Comparative study of the effects of thermal and mechanical treatments on the structures of clay minerals

Controlled heating (in air) of clay minerals leads to transformations into disordered structures and recrystallization into new phases at high temperatures. These phase transformations are of topotactic nature. On the other hand, prolonged dry-grinding treatments of the same silicates causes structural amorphization with no recrystallization into new phases. The mechanical energy relaxation mechanism, invoked to explain these differences, accounts for the particle size changes and the large growth of dislocations. The latters affect sensibly the reactivity of the materials submitted to such solid-state treatments.These processes are monitored mostly by XRD, DTA and IR spectroscopy. The latter method allows to study in some silicates submitted to progressive grinding the location and nature of OH groups and water molecules.Plenary lectures.     

The estimation of structural component contents in lead silicate glasses

The contents of structural components in lead silicate glasses were determined by the X-ray electron spectroscopy and thermodynamic analysis methods. Lead silicate glasses were described as solid solutions of lead and silicon oxides and lead silicates. The thermodynamic properties of metastable lead silicates were estimated. Thermal treatment in an inert or oxidizing atmosphere led to identical glass structures. High-lead glasses are based on metastable lead-oxygen structures containing 3–7 lead atoms. Such structures are absent in crystalline silicates.     

Asbeste

Asbestos minerals are naturally occurring fibrous silicates. From the structural chemistry point of view they belong to two different classes: The group of serpentines, to which the primarily used chrysotil (white asbestos) belongs, consists of curled up layer silicates. On the other hand in the amphiboles (for example blue asbestos (crocidolite)), which belong to the band silicates, the fibrous form is pre-formed in their silicon-oxygen partial structure. The characteristic properties of asbestos (its fibrous form, thermal and chemical inertness, good electric and thermal insulation, spinability…) led to extensive application of the mineral fibers in nearly all fields of engineering and everyday life. Because of the health effects of asbestos dust – fibrogene and cancerogene diseases – the application of asbestos containing products will cease with the end of this century. The search for substitute materials (especially fibers) is currently an area of interest in material sciences.     

Effect of organomodified layered silicates on the properties and structure of polytetrafluoroethylene

The physicomechanical and triboengineering properties and the structures of polymer composite materials based on polytetrafluoroethylene and layered silicates are studied. The triboengineering characteristics are substantially improved by the introduction of a small amount of layered silicates (2–5 wt %). It is found that the introduction of organomodified layered silicates leads to a considerable reduction in the friction coefficient, by an order of magnitude, and causes an increase in wear resistance (2000-fold). With the use of X-ray structural analysis and scanning electron microscopy, it is shown that, during friction loading, filler particles are localized on the friction surface, thereby hampering wear of the material.     

Novel microporous lanthanide silicates with tobermorite-like structure

The synthesis and structural characterization of microporous lanthanide silicates (Na(1.08)K(0.5)Ln(1.14)Si(3)O(8.5).1.78H(2)O, Ln = Eu, Tb, Sm, Ce) are reported. The structure of these solids is closely related with the structure of hydrated calcium silicate minerals known as tobermorites and was solved by powder X-ray diffraction ab initio (direct) methods and further characterized by chemical analysis, thermogravimetry, scanning electron microscopy, (23)Na and (29)Si MAS NMR and luminescence spectroscopy. These materials combine microporosity with interesting photoluminescence properties, and their structural flexibility allows fine-tuning of luminescence properties, by introducing a second type of lanthanide ion in the framework. Thus, they may find applications in new types of sensors.     

Une micro-reaction pour le manganese dans les minerais et dans les silicates

A rapid and sensitive method is described for the detection of manganese in rocks, silicates, ores and minerals. The reagent is a 5 % alcoholic solution of 8-hydroxyquinoline. A drop of the solution to be tested suffices for the test. Other elements present in the substance under examination do not interfere. The method can be used for field tests.     

A Universal Geochemical Scenario for Formamide Condensation and Prebiotic Chemistry

The condensation of formamide has been shown to be a robust chemical pathway affording molecules necessary for the origin of life. It has been experimentally demonstrated that condensation reactions of formamide are catalyzed by a number of minerals, including silicates, phosphates, sulfides, zirconia, and borates, and by cosmic dusts and meteorites. However, a critical discussion of the catalytic power of the tested minerals, and the geochemical conditions under which the condensation would occur, is still missing. We show here that mineral self-assembled structures forming under alkaline silica-rich solutions are excellent catalysts for the condensation of formamide with respect to other minerals. We also propose that these structures were likely forming as early as 4.4 billion years ago when the whole earth surface was a reactor, a global scale factory, releasing large amounts of organic compounds. Our experimental results suggest that the conditions required for the synthesis of the molecular bricks from which life self-assembles, rather than being local and bizarre, appears to be universal and geologically rather conventional.     

Chemical evaluation of hypothetical uninodal zeolites

Optimized structural parameters, framework energies relative to alpha-quartz, and volumes accessible to sorption have been calculated for the systematically enumerated hypothetical uninodal zeolitic structures (structures in which all tetrahedral sites are equivalent). The structures were treated as silica polymorphs, and their energies were minimized using the GULP program with the Sanders-Catlow silica potential. Results are given for 164 structures, which include all 21 known uninodal zeolites, two known minerals (tridymite and cristobalite), and 78 unknown zeolite topologies. Twenty-three hypothetical structures were identified as chemically feasible. Complete structural information is provided, and several structures are discussed in detail. The results will assist in the design of new synthetic routes and in the identification of newly synthesized materials.     

Molecular manipulation of two- and three-dimensional silica nanostructures by alkoxysilylation of a layered silicate octosilicate and subsequent hydrolysis of alkoxy groups

A novel methodology for constructing molecularly ordered silica nanostructures with two-dimensional (2-D) and three-dimensional (3-D) networks has been developed by using a stepwise process involving silylation of a layered silicate octosilicate with alkoxytrichlorosilanes [ROSiCl(3), R = alkyl] and subsequent reaction within the interlayer spaces. Alkoxytrichlorosilanes react almost completely with octosilicate, bridging two closest Si-OH (or -O(-)) sites on the silicate layers, to form new five-membered rings. The unreacted functional groups, Si-Cl and Si-OR, are readily hydrolyzed by the posttreatment with a water/dimethyl sulfoxide (DMSO) or water/acetone mixture, leading to the formation of two types of silicate structures. The treatment with a water/DMSO mixture produced a unique crystalline 2-D silicate framework with geminal silanol groups, whereas a water/acetone mixture induced hydrolysis and subsequent condensation between adjacent layers to form a new 3-D silicate framework. The 2-D structure is retained by the presence of DMSO molecules within the swelled interlayer spaces and is transformed to a 3-D silicate upon desorption of DMSO. The structural modeling suggests that both of the 3-D silicates contain new cagelike frameworks where solvent molecules are trapped even at high temperature (up to 380 degrees C, in the case of acetone). Both 2-D and 3-D silica structures are quite different from known layered silicates and zeolite-like materials, indicating the potential of the present approach for precise design of various silicate structures at the molecular level.     

Trimethylsilylation of Mineral Silicates

The application of the technique of trimethylsilylation to mineral silicates is reviewed. Information can be obtained on the molecular size distribution of the silicate anions and also on the aluminium distribution in a silicate backbone. Examples of the use of trimethylsilylation in the structural determinations of silicates include examination of sodium silicates and their solutions, calcium silicates, various glasses, cements and concretes. Included within the review is a discussion of the attempts to prepare new polyorganosiloxane materials. Trimethylsilylation of certain micas, for example, give polyorganosiloxane fluids and greases; and attempts to physically and chemically modify the polyorganosiloxane products are also reviewed.     

Hydrothermal Synthesis of Metal Silicates

Organically templated metal phosphates have been extensively studied because of interesting structural chemistry and potential applications in catalysis. However, in most cases the organic templates cannot be removed without collapse of the frameworks. This is in contrast to the high thermal stability and extensive applications of zeolites in refinery and petrochemical processes.Therefore, studies have been directed to the synthesis of transition metal silicates to produce more stable frameworks. Our synthetic methods are twofold, namely mild hydrothermal reactions in Teflon-lined autoclaves at 100-200 ℃ using organic amines as templates and high-temperature,high-pressure hydrothermal reactions in gold ampoules contained in a high-pressure reaction vessel at ca. 550 ℃ and 150 Mpa using alkali metal cations as templates. In this presentation I will report the high-temperature, high-pressure hydrothermal synthesis, crystal structures, and solid-state NMR spectroscopy of a number of new silicates of indium, uranium, and transition metals.     

Application of differential thermal analysis in mineralogy

In mineralogical research differential thermal analysis can be applied as either a single or a combined method for three purposes:

1. for the qualitative identification of minerals and the (semi-)quantitative determination of the components of rocks and soils,
2. for the characterization of crystal-physical and crystal-chemical properties, including the study of kinetics and the determination of thermodynamic data, phase and reaction equilibria,
3. for special petrogenetic investigations concerning the interrelation of mineralogical properties with the formation, decomposition or recrystallization of minerals.

The presented results were obtained under strongly standardized and reproducible conditions of analysis and by means of some methods recently developed by the author. They include the construction of key diagrams ofΔT versus peak temperatures to identify unknown minerals; the dependence of dehydration and decomposition peak temperatures on the partial pressure of gaseous reaction products (= PA curves); the determination of chemical and crystal-chemical parameters of minerals by using the dependence of dehydroxylation peak temperatures on solid solution (chlorites, smectites) or by using the interrelation between the peak temperatures of structural transformations and crystal-chemical substitutions (sulfides, carbonates, phosphates, silicates); the determination of the degree of disorder of minerals formed in sedimentary environments by using the dehydration or inversion peak temperature and peak shape dependence on the defect character of these crystal structures (clay minerals, quartz, cristobalite, hydroxides); the subdivision of mineral groups by using the interrelation of crystal chemistry and dehydration behaviour (zeolites, mixed-layer clay minerals); the characterization of ferromagnetic minerals by studying the Curie temperatures as a function of the chemical composition; the development of a DTA method to measure the degree of weathering of granitic rocks, etc. Data of 150 minerals are given.     

Thermal decomposition of different types of asbestos

Given the known carcinogenic effects, asbestos minerals are considered as general health hazard. Therefore, the elimination of asbestos materials from the environment is necessary. Asbestos minerals should be entirely transformed to a non-hazardous material. One of these methods is destructing the fibers structure of asbestos minerals by thermal treatment. Asbestos minerals are naturally occurring hydrous silicates, so that they decompose to release water by heating at high temperatures which may lead to changes in crystal structure and the formation of new phases without the dangerous properties. In this article, thermal behavior of asbestos minerals is investigated to observe the disappearance of this hazardous structure and to characterize products obtained by this way. Ten samples of asbestos minerals (six chrysotile samples from different locations, two samples of crocidolite, one amosite, and one tremolite) from different locations were tested. Mineralogical and morphological data (X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy) were obtained before and after differential thermal analysis.     

Systematic synthesis of heterometallic Ni/Fe/S and Cu/Fe/S clusters with a pentlandite-like M8S6 core

The systematic synthesis of heterometallic Ni/Fe/S and Cu/Fe/S clusters with a M8S6 core structure that resembles that of pentlandite minerals is described. The chemical properties and electronic structures of the new clusters have been investigated and are reported in this communication.     

Chemical and spectroscopic investigation on the plaster of a Byzantine church

The discovery of a Byzantine church under the floor of one of the oldest churches of Salerno (Italy) has given us the opportunity to investigate the fine composition of the plaster through chemical and spectroscopic methods. In particular, considering that plasters are generally formed by a carbonate phase (carbonates) and an inert phase (silicates), the characterization, performed on the carbonate phase by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), has revealed the presence of dolomite minerals. This information indicates that, during the building process, some accessory minerals of marble were added in the mortar to be used like fillers. The carbonate phase makes too difficult the spectroscopic characterization of the inert phase. Only by acid attack of the plaster the inert phase has been isolated and its characterization, by XRD and optical microscopy, has indicated the presence of olivine minerals and other typical silicates of river sands.     

Formation and Properties of Medium and High Molecular Weight Compounds of Elements of Groups 3, 4, and 5

The differences between compounds of carbon and of silicon are discussed, and the role of silicon as a fundamental structural component in inanimate nature is described. Starting with silicic acid and silicates, the structure and properties of the silicones and of compounds having Si? C? Si skeletons are described. The possibilities for the formation of high molecular-weight compounds between elements of Groups 3 and 5 of the periodic system are considered, with particular reference to boron nitride and to aluminum-phosphorus compounds.     

Ni[B2(SO4)4] and Co[B2(SO4)4]: Unveiling Systematic Trends in Phyllosilicate Analogue Borosulfates

Borosulfates are compounds analogous to silicates, with heteropolyanionic subunits of vertex-linked (SO₄)- and (BO₄)-tetrahedra. In contrast to the immense structural diversity of silicates, the number of borosulfates is yet very limited and the extent of their properties is still unknown. This is particularly true for representatives with phyllosilicate and tectosilicate analogue anionic substructures. Herein, we present Ni[B₂(SO₄)₄] and Co[B₂(SO₄)₄], two new borosulfates with phyllosilicate analogue topology. While the anionic subunits of both structures are homeotypic, the positions of the charge compensating cations differ significantly: Ni^II is located between the borosulfate layers, while Co^II—in contrast—is embedded within the layer. Detailed analysis of these two structures based on single-crystal X-ray diffraction, magnetochemical investigations, X-ray photoelectron spectroscopy, and quantum chemical calculations, unveiled the reasons for this finding. By in silico comparison with other divalent borosulfates, we uncovered systematic trends for phyllosilicate analogues leading to the prediction of new species.     

A total dissolution method for determination of the α-emitting isotopes of uranium and thorium in deep-sea sediments

A method is described for the determination of specific activities of the natural α-emitting isotopes of uranium and thorium in deep-sea marine sediments. Such information is needed primarily for estimation of sediment accumulation rates. The method is based on total dissolution of sample after fusion with potassium fluoride (to dissolve silicates) and with sodium/potassium pyrosulphate (to dissolve other refractory minerals and convert to the sulphate system). Uranium and thorium fractions are purified for electroplating by anion exchange, with elements naturally present used as carriers. Final determinations are made by α-spectrometry by means of isotopic tracers added at the outset of processing. Experience gained with different types of deep-sea sediment is discussed.     

Borosulfates—Synthesis and Structural Chemistry of Silicate Analogue Compounds

Borosulfates are oxoanionic compounds consisting of condensed sulfur- and boron-centered tetrahedra. Hitherto, they were mostly achieved from solvothermal syntheses in SO₃-enriched sulfuric acid, or from reactions with the superacid H[B(HSO₄)₄]. The crystal structures are very similar to those of the corresponding class of silicates and their substitution variants, especially regarding the typical structural motif of corner-sharing tetrahedra. However, the borosulfates are supposed to be even more versatile, because (BO₃) units might also be part of the anionic network. The following article deals with detailed reports on the different synthesis strategies, the crystal chemistry of borosulfates in comparison to silicates, and their hitherto identified properties.     

Solid-state NMR study of geopolymer prepared by sol-gel chemistry

Geopolymers are a new class of materials formed by the condensation of aluminosilicates and silicates obtained from natural minerals or industrial wastes. In this work, the sol-gel method is used to synthesize precursor materials for the preparation of geopolymers. The geopolymer samples prepared by our synthetic route have been characterized by a series of physical techniques, including Fourier-transform infrared, X-ray diffraction, and multinuclear solid-state NMR. The results are very similar to those obtained for the geopolymers prepared from natural kaolinite. We believe that our synthetic approach can offer a good opportunity for the medical applications of geopolymer.