Morphological analysis of quartz grains of two horizons in a Ultisol , Paraná – Brazil

Aratu stream basin, in the city of Floraí, Paraná. The Ultisol is derived from sandstones of the Caiuá Formation and it is subjected to mesothermal humid subtropical climate. The materials from the E horizon showed abundant porosity compared to the Bt horizon ones, due to the filling of pores by coating of clay and iron oxides. On the surface of the quartz grains of the E horizon, several features were identified such as "craters", pyramidal microfeatures, gulfs of dissolution, elongated cavities, and silica reprecipitation, resulting from the chemical change, and fragmented grains resulting from a phenomenon called "plasma infusion." In the Bt horizon, nearly all the detrital grains were surrounded by clay and iron oxide.The detrital grains of the Bt horizon were better preserved, showing little corrosion features and rare fragmented grains, compared to the E horizon. These analyzes showed that the water flow accounts for the superficial alteration of detrital quartz grains when they are under the influence of humid subtropical climate.


Introduction
The chemical weathering is the major factor in the processes of geochemistry erosion.In this sense, the pedogenesis from dissolution mechanisms, hydrolysis and leaching is able to promote changes in the morphology of slopes.This occurs in regions where there is soil bioclimate imbalance, which promotes the separation of plasma or fine material (in this case, the association between clay and iron oxide) and the skeleton or coarse fraction (sand).Many studies dealing with issues related to pedogenesis consider the skeleton (usually consisting of quartz in the case of regions of hot and humid climate) as final result of chemical weathering, that is, the unchanged waste material resulting from geochemical processes.Nevertheless, this is not what we found in our study.
We analyzed the soil of a slope, which was developed from the alteration of the Caiuá Zaparoli, F. C. M.; Gasparetto, N. V. L.
Formation sandstones, as well as its relationship with the evolution of landscape in Northern Paraná, Brazil.Along the slope, there is a sequence of soils formed by Oxisol, Ultisol, EntisolPsamment and Inceptsol.These soils are in soil bioclimate imbalance, presenting at least two transformation fronts in the slope.The first front occurs in the upper slope, where the Ultisol advances upward over the latosol.The second front is located in the lower slope, where EntisolPsammentadvances sideways toward the Ultisol, due to the oscillation of the groundwater.There is also a vertical transformation process with e-illuviation, in which clay and iron oxides dissociate the structures of the soil and translocate vertically to the base of the profile, and what remains in the upper horizons is a large amount of fine and medium sand mostly of quartz grains.
The surface horizons of Ultisols A and E contain large amounts of sand with simple stacking microstructure.This structure is fragile and prone to mechanical breakdown caused by surface and subsurface runoff during periods of precipitation.This process, coupled with geochemical processes, accounts for the changes in the modeling of the profile of the slope.Conversely, the Bt horizon has reduced porosity, due to the fillers and / or coating of the pores by clay and iron oxide translocated from overlying horizons, thus hindering the water flow into this horizon.This promotes water flow differential: one more intense in A and E horizons of Ultisols and another with greater difficulty of movement in the Bt horizon due to the coatings and fillings with clay that reduce porosity.
This study does not intend to discuss the lateral transformations, nevertheless emphasizes the processes of geochemical alterations occurring on the surface of detrital quartz grains after the vertical translocation of clay and iron oxides between the E and Bt horizons.

Context of the study area
The study area is located in the watershed of the stream Aratu in the city of Floraí, North Central in the middle region of Paraná State, in soils derived from sandstones of the Caiuá Formation (Figure 1).
The sandstones of the Caiuá Formation are composed by sand of fine to medium roughness, moderately to well sorted, aggregated by a siliceous, carbonated, and ferruginous cement and brown to reddish-purple clay.The sandstones are composed of quartz grains (dominant), and in lower proportion of feldspar, mica, chalcedony and opal.The grains range from rounded to under rounded, matte and covered by a film of an association of clay and iron oxide.The textural homogeneity and large cross-shape stratification configures an aeolian deposition for this formation (Soares et al, 1980).According to Fernandes (1992), the purple tone of sandstones is associated to the presence of clayey cement, while the reddish-brown color is due to the presence of iron oxides.
The slopes of the area are low, ranging between 2% and 15%, setting thus wavy landscape.Slopes between 5% and 10% dominate the largest area of the stream Aratu basin, while larger slopes, between 10% and 15%, are located near the break of slopes, where lower levels are shaped.
The climate is classified as humid mesothermal subtropical with hot and humid summers.The average temperature of the warmest quarter is larger than 28°C, the average temperature of the coldest quarter is lower than 18°C, and the average annual precipitation is about 1.485mm, ranging from 225mm in the driest quarter to 600mm in the wetter quarter.Gasparetto and Carvalho (2001) identified, in the soils derived from the Caiuá Formation, various classes of roundness and sphericity of the detrital quartz grains: detrital grains rounded with high sphericity classes comprised in the medium sand and coarse sand classes; underrounded and subordinate subangular grains, with average low spherical fractions, comprised in the fine sand and very fine sand classes.Using scanning electron microscopy (SEM), Gasparetto and Carvalho (2001) identified different features on the surface of the quartz grains "responsible for important changes in its morphology."Some features, such as fractures and crenulations, were generated during the process of wind transport and aeolian deposition that originated the lithology.The authors state that, with pedogenesis processes, chemical modification settle preferentially in the micro-cracks or conchoidal fractures, where deposits of clay particles and/or iron oxides usually occur.They also identified different forms of dissolution as parallel cavities of varying size, resembling tetrahedrons and other triangular shapes.In the in fine and very fine sand fractions of the horizons located from the middle part towards downstream of the slope, where soil moisture is the highest, they found both dissolution features and quartz grains nearly sectioned by chemical changes.Also, prismatic like structures were described, with filaments of silica reprecipitated showing the aspect of lace.Features similar to those found by Gasparetto and Carvalho (2001) had already been described by Fritsch (1988) in French Guiana soils.

Material and methods
The work presented here is part of a master thesis that addressed the steps of the Structural Analysis of Soil Cover systematized by Boulet et al. (1982 a, b, c).This methodology allows for the reconstruction of the soil cover through the geometric approximation of the spatial organization of soild cover at the scale of elementary interfluve (Boulet, 1988).Thus, a toposequence in a slope located near the headwaters of the stream drainage Aratu in the city of Floraí, Paraná, was studied.
In order to study the morphology of the quartz grains, two horizons were analyzed distributed between the middle and lower part of the slope of the toposequence.The horizons were an eluvial horizon (E horizon), where subsurface water flow is heavy, and another iluvial (Bt horizon), with slower water flow due to accumulation of clay and iron oxides that fills the pores.
Undisturbed samples were collected from the walls of trenches dug along a toposequence.In this study we used samples from two profiles located between the middle and lower slope and analyzed the E horizon at 40cm depth, and the Bt horizon, at 120cm depth.
For the micromorphological study, undisturbed soil samples were impregnated with resin and then they were sectioned.For micromorphological descriptions, a petrographic polarizing light microscope was used, and particular features of the quartz grains of both horizons were discriminated.The terminology used for the identification of features of the quartz grains was has the same as Fritsch (1988).
At a later stage, an analysis of the sand fraction of grains was performed, under scanning electron microscopy (SEM).Samples were collected in the disturbed E and Bt soil horizons of the same points where samples were collected for light microscopy.These samples were washed in a solution with distilled water and 20% sodium hydroxide.Soil samples were left to stand in this solution for 24 hours and subsequently were stirred for 10 minutes in a propeller stirrer and washed again.After drying at 105°C, they were sieved and separated into medium, fine and very fine fractions, and analyzed separately.
The scanning electron microscopy (SEM) allowed for three-dimensional imaging of the quartz grains with intensifications that ranged from 40 to 20,000 times, allowing for the observation of the various stages of surface geochemistry erosion, as well as for the comparison between the detrital grains of the two horizons studied.From these two techniques it was possible to perform qualitative comparisons between the detrital quartz grains from the two horizons.

Morphology of detrital quartz grains
Research about the forms of quartz dissolution from image analysis began in the mid-1970s, mainly with the spread of the Scanning Electron Microscope (SEM) technology.This equipment provided additional evidence about the surface morphology of detrital grains of quartz, showing different stages of dissolution and reprecipitation of silica (Leneuf, 1973;Eswaran and Bin, 1978;Eswaran and Stoops, 1979;Flageollet, 1981;Chalcraft and Pye, 1984;Fritsch, 1988, Howard et al., 1995;Marcelino et al., 1999).
Several studies regarding grains quartz dissolution show that, despite being one of the most resistant minerals to chemical weathering, the harsh conditions of the tropical climate can take it to complete dissolution, giving rise to monosilicilic acid (McBride, 1994;Thomas, 1994).The natural solubility of silica is greater in alkaline environments with high temperatures and high precipitation and also in soil with efficient drainage, which allow for the solutions to remain unsaturated due to the relatively quick percolation of water (McBride, 1994;Thomas, 1994).However, according to Howard et al. (1995), a sufficiently long time of exposure to the chemical weathering may be enough to generate dissolution features in not so aggressive environments, such as those on temperate weather.Fritsch (1988) studied the morphology of quartz grains of soil cover in French Guiana, where the temperature reaches 30°C and rainfall reaching 3,500mm year -1 .The soils of these areas are developed over metamorphic rocks with intrusions of pegmatite.The author, supported hydropedology knowledge, considered the quartz grains as remaining of geochemical evolution in situ.
The main characteristics of the surface of the quartz grains that show the changes shown by Fritsch (1988) are: the dissolution features of quartz grains, defined by the author as "tooth decay" and elongated cavities with increasing dissolution that end by individualizing the grains into smaller fragments; pyramidal microfeatures from the dissolution that in some situations are organized in the form of larger polyhedra, and reprecipitation of silica.Tetrahedral cavities were also identified in several stages: newly formed ones with sharp edges, and old, already degraded ones, which release silica that reprecipate forming coating layers.Furquim (2002), studying the relationship between soil and landscape, identified features of quartz dissolution in a Neosol Quartz Sandy, situated on the upper end of a slope in São Pedro (SP).The author attributed the dissolution to the degree of weathering of the soil, emphasizing that there was a possibility of a prolonged action of the dissolution process and, even in an acid environment, there were conditions for the silica to remain soluble.It must be remembered that, in this case, the solubility is much lower than at pH larger than eight.The author adds that "as commented by Eswaran and Stoops (1979), the quartz dissolution is slow, but steady, and may account for a low but constant supply of silica in the soil solution, which promotes the removal of material in solution and may even decrease the volume of soil overtime." The findings of Furquim (2002) are corroborated by the analysis of the graph in which Thomas (1994) demonstrates the solubility of minerals on the soil (Fig. 2).The author states that, among some others ions, Si 4+ is sensitive to the different properties of the soil solutions, in particular for pH values, and it is more susceptible to changes in ambient with pH above 8.As seen in Figure 2, the solubility of quartz is low, but constant, in an environment with a pH below 8.This explains the change of the quartz in slightly acidic environments.
Figure 2: Graphic extracted of Thomas (1994).Relationship between pH and solubility of aluminum, iron, amorphous silica and quartz.The data about amorphous silica and quartz are from Krauskopf (1995) studies, and iron and aluminum, from Black (1957) (Reprinted with permission from Oxford University Press Birkeland, 1984).Furquim (2002) identified several features on the surface of the quartz grains such as craters, dissolution gulfs and fissures filled with plasma generally of dark red color, as well as particles broken into smaller pieces.
These types of features have also been described by Eswaran and Bin (1978) and Eswaran and Stoops (1979) in tropical soils.The authors observed that the surfaces of quartz grains were often dominated by clay or by crystallization of iron oxides/hydroxides, usually associated with fissures, gulfs and dissolution craters, forming a phenomenon called plasma infusion, considered by Furquim (2002) "as a major ally of disintegration of quartz grains, since the presence of plasma contributes with the breaking of particles, mainly by mechanical stress."This leads to the production of smaller fragments from the disintegration of the larger ones, and it can enhance both the mechanical removal, since it produces particles more easily carried by water flow, and the dissolution, because there is an increase of the specific surface of the particles.

Presence of suspended groundwater and the change of quartz grains
The structure and texture of the soil are considered to be the main factors that control several processes within the soil profile, specifically the water percolation processes, because the porosity is closely related to these two properties.Sandy soils have a higher saturated conductivity than clay soils, while soils with polyhedral structures and rich in clay, when in contact with water, hydrates and reduce its pore size and impair the hydraulic conductivity (Brady, 1989).
The macropores are responsible for the percolation of water into the soil, especially during rainfall, whereas the micropores are arranged as short continuous capillaries distributed into many directions.Due to its smaller diameter than macropores, the micropores are better able to withstand the loss of water because of its intermolecular and ionic bonds.Furthermore, the speed of groundwater flow along these pores is much slower.
In soils with strong textural gradient, as in the E and Bt horizons of the Ultisol that have very dissimilar pore size distribution, Robain and Curmi (1986) showed that two soil horizons characterized by the same components, but with very differentiated structures, had retention curves with marked differences.Grimaldi and Boulet (1989), when studying soils of Guyana, showed that hydraulic soil behavior is modified according to their structure, which highlights the importance of macropores for subsurface water flow.
Many studies point to the hydraulic conductivity of Ultisols, noting high percentage of macropores in the upper horizons (Ap and E) and low percentage in the Bt horizons, that is, with the highest concentration of micropores in the Bt horizon and the lowest in the upper horizons.Others call attention to the spatial arrangement of the pores in these horizons, which have high connectivity in the A and E horizons and low connectivity in Bt horizons (Zago, 2000;Castro, 1989;Salomão, 1994;VidalTorrado, 1994;Santos, 1995;Cooper, 1999;Cunha andCastro, 1996, Dias Ferreira, 1997).Another key fact is the presence of strong and coalescent porous cavity, configuring a "fusion" between a pore and another, and denoting the loss of material in the transition between the E and Bt horizons (Castro, 1989;Vidal Torrado, 1994;Cooper, 1999;Cunha, 1996, Oliveira, 1997).
All these differences in porosity among A, E and Bt horizons, lead to the formation a suspended groundwater in the transition between the E and the Bt horizons.Zago (2000) found that the suspended groundwater, at the contact between the two horizons, remained for two days after the rainfall ceased in soils derived from Caiuá Formation.Cunha (2002) found that the vertical gradient of podzolic cover creates conditions for the existence of flows with greater speed in the upper portion (A, E horizon and top of the Bt) and lower speed down, thus creating side to side water flows on the first ones, which certainly affect the field ability and even saturate before the underlying horizons.
Both Zago (2000) and Cunha (2002) drew attention to the suspended groundwater and to the percolation of water in the pores after rains, recognizing several fronts of vertical wetting and drying.They also found in increments of water flows as one advanced towards downstream of the slope, which characterized subsurface lateral flows.
The authors show that the stay of the water in the form of suspended groundwater and the lateral flow are largely responsible for both the destabilization of the surface structures of the Bt horizon and the eluviation of its constituents (clay and iron oxides), generating the side transformations.Thus, there is an increase of the porosity at the top of the Bt, and the quartz grains become more susceptible to geochemical attack due to an intense water flow on the E horizon.

Results and discussion
The soils of the study area are mildly acidic, with pH ranging between 5.50 and 6.50.The Ultisol shows an obvious increase in clay content from surface horizon to horizon B, containing therefore an eluvial horizon (E horizon) and other underlying eluvial (Bt horizon).The transition between these two horizons is abrupt on the medium slope and it is clear the lower slope, while the texture ranges from sand, in the E horizon, to frank sandy-clay, in the Bt horizon (Table 1).These features influence the percolation of water into subsurface soil.Because the E horizon is more porous and has low clay content, the water percolates quickly, in contrast to the Bt horizon that has pores filled or coated with clay and iron oxides and the water flow is poor.In this case, the Bt horizon is slightly permeable, which encloses a suspended groundwater at the interface between the E and Bt horizons.At this suspended level, higher saturation occurs in water and the lateral water flow is large.The water saturation contributes to the dissociation of iron oxide and clay, disrupting the soil aggregates; iron oxide becomes soluble and both (iron oxide and clay) migrate vertically through the pores of the profile and laterally towards the foot the slope.Thus, a lot of sand and pores remain on the E horizon, as well as remnants of the Bt horizon that originate lamellae.As this level is characterized by the presence of suspended groundwater due to textural contrast with the Bt horizon, the sand fraction of the E horizon remains for long periods in a saturated environment, where the water flow occurs laterally in the slope.This situation generates different appearances between the quartz grains of the E and Bt horizons.
From the observations of thin sections of soil with an optical microscope, many features were identified on the E horizon that indicated a chemical change in the surface of the quartz grains such as craters, dissolution gulfs, and elongated cavities, as well as fragmented grains surrounded by clay particles and iron oxides.These corrosion features are installed mainly in crenulation and conchoidal fractures, some of which with mechanical origin generated during aeolic transportation of the source rock (Gasparetto and Carvalho, 2001).
In the Bt horizon the quartz grains are better preserved, with few corrosion features on the surface of the grains and without the presence of fragmented grains, like those seen on the E horizon.The analysis of the micrographs of soil thin sections revealed smooth surfaces in grains of the sand fraction of this horizon.Usually these grains are surrounded by fine material comprising of clay and iron oxide (Figure 3).
During the analysis of quartz grains under scanning electron microscopy (SEM), it was possible to identify the corrosion features in detail and to confirm the evidences found in optical microscopy.
On the E horizon, the quartz grains showed "washed" surfaces, with no signs of coating iron oxide and clay, typical of the sandy fractions of Caiuá Formation sandstones.In the three analyzed fractions (medium sand, fine sand and very fine sand) dissolution features of quartz were found.This dissolution does not occur uniformly on the surface of the grains, but in a selective way; its takes advantage of fissures and irregularities of grains; besides, the corrosion goes along the crystal structure of the mineral.In the very fine sand fraction (0.125 and 0.062 mm) the corrosion features are at a more advanced stage of corrosion due to its grain size, because the smaller the grain size, the larger the specific surface, which let it more vulnerable to geochemical attack.In these grains, gulfs in an advanced dissolution stage are observed, often almost splitting the grains and causing nearly dissolution craters.
These shapes are similar to orthorhombic or tetrahedral cavities; they are deep and prominent (Figure 4, C 1-1-D).All corrosion features have orthorhombic or tetrahedral cavities with sharp edges; they are conspicuous, regardless of the size of quartz grains and of its evolution stage.
In the Bt horizon, the quartz grains also have some corrosion features, but rarest.Most likely the most corroded grains have migrated from the E horizon trough the porosity.Unlike the E horizon, in the Bt horizon there is no differentiation in the degree of corrosion between the different san fractions.Largely, the grains from this horizon show conchoidal fractures, most likely originated during the transport that gave rise to the Caiuá Formation (Figure 5, 1-B, 1-C, 1-D, 2-B, 2-C and 2-D).The grains are partially coated by the association of iron oxides and clay, as seen in Figure 5, 1-C, either from remnants of the matrix that covered the sandstone of Caiuá Formation, or from coating of clay and iron oxides resulting from illuviation.It is possible to identify crenulations and fractures filled with clay and iron oxides on the surface of some grains.These fractures have parallel walls; this is probably due to the contraction and expansion abilities of clay (Figure 5, C-3 and 3-D).Generally, the surfaces of the grains are preserved.The analysis of the images on Figures 4  and 5 supports that the dissolution of quartz grains occurs mainly in the horizons where the subsurface water flows are the most intense, following the crystallographic directions of the minerals.Generally the dissolution shapes begin from features originated from physical friction, such as cracks that are usually filled with clay, which expands in the presence of moisture and cause the increase of the fissures.The geochemical dissolution makes cavities, which increase the contact area with the solutions that percolate the soil and speed up the process.

Conclusion
The results corroborate the importance of lateral and vertical transformations in the distribution of different soils types along the slope, which cause themarked differences of clay contents found between the E and Bt horizons.This textural contrast accounts for the degree of aggressiveness of geochemical alterations under which detrital quartz grains are exposed.
On the E horizon, where the water flow is the most intense during the rainfalls season, the Zaparoli, F. C. M.; Gasparetto, N. V. L.
quartz grains are more easily altered, while one the Bt horizon, that has limited water percolation, the quartz grains are better preserved.
It is verified also that, despite the slight acidic pH of the soil, and thus adverse to geochemical changes of quartz, the grains, mainly of fine sand fraction, were altered easily in the horizons with intense water flow.It is verified that in humid intertropical locations it is not necessary to have an alkaline environment for this transformations to occur, the intense water flow is sufficient to cause the degradation of detrital quartz grain.

Figure 1 :
Figure 1: Location of study area.
Among the medium sand (0.25 to 0.5 mm) and fine sand (0.125 and 0.25 mm) fractions (Figure4, 2-A, 2-B, 2-C, 2-D, 3-A, 3-B, 3-C and 3-D), the corrosion features are alike.Several cavities occur in the form of elongated gulfs of dissolution, most likely derived from pre-existing cracks.In such cases, the corrosion figures are evolved and easily identified.Alternatively, these cavities are filled with fine material, possibly a combination of clay and iron oxide.Sometimes this material also fills the fissures of the grains observed under optical microscopy.The evolution of the dissolution within these cavities may result in splitting of the grains.Most corrosion cavities have orthorhombic or tetrahedral shapes, following the quartz crystal structure (Figure4, 2-C, 2-D and 3-D).

Figure 3 :
Figure 3: Photomicrographs of thin sections of soil under optical microscope and natural light -(A) Microstructure of the E horizon in a region with lamellae that still contains fine material (clay and iron oxides) within a matrix background with relative gefuric distribution.At the right bottom of the picture, is noteworthy that the sand grains fractions are cracked and corroded; (B) Dominant microstructure of E horizon, showing the matrix background with monic relative distribution, with grains of the sand fraction with cracks and signs corrosion (bottom left corner); (C) Appearance of the microstructure of the Bt horizon in a region with high occurrence of pores and porphyry-chitonic-gefuric relative distribution; well-preserved of quartz grains are evident; (D) detail of the microstructure of the Bt horizon in a region with incipient porosity, porphyry relative distribution (quartz grains are well preserved).

Figure 4 :
Figure 4: Photomicrographs of Scanning Electron Microscopy (SEM) of the sand fractions of the E horizon of Ultisol derived from sandstones of the Caiuá Formation: (1-A, 1-B, 1-C and 1-D) very fine sand fraction shown under increasing magnifications depicting the quartz grains on an advanced degree of geochemical corrosion, with tetrahedral cavities; (2-A, 2-B, 2-C and 2-D) Fine sand fraction shown under increasing magnifications displaying quartz grains on an advanced degree of geochemical corrosion, with tetrahedral cavities and fissure filled with fine material (clay and iron oxides); (3-A, 3-B, 3-C and 3-D) Medium sand fraction shown under increasing magnification, depicting quartz grains with geochemical erosion, orthorhombic and tetrahedral cavities and fissure filled with fine material (clay and iron oxides).

Table 1 :
Particle size of the E and Bt horizons.