Palygorskite Fibers Research Articles

Palygorskite and SnO2–TiO2 for the photodegradation of phenol

The photocatalytic removal of phenol was studied using palygorskite-SnO2–TiO2 composites (abbreviated as Paly-SnO2–TiO2) under ultraviolet radiation. The photocatalysts were prepared by attachment of SnO2–TiO2 oxides onto the surface of the palygorskite by in situ sol–gel technique. The products were characterized by XRD, TEM and BET measurements. SnO2–TiO2 nanoparticles, with an average diameter of about 10nm, covered the surface of the palygorskite fibers without obvious aggregation. Compared with palygorskite-titania (Paly-TiO2), palygorskite-tin dioxide (Paly-SnO2), and Degussa P25, Paly-SnO2–TiO2 and SnO2–TiO2 exhibited much higher photocatalytic activity. The photodecomposition of phenol was as high as 99.8% within 1.5h. The apparent rate constants (kapp) for Paly-SnO2–TiO2, TiO2, and P25 were measured. Paly-SnO2–TiO2 showed the highest rate constant (0.03435min−1). The chemical oxygen demand (COD) of the phenol solution was reduced from 220.2mg/L to 0.21mg/L, indicating the almost complete decomposition of phenol. Reusability of the photocatalyst was proved.

Synthesis and characterization of ZnO/palygorskite

This paper reports the synthesis of ZnO/palygorskite (ZnO/PGS) composites prepared by coating palygorskite (PGS) fibres with ZnO nanoparticles. The samples were characterized by thermogravimetric-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The PGS fibres were uniformly coated with well-crystallized ZnO nanoparticles with an average size of 15 nm. The composites still retained the fibre-like morphology of PGS and showed excellent antibacterial activity against E. coli. A possible antibacterial mechanism was proposed.

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.

Palaeoenvironmental approach to the geology, mineralogy and geochemistry of an Early Miocene alluvial-fan to cyclic shallow-lacustrine depositional system in the Aktoprak Basin (central Anatolia), Turkey

AbstractThe Aktoprak Basin, Turkey, provides important clues regarding regional palaeoclimatic changes which have not so far been elucidated. The purpose of the present study is to fill this gap using mineralogical, geochemical and isotopic characterization of cyclic sedimentation within Neogene sedimentary units. Early Miocene alluvial-fan to cyclic shallow-lacustrine sediments of the Aktoprak Basin consist of conglomerate, sandstone, mudstone, marl, limestone and palaeosol. The increasingly fine upward grain size and bed thicknesses suggest that the alluvial fan was fed by an ephemeral braided river. The cyclic sandflat–mudflat–shallow-lake succession is predominantly composed of small-scale sandstone-mudstone beds. Sandstone intervals in the sedimentary cycles are interpreted to be a result of sedimentation from shallow, ephemeral braided-river channels, the deposits having evolved under water-saturated conditions in response to climate changes. The mudstone intervals are interpreted as having been laid down via sedimentation during distal sheet floods which developed either under sub-aerial exposure or in a water-saturated environment. Palaeocurrent measurements indicate that the general transport direction in the Early Miocene was from east to west. The lacustrine sediments of the Aktoprak Basin consist of calcite + quartz + feldspar + serpentine coexisting mainly with smectite ± palygorskite ± illite ± chlorite. Illite and chlorite are either detrital, having originated from basement units, or the illite formed authigenically from smectite. Calcite and clay appear as cement-building conglomerate, sandstone and mudstone. The clay content decreases with increasing calcite and the presence of an inverse relationship between CaO and other oxides may correspond to an inverse relationship between calcite and other minerals. Furthermore, Al2O3, Fe2O3, TiO2, MgO, Ni, Co, Cu, Sr, Ba and Zr values in the smectite-bearing mudstone and sandstone are consistent with the chemical compositions of rocks in the surrounding area, mainly ophiolitic and volcanogenic associations. Micromorphologically, the development of smectite as a cement on leached feldspar, the occurrence of palygorskite fibres as bridges between crystals and rock fragments, as well as the presence of ‘edging’ smectite flakes suggest an authigenic origin. Field observations, mineralogy, geochemistry and the results of δ18O and δ13C isotopic determinations reveal that periodic palaeoclimatic changes were controlled by alternating wet and dry periods that resulted in a relative upward decrease in precipitation of authigenic smectite ± palygorskite within the sections, under the influence of geochemical and pH fluctuations in meteoric water within the shallow-lake environment.

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 <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 (<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.

A study of adsorption and absorption mechanisms of copper in palygorskite

AbstractDesorption experiments performed on four Cu-adsorbed palygorskites suggest that the leached Cu2+ ion originates at the surface and/or net-like interstice of the palygorskite fibres. The leached fraction, calculated from the quantities of adsorbed Cu2+ before and after desorption, is <1%. This may indicate that the majority of Cu is in inaccessible structural sites. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared (FTIR) spectroscopy and electron spin resonance (ESR) were used to determine the mineralogical character of the Cu-adsorbed palygorskite. Two photoelectron lines at 932.5 and/or 933.7 eV in the narrow scan Cu 2p3/2 spectra show that Cu adsorbed on the surface of palygorskite is in the Cu+ and Cu2+ state. The stretching vibrations of the octahedral cation shift ~3–5 cm–1 towards a greater wavenumber in the FTIR spectra of Cu-adsorbed palygorskite. It can be deduced that the Cu2+ is trapped in the channel of the palygorskite structure. The ESR spectra of the palygorskite give g values of 2.34, 2.12, 2.08 and 2.05, suggesting that some Cu ions cannot be reached by H+. These results confirm that Cu is adsorbed by palygorskite via three possible mechanisms: (1) the Cu is adsorbed onto the surface or in a net-like interstice, and its oxidation states are +1 and +2; (2) Cu forms a complex ion – [Cu(H2O)4]2+ or [Cu(H2O)6]2+, and is trapped in the channel; or (3) Cu enters into the hexagonal channel of the tetrahedral sites or the unoccupied octahedral sites of palygorskite.

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.

Crystallochemical characterization of the palygorskite and sepiolite from the Allou Kagne deposit, Senegal

AbstractThe Allou Kagne (Senegal) deposit consists of different proportions of palygorskite and sepiolite, and these are associated with small quantities of quartz and X-ray amorphous silica as impurities. No pure palygorskite or sepiolite has been recognized by X-ray diffraction. Textural and microtextural features indicate that fibrous clay minerals of the Allou Kagne deposit were formed by direct precipitation from solution. Crystal-chemistry data obtained by analytical/transmission electron microscopy (AEM/TEM) analyses of isolated fibers show that the chemical composition of the particles varies over a wide range, from a composition corresponding to palygorskite to a composition intermediate between that of sepiolite and palygorskite, but particles with a composition corresponding to sepiolite have not been found. Taking into account the results from selected area electron diffraction and AEM-TEM, fibers of pure palygorskite and sepiolite have been found but it cannot be confirmed that all of the particles analyzed correspond to pure palygorskite or pure sepiolite because both minerals can occur together at the crystallite scale. In addition, the presence of Mg-rich palygorskite and very Al-rich sepiolite can be deduced.It is infrequent in nature that palygorskite and sepiolite appear together because the conditions for simultaneous formation of the two minerals are very restricted. The chemical composition of the solution controls the formation of the Allou Kagne sepiolite and palygorskite. The wide compositional variation appears as a consequence of temporary variations of the chemical composition of the solution.

Mineralogy, geochemistry and genesis of smectite in pliocene volcaniclastic rocks of the doğanbey formation, beyşehir basin, Konya, Turkey

AbstractPliocene volcaniclastic lacustrine rocks of the Doğanbey formation in the Beyşehir region (central Anatolia) are composed of organic-matter-bearing claystone, clastic units and dolomite interbeds, suggesting an anaerobic, shallow swampy, lacustrine depositional environment. The depositional environment was subjected to periodic climatic change during which diagenesis occurred, and smectite, and locally palygorskite, were precipitated. Smectite flakes formed authigenically on feldspar and palygorskite fibers between dolomite rhombs and at the edges of smectite flakes. Increases in leaching of Na, K, Sr, Ba and Rb, increasing Al/Si ratios, and Fe with increasing degree of alteration reveal that hydration of volcaniclastic grains (feldspar, glass) by meteoric water — determined from O and H isotopic values — was the main cause of precipitation of beidellite and montmorillonite based on the tetrahedral charge/octahedral charge ratio, with average structural formulae of (Si7.72Al0.28)(Al3.20Fe0.53Mg0.25 Mn0.02Ti0.04) (Ca0.11Na0.09K0.11), and (Si7.88Al0.13)(Al3.18Fe0.53Mg0.18Mn0.02Ti0.05)(Ca0.11Na0.11K0.09), respectively. Therefore, the Doğanbey-area smectite is presumed to have formed by chemical weathering and dissolution-precipitation from feldspar and glass during diagenesis; palygorskite formed by direct precipitation from Mg-rich solutions during dolomitization, and by transformation from smectite in an alkaline lacustrine environment.

Microscopy Studies of the Palygorskite-to-Smectite Transformation

AbstractThe transformation process between palygorskite and smectite was studied by examining the morphological and structural relationships between these two minerals in an assemblage from the Meigs Member of the Hawthorne Formation, southern Georgia. Studied samples were related to an alteration horizon with a tan clay unit above and a blue clay unit below. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to study the mechanism of transformation.From AFM data, both clay units contain euhedral palygorskite fibers. Many fibers are found as parallel intergrowths joined along the [010] direction to form ‘raft-like’ bundles. Degraded fibers, which are common in the tan clay, have a distinctly segmented morphology, suggesting a dissolution texture. Many of the altered palygorskite fibers in the tan clay exhibit an oriented overgrowth of another mineral phase, presumably smectite, displaying a platy morphology. This latter mineral forms along the length of the palygorskite crystals with an interface parallel to {010} of the palygorskite. The resulting grain structures have an elongate ‘wing-like’ morphology.Imaging by TEM of tan clay material shows smectite lattice-fringe lines intergrown with 2:1 layer ribbon modules (polysomes) of the palygorskite. These features indicate an epitaxial overgrowth of smectite on palygorskite and illustrate the structural relationship between platy overgrowths on fibers observed in AFM data. The epitaxial relationship is described as {010} [001] palygorskite ‖ {010} [001] smectite.Energy dispersive spectroscopy indicates that the smectite is ferrian montmorillonite. Polysomes of palygorskite fibers involved in these textures commonly vary and polysome widths are consistent with double tetrahedral chains (10.4 Å), triple tetrahedral chains (14.8 Å), quadruple tetrahedral chains (21.7 Å) and quintuple tetrahedral chains (24.5 Å).The transformation of palygorskite to smectite and the resulting intergrowths will cause variations in bulk physical properties of palygorskite-rich clays. The observation of this transformation in natural samples suggests that this transformation mechanism may be responsible for the lower abundance of palygorskite in Mesozoic and older sediments.

Improved Constraints on Sedimentary Environments of Palygorskite Deposits of the Hawthorne Formation, Southern Georgia, from a Detailed Study of a Core

AbstractThe sedimentology and mineralogy of a 2.5 m core from a palygorskite deposit of the Miocene Hawthorne Formation, southern Georgia is described. The lithology involves laminated clay-rich sediment composed of ∼90% clay and 10% sand, with six clay-pebble layers present. Sand to pebble-size clasts of phosphate material are common throughout the core. The sand laminations are probably flood-related and the clay-pebble layers are storm deposits, with the pebbles being derived locally from subaerial environments. Phosphate clasts are reworked bone material.The sands are quartz-rich and are subarkosic in composition with average quartz counts of 86.50% and average total feldspar counts of 11.50%. Heavy minerals observed include orthopyroxene, clinopyroxene, amphibole, zircon, rutile, garnet, tourmaline, kyanite, muscovite, biotite, spinels and opaques. Palygorskite fibers dominate the clay-size fraction of the samples and comprise ∼80–90% of sample material with smectite comprising the remainder. Hydroxylapatite comprises ∼3% of sediment volume and occurs as individual euhedral hexagonal crystals and as clusters of crystals.Investigation of this core suggests that the palygorskite deposit represents a dynamic system with regular flooding and storm deposition being common. Mineral composition of sands may be useful for stratigraphic correlation of palygorskite deposits in the Apalachicola Embayment. This study supports the general environmental interpretations of previous workers for the palygorskite deposits of southern Georgia, but provides greater detail.

Polyethylene‐Palygorskite nanocomposite prepared via in situ coordinated polymerization

AbstractA polyethylene/palygorskite nano‐composite (IPC composite) was prepared via an in‐situ coordinated polymerization method, using TiCl4 supported on palygorskite fibers as catalyst and alkyl aluminum as co‐catalyst. These composites were compared with those prepared by melt blending (MBC composites). It was found that in the IPC composites, nano‐size fibers of palygorskite were uniformly dispersed in the polyethylene matrix. In contrast, in the MBC composites, the palygorskite was dispersed as large clusters of fibers. Regarding the mechanical properties of the IPCs, the tensile modulus increased and the elongation at break decreased with increasing fiber content, while the tensile strength passed through a maximum. The tensile strength and elongation at break were much smaller for the MBC composites. The final degree of crystallinity of the IPC composites decreased with increasing palygorskite content. Regarding the kinetics of crystallization, the ratio between the degree of crystallinity at a given time and the final one was a universal function of time. It was found that large amouns of gel were present in the IPC composites and much smaller amountes in the MBC composites.

Novel organic/inorganic nanocomposite of polyethylene. I. Preparation via in situ polymerization approach

AbstractA novel methodology for the preparation of polyethylene/palygorskite, “in situ coordinated polymerization,” was proposed. A Ziegler–Natta catalyst was first supported on the surface of nanoscale crystal fibers of palygorskite and subsequently used to initiate ethylene polymerization on the surface of the fibers. The palygorskite was encapsulated by polyethylene and became reinforcement fibers of the composite. The palygorskite fibers were heat‐treated to remove water, to ensure that the supported titanium species possessed proper activity for the polymerization of ethylene. The effect of the conditions of heat‐treating, titanium‐supporting, and other factors such as temperature and type and amount of the cocatalyst on the initiation activity of the palygorskite fibers were investigated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1829–1837, 2001

Characterization of mural paintings from Cacaxtla

One of the most remarked customs of ancient mesoamerican cultures was that of record historical events by painting murals. It was used a rich variety of colours which is still visible in some mesoamerican buildings. In this work is reported the analysis by SEM, TEM, XRD, and FTIR, of the materials used to produce mural paintings of the archaeological zone of Cacaxtla Mexico. The results show needle-like fibers, granular aggregates and large crystalline areas. The elemental analysis reveal a substantial concentration of O, Ca and C, followed by Si, Mg, Al, Fe, and K in lower concentration. It was found the main component in the substrate is calcite. It is also relevant the presence of palygorskite in blue and green pigments, hematite in red and pink, and calcite in white. It was also possible to identify long fibers of palygorskite on whose surface there were observed nanoparticles of metallic oxides attached.

Rheological Properties of Aqueous Suspensions of Palygorskite

The rheology of aqueous suspensions of six palygorskites with different particle morphologies has been investigated as a function of clay concentration, adsorbed ion, pH, and electrolyte concentration, using a rotary viscometer. Rheological parameters (plastic viscosity, Bingham yield value, and apparent viscosity) increase with the length/width ratio of individual palygorskite fibers. The plastic viscosity increases linearly with clay concentration within the range investigated (up to 5% w/v). The rheological parameters are higher for clays saturated with divalent ions than those for clays saturated with monovalent ions. For ions of the same valency the rheological parameters tend to increase with decreasing radius of the hydrated ion. The flow of suspensions is pseudoplastic regardless of the nature of the adsorbed ion. Measurements of electrophoretic mobility in very dilute suspensions suggest that the point of zero charge of the palygorskite surface is at pH 4 to 4.5. Scanning electron microscopy indicates significant differences in fiber arrangement between low and high pH values. Face‐to‐face particle association occurs at low pH values, giving rise to close‐packed domains of fibers, while at high pH values the fibers adopt a random orientation. The flow of suspensions is pseudoplastic at pH ≤ 7 and changes to Newtonian at pH ≥ 9. At pH ≤ 7 the rheological parameters remain relatively constant even at high electrolyte concentrations, but at pH ≥ 9 they are influenced significantly by electrolyte addition. Electrolyte‐free palygorskite suspensions at pH 7 are antithixotropic. When the pH falls below 7 or when electrolyte is added, the suspensions coagulate and become thixotropic. Some rheological properties of palygorskite are similar to those of platy clay minerals such as kaolinite and montmorillonite, while other rheological properties deviate considerably. The models developed to explain the rheological behavior of platy clay minerals do not always account for the behavior of palygorskite, because of differences in particle morphology and surface structure.

Clay mineralogy as affecting disaggregation in some palygorskite containing soils of the Jordan and Bet-She'an Valleys

The influence of clay mineralogy on disaggregation in some palygorskite-containing soils of the Jordan and Bet-She'an Valleys was examined. The disaggregation potential of different minerals in the soil clay fraction was investigated by establishing differences in the mineral suite between the original and disaggregated clay fractions. By shaking the soil with distilled water, calcite, dolomite, feldspar, and palygorskite were disaggregated preferentially. The pattern for quartz was inconsistent. Among phyllosilicates, palygorskite was the most strongly, disaggregated, while smectite was the least disaggregated mineral. The disaggregation potential of kaolinite was of intermediate value. By shaking the soil with 0 ⋅ 01 N NaCl, calcite and dolomite were released preferentially, and were the dominant minerals in the disaggregated clay fraction. Scanning electron microscopy observations indicate that palygorskite fibres do not associate into aggregates in soils and suspensions, even when saturated with calcium ions. The present findings are relevant for soils with low exchangeable sodium percentage. These soils can be expected under rain-fed agriculture or irrigation with high quality water which has a low sodium adsorption ratio.

Stabilization of aqueous suspensions of palygorskite by thermal vapour pressure shock explosion (TSE) treatment

A device was constructed in which a clay suspension is hermetically heated at 220°C for a few minutes. This thermal treatment is accompanied by a pressure increase in the cell. Once the valve is opened, there is a fast release of the pressure inside the cell and a sudden evolution of the interparticle water. This shock leads to a quasi explosion of the clay particle. This technique was named thermal vapour pressure shock explosion (TSE). The effect of TSE treatment on the properties of palygorskite suspensions was investigated. Palygorskite suspensions in water are rather unstable and particles smaller than 3 μm in size are not found before a TSE treatment. Stabilization of the suspension can be obtained by TSE treatments and/or by using a dispersing agent such as pyrophosphate, or both. As a result of TSE treatments smaller particles are obtained, the dispersiveness of the particles is improved and electrophoretic mobility is increased. Electron microscopy scans showed that the aggregates of needles which form the palygorskite fibres, disintegrate to separated thin needles as a result of the TSE treatment.

The Coordination of Aluminum Ions in the Palygorskite Structure

AbstractNMR spectra of PF1-1 Floridan palygorskite strongly suggest that Al3+ occurs only in octahedral coordination. X-ray microanalysis of the palygorskite fibers indicate a chemical composition defined by the atomic ratios: Mg/Si = 0.34, Al/Si = 0.27, and Fe/Si = 0.04. Considering the NMR and CEC data in this report along with the previously published results of IR and Mössbauer spectroscopic studies, the following structural formula is proposed for PF1-1 palygorskite: $${\rm{(M{g_{2.12}}A{l_{1.68}}F{e^{3 + }}_{0.24}{\square _{0.96}})S{i_8}{O_{20}}{(OH)_2}{(O{H_2})_4}}}$$ where □ represents the vacant Ml octahedral sites in the structure, and Al3+ and Fe3+ are all exclusively assigned to the octahedral sites.

Neoformation of Palygorskite in a Southern New Mexico Aridisol

AbstractPalygorskite is the dominant clay mineral in a petrocalcic horizon of the Rotura soil series (a coarse‐loamy, mixed, thermic, Typic Haplargid), a soil of middle Pleistocene age in southern New Mexico. The purpose of this study was to determine if palygorskite is eolian in origin, inherited from its alluvial parent material, or neoformed in the soil. The origin of palygorskite was determined by examining clay mineral distribution, soil‐water extract chemistry, and palygorskite fiber morphology. An eolian origin of palygorskite is unlikely because it is absent in upper soil horizons and no palygorskite was captured in dust traps during a 10‐yr study. An inherited origin is also unlikely because palygorskite is not an inherent component of Rio Grande alluvium. Scanning electron microscopy revealed that delicate palygorskite fibers radiate perpendicularly into pore spaces, which indicates neoformation. The petrocalcic horizon may foster palygorskite neoformation because it is the site of high Mg content and abundant sand‐ and silt‐grain dissolution.

Weathering of the Villamayor arkosic sandstone used in buildings, under a continental semi-arid climate

The Villamayor Sandstone is an arkosic sandstone, the cementing material of which is principally composed of palygorskite, smectite and by small amounts of other clay minerals, and of oxides. This stone has been used as a building material since the Roman period in Salamanca (Spain). This city is located in the plains of Castilla at an altitude of 800 m, characterized by a continental semi-arid climate with a mean rainfall of 420 mm/year and 2679 sunshine hours per year on the average. A comparative study of fresh and weathered sandstones collected from different areas of the city shows, through X-ray diffraction, DTA, DTG, SEM and chemical analyses, the existence of two-well defined weathering types. The first occurs in well drained locations where humid conditions following rainfall do not last long. It is characterized by a small loss of the finest fractions, especially smectite, the breakage of palygorskite fibres covering the pores, and oxidation of the ferrous components that produces the characteristic golden colour. The second type, observed in zones with fairly constant humid conditions, is characterised by a high degree of surficial salt precipitation which provokes a high physical degradation as compared to the first type.