Mercedes Suárez

Comparative FT-IR study of the removal of octahedral cations and structural modifications during acid treatment of several silicates

Abstract The acid activation of different silicates of the clay group has been carried out. Sepiolite, palygorskite and two saponites, one having a high iron content, have been studied. Thus, different structures (layered or fibrous) and different chemical compositions (aluminic, magnesic and ferromagnesic) of the clays can be compared. The FT-IR spectra of solids obtained after the acid treatment permit the observation of structural changes occurring during the acid treatment. Thus, the depopulation of the octahedral sheet can be clearly observed from the change in intensity of bands related to octahedral cations. The changes result in the formation of amorphous silica gel, which can be observed from the change in the bands corresponding to the SiO bonds and in new bands corresponding to free silica.

Structural and textural modifications of palygorskite and sepiolite under acid treatment

Sepiolite from Mara (Zaragoza, Spain) and palygorskite from Attapulgus (Georgia, USA) were activated by treatment at different concentrations with solutions of boiling HCl under reflux conditions. The natural and treated solids were characterized by mineralogical, chemical and textural analyses.Acid attack resulted in a progressive dissolution of the octahedral layer of these silicates. Silica contents increased and octahedral cations (Al, Mg and Fe) decreased with the intensity of the acid attack. In both cases, fibrous free silica was obtained.The sepiolite was destroyed more rapidly than palygorskite under the conditions used because of its magnesic composition and the larger size of its structural microchannels. The maximum increase in specific surface area was obtained for sepiolite at 3 N and for palygorskite at 9 N. Cleaning and disaggregation of the particles and the increase in the number of micropores were responsible for this increase in specific surface area.

ON THE CHEMICAL COMPOSITION OF SEPIOLITE AND PALYGORSKITE

Many studies of the chemical composition of sepiolite and palygorskite have been carried out using analytical electron microscopy (AEM). According to the literature, a compositional gap exists between sepiolites and palygorskites, but the results presented here show that they may all be intermediate compositions between two extremes. The results of >1000 AEM analyses and structural formulae have been obtained for the samples studied (22 samples of sepiolite and 21 samples of palygorskite) and indicate that no compositional gap exists between sepiolite and palygorskite. Sepiolite occupies the most magnesic and trioctahedral extreme and palygorskite the most aluminic-magnesic and dioctahedral extreme. Sepiolite and palygorskite with intermediate compositions exist between the two pure extremes: (1) sepiolite with a small proportion of octahedral substitution; (2) palygorskite with a very wide range of substitution (the pure dioctahedral extreme is unusual); and (3) intermediate forms, Al-sepiolite and Mg-palygorskite with similar or the same chemical composition. The chemical compositions of the intermediate forms can be so similar that a certain degree of polymorphism exists between Al-sepiolite and Mg-palygorskite.

Octahedral cation distribution in palygorskite

Abstract The OH speciation of 18 palygorskite samples from various localities were evaluated by near infrared spectroscopy (NIR) and compared to the corresponding octahedral composition derived from independent, single-particle analytical electron microscopy (AEM). NIR gives evidence for dioctahedral-like (AlAlOH, AlFe3+OH, Fe3+Fe3+OH) and trioctahedral-like (Mg3OH) species. Therefore, palygorskite can be approximated by the formula yMg5 Si8O20(OH)2·(1 - y)[xMg2Fe2·(1 - x)Mg2Al2] Si8O20(OH)2, where x is the Fe content of the dioctahedral component, and y is the trioctahedral fraction. The values of x estimated from the NIR data are in excellent agreement with the Fe/(VIAl + Fe) ratio from AEM (R2 = 0.98, σ = 0.03), thus suggesting that all octahedral Al and Fe in palygorskite participate in M2M2OH (dioctahedral-like) arrangements. Furthermore, y values from AEM can be compared to NIR (R2 = 0.90 and σ = 0.05) after calibrating the relative intensity of the Mg3OH vs. (Al,Fe)2OH overtone bands using AEM data. The agreement between the spectroscopic and analytical data are excellent. The data show that Fe3+ for Al substitution varies continuously in the analyzed samples over a broad range (0 < x < 0.7), suggesting that fully ferric dioctahedral palygorskites (x = 1) may exist. On the other hand, the observed upper trioctahedral limit of y = 0.50 calls for the detailed structural comparison of Mg-rich palygorskite with sepiolite.

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

The 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.

Variability in sepiolite: Diffraction studies

Abstract Twenty sepiolites of known composition from different origins were analyzed to quantify the variability in structural parameters and its possible relationships with composition and morphology. Morphology SEM analyses led to classify the sepiolites into several groups, beyond the two simple macroscopic or clay groups. X‑ray powder diffraction with synchrotron light was used to discuss the variability of the a and b cell parameters with the nature of the cations and occupancy of the octahedral shell. Rietveld refinement using the ideal sepiolite model is performed on sepiolites at two temperatures: 225 °C (for zeolitically dehydrated sepiolite) and 25 °C (for hydrated ambient sepiolite). The latter permitted to locate ca. six molecules of the zeolitic H2O within the tunnels. A few samples were selected to evaluate the feasibility and potential of single-crystal diffraction methods: X‑ray microdiffraction and electron diffraction. The macroscopic sepiolites gave wellstructured and rich X‑ray fiber diffraction patterns, in excellent agreement with ab initio simulations. High-quality single-crystal electron diffraction patterns for three axis zones are indexed and compared with simulations. The experimental and modeling results for X‑ray microdiffraction and electron diffraction open a new path for quantitative crystallography on sepiolite and other fibrous clays from the sepiolite-palygorskite group.

SEPIOLITE PALYGORSKITE: A CONTINUOUS POLYSOMATIC SERIES

A compositional gap between sepiolite and palygorskite has long been accepted even though they have similar structures, but recent studies found that such a gap does not exist and that the compositional series is continuous between them. If this is true, intergrowths between these two minerals should exist. The purpose of the present study was to demonstrate the existence of sepiolite-palygorskite intergrowths in all possible proportions, in order to establish the compositional links between ideal sepiolite and ideal palygorskite and to define the compositional limits of these two minerals. Sepiolite and palygorskite have similar structures but different chemical compositions: sepiolite is the most trioctahedral and magnesic extreme, while palygorskite is dioctahedral with Al and Mg in its octahedral sheet. The existence of all intermediate compositions between these two pure extremes has led to the definition of the intermediate minerals, Al-sepiolite and Mg-palygorskite, which can have similar chemical composition. The structural relations between the different minerals of the palygorskite—sepiolite series were studied here by powder X-ray diffraction (XRD), and continuous variation in the chemical composition is explained by the existence of intergrowths of sepiolite and palygorskite polysomes forming a continuous polysomatic series. The existence of intergrowths by mixtures of polysomes in modulated structures can be considered in the same way that the mixed-layer minerals in layered structures are considered. The continuous polysomatic series of sepiolite-palygorskite can be expressed by the general formula: [Si12Mg8O30(OH)4(OH2)4]y∙[Si8O20(Mg2Fe2)x(Mg2Al2)(1-x)(OH)2(OH2)4](1-y)·nH2O, where sepiolite and palygorskite are the end-members. They and xvalues can be calculated using a ternary plot with the oxide contents of the main octahedral cations (Al2O3, Fe2O3, and MgO). The proposed model, which is based on the intergrowth of sepiolite and palygorskite polysomes, explains both the variability in the chemical composition and the compositional limit for the identification of these minerals by X-ray diffraction.

FAULT-HOSTED PALYGORSKITE FROM THE SERRATA DE NÍJAR DEFORMATION ZONE (SE SPAIN)

Palygorskite fibers growing along fault planes in the outcrops of a large fault zone in SE Spain (Carboneras Fault Zone: CFZ; Serrata de Níjar) were studied by X-ray diffraction, scanning electron microscopy-energy dispersive X-ray analysis, and transmission electron microscopy-analytical electron microscopy. The structural formulae, calculated per half unit-cell, is: Si7.95Al0.05O20(Al1.93Fe0.08Mg1.92) (OH)2(OH2)4Na0.09K0.01Ca0.034(H2O). The samples have minor tetrahedral substitutions, with Mg/Al ratios close to one, and contain very small amounts of Fe3+. The number of octahedral cations per half unit-cell is 3.93. The fault-hosted palygorskite shows macroscopic ductile features including incipient foliation. Based on field and laboratory observations, as well as on regional geological evidence indicating the existence of widespread hydrothermal processes along the Serrata de Níjar and surrounding areas, we suggest that palygorskite may have formed during ongoing deformation in the CFZ, as a precipitate from Mg-rich hydrothermal fluids.

Sepiolite-palygorskite polysomatic series: Oriented aggregation as a crystal growth mechanism in natural environments

Abstract A detailed microscopic study of sepiolite and palygorskite natural samples reveals that independently of the fiber length they all are composed of other minor width fibers until the minor units or the true crystals. They are prismatic crystals elongated along [001] with rhombus-like morphology in cross section, designated laths. Although their length can vary greatly, their width is always nanometric (~10-30 nm). The laths are grouped in a crystallographic arrangement, sharing edges or faces forming rods. Several rods, parallel to the c-axis of the fiber, form bundles. The laths are the smallest stable nucleated crystals. After the nucleation, the process of growth continues via a nonclassic crystal growth mechanism (aggregation of the nanolaths). Subsequently, the aggregated sepiolite and palygorskite natural crystals can continue growing, along the c-axis at the expense of the diffusion of molecular scale species throughout the solution. The morphology of the faces is controlled by the highly different attachment energy of the faces.

Macroscopic palygorskite from Lisbom Volcanic Complex

The palygorskite of the Volcanic Complex near of Lisbom (Portugal) is particular both in the size of the fibres and in chemical composition. It appears as veins of very pure mineral. From hand specimen and optical observations it can be described as a macroscopic palygorskite. The crystals are large with exfoliation traces of several hundred microns to few millimetres in length. In thin section, this palygorskite is colourless, translucent, negative biaxial, with positive elongation and parallel extinction. The optically homogeneous fibres and laths are shown by the selected area electron diffraction to be composed of aggregates of much thinner fibres rotated differently around the c crystal axis which represents their common elongation direction. The chemical formula obtained by the X-ray EDS is Si8.02O20 (Al1.91 Fe0.04 Mg 2.01) (OH)2 (OH2)4 Ca 0.01 Na 0.07 4(H2O) very close to the ideal formula of a pureMg-Al palygorskite,with almost no octahedral Fe, and noAl in tetrahedral sites. The cell parameters are a0 or a0 sin q = 12.64 A° , b0 = 17.84 A° and c0 = 5.3 A° .