Formation Of Palygorskite Research Articles

Mineralogy of Palygorskite Occurrences from Guadalupe, Piauí, Parnaíba Basin

Palygorskite is a clay mineral with structural units forming ribbons, whose economic importance is directly related to its unusual structure and physicochemical properties, which give it a wide variety of uses in industry. The formation stability field of palygorskite is very limited, which makes deposits of this clay rare around the world. In Brazil, the only exploitable deposits of this mineral are located in the municipality of Guadalupe, Piauí state, inserted in the geological context of the Parnaíba Basin, specifically in the Carboniferous and Permian sequence represented by the Piauí, Poti and Pedra de Fogo formations. The objective of this study was the geological and mineralogical description of palygorskite occurrences in the Guadalupe region. Six occurrences were studied, and 25 samples were collected. These raw samples were processed to concentrate particles with size smaller than 44 µm. XRD, SEM/EDS, density and particle size analysis were performed. Most of the samples were composed of palygorskite, quartz and kaolinite; however, according to the diffractograms, there are three samples with the absence of palygorskite, two with the absence of kaolinite and one with presence of dolomite. From these results, it is possible to correlate hypotheses for palygorskite formation with the local geological context. The environmental conditions of Piauí and Pedra de Fogo formations were favorable to this clay mineral formation, the association of dolomite and palygorskite in an upper layer may indicate a greater association with Pedra de Fogo Formation, however the field descriptions, as the levels of silexite, and the analyses results are not enough to define in which one of these formations the occurrences of palygorskite are inserted.

Palaeoclimate, palaeosalinity and redox conditions control palygorskite claystone formation: an example from the Yangtaiwatan Basin, northwest China

AbstractPalygorskite-bearing claystones and mudstones were deposited in a salt lake in the middle and lower parts of the Neogene Baiyanghe Formation in the Yangtaiwatan Basin, China. The petrological, mineralogical and geochemical characteristics of the sediments were investigated to determine the factors controlling palygorskite formation. The palygorskite claystones and mudstones have distinctly varying mineral compositions. The claystones are composed of detrital minerals, palygorskite and illite, whereas the mudstones consist mainly of mixed-layer illite/smectite and illite. The palygorskite crystals were intact with sharp edges and interwoven with other minerals, indicating an authigenic origin. The chemical characteristics indicate that the palygorskite claystones in the middle part of the Baiyanghe Formation were deposited in a salt lake environment in an arid and hot climate. As the salinity of the lake gradually increased, the detrital minerals such as quartz, feldspar, dolomite and detrital clay minerals dissolved in the alkaline medium, thus providing Si4+, Mg2+and Al3+for the crystallization of palygorskite. The palygorskite coexists with certain amounts of detrital quartz and feldspar with limited roundness and sorting, indicating that the shallow lake of the basin under an oxidation environment may represent a favourable environment for the crystallization of palygorskite.

Palaeoenvironmental reconstruction of Kathu Pan, South Africa, based on sedimentological data

Several studies have highlighted the importance of collecting well-dated records of palaeoenvironmental changes on desert margins, particularly in the semi-arid interior of southern Africa. Here we describe sediments at three sites in Kathu Pan, a pan complex overlying karstic bedrock at the southern edge of the Kalahari Basin. Sinkholes have exposed sedimentary sequences dating back to the Pleistocene, making it possible to document multiple palaeoenvironmental regimes in this landscape. Using a multi-analytical sedimentological approach, including micromorphology, energy dispersive X-ray spectrometry, electron microprobe, loss-on-ignition, laser diffractometry and luminescence dating, we demonstrate that the sediments at Kathu Pan represent an environment fluctuating among a number of thresholds for accumulation of different kinds of transported and authigenic sediments. Within the Pleistocene sands at the base of our studied sedimentary sequence, the presence of palygorskite, indicative of less extreme evaporative control, represents a semi-arid environment from 156 ± 11 ka to 32 ± 2 ka (Marine Isotope Stages 6–3). Increased moisture availability at 84 ± 4 ka and at 96 ± 5 ka (possibly during Marine Isotope Stages 5a and 5c, respectively) is reflected in the sands by the formation of palygorskite in concert with the deposition and oxidation of organic material. As the climate became increasingly arid, possibly in response to global cooling during the Last Glacial Maximum (21 ± 2 ka), Kathu Pan crossed a threshold, and deposition of CaCO3 dominated over that of palygorskite. In the Holocene beds within our studied unit, alternating CaCO3- and organic-dominated deposits, sometimes in very fine laminae, reflect an increase in the amplitude of either seasonal or annual changes in water availability.

Zircon in sak lu'um: Evidence of multiple Eocene silicic tephra layers as palygorskite precursors in the Yucatán peninsula, Mexico

The provenance and nature of palygorskite clay used by Mayas for the creation of the outstanding Maya Blue pigment and pottery in the Yucatán peninsula has not been analyzed thoroughly. Especially the hypothesis of palygorskite being a diagenetic product of volcanic glass has remained uncertain. Here, we present a detailed study of zircon extracted from palygorskite-rich materials (sak lu'um) collected in the interior of the Sacalum cenote and near Chapab village, to evaluate the potential of zircon as an indicator mineral for tephra-derived palygorskite or clay-rich deposits intercalated with carbonate rocks. U-Pb zircon geochronology and correlative trace element analyses from stratigraphically different palygorskite-rich horizons indicate a restricted Eocene age range, which is coeval to the deposition of the host carbonates, and discrete airborne emplacement events of silicic tephra into warm and shallow waters at ca. 50 Ma and 41 Ma, likely derived from volcanoes in a continental magmatic arc. Our findings also indicate that palygorskite deposits in the Yucatán peninsula are not inherited from continental sources but rather formed in-situ partly from weathering of volcanic glass that sourced Si and Al required for palygorskite formation.

Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

AbstractPalygorskite is a fibrous, magnesium‐bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolution–precipitation reaction or solid‐state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene–Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre‐scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg2+ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co‐occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form.

Heavy Metals Distribution and Their Correlation with Clay Size Fraction in Stream Sediments of the Lesser Zab River at Northeastern Iraq

Heavy metals (i.e. Cr, Co, Ni, Cu, Zn, Rb, Sr, Ba, Pb, V and Ga) distribution and their correlation with clay fraction were investigated. Fifteen samples of stream sediments were collected from the Lesser Zab River (LZR), which represent one of three major tributaries of the Tigris River at north-eastern Iraq. Grain size distributions and textural composition indicate that these sediments are mainly characterized as clayey silt and silty sand. This indicates that the fluctuation in the relative variation of the grain size distribution in the studied sediments is due local contrast in the hydrological conditions, such as stream speed, energy of transportation and geological, geomorphological and climatic characterizations that influenced sediments properties. On the other hand, clay mineral assemblages consist of palygorskite, kaolinite, illite, chlorite and smectite, which in turn reveals that these sediments were derived from rocks of similar mineralogical and chemical composition as it is coincided with other published works. The clay mineral assemblages demonstrate that major phase transformations were not observed except for the palygorskite formation from smectite, since the minerals pair exhibit good negative correlation (-0.598) within the Lesser Zab River (LZR) sediments. To determine interrelation between the heavy metals and the clay fractions in the studied samples, correlation coefficients and factor analysis were performed. Heavy metals provide significant positive correlation with themselves and with Al2O3, Fe2O3 and MnO. In addition, the results of factor analysis extracted two major factors; the first factor loading with the highest percent of variation (60%) from the major (Fe2O3, Al2O3 and MnO in weight %), heavy metals and clay fraction. While the second factor with the (14%) of variance includes Cr and silt fraction, which indicate the affinity of the heavy metals being adsorbed onto solid phase like clay particles. These observations suggest that a common mechanism regulates the heavy metal abundance, and that their concentrations are significantly controlled by fine clay fractions, clay mineral abundance and ferro manganese oxides-hydroxides.

Palygorskite occurrences and genesis in calcisol and groundwater carbonates of the Tensift Al Haouz area, Central Morocco

Palygorskite has been identified as a major constituent of the clay fraction in carbonate soils of arid and semi-arid regions. This fibrous clay has a great significance as a paleoclimatic indicator and is of fundamental importance for a more complete understanding of conditions under which petrocalcic horizons and groundwater carbonates precipitate. The present study was undertaken in order to evaluate the occurrence, the genesis as well as the micromorphological aspects of palygorskite in calcisol and groundwater carbonates. The samples were analyzed by X-ray Diffraction (XRD), scanning electron microscopy (SEM), and chemical analysis (XRF). Two modes of genesis for palygorskite were identified: (1) In the petrocalcic horizons, the palygorskite is the dominant clay mineral; palygorskite fibers and fiber bundles extend from euhedral or subhedral calcite crystals showing an authigenic origin of palygorskite. The palygorskite is formed in the vadose environment from infiltrating soil water through the intense evaporation of subsurface water after calcite precipitation. (2) In the groundwater carbonates, smectite flakes appear to terminate palygorskite fibers suggesting formation of palygorskite through the alteration of precursor smectite. The increasing aridity would have caused the intense evaporation conducive to the precipitation of calcite and an increase in pH, which lead to dissolution of smectite and formation of palygorskite.

Microstructure and palygorskite neoformation in pedogenic calcretes of central Morocco

The aim of this research was the micromorphological and mineralogical characterization of calcrete outcrops located in central Morocco, with particular focus on microstructure development and palygorskite formation processes. Two study sites in the Al Haouz Plain (ca 30km west to Marrakech city) were identified, in an area dominated by Quaternary sedimentary formations including layers cemented by carbonate and widespread calcareous crusts at the surface. The calcrete samples were examined by means of polarizing microscopy from thin sections, scanning electron microscopy / energy-dispersive X-ray spectroscopy (SEM/EDX) from undisturbed samples, and X-ray diffraction (XRD) methods. The matrix of the calcrete material cementing the schist fragments composed of abundant calcite was associated mainly with palygorskite, quartz, and to a lesser extent illite/mica, feldspar, and chlorite. SEM observations indicated that palygorskite mainly occurred as fibres, fibre bundles and networks mainly grown on and/or enclosing calcite rhombohedrons. The EDX spectra of the palygorskite fibres exhibited strong silicon (Si) peaks followed by Al, Mg and Fe peaks. The EDX analysis of calcite rhombohedric crystals exhibited a strong Ca peak, followed by weak peaks of Si, Mg, Al and Fe possibly due to nearby or overlapped palygorskite crystals. One main palygorskite neoformation process was described. Palygorskite was authigenically formed (neoformed) during and after precipitation of calcite from percolating soil solutions in a near-surface setting. Peculiar sub-rounded microstructural units, here designated as pedogenic ooids/pisoids (according to the size), were determined in the matrix of the studied calcretes, associated to pendants and clay-rich rims surrounding them.

Textural evidences for dissolution of silica-rich rocks of the Ypresian phosphatic series, Gafsa-Metlaoui basin, southwestern Tunisia: implication of biogenic silica supply on genesis of fibrous clays

The present work aimed to determine the mineralogical composition of Ypresian series and to clarify the influence of the dissolution of siliceous frustules on the genesis of fibrous clay minerals. The specimens sampled from CPG trench are mainly constituted of silica-rich rocks at Mides area located at the western part of Gafsa-Metlaoui basin. The samples were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to determine texture of constituents. The data obtained indicate that the bulk rock samples are mainly made up of opal CT and clay minerals. The latter consists of palygorskite-sepiolite minerals associated with smectite and few amount of illite. The trend of mineralogical composition indicates that fibrous clay minerals are more concentrated at the upper part. SEM observations indicated that palygorskite mineral appears as thread-like facies, which surround foliated texture of smectite in the lower part of the Mides section, although with the low Mg activity confirmed by the absence of dolomite. But, at the upper part of the Mides section, SEM observations revealed the occurrence of siliceous frustules, which have numerous dissolved areas and replacement of carbonate tests by silica. The dissolution saturated the depositional environment with silica which is required for the formation of palygorskite and sepiolite minerals, in addition to high Mg activity confirmed by the presence of dolomite in the bulk rock, which is required basically for the formation of sepiolite. Although the genesis mode of palygorskite and sepiolite is similar with very little difference, the genesis of sepiolite needs a high alkalinity than the formation of palygorskite.

Late Pleistocene calcretes from Central Anatolia (Lakes Eymir and Mogan, Gölbaşı Basin): Comparison to quaternary calcretes from Turkey

This study is the first to report the radiocarbon ages, mineralogical, morphological, geochemical and stable isotope compositions of calcretes from Neogene sediments around lakes Eymir and Mogan in the Golbasi Basin of Central Anatolia. Morphologically different forms of calcretes in the Golbasi Basin include powdery, nodular, fracture infill, laminar and hardpan types. Calcite is the dominant mineral of calcrete compositions; the diagnostic features of dessication cracks, random fractures, MnO linings and dense sparitic infillings are observed. Chemical analyses show arid conditions with mean annual precipitation of <50 mm. δ13C compositions of the calcretes range from -6.77‰ to -9.32‰ PDB, typical for most pedogenic calcretes, reflecting the development under seasonally arid climates and C3-dominated vegetation cover. δ18O values are between -5.57‰ and - 7.80‰ PDB, indicating the formation from meteoric water in a vadose zone environment. The results suggest that the Middle Pleistocene was arider and warmer, favouring the formation of palygorskite in association with the different forms of calcrete occurrences, whereas the Late Pleistocene was dryer and cooler, supporting the development of calcretes.

A microscopic approach to the pedogenic formation of palygorskite associated with Quaternary calcretes of the Adana area, southern Turkey

Quaternary calcretes in the Adana area (southern Turkey) are widespread and occur in a variety of forms such as nodular, columnar/tubular, fracture-infill, laminated hardpan, and conglomeratic crusts. Palygorskite associated with calcrete nodules, tubes, and fracture-infills in the columnar horizon and adjacent host-rock mudstones suggests a cogenetic formation. The Beta-fabric (biogenic) constituents and stable isotope values of calcretes support a pedogenic origin for calcretes and also palygorskite. Extensions of palygorskite fibre and fibre bundles from euhedral to subhedral calcite crystals in calcretes and from smectite flakes in adjacent host-rock mudstone suggest an authigenic origin related to calcretisation in which precipitation and dissolution/precipitation are the main processes for palygorskite formation, respectively. The presence of palygorskite in subspherical aggregates containing calcite crystals, and with smectitic clay coatings around the palygorskite aggregate, similarly suggests a relationship of pedogenic evolution between the 2 clays. Palygorskite precipitated in a vadose zone from strongly evaporated alkaline water, rich in Si and Mg and low in Al and Fe, at elevated temperatures and at an advanced stage of calcrete formation. Development of palygorskite mainly on relict calcite crystals indicates its formation during or shortly following calcite formation in calcretes. Palygorskite precipitation on relative large crystal surfaces has resulted in encrusted and platy fan-like crystal morphologies, whereas its nucleation on the crystal edge relief triggers fibre-like morphologies from crystal edges.

Mechanism of palygorskite formation in the Red Clay Formation on the Chinese Loess Plateau, northwest China

The morphological characteristics and microstructures of the authigenic palygorskite occurring with smectite and carbonate minerals in the Late Miocene Red Clay Formation, on the Chinese Loess Plateau, have been investigated with the powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM). The XRD results indicate that the red clay sediments contain quartz, feldspar, calcite, dolomite, illite, kaolinite, chlorite, palygorskite, and smectite, and SEM analyses show that palygorskite is a common component. Delicate palygorskite fibers radiate from the margins of platy smectite, suggesting a pedogenic origin during the weathering of pre-existing smectite, a process that involved intense dissolution of the smectite and an adjustment in its structure. Moreover, palygorskite occurs as coatings, pore-fillings, and pore-bridging cements among dolomite rhombs, suggesting that the fibrous palygorskite was formed by a direct dolomite precipitation from pore fluids, mostly after dolomite crystallization. There is evidence to show that these two mechanisms took place synchronously in the Red Clay Formation, and that the two processes are related. Under alkaline conditions, with high-pH (~8.0) fluids rich in magnesium, magnesium ions occupy the interlayer positions in the smectite, and this results in a misfit between the Mg octahedral sheet and the smectite layer, thereby allowing interaction between the magnesium ions and smectite layers and a reorganization of the structure to form palygorskite. Then, with this kind of palygorskite as a crystal nucleus, more palygorskite grows through a direct chemical deposition process. According to previous studies, the red clay sediments experienced a steady dry–warm climate with periods of high rainfall and evaporation. The warm climate would have increased the rate of weathering of minerals, and subsequently enhanced the ions content and pH in the soil solutions during dry periods. This soil environment is favorable for smectite transformation and palygorskite deposition. This study has improved our understanding of the environmental and paleoclimatic changes during the time that palygorskite formed in the arid soils of the Chinese Loess Plateau.

Quaternary alluvial sinkholes: Record of environmental conditions of karst development, examples from the Ebro Basin, Spain

The central Ebro Basin is an exceptional region for studying karstification through time and under different environmental conditions, as sinkholes have been developing since the Early Pleistocene. Knowledge of active sinkholes is complemented with research on paleosinkholes and contemporary deposits. Sedimentological, min- eralogical, geomorphological and structural approaches permit interpretation of the natural environmental conditions that favored karst in the past and the main genetic mechanisms involved. The sedimentary features of Pleistocene terraces indicate that they were deposited by a gravel braided fluvial system characterized by higher water and sediment availability than today, probably related to meltwater flows coming from glaciated source areas, mainly in the Pyrenees. Genesis of paleosinkholes was mainly linked to this high water supply. Some of them acted as small lakes where fine sediments are exceptionally well conserved to give clues about environmental conditions. The neo- formation of palygorskite and sepiolite suggests arid to semiarid climatic conditions, in agreement with the idea of cold glacial episodes. During Pleistocene times, development of sinkholes was influenced by tectonics. Currently, the genesis and evolution of numerous sinkholes are also influenced by water supplies from human activities such as irrigation or urbanization, sharply changing the nearly steady state exhibited in the past.

Palygorskite Formation Under the Influence of Saline and Alkaline Groundwater in Central Iranian Soils

Few studies have investigated the formation or instability of palygorskite under the influence of groundwater. This study aims to identify the effects of a saline and sodic groundwater on the formation of palygorskite and the associated clay minerals. Four pedons along a calcareous catena with different depths of groundwater were sampled and analyzed using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and inductively coupled plasma mass spectrometry. Chemical compositions of both the groundwater and soil samples were also determined. Results revealed the presence of palygorskite, smectite, illite, chlorite, quartz, and feldspar in the soils studied. The highest amount of palygorskite was found in horizons with permanent groundwater. In surface horizons, palygorskite could not be detected by X-ray diffraction, but its presence, although in trace amounts, was confirmed by transmission electron microscopy. In most of the pedons studied, smectite showed an inverse trend, decreasing with depth, as compared with palygorskite. Stability diagrams for the smectite-palygorskite system showed that geochemical conditions were conducive to the formation of palygorskite from smectite under the influence of Mg-rich groundwater. Growth of palygorskite fibers on halite and calcite crystals, as revealed by scanning electron microscopy observations, clearly indicated in situ formation of palygorskite. The chemical composition of groundwater in the study area showed high concentrations of soluble Mg2+ and silica, both of which are necessary for the formation of palygorskite from solution. In conclusion, palygorskite seems to have been formed in partially or entirely submerged horizons through smectite transformation or in situ formation.

Pleistocene calcretes from eastern Tunisia: The stratigraphy, the microstructure and the environmental significance

This paper is meant to study the stratigraphy, the mineralogy, the microstructure and the geochemistry of Pleistocene calcretes from eastern Tunisia in order to infer the environmental factors intervening in their formation. Samples of eight profiles of Pleistocene calcretes from eastern Tunisia were examined on the basis of a variety of techniques including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), chemical analysis and Atomic Absorption Spectrophotometer (AAS) techniques. Then, the obtained data underwent a statistical analysis on the basis of Factor Analysis (FA) and Principal Component Analysis (PCA). On the basis of field missions, five different horizons have been differentiated from bottom to top of all profiles: nodular, powdery, massive Brecciated and laminar horizon. The mineralogical study shows two minerals categories inversely proportional: calcite and (quartz and the clay). It shows also shows that Palygorskite is the dominant clay mineral. The escarpment edge is capped by a limestone containing fibrous palygorskite. Finally, superficial calcrete are described: a brecciated horizon which occurs in pockets on the plateau surface. This study about eastern Tunisia revealed the occurrence of successive cycles of calcretisation. Pedogenesis, water table oscillation, sedimentogenesis and stromatogenesis are the intervening factors in the calcretisation process. During the Pleistocene, they interfered with each other according to the climatic pulsations. From the studied case, it may be noticed that the formation of each calcrete horizon is the result of a dominating process that takes place during a distinguishable stage. In the first stage, the pedogenic process is developed by palygorskite formation including authigenic replacement or formation from a precursor mineral, neoformation from the breakdown products of such minerals or neoformation from suitable solutions. In the second stage, the powdery horizon is formed in the slope of the distal zone which presents a drained environment. In the third stage, several diagenetic processes (cementing, compaction, dissolution...) contribute to the formation of the laminar and massive horizon. Since it is exposed to dryness for a long period, the massive horizon is harder and more compact. In the fourth stage, the banding of light–dark in the laminar horizons reflects a dry-wet season alternation seasons. Dark beds are formed by the stromatolitic cover were developed during the wet season, whereas light beds were developed in an extremely arid climate argued by the presence of the detrital grains. In the fifth stage, the brecciated horizon, which occupies the channels, is formed by well rolled concretions, which present a dismantling material of Early and Middle Pleistocene calcretes after the Post-Villafranchian compressive phase. Thus, calcretisation seems to have been controlled by periods of uplift and stability of the slope, given that calcrete formation might be inhibited by the activation of the sedimentation of colluvial materials as a consequence of the tectonic activity. We also suggest that groundwater and biological activity may play a significant role in the development of pedogenic, sedimentological and polygenetic calcrete cycles within the same sedimentary basin. The alternation of dry and wet climatic periods may be responsible for the calcrete genesis.

Contemporary Pedogenic Formation of Palygorskite in Irrigation-Induced, Saline-Sodic, Shrink-Swell Soils of Maharashtra, India

AbstractIncreasing use of irrigation in India has exacerbated the problems of soil salinity and sodicity. The present study was undertaken on shrink-swell soils from Maharastra State to determine if changes in soil chemistry due to irrigation have affected the clay mineralogy. Twenty six samples (15 locations) of irrigation-induced, saline-sodic, shrink-swell soils and 27 samples (22 locations) of normal un-irrigated (rain-fed) shrink-swell soils were studied using X-ray powder diffraction (XRPD), infrared spectroscopy (FTIR), and scanning and transmission electron microscopy (SEM, TEM). The XRPD analysis of the &lt;0.2 µm fraction of rain-fed, shrink-swell soils indicates a predominance of dioctahedral smectite with minor to trace amounts of kaolinite and chlorite. Traces of palygorskite (1–4%) were detected in three samples. In contrast, palygorskite is a common component (1–20%) of the fine-clay fraction of saline-sodic soils. Quantitative analysis of palygorskite by XRPD in whole-soil (&lt;2 mm) samples showed that saline-sodic soils contain up to 20 wt.% of palygorskite, whereas palygorskite was only detectable (1.5 wt.%) in one sample of the rain-fed set. The SEM, TEM, and FTIR confirm the presence of Fe-rich palygorskite in saline-sodic soils and demonstrate that the fibrous palygorskite crystals are exceedingly small (∼0.5 µm long). Delicate palygorskite fibers radiate from the margins of smectite plates suggestive of a pedogenic origin and a close genetic relationship between smectite and palygorskite. The compositions of saturation-paste extracts display a shift from the stability field of smectite in rain-fed soils to that of palygorskite in saline-sodic soils. Thus the occurrence and formation of palygorskite appears to be related to the change in land management from rain-fed to irrigated agriculture. This change has occurred over a period of no more than 40–50 y, implying that palygorskite formation in the irrigated, saline-sodic soils has been an extremely rapid process.

The occurrence and genesis of clay minerals associated with Quaternary caliches in the Mersin area, southern Turkey

AbstractCaliche in various forms, namely powdery, nodule, tube, fracture-infill, laminar crust, hard laminated crust (hardpan), and pisolitic crust, is widespread in the Mersin area in southern Turkey. It generally occurs within and/or over the reddish-brown mudstone of the Kuzgun Formation (Tortonian, Miocene) and alluvial red soils of the Quaternary. The mineralogical distribution along representative caliche profiles was examined by X-ray diffraction, scanning electron microscopy, differential thermal analysis-thermal gravimetry, and chemical techniques. Calcite is the most abundant mineral associated with minor amounts of palygorskite in caliche samples, whereas smectite is prevalent mainly in the reddish-brown mudstone and alluvial red soils of the caliche parent materials and is associated with appreciable amounts of palygorskite. These minerals are also accompanied by trace amount of illite, quartz, feldspar, and a poorly crystalline phase. Palygorskite fibers and fiber bundles were developed authigenically on euhedral or subhedral calcite crystals of the caliche units and at the edges of smectite flakes in the caliche host-rocks or sediments. Intense, continuous evaporation of subsurface soil-water resulted in an increase in pH and the dissolution of detrital smectite within the red mudstones and alluvial red soils that enclose the isolated caliche forms, and caused an increase in the Al+Fe and Mg/Ca ratio, favoring the formation of palygorskite under alkaline conditions. The calcium required for caliche formation may have originated from eolian dust, detrital carbonate minerals, and/or other caliche materials, which are dissolved by carbonic acid.

Fibrous-clay mineral formation and soil evolution in Aridisols of northeastern Patagonia, Argentina

Recent studies of soil–landscape relationships in northeastern Patagonia identified fibrous-clay minerals in calcic and petrocalcic horizons developed on old fluvio-glacial plains called “Rodados Patagónicos” (RP). The objectives of this study were: i) to evaluate the occurrence of fibrous-clay minerals in the arid soil environment, and ii) to establish the relationship between the soil properties and degree of the calcic horizon development, including the age of the soils containing fibrous-clay minerals in extra-Andean Patagonia. The soil studied were Calciargids, Natrigypsids, and Petrocalcids, located at elevations of 50, 70, and 130 m a.s.l., respectively. The soils are polygenetic, where the clay mineralogy is related to the age of the pedogenetic periods that affected the formation of geomorphic surfaces. In the surface horizons, illite proved to be the dominant clay mineral and was slightly altered to interstratified illite–smectite and smectite. An older pedogenic episode was identified in argillic and calcic horizons, in which smectite was prevalent. The following soil-formation period was recorded in calcic and calcic-gypsic horizons appearing in the upper limit of the RP. In these horizons, palygorskite is the dominant clay mineral. Pedogenetic carbonate was qualified as low-Mg calcite, indicating that during its precipitation, the Mg 2+ activity increased in the soil solution, favoring the smectite → palygorskite transformation. The soil environment, favorable for this transformation, was the textural transition between the fine materials of sub-surface horizons and the coarsest deposit of RP, where temporary waterlogging occurred. The petrocalcic horizons, and their re-transported fragments, represent the oldest pedogenetic period, where sepiolite was the dominant clay mineral. During the calcretization processes, the sepiolite was precipitated from the soil solution following the formation of palygorskite. Fluorite was identified in the petrocalcic horizon, and its association with calcite and sepiolite indicated a successive precipitation of these minerals under alkaline conditions during evaporation processes.

Micromorphological and geochemical characterization of Tertiary ‘freshwater carbonates’ locally preserved north of the edge of the Miocene Molasse Basin (SW Germany)

Miocene carbonates intercalated in a deposit of Upper Freshwater Molasse (Middle Miocene), exposed in a limestone quarry at Heidenheim-Mergelstetten (Germany), were analysed geochemically and micromorphologically. The heavy minerals (fine sand) and clay minerals confirm the sedimentary interpretation of the formation as a sequence of Paleogene soil residues overlain by younger Neogene soil horizons developed in sediments of the Upper Freshwater Molasse (OSM). The supposed continental origin of the massive, bedded, pisolitic limestone and white, chalky, columnar limestone indicated by terrestrial gastropods, is confirmed by analysis of stable carbon and oxygen isotopes. The microfacies show the characteristics of terrestrial carbonates: pisoids, nodules and pedodes. Nevertheless, the occurrence of intraclasts, reworking features, grey/black concretions, indications of swampy conditions, and carbonate enrichment restricted to the depression area cannot be explained by terrestrial pedogenetic processes. Cathodoluminescence suggests the meteoric character of the sparitic cements, which could be differentiated into three phases of cementation corresponding to conditions during lake regression and subsequent transgression. X-ray diffraction, differential thermal and transmission electron microscope analysis of the ammonium acetate insoluble residue of the carbonates indicate neoformation of palygorskite within the pisolitic limestone. Si and Al for formation of palygorskite were probably supplied by degradation of clay minerals (illite) under alkaline conditions. Mg was externally supplied to the system. The results suggest formation of the Miocene carbonates in a marsh or shallow ephemeral lake environment with repeated superimpositions of pedogenetic processes.

Submicroscopy and stable isotope geochemistry of carbonates and associated palygorskite in Iranian Aridisols

SummaryPedogenic carbonates in arid and semi‐arid regions of the world have a great significance as palaeoecological and palaeoclimatological indicators and form a major pool in the carbon cycle. We analysed the ultra‐microfabric and the stable isotope composition of C and O in pedogenic carbonates in colluvial soils derived from limestone in an arid region of central Iran. Our objective was to determine the conditions for the formation of soft pedogenic carbonate nodules and their co‐existence with palygorskite in the palaeo‐argillic horizon. Scanning electron microscopy revealed that the calcite aggregates were matted with palygorskite. Ultra‐microtome cuts, examined using transmission electron microscopy, provided more detailed information about the fundamental particle association of secondary carbonates and palygorskite. Although less abundant, other silicate clays were detected in both the acid‐insoluble clay fractions and in ultra‐cuts, mostly in fine clay size, suggesting the engulfing of palygorskite by growing calcite or illuviation of palygorskite during or after formation of the calcite. Coatings of illuvial clays on calcite crystals support the hypothesis that palygorskite was trapped by pedogenic carbonate when the climate was wetter than it is today to form an argillic horizon. However, electron microscopic evidence of the occurrence of fibres on the immediate pedogenic carbonate particle surfaces suggests the in situ formation of palygorskite. The δ13C and δ18O values of pedogenic carbonates suggest that these carbonates were formed in an environment with more available moisture and more C4 plants than now.