increasing trend of the oxides towards the top of the collected sequences, except for SiO
2
. This trend is
also observed towards the center of the lakes. Therefore, the concentrations of the oxides are higher
in the center of the lakes and in the top of the sequences (in contact with the water), except for SiO
2
,
which demonstrates a reverse trend.
The clay formulae obtained from chemical analyses by AEM. All samples were collected at
Barranco Alto farm in 2017. Contrasting the large dimensions of the areas of analysis (100
×
20 nm
2
)
with the small particle sizes of clays (5–20 nm) and considering the presence of intimate intergrowth of
smectites, non-crystalline phases and mixed-layers, the results may not represent a single crystal of
clay. The results have been discriminated as illite, I/S and smectite (Table 6).
Minerals 2020,10, 718 18 of 26
Table 6. Criteria to determinate the chemical formulae by AEM analysis [32,37,38].
Detrital Micas Illite I/S Smectites
Si =3.05–3.15 Si =3.20 K =0.40–0.52 Si =3.50–4.00
Mg: 0.10–0.25 Mg: 0.30–0.70
Fe: 0.05–0.10 P(VI): 1.95–2.20 or 2.80–3.05
K: 0.57–0.70
P(XII):<0.65
P(VI): Sum of octahedral cations; P(XII): Sum of interlayer cations.
The I/S species with >90% of illite show K >0.65 atoms per formula unit (apfu) and Si =3.34–3.49
apfu [
39
]. Some of the interlayer charge, which results from the summation of interlayer cations
multiplied by their respective charges (K +Na +2Ca), showed particularly high values (0.21–0.83 apfu).
The larger interlayer charge values are normally associated with illite, indicating a mixed-layered I/S
or an inclusion of amorphous materials in the analyses [32]. The clay formulae are listed in Table 7.
Table 7.
Chemical analyses obtained by analytical electron microscopy (AEM) and expressed as smectite
formulae *. MM—Mara Maravilha; P1—crust; P2—border of the lakes.
Lakes Si Al(IV) Al(VI) Fe ** Mg Mn P(VI) K Na Ca P(XII)
MM
(P2) 3.58 0.42 1.82 0.18 0.18 0.02 2.20 0.02 0.00 0.00 0.02
Coraç
ã
o
(P1)
3.75 0.25 1.04 0.67 0.31 2.02 0.40 0.05 0.03 0.48
3.80 0.20 1.37 0.36 0.25 1.98 0.29 0.16 0.03 0.48
4.00 1.24 0.31 0.40 1.95 0.19 0.03 0.22
3.97 0.03 0.39 1.01 0.59 0.13 2.12 0.34 0.02 0.36
3.64 0.36 0.80 0.79 0.59 0.09 2.27 0.26 0.02 0.28
4.00 0.32 0.88 0.56 0.13 1.89 0.24 0.01 0.25
3.73 0.27 0.67 0.90 0.32 0.03 1.92 0.77 0.04 0.81
4.00 0.39 0.64 0.75 0.08 1.86 0.27 0.01 0.28
3.91 0.09 0.30 1.03 0.64 0.12 2.09 0.55 0.02 0.57
3.45 0.55 1.79 0.16 0.25 2.20 0.15 0.02 0.17
Burro
Branco
(P2)
4.00 0.75 0.59 0.39 0.08 1.80 0.42 0.11 0.53
4.00 0.44 0.81 0.49 0.08 1.81 0.50 0.07 0.01 0.58
4.00 1.06 0.18 0.18 1.41 0.23 0.10 0.01 0.34
4.00 0.78 0.49 0.27 0.03 1.57 0.36 0.09 0.01 0.46
4.00 0.95 0.43 0.26 0.05 1.69 0.26 0.08 0.02 0.36
4.00 1.29 0.27 0.21 0.02 1.80 0.19 0.11 0.02 0.31
Tubar
ã
o
(P2)
3.64 0.36 1.47 0.4 0.24 0.02 2.13 0.19 0.02 0.21
3.66 0.34 1.31 0.35 0.31 0.02 1.99 0.62 0.08 0.70
3.94 0.06 0.7 0.82 0.37 0.1 1.99 0.5 0.04 0.54
4.00 0.68 0.63 0.41 0.05 1.77 0.44 0.03 0.47
3.91 0.09 0.41 0.97 0.53 0.10 2.01 0.62 0.04 0.66
3.26 0.74 1.77 0.08 0.17 0.01 2.03 0.83 0.83
Sete B
(P2)
3.78 0.22 0.36 1.1 0.5 0.05 2.01 0.62 0.07 0.69
3.82 0.18 0.43 1.03 0.45 0.04 1.95 0.51 0.16 0.67
3.74 0.26 0.18 1.35 0.47 0.05 2.05 0.55 0.04 0.59
3.83 0.17 0.4 1.1 0.45 0.06 2.01 0.58 0.03 0.61
3.75 0.25 0.57 1.0 0.45 0.03 2.05 0.49 0.05 0.54
3.61 0.39 1.45 0.41 0.26 0.03 2.15 0.18 0.03 0.21
3.4 0.60 1.08 0.68 0.22 0.11 2.09 0.27 0.2 0.47
Negro
River
(P2)
3.74 0.26 1.49 0.2 0.41 2.10 0.17 0.17
3.73 0.27 1.56 0.18 0.42 2.16 0.05 0.08 0.13
3.54 0.46 1.27 0.49 0.35 2.11 0.4 0.05 0.45
3.5 0.50 1.35 0.59 0.19 2.13 0.08 0.11 0.19
3.39 0.61 1.57 0.22 0.4 2.19 0.47 0.47
3.51 0.49 1.38 0.46 0.36 2.20 0.17 0.05 0.22
* Units: apfu (atoms per formulae unit). Normalization to 22 charges. ** Total Fe expressed as Fe
3+
.Values in bold
represent octahedral and interlayer sums. Red values represent octahedral sum <1.92 apfu and interlayer sum >
0.65 apfu.
Minerals 2020,10, 718 19 of 26
Despite the wide range among the elements, illite species will be considered in this work as having
K>0.62 pfu. Analyses corresponding to smectites follow the criteria described above. Only five points
show K values equal or higher than 0.62 pfu. Besides, almost all Si and
P(VI)
values correspond to the
range described for smectites [
37
]. Fourteen analyses (highlighted in red), including all points from
Burro Branco lake, however, do not correspond to this criterion. The sum of octahedral atoms is lower
than 1.92 apfu and they are hereby classified as illite.
5. Discussion
5.1. The Distinct Clay Minerals from Pantanal
The clays identified in Barranco Alto farm are mainly of illite and kaolinite composition,
with d-spacings at 10–10.1 and 7–7.1 Å, respectively [
28
,
35
]. Some illite 001 peaks, however, varied
toward 12 Å in air-dried condition, remained constant under EG treatment and shifted to 10 Å
after calcination, indicating the ocurrence of illite-smectite interstratification (I/S) [
28
,
40
]. The I/S
was mainly observed at the Mara Maravilha, Burro Branco, Tubar
ã
o, Coraç
ã
oand Sete B lakes in 2017
(mostly saline and oligosaline lakes), whereas in 2018, it was observed at the Sete C and Mineira lakes
(Table 8). Clays from the Coraç
ã
oand Mineira lakes exhibited a specific feature in their 001 peaks
in air-dried condition: between 10 and 15 Å, two higher intensity peaks occur, confirming the I/S
interstratification interpretation.
In the Negro River, on the other hand, the smectite diraction pattern was clearly observed in both
campaigns, as shown in Figure 6. Their 060 patterns showed values of 1.541 Å, typical of trioctahedral
smectites [
28
,
35
]. Dierently, the 060 diraction peaks of the Mineira and Sete C lakes are at 1.504
and 1.529 Å, respectively, which could be consistent with a mixture of di- and trioctahedral character
clays [35,36].
Petrographic analyses, SEM and TEM images have shown dierent types of clay minerals, based
on their morphologies and chemical composition. Figures 5f and 7c exhibit clay crystals forming a
“bridge” between quartz grains and Figure 5g shows a pore-lining morphology growing up from
a core sample to form a cotton-like structure, indicating a characteristic typical of smectites [
30
].
The aggregates of clay minerals at Figure 7a and the subhedral and elongated shapes recognized
at HRTEM images (Figure 8a–f are common in detrital clays. The dierent morphologies, however,
do not show a specific chemical composition, demonstrating that clay crystals are comprised by
distinct components.
Although the calculated chemical formula of some samples showed a very small amount of
tetrahedral Al, such a low amount could indicate the presence of trioctahedral smectite [
7
]. The sum of
the octahedral sites, however, does not correspond to the average value of 2.80 apfu for trioctahedral
smectite, possibly due to the low amount of Mg [7,37].
Table 8.
Interpretation of 001 and 060 peaks from XRD analyses of the clay fraction from the Barranco Alto
and Nhumirim farms (Figure 6). I/S—illite/smectite; Sme—smectite; di—dioctahedral; tri—trioctahedral;
d—d-spacing; (-) not observed.
Farm Year Lakes d(001)Å d(060) Å Clay Minerals
Air-Dried Ethylene-Glycol 550 C
Barranco
Alto
2017
Burro Branco 10.0–12.0 No change 10 Å more intense - I/S
Coração10.0/13.0 No change 10 Å more intense 1.541 I/S
Negro River 15.0 17.0 10.0 - Sme
2018
Sete C 10.0–12.0 17.0 10 Å more intense 1.529 I/S
Mineira 10.0/14.0 No change 10 Å more intense 1.504 I/S
Negro River 15.0 17.0 10.0 1.541 Sme (tri)
Nhumirim 2018
Meio Lake—P2
15.0–16.0 No change 10.0 - I/S
Meio Lake—P3
15.0–16.0 17.0 10.0 1.521 Sme (di/tri)
Meio Lake—P4
15.0–16.0 No change 10.0 1.524 I/S
Minerals 2020,10, 718 20 of 26
The di or trioctahedral character in a solid solution is not continuous between its end members in
low temperatures [
7
,
37
]. The trivalent cation cannot represent less than 65% of the octahedral sites in
dioctahedral smectite, which represents 1.3 R
3+
cations on a Si
4
O
10
calculation basis, and the natural
trioctahedral smectite exhibits a minimum number of 1.83 R
2+
[
41
,
42
]. According to the diagram of
octahedral occupancy, AEM analyses mainly display a dioctahedral composition (Figure 9a) [
2
,
7
,
41
,
42
].
In the Si-Al-Fe triangular composition diagram, the points analyzed formed a gradual trend from
montmorillonite to nontronite (Figure 9b). Despite this trend, the analyses of each lake are widely
scattered, and a geographic distribution is not feasible. Smectite composition shows a high variation
regarding Si, Al, Fe and Mg contents (specially Si and Al). Analyses plotted in the AlAl-AlFe-AlMg
ternary diagram (Figure 9c) show they can be designated as Fe-rich beidellite [37].
Minerals 2020, 10, x FOR PEER REVIEW 16 of 32
Figure 9. (a) Diagram showing dioctahedral, trioctahedral and intermediate character of the smectite
formulae; (b) triangular Fe-Si-Al composition diagram showing a trend from montmorillonite to
nontronite; (c) triangular diagram AlAl-FeAl-AlMg [37] showing that smectite corresponds mainly to
a Fe-rich beidellite. Bl—beidellite, Mo—montmorillonite, No—nontronite, FeBl—Fe-rich beidellite,
FeMo—Fe-rich montimorillonite.
5.2. A Primary Precursor as a Responsible for the Precipitation of Clay Minerals Associated with Calcite
The subhedral morphologies observed in the SEM and HRTEM images pointed out the
occurrence of detrital clays, whose mineralogy could include illite, kaolinite and smectite minerals
[2,30,33]. The cotton-like morphologies, however, are associated with transformation and authigenic
processes, both related to physical-chemical conditions in the environment, generally in the presence
of smectite clays [30] or non-crystalline substances [32].
Figure 9.
(
a
) Diagram showing dioctahedral, trioctahedral and intermediate character of the smectite
formulae; (
b
) triangular Fe-Si-Al composition diagram showing a trend from montmorillonite to
nontronite; (
c
) triangular diagram AlAl-FeAl-AlMg [
37
] showing that smectite corresponds mainly to
a Fe-rich beidellite. Bl—beidellite, Mo—montmorillonite, No—nontronite, FeBl—Fe-rich beidellite,
FeMo—Fe-rich montimorillonite.
Minerals 2020,10, 718 21 of 26
Clays from Nhumirim farm are generally composed of illite and kaolinite, with the exception
of the Salvina Doce and Salitrada lakes, which exhibited just kaolinite and quartz in the clay fraction.
In the Seis, Oito and Meio lakes, calcite appears in the top of sequences, essentially in the regions
closest to the center of the lakes. In Meio lake, the 060 diractogram pattern of the sample P3 occurs
at 1.521 Å, which could indicate a possible intermediate di- and trioctahedral composition [
7
,
43
].
Previous research in Meio Lake had indicated 060 XRD patterns typical of trioctahedral minerals in
their samples [
7
]. However, most of their chemical structural formulae displayed an intermediate
di- and trioctahedral composition, allowing the authors to indicate that the Mg-rich smectites had a
dominant trioctahedral character, but presented a dioctahedral component [7].
5.2. A Primary Precursor as a Responsible for the Precipitation of Clay Minerals Associated with Calcite
The subhedral morphologies observed in the SEM and HRTEM images pointed out the occurrence
of detrital clays, whose mineralogy could include illite, kaolinite and smectite minerals [
2
,
30
,
33
].
The cotton-like morphologies, however, are associated with transformation and authigenic processes,
both related to physical-chemical conditions in the environment, generally in the presence of smectite
clays [30] or non-crystalline substances [32].
Clays identified in Meio lake are genetically related with transformation processes on the upper
zone, where ferribeidellite, vermiculite and illite minerals dominate [
7
]. In the lower zone, however,
saponitic and stevensitic-like smectite minerals prevail, and they are normally related to the authigenic
process [
7
]. This argument was established on the basis of rare-earth elements (REE) concentrations,
which are smaller in minerals formed by chemical precipitation, because most natural waters have small
REE concentrations in contrast to rocks, generally [
7
,
44
]. Magnesium-rich phyllosilicate minerals are
commonly precipitated directly from surface water, normally in alkaline lake systems, and parameters
like salinity, pH and Mg/Si ratios may play an important role in the nucleation of such minerals [
5
,
8
,
45
].
At high pH (8.7–9.4), high salinity and high Mg/Si ratio (0.67–6.0), kerolite, sepiolite and stevensite-like
products are favored [5].
As well as other works already discussed [
13
,
18
,
21
,
23
25
], we also observed that the lakes
from Pantanal show high levels of pH and salinity, especially those classified as oligosaline and
saline lakes. The Mg/Si rate, however, reached the maximum value of 0.04 in our studies (based on
XRF concentrations), and, therefore, do not reach the necessary chemical conditions [
5
]. Moreover,
the lacustrine Mg-silicate minerals studied in previous works did not contain Al
3+
, and our results
display high levels of aluminum [
45
]. Nevertheless, research carried out in Pantanal [
7
,
13
,
14
] and
our work show local occurrences of trioctahedral smectites on XRD patterns, indicating that other
physico-chemical agents may be playing a larger role in the studied lakes.
The nucleation of Mg-silicates can take place on pre-existing surfaces, like amorphous silica,
detrital clays and biological materials, such as EPS [
45
,
46
]. Mg-clay minerals’ primary nucleation
processes are generally related to a gel-like material, which promotes the binding of Fe, Al, Si, Ca,
Mg, Na, K and other elements, increasing the concentration of cations and degrading organic matter
and/or particulate sediments [
32
,
44
,
45
,
47
]. In this work, EPS have been recognized in the surface
water, generally with algae and often associated with alkaline-saline lakes. The samples from the
top of the sequences in these lakes showed features on SEM and TEM images that could refer to
authigenic characteristics, such as the thin laminae covering quartz grains (Figure 7b), filamentous
structures bridging quartz grains together (Figure 7c) and a cotton-like material involving organic
nuclei (Figure 8b) and other clays (Figure 8a,e) [30,32].
In Figure 7e (Meio lake), there are thin and elongated structures (1) composed of Si, Al and Mg on
the surface of quartz grains. This structure seems to develop to a more complex material (2), composed
of Si, Al, Mg, K and Na. These structures give rise to the rounded material (3), formed only by Ca, C and
O. This chemical composition and the morphology resemble spherulitic calcite minerals, which have
already been observed in Santos Basin carbonates [
45
]. Imagery and EDX analysis (Figure 7e) allow
to determine that spherulitic-shaped calcite was precipitated after Al-Mg silicates using them as a
Minerals 2020,10, 718 22 of 26
template, which, in turn, precipitated probably as a response of the biological and chemical conditions
of the water, using a detrital grain as a substrate [48].
Spherulitic calcite crystals might have formed as cement in a stevensitic-like gel, which acted as an
ideal substrate, but with no clear evidence of microbial influence in their formation [
48
]. An association
of stevensite and microbialites from Lake Clifton (Western Australia) suggested that Mg-silicates are
mediated by high silica activity from dissolving diatoms through biological processes [
49
]. At Pantanal,
for instance, previous work has proposed that the microorganisms in the lakes play an important role
in increasing the pH, favoring quartz dissolution and promoting the precipitation of amorphous silica
and carbonate during the dry seasons [
24
]. Other several works in ancient and recent sedimentary
basins have identified the association of Mg-silicates with carbonates and evaporitic sequences forming
by means of a concentration of elements by EPS in microbial mats [43,5054].
5.3. Genesis of di and Trioctahedral Clay Minerals
In this work, XRD patterns have demonstrated the predominance of illite, kaolinite and even
quartz in the clay fraction. Selected samples, which suggested the presence of smectite, from the first
campaign, analyzed by AEM/HRTEM analysis, have shown that most of the smectite minerals have a
dioctahedral character, classified as Fe-rich beidellite. The pure crystals of beidelite identified on the
Negro River may have been transported by the river, as mentioned in previous researches [
2
], where the
colloidal particles on the Amazon River were composed essentially of smectite.
Smectite minerals identified in Barranco Alto are mainly beidelite, a mineral comprised of aluminum
in the octahedral sheets and mainly by Na and Ca in the interlayers. In Nhumirim, only at Meio lake,
a trioctahedral character was firstly identified. The 060 reflection from one sample of the lake, however,
revealed a possible mixture of di- and trioctahedral components, which is in accordance with the
structural formulae obtained in the AEM analysis [7].
The two distinctive characteristics of the smectite minerals in the farms are correlated to the types
of water found in them. In both farms, sodium bicarbonate water predominates, but in Nhumirim,
Ca and Mg have higher concentrations than in Barranco Alto. This characteristic is compatible with
the presence of the trioctahedral components in Nhumirim, while in Barranco Alto, the smectite is
exclusively dioctahedral, rich in Al and Na. Therefore, smectite minerals that have been formed in
the lakes should be related to the biological and chemical conditions of the water [
8
]. Furthermore,
the dierences in cation concentration and detrital input may control the species of smectite formed:
from the least to the most ionic concentrated solution and from the most to the least detrital input,
the sequence of smectite formation would be: beidellitic, saponitic and stevensitic components [10].
Although most of the works mention only Mg-silicates with authigenic processes [
1
,
5
,
7
,
13
,
45
],
the morphologies observed in the SEM and HRTEM images, associated with the EDX analysis, have
demonstrated that there are authigenic structures, even though the chemical composition does not
correspond to the expected Mg-silicates [
8
]. According to this author, in an environmental setting
with high levels of alkalinity, pH and silica activity, detrital clays favor the authigenic Mg-rich
smectite precipitation. Meio lake in Nhumirim shows an unusual concentration of Mg on the pore
water, which enhances the precipitation of trioctahedral clay components. The composition and the
biological-physical-chemical characteristics of the saline-alkaline lakes in the studied area also provide
the minimum conditions for intermediate di- and trioctahedral smectites to precipitate directly from
the water, using the elements available in the water, such as Al, Na and Ca [14,43].
As other research already suggested [
48
,
49
,
53
], calcite has been found to precipitate within Mg-rich
silicates. Such silicates probably formed as a primary bacterial gel rich in Si and Mg, giving rise to labile
minerals, identified only in the water/sediment interface, where biological and chemical conditions
determine the mineral precipitation.
Minerals 2020,10, 718 23 of 26
6. Conclusions
The clay minerals from the Pantanal lakes are controlled by distinctive processes of transportation,
transformation and neoformation. The biological, physical and chemical conditions of the lakes
induce the supersaturation of the water and EPS act as a binding agent, promoting the precipitation
of various elements, first as a labile precursor gel and then developing to minerals. Although most
of the authigenic minerals have been related to Mg-silicates, an intermediate composition of di-
and trioctahedral smectites can also be formed directly from the water, considering the presence of
microorganisms and chemical conditions available. As well as other works already observed in ancient
deposits, calcite precipitates as smectite and I/S minerals at the points with contact with the water,
showing that both minerals are genetically related.
Barranco Alto and Nhumirim are located in the same geological environment, but display dierent
mineral and geochemical characteristics. The water of Barranco Alto has shown higher values of
Na, whereas Nhumirim has shown significant concentrations of Mg. These features, respectively,
are consistent with the dioctahedral and intermediate di- and trioctahedral character of smectites of
each farm. The cause of this dierence might be related to sediment input, rocks in the basement,
fractures and/or faults in the basin, degrees of isolation of each lake, geomorphological features of
the environment and the great annual climatic variation which promotes mineral dissolution and
precipitation. To evaluate what is promoting the mineralogical and geochemical dierences in these
two near regions, more studies should be carried out to properly understand them.
Supplementary Materials:
The following are available online at http://www.mdpi.com/2075-163X/10/8/718/s1,
Table S1: Major oxides (%) of the bulk sediments in dierent positions of the lakes in distinct depths.
Author Contributions: Conceptualization, A.M.B.R.; G.B.A.; L.F.C.; methodology, software and formal analysis,
B.G.T.; G.F.; C.d.V.M.A.; A.P.S.; L.S.; investigation and resources, L.F.C.; G.B.A.; writing—review and editing,I.A.D.;
L.d.R.S.; B.G.T.; A.M.B.R.; C.d.V.M.A.; A.P.S.; visualization and supervision, A.M.B.R.; project administration and
funding acquisition, A.M.B.R.; L.F.C. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the Microbial Project, sponsored by PETROBRAS, grant number 23075.120
789/2016-110 (UFPR) and SIGITEC number 2016/000141-1.
Acknowledgments:
The authors thank the National Agency for Petroleum, Natural Gas and Biofuels–ANP,
and the Sedimentology and Stratigraphy Division of CENPES/Petrobras for the support in the research, for the
master fellowship and for financial support for the research, including analysis and field trips. They are also
grateful to Edi Guimar
ã
es (UNB), Vander Mello (UFPR), Lucas Warren (UNESP), Alm
é
rio Barros França (iLAMIR)
and Francisca Martinez Ruiz (Universidad de Granada) for the important contributions to the study. They are
grateful to EMBRAPA for permission and logistic support to access and sampling the lakes at Nhumirim farm
and, Barranco Alto farm for the access to the study areas. Further, to the geologists, chemists and technicians from
iLAMIR for general help in the laboratory.
Conflicts of Interest:
The authors declare that the mainly results of this research are part of the thesis “Mineralogia
e Geoqu
í
mica de argilominerais da Nhecol
â
ndia–Pantanal Sul-Mato-Grossense” from graduation program at UFPR
(Curitiba, Brazil). The authors declare no conflict of interest. The funders had no role in the design of the study;
in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish
the results.
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... The low levels of SiO 2 verified in the profiles P2 and P3, in function of the processes of dissolution of silicates in alkaline medium, also contributed to the reduction of the ki and kr ratios, mainly in the superficial horizons (Dias et al., 2020;Furquim et al., 2010a;Oliveira et al., 2021). The dissolution of silicates in an alkaline medium also contributes to the accumulation of amorphous silica and the formation of cemented horizons such as Ortstein (Furquim et al., 2010a;Schiavo et al., 2012Schiavo et al., , 2020. ...
... As for mineralogy, through the analysis of DRX patterns, it appears that the predominance of micas, smectites, calcite and kaolinite and even quartz in the clay fraction are controlled by different processes of transport, transformation and neoformation (Coringa et al., 2012(Coringa et al., , 2014Dias et al., 2020;Furquim et al., 2008Furquim et al., , 2010bOliveira Junior et al., 2017;Oliveira et al., 2021). The minerals come from sediments from surrounding geological formations (Coringa et al., 2012;Oliveira Junior et al., 2017;Oliveira et al., 2021). ...
... The minerals come from sediments from surrounding geological formations (Coringa et al., 2012;Oliveira Junior et al., 2017;Oliveira et al., 2021). The intense process of evapotranspiration of the saline lagoon water promotes the precipitation of Ca, Mg and K and consequent participation in autigenic mineral phases, such as carbonates, smectites and probably micas around the saline (Barbiéro et al., 2002;Dias et al., 2020;Furquim et al., 2010b). ...
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The Nhecolândia region is formed by sandy sediments, coming mainly from the Taquari river basin, with soil sodification and the accumulation of organic matter in the subsurface being the main pedogenic processes observed in this environment. This research aimed to study the genesis of soils with spodic features in the subsurface horizons in an alkaline environment in the saline lagoons of the Pantanal of the State of Mato Grosso do Sul, Brazil. Three trenches were excavated, and the soils were morphologically described and classified according to Brazilian Soil of Classification System in Espodossolos and WRB in Arenosols. In addition, deformed samples from all soil horizons were sampled and used in physical, chemical, mineralogical and micromorphological analyses. In all profiles, the sandy texture predominates (sand ≥879 g kg⁻¹) the colors vary from light olive-brown on the surface horizons to grayish-brown-very dark on the subsurface horizons, with hues ranging from 2.5Y to 5Y. In the profiles, the occurrence of the E horizon was observed, followed by the subsurface horizon with cementation, forming ortstein. The profiles showed high pH values (6.8–10.2), electrical conductivity (EC) > 4 dS m⁻¹, and high saturation for Na⁺ (up to 60.6%), characterizing them as saline-sodic soils. In the soils around the saline lagoon, it was observed the accumulation of Fe (Fe2O3), Al (Al2O3) and dissolved organic carbon (DOC) in the horizons with spodic features. Among the humic substances, the humin fraction presented the highest levels, mainly in the B horizons, with variations from 0.1 to 6.2 g kg⁻¹. In the thin sheets, coatings and fillings were observed, consisting of organo-ferruginous colloidal material of brown to opaque color with monomorphic characteristics, covering and binding the quartz grains. Alkaline soil conditions in saline lakes contribute to the dissolution of clay minerals and surface organic matter, and later, to the dispersion of dissolved organic compounds together with Al2O3, Fe2O3, and amorphous silica, forming subsurface horizons rich in organic matter, Al, neoformed clay minerals and ortstein cemented horizons.
... The few published studies about the mineralogy of Pantanal use material from the sub-regions of north Pantanal (Coringa et al. 2012(Coringa et al. , 2014Couto et al., 2017;Olivera Junior et al. 2017. In south Pantanal a few studies have been carried out in the sub-region of Nhecolândia; although they are very specific to saline soils with a sandy texture (Schiavo et al., 2012;Barbiero et al. 2008Barbiero et al. , 2016Barbiero et al. , 2016Furquim et al. 2008Furquim et al. , 2010aFurquim et al. , 2010bFurquim et al. , 2017Furquim et al. , 2010aFurquim et al. , 2017McGlue et al., 2017;Dias et al., 2020;Andrade et al., 2020). However, in other sub-regions of south Pantanal, there are soils formed by felsic and carbonate minerals, which present a morphology with a prismatic structure, hard consistency when dry, plastic and sticky when wet, a vertic character and elevated fertility. ...
... These characteristics contributed to the formation of illite/smectite minerals with some differences in relation to profiles P1 and P2. The high levels of MgO and the low levels of Al 2 O 3 identified in the EDS suggest the formation of Fe Mg-smectites trioctahedral in this profile (Furquim et al., 2010a;Oliveira Junior et al., 2019;Dias et al., 2020). The activity of the high clay fraction in the ABk horizon of the P3 profile, much higher than the clay activity of the other profiles, is probably the result of the formation of trioctahedral smectites. ...
... The chemical characteristics of water from rivers and/or underground sources, with high pH rich in cations such as Na + , Ca 2+ K + and Mg 2+ associated with soil solution with low levels of Fe 3+ , Fe 2+ and Al 3+ , favor the formation of silicates rich in Mg such as the trioctahedral Mg-smectite (Barbiero et al., 2016;Oliveira Junior et al., 2019;Dias et al., 2020;Furquim et al., 2021). Furquim et al. (2008Furquim et al. ( , 2010a proposed the hypothesis of authigenic Mg-smectite formation by chemical precipitation directly from the lake and not by the recrystallization of a clay precursor. ...
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The genesis of clays in Pantanal soils is influenced by parent material, water seasonality and elevated temperatures, resulting in soils with attributes not so common in other tropical regions. This research aimed to study the genesis of minerals in the clay fraction present in three soil profiles developed from carbonate sediments in Pantanal of the State of Mato Grosso do Sul, Brazil. Three trenches were excavated, and the soils were morphologically described and classified according to WRB in Chernozem (profiles P1 and P3) and Gleysol (P2). In addition, deformed samples from all soil horizons were sampled and used in physical, chemical, and mineralogical analyses. Clay fraction obtained from the diagnostic subsurface horizons of the profiles was submitted to identification of the mineral phases using X-ray diffraction, morphological analysis in a scanning electron microscope and microchemical analysis using energy dispersive X-ray spectroscopy. In the three profiles, the presence of interlayered minerals of kaolinite-smectite and illite-smectite; quartz and calcite in profile P3, was confirmed in the clay fraction. The clays from the three profiles presented high levels of SiO2 (ranging from 35.4 to 56.7%). The clays from profiles P1 and P2 presented high levels of Al2O3 (P1 = 24.6% and P2 = 15.2%), Fe2O3 (P1 = 14.0% and P2 = 11.8%), which suggests the formation of dioctahedral clay and illite rich in Fe. The high levels of MgO (11.1%) and Fe2O3 (7.2%) in clays of the P3 profile indicated the presence of trioctahedral Mg-smectite and illite rich in Fe. In the studied soils, the condition of moderate to imperfect drainage, water from the water table rich in cations and the alkaline environments of soils, constitutes the principal factor responsible for the current dynamics of neoformation of dioctahedral illite rich in Fe and trioctahedral Mg-smectite and dissolution of kaolinite.
... One of the papers focuses on inclusions remaining from a high P metamorphism in rocks affected by a complex metamorphic history, which includes prograde as well as retrograde processes [59]; another [60], with garnet nucleation and growth, which record the initial steps of dehydration within the subduction zone. Two of them present mineral transformations in the smectite-illite system during diagenesis, either of chemical [61] or burial [28] origins, and a third the incorporation of B, coming from the transformation of organic matter, into the illitic tetrahedral layers [62]. The last one [63] uses the clay transformations and related parameters to Minerals 2020, 10, 879 3 of 7 establish the pressure/temperature conditions in a key region for the interpretation of the Eurasia-Africa collision in Cenozoic times. ...
... One of the papers focuses on inclusions remaining from a high P metamorphism in rocks affected by a complex metamorphic history, which includes prograde as well as retrograde processes [59]; another [60], with garnet nucleation and growth, which record the initial steps of dehydration within the subduction zone. Two of them present mineral transformations in the smectite-illite system during diagenesis, either of chemical [61] or burial [28] origins, and a third the incorporation of B, coming from the transformation of organic matter, into the illitic tetrahedral layers [62]. The last one [63] uses the clay transformations and related parameters to establish the pressure/temperature conditions in a key region for the interpretation of the Eurasia-Africa collision in Cenozoic times. ...
... The work by Armstrong and co-authors presents a study of the clay mineralogy of sediments from several lakes from the Nhecolândia region [61], the southernmost region of the Pantanal Wetland (Brazil). Sediment samples were analyzed by X-ray diffraction and fluorescence as well as scanning and transmission electron microscopy, together with the geochemical characterization of the lake waters, including in situ determination of pH, EC, and TDS. ...
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Rocks affected by pressure–temperature conditions in the transitional field between diagenesis and low-grade metamorphism make up large domains of the Earth’s upper continental and oceanic crust [...]
... Although magnesium is an element that can be easily diluted by water and thus forms part of the smectite structure when the magnesium fluid interacts with the volcanic glass, an extra Mg contribution to the system is required. Smectites with high Mg-contents (like saponites and stevensites) have been associated whit alkaline lakes systems (Furquim et al., 2008;Dias et al., 2020). The enrichment of cations such as Mg and Si provided from detrital phases is usually propitiated, among other things, by the confluence of seasonal rains and intense evaporation due to long periods of drought (Meunier, 2005). ...
... Fig. 6. VI Al + Fe vs VI Mg plot for smectites, showing fields corresponding to Dioctahedral, Trioctahedral and Intermediate character (modified from Dias et al., 2020). Microanalysis were obtained by SEM-EDS (reported in Table 5). ...
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Clay mineral characterization is a valuable tool for unraveling the evolution of continental sedimentary basins. The Fiambalá basin is a foreland Andean basin located in the Southwest of the province of Catamarca (Argentina), on the flat subduction segment. In order to characterize its paleoenvironment and post-depositional evolution, petrographic, X-ray diffraction (XRD), and scanning microscopy (SEM-EDS) studies were carried out in detrital and volcaniclastic samples from the Tambería, Guanchín, and Rodados de la Puna Fms. Petrographic and XRD analyses show a predominance of phyllosilicates, quartz, plagioclase, and low proportions of feldspar and anhydrite, sporadic calcite, analcime, heulandite, and hematite. In the XRD fraction <2 μm, minerals from the smectite and illite groups dominate, with lower proportions of chlorite and kaolinite. An analyzed tuff level presents smectite solely in this fraction. The textural-compositional analyses of SEM-EDS show that illite and chlorite have a detrital origin. Their preservation would be consistent with the dominance of an arid climate in the region during the erosion and deposition of material from the source areas. The smectites are of the magnesium-rich beidellite-montmorillonite type and, together with the zeolites would be authigenic as a product of the alteration of both the volcanic material and magnesian detrital phases (chlorite and biotite), possibly under the influence of an alkaline environment related to the arid climate. From a thermal point of view, the presence of smectite throughout all stratigraphic succession allows interpreting maximum temperatures that, even in the deepest levels of the basin, inhibit the development of prograde phases (such as smectite/illite and smectite/chlorite mixed-layered) and allow the preservation of smectite. Based on the authigenic clays present in the basin, paleogeothermal gradients of between 13 and 18 °C/km (considered a fill of 4000 m thick) and between 8 and 11 °C/km (considered a fill of 6000 m thick) could be estimated.
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