The thermal decomposition of a fibrous talc was studied by transmission electron microscopy (TEM) and selected-area electron diffraction (SAD). Small changes in the lengths of a and b unit-cell parameters were noticeable at 500°C, but the talc laths did not change morphologically until 800°C. At this temperature striations began to appear in the TEM image, and the talc SAD reflections began to develop faint satellite streaks. At 900°C the striations appeared to be crystallites, and reflections of orthorhombic enstatite were noted. Both TEM and SAD evidence showed that the enstatite crystallites were formed in three orientations corresponding to the three pseudohexagonal a axes of the talc. Thus, triple superposition of the electron diffraction pattern at the three equivalent angles explains the high symmetry star-shaped pattern. At 1000° to 1100°C the enstatite crystallites were shorter and thicker, and the streaks in the SAD pattern were nearly absent. Above 1200°C only one orientation of the enstatite crystallites was found. Noncrystalline regions were also detected at 900°C and became progressively scarce at 1000° and 1100°C. They were not detected at 1200°C. At 1300°C cristobalite was detected in some SAD patterns. The crystallographic axes and unit-cell parameters of the talc and enstatite were also topotactically related as follows: at (5.3 Å)//ce (5.2 Å); bt (9.16 Å)//be (8.8 Å); d(001)t (18.84 Å)//ae (18.2 Å). These results are compatible with an inhomogeneous decomposition mechanism as proposed by earlier workers.

Lode, a dioctahedral illite-rich clay from Latvia belonging to the mica group of clay minerals, undergoes thermal transformation via a series of structurally disordered intermediate phases. Despite containing high levels of paramagnetic Fe substituted into the octahedral sites, 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of sufficient quality are obtained to resolve different structural units, showing clearly defined structural changes which occur in the sample during calcination to 1200 °C. However, Fe plays a significant role in broadening the Al signal, with integrated peak intensities decreasing as temperature increases. Significant differences are revealed in the thermal decomposition process by NMR spectra between pyrophyllite, Ca-montmorillonite and illite clays, possibly due to the different cations present in the interlayer. It has also been shown for illite that no structural differences at the atomic level occur when the dwell time at a particular temperature is varied and no difference is observed between samples that have different thermal histories; however, a minor effect of particle size and surface area is visible in the NMR data.

The production and industrial use of asbestos cement and other asbestos-containing materials have been restricted in most countries because of the potential detrimental effects on human health and the environment. Chrysotile is the most common form of asbestos and investigations into how to recycle this serpentine phyllosilicate mineral have attracted extensive attention. Chrysotile asbestos tailings can be transformed thermally, at high temperature, by in situ carbothermal reduction (CR). The CR method aims to maximize use of the chrysotile available and uses high temperatures and carbon to change the mineral form and structure of the chrysotile asbestos tailings. When chrysotile asbestos is employed as the raw material and coke (carbon) powder is used as the reducing agent for CR transformation, stable, high-temperature composites consisting of forsterite, stishovite, and silicon carbide are formed. Forsterite (Mg2SiO4) was the most abundant crystalline phase formed in samples heat treated below 1500ºC. At 1600ºC, forsterite was exhausted through decomposition and β-SiC formed by reduction of stishovite. A larger proportion of β-SiC was generated as the carbon content was increased. This research revealed that both temperature and carbon addition play key roles in the transformation of chrysotile asbestos tailings.

The objective of the present study was to investigate changes in the structural, textural, and surface properties of tubular halloysite under heating, which are significant in the applications of halloysite as functional materials but have received scant attention in comparison with kaolinite. Samples of a purified halloysite were heated at various temperatures up to 1400°C, and then characterized by X-ray diffraction, electron microscopy, Fourier-transform infrared spectroscopy, thermal analysis, and nitrogen adsorption. The thermal decomposition of halloysite involved three major steps. During dehydroxylation at 500–900°C, the silica and alumina originally in the tetrahedral and octahedral sheets, respectively, were increasingly separated, resulting in a loss of long-range order. Nanosized (5–40 nm) γ-Al2O3 was formed in the second step at 1000–1100°C. The third step was the formation of a mullite-like phase from 1200 to 1400°C and cristobalite at 1400°C. The rough tubular morphology and the mesoporosity of halloysite remained largely intact as long as the heating temperature was <900°C. Calcination at 1000°C led to distortion of the tubular nanoparticles. Calcination at higher temperatures caused further distortion and then destruction of the tubular structure. The formation of hydroxyl groups on the outer surfaces of the tubes during the disconnection and disordering of the original tetrahedral and octahedral sheets was revealed for the first time. These hydroxyl groups were active for grafting modification by an organosilane (γ-aminopropyltriethoxysilane), pointing to some very promising potential uses of halloysite for ceramic materials or as fillers for novel clay-polymer nanocomposites.

The thermal transformations of kaolinite have been studied using 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TG). The experimental results show that this sample is a pure kaolinite which contains ∼3% carbonaceous material as impurity. 27Al and 29Si MAS NMR spectra show that the microstructural evolution of the thermal transformation of kaolinite at 450 –1050ºC is similar to that of the kaolinite– mullite reaction series previously published by many authors. 29Si MAS NMR spectra of mullite at 1190 and 1250ºC display two resonances at ∼ – 87 and –92 ppm, corresponding to sillimanite-type geometry around Si and the typical Si environment of mullite, respectively. At 1350ºC, the splitting of (hk0) and (kh0) of mullite occurs, indicating that the primary mullite transforms into orthorhombic mullite. Simultaneously, the resonance at ∼ – 92 ppm splits into two signals at ∼ –90 and –94 ppm. 27Al MAS NMR spectra of the mullite consist of three signals centred at ∼ –4, 45 and 60 ppm, corresponding to octahedral, distorted tetrahedral and tetrahedral Al sites, respectively.

A ‘metamorphic vermiculite’-like phase was identified in Ca-rich ancient ceramic sherds from excavations in NW Peloponnese, Greece. Archaeometric investigations have shown that the raw materials used in the production of ancient ceramics were derived from local Plio-Pleistocene sediments. Analysis by X-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy showed that ‘metamorphic vermiculite’ is a common constituent phase of these sediments occurring in three texturally different types: as interstratified white mica-chlorite and chlorite-vermiculite, and white mica-(white mica-chlorite) stacks. In an attempt to elucidate the thermal transformation of ‘metamorphic vermiculite’ present in the calcareous raw materials, we produced experimental ceramics made from these local raw materials and fired them at 700, 750, 850, 950 and 1050°C in a static air furnace. The mixed-layered chlorite-vermiculite was transformed completely at ∼800°C contributing to the formation of new high-T minerals. The two other types of ‘metamorphic vermiculite’ retain their original lath-like shape up to 1050°C and only a few crystals show that they have undergone complete transformation at this temperature. In the latter crystals, numerous nanocrystals were formed sub-parallel to the former cleavages of ‘metamorphic vermiculite’ pseudomorphs, suggesting their contribution to the nucleation of high-T minerals (i.e. ferrian aluminian diopside, spinel, Fe oxides) by reactions with the available CaO. It is suggested that the firing conditions (i.e. maximum reaction temperature of 1050°C, reaction time of 1 h, oxygen atmosphere) which promote disequilibrium reactions, and the greater contribution of the white mica constituent against chlorite in some of the initial structures of ‘metamorphic vermiculite’ may be responsible for the delay of its thermal decomposition at 1050°C.

Selected samples of asbestos-containing material (ACM) with different Ca/Si ratios have been treated thermally at 1200°C for 15 min to obtain an ‘end of waste geo-inspired material’. Before and after treatment, micro-Raman spectroscopy allowed the investigation of both powdered and massive samples by directing the laser beam onto crystals with elongated morphology, thin fibres and the matrix. In the raw samples, chrysotile and/or crocidolite were detected. After the thermal treatment, no asbestos phases were identified in the Raman spectra collected on fibrous or fibre-like morphologies. The scanning electron microscopy/energy dispersive spectroscopy investigations confirmed the onset of a pseudomorphic process during annealing, leading to the complete transformation of asbestos minerals into non-hazardous magnesium or calcium magnesium silicates such as forsterite, monticellite, a˚kermanite and merwinite.

The identification of such mineral assemblages was inspired by the close inspection of a natural counterpart, the high-temperature contact metamorphic imprint due to the intrusion of a sill into carbonate rocks. The process turned out to occur largely at the solid state and involved substantial mobilization of Ca and Mg to form a spinel phase (namely MgFe2O4) which was recognized in the matrix and within, or close to elongated morphologies.

The fate of 14C-sethoxydim {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} in an aqueous solution and on glass exposed to light was evaluated. Within 1 h, more than 80% of the 14C-sethoxydim was transformed to six major products. Qualitatively the transformations were similar for both systems, and a single end product constituted the majority of 14C after 72 h. Two transitory compounds were found to be phytotoxic to barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCG] and were more stable than sethoxydim. A nonphytotoxic compound isolated and identified by mass spectroscopy was desethoxy-sethoxydim.

Time-resolved synchrotron powder diffraction was used to follow the thermal transformation of cement-asbestos. Thermal transformation of asbestos fibers into nonfibrous crystalline phases is a promising solution for the elimination of these hazardous minerals. Time resolution offered by the use of an imaging plate detector with a high-brightness X-ray source allowed for the observation of metastable phases, commonly not detectable with conventional instrumentation. In addition, the use of a closed capillary as a sample holder mimicked the real, novel industrial reactor where cement-asbestos slates are sealed in a tunnel kiln. The changing gas atmosphere in the closed system was shown to affect the final composition of the recrystallized product. This study demonstrates the importance of advanced powder diffraction techniques in this field of applied research.