Structural and textural evolution under thermal treatment of natural and acid-activated Al-rich and Mg-rich palygorskites

A B S T R A C T: IR spectra of antigorite and sepiolite show an absorption band at ~ 1200 cm-1. Spectra of palygorskite show several small absorptions between 1140-1200 cm -t. Since these three minerals-arethe only layer-silicates which show this absorption this band was assigned to Si4)-Si groups which serve as bridges between alternating alumino-Mg-silicate ribbons. By comparing the location of this band to bands observed in the spectra of P=O groups it was concluded that the contribution of n bonding in the bridging Si-O-Si group is of a high order. Similar conclusions were drawn after comparing the spectra of antigorite, sepiolite and palygorskite with those of various pyrosilicates. An inversion of the tetrahedral sheet requires an Si-O--Si angle of 180 ~ which can be stabilized by d~-p~ bonding in the Si--O-Si group. Due to the inductive effect of the octahedral sheet this type of bonding may occur in trioctahedral clay silicates, but is not to be expected in dioctahedral clay silicates. The extent to which n orbital systems in the Si-O bonds contribute to the overall stability and structure of the silicates is still not clear. The observed bond lengths in a large number of compounds can be cited as evidence for double-bond character. Deviations can be calculated by comparing the experimental and theoretical Si ~ O distances (Yariv & Cross, 1979). In the different varieties of SiO2, as well as in most silicate minerals, the measured distances are shorter than those calculated from the corresponding radii of the elements for either electrostatic or single covalent bonds. Silicon, unlike carbon which forms stable p~-p~ bonds with oxygen, rarely forms p~-p~ bonds. This may be due to its greater size, which leads to a much weaker n overlap between its valence p-orbital and that of oxygen. The double-bond character of the Si-O bond results from the involvement of valence dorbitals of the silicon atom. Since the d-orbitals possess a considerable sideways extension, they can overlap with 2p-orbitals of the oxygen atom to form d~-p~ bonds. The nature of the d~-p~ bond in the XO4 ~- ions (in which X = Si, P, S or CI) was discussed by Cruickshank (1961). The combinations of the X d- and oxygen 2p-orbitals were settled by group theory applied to the point group Td. There proved to be two strongly n-bonding molecular orbitals in the representation E, three weakly 7t-bonding molecular orbitals in /'2, which are effectively lone-pair combinations, and three non-bonding combinations of oxygen orbitals in T1, thus giving eight molecular orbitals. The orbitals of the T2 representation play a very small effective part in the n bonding system of the XO4 "- ions. Only two orbitals of the E representation play the principal effective part in the n bonding system of the tetrahedral ion. Due to this fact the n bonding system in XO4 n- is defined as a double system of n orbitals. The

Infrared evidence for the occurrence of SiO groups with double-bond character in antigorite, sepiolite and palygorskite

The dehydroxylation reactions of chrysotile Mg3Si2O5(OH)4 and brucite Mg(OH)2 were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO2 in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (‘α’-resolved) course of the apparent activation energy E
 a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E
 a (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing E
 a (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E
 a versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO2 delivers a dynamic course with a much higher apparent E
 a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E
 a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E
 a versus α plots.

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Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

One of the most important mechanisms releasing water in subducting slabs of oceanic crust is connected to the dehydration of serpentinized oceanic rocks. This study reports on a detailed investigation of the transition from chrysotile—an important serpentine mineral—to forsterite through the release of water.

The dehydroxylation of natural chrysotile and the subsequent phase change to forsterite was studied by in situ micro-Raman and micro-FTIR spectroscopy in the temperature range of 21 to 871 °C. Comparisons were made with previously published data of lizardite-1 T . Micro-Raman spectra obtained in the low-frequency (100–1200 cm−1) and high-frequency ranges (3500–3800 cm−1) were complemented by micro-FTIR measurements between 2500 and 4000 cm−1 to study changes in the chrysotile structure as a function of dehydroxylation progress. In general, room-temperature chrysotile bands lie at higher wavenumbers than equivalent bands of lizardite-1 T except of three bands positioned at 301.7, 317.5, and 345.2 cm−1. Different band assignments of chrysotile and lizardite-1 T Raman spectra from literature are compared. The most striking assignments concern the three aforementioned Raman bands and those lying between 620 and 635 cm−1. The present data support a chrysotile- or at least curved TO layer related origin of the latter. Deconvolution of overlapping OH stretching bands at room temperature revealed the presence of five (FTIR) and four (Raman) bands, respectively. A slight change in the ditrigonal distortion angle α during heating and the effects of a radius-dependent dehydroxylation progress can be shown. Furthermore, it was possible to identify a quenchable talc-like phase immediately after the onset of the dehydroxylation at 459 °C. Main bands of this phase are positioned at 184.7, 359.2, and 669.1 cm−1 and a single OH band at 3677 cm−1, and are thus quite similar to those reported for dehydroxylating lizardite-1 T . Their appearance coincides with the formation of forsterite. A maximum in the integral intensity of the talc-like intermediate is reached at 716 °C. At higher temperatures, the intermediate phase breaks down and supports the accelerated growth of forsterite. The lack of OH bands with the concomitant appearance of broad chrysotile-related modes in the low-frequency range after heating the sample to 871 °C indicates the presence of a heavily disordered phase still resembling chrysotile. However, there are no spectral evidences for further Si- and/or Mg-rich amorphous phases during the dehydroxylation and no indications for a relationship between the breakdown of the talc-like phase and the growth of enstatite as previously reported in literature.

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The dehydroxylation of chrysotile: A combined in situ micro-Raman and micro-FTIR study

Dehydration and phase transformation in sepiolite

Structural changes accompanying dehydration and thermal transformations in palygorskite have been critically reviewed. Salient features of the dehydration mechanisms proposed by various authors have been discussed. Results of some X-ray and electron microscopic studies made by the authors on this problem, clarifying some points in greater detail, have also been included.

Dehydration Transformation in Palygorskite

Radial distribution analysis of x-ray intensities diffracted by chrysotile samples untreated and treated at different temperatures upto 900°C has been carried out. Interatomic distances, coordination numbers, mean square displacements and the interatomic coupling constants for different pairs of atoms have been calculated from the radial distribution curves. The interatomic distances and octahedral co-ordination number is found to decrease marginally upto 640°C and thereafter decrease steadily upto 800°C. The hydroxyl water is completely expelled from the structure and the original chrysotile structure breaks down. The entire process of dehydration has been interpreted in terms of RDF data.

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Dehydration and phase transformation in chrysotile asbestos—A radial distribution analysis study

Structural changes in kaolinite during dehydration have been studied by calculating the radial distribution function from the X-ray diffraction intensities. An estimate of the inter-atomic distances, relative strength of the couplings and the coordination numbers has been obtained for the samples at room temperature and heated at higher temperatures. The results show high stability of the silicon tetrahedral layer compared to the Al-O-(OH) octahedral one.

Study of Dehydration Transformation in Kaolinite by RDF Analysis

The dielectric constant (K) and loss (tan δ) and AC conductivity of a ripidolite sample have been measured as a function of frequency in the range 102–107 Hz and in the temperature range 30°–400°C. K and tan δ appear to decrease with increase of frequency and their values at 106 Hz are found to be 4.2 and 0.105 respectively. Variational patterns of K and tan δ reveal relatively larger changes at higher temperatures and appreciable frequency dependence, the values being larger at lower frequencies. Temperature variation of AC conductivity at different frequencies and the value of activation energy in the intrinsic range (∼ 1.0 eV) suggest that the conduction process is mainly associated with ions.

Dielectric Properties of Ripidolite, a Chloritic Clay Mineral

Temperature and frequency variations of the dielectric constant (K), loss (tan δ) and conductivity of a di-trioctahedral chlorite (cookeite) were measured in the temperature range 30°—400°C and frequency range 102—107 kHz. The variational patterns of these parameters of cookeite have been compared with those of ripidolite, a tri-trioctahedral chlorite and some interesting results have been obtained. The sample was also characterized by X-ray diffraction, chemical and differential thermal analyses.

S. Bhattacherjee

Papers

Dehydration Transformation in Palygorskite

Dehydration and phase transformation in chrysotile asbestos—A radial distribution analysis study

Study of Dehydration Transformation in Kaolinite by RDF Analysis

Dielectric Properties of Ripidolite, a Chloritic Clay Mineral

Dielectric Properties of a Di-trioctahedral Chlorite: Cookeite