J. Bastida

XRD microstructural analysis of mullites obtained from kaolinite–alumina mixtures

Abstract A microstructural study of mullite obtained by the reaction sintering of kaolinite–α–alumina mixtures in the range 1150–1700°C has been performed by using X-ray line profile analyses together with scanning and transmission electron microscopy equipped with microanalysis by energy dispersion (SEM-EDS, TEM-AEM). Two kinds of morphology corresponding to primary (elongated grains) and secondary (equiaxed grains) mullite have been observed. A bimodal crystallite size distribution has been detected through XRD microstructural analysis from 1300°C. The results obtained by this method are compared with SEM/TEM data.

Microstructural evolution and growth of crystallite size of mullite during thermal transformation of kyanite

Abstract The microstructural evolution of mullite during the thermal transformation of kyanite has been studied in the temperature range 1200–1600 °C. The shape and size of the grains were analysed by means of SEM-EDS while crystallite size evolution was studied by X-ray line profile analyses. The results obtained showed that total transformation of kyanite to mullite takes place between 1350 and 1400 °C. At temperatures below 1350 °C needle-like mullite grains are always produced. At higher temperatures the mullite grains reveal rounded end platelet morphology. Evolution from needle-like to platelet shape was correlated with the X-ray data.

Crystallinity of Lower Cretaceous kaolinites of Teruel (Spain)

Abstract The crystallinity criteria based on X-ray diffraction powder data of unoriented samples proposed to date for kaolinite, are usually difficult to apply to polimineralic samples because of overlapping peaks. The aim of this work is to avoid this problem by using X-ray diffraction data of oriented samples to measure crystallinity. Fast X-ray diffraction patterns registering the 001 and 002 peaks and the 02,11 bidimensional band have been performed and the crystallinity has been obtained by comparing the fast patterns with classifications based on the appearance of X-ray diffraction patterns, which are related to the definition of the peaks in the 02,11 band and to the width and height of the 001 and 002 peaks (Thorez, 1976). The 001 and 002 reflections have also been studied in X-ray diffraction patterns of oriented powder aggregates with a slow goniometer speed; the obtained data have been used to calculate the crystallite size in the [001] direction by the methods of Williamson and Hall (1953) and Warren and Averbach (1950), and the results of both methods have been compared. The extent of the hydrazine intercalation has been tested by the Tchoubar et al. (1982) index. The particle size has been observed by TEM. The methods of Williamson and Hall and Warren and Averbach used for measuring crystallite size in the [001] direction offer consistent results and with a good correlation. These results have been compared with evaluations of crystallinity based on the appearance of the X-ray diffraction pattern of the unoriented sample and with the corresponding values of the Tchoubar et al. (1982) index. The 001 and 002 reflections are affected by the general diminution of crystallinity as shown in powder patterns of the unoriented sample, and thus a line broadening is observed. The hydrazine intercalation capacity seems to be related to the crystallite size in the [001] direction (a general tendency to the greater Tchoubar rate the lower size can be observed) but not with the particle size and morphology of the (001) plates observed by TEM.

Crystalline microstructure of sepiolite influenced by grinding

The crystalline microstructure of ground sepiolite has been investigated. A reference sample of sepiolite and products of its comminution by dry grinding were studied through X-ray diffraction pattern analysis, specific surface measurements by nitrogen adsorption and complementary analysis of field emission scanning electron microscope images. A statistical model of polycrystals was applied to describe and determine the crystalline microstructure of the studied specimens. The model parameters characterizing the microstructure were prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice strain distribution, and they were determined for each sample by modelling a selected part of the X-ray diffraction pattern and fitting the simulated pattern to a measured one. A strict correlation of microstructure parameters with grinding time and with specific surface magnitudes was observed. A parallelepiped with edge-length ratios almost independent of grinding time (for longer times) was found to be the predominant crystallite shape. The crystallite size distributions were found to be close to logarithmic normal ones, with the mean values decreasing with increasing grinding time and the standard-deviation-to-mean-value ratios approximately constant. The second-order crystalline lattice strain distributions were found to be close to some simple function with the mean value equal to zero, the mean deviation increasing with increasing grinding time and the standard-to-mean-deviation ratios approximately constant. It was demonstrated that the specific surface can be calculated on the basis of the microstructure characteristics. Some details of the relation between crystallites and crystalline grains were explained by comparing the results of analyses via X-ray diffraction and scanning electron microscopy.

X-RAY DIFFRACTION LINE-BROADENING STUDY ON TWO VIBRATING, DRY-MILLING PROCEDURES IN KAOLINITES

Due to the great technological importance of the microstructure of kaolinite, characterizing its evolution during dry milling of kaolin and analyzing the microstructural information obtained from different methods were the main aims of this work. The microstructural alteration of kaolinite is evaluated by X-ray diffraction and electron microscopy methods, comparing the results obtained and analyzing the correlations between them. The Warren-Averbach and Voigt-function methods of X-ray diffraction microstructural analysis have been applied successfully to the study of the effects of two different, vibrating-cup dry-milling configurations in the microstructure of kaolinite from the reflections corresponding to (001) diffracting planes. The crystallite-size estimates obtained from the two methods correlate well. Field emission scanning electron microscopy measurements of kaolinite particle thickness are in good agreement with the crystallite size estimated by the two methods. The Warren-Averbach method also provided estimates of the contribution to line broadening. Vibrating-cup milling has been proved to be a more efficient method of strain comminution of kaolinites than other milling techniques, reaching extensive microstructural degradation within seconds.

X-RAY DIFFRACTION LINE BROADENING ON VIBRATING DRY-MILLED TWO CROWS SEPIOLITE

A reference sample of sepiolite and products of its comminution by vibrating dry-milling have been studied using X-ray diffraction (XRD) line-broadening analysis, complementary field emission scanning electron microscopy (FESEM) images and surface area measurements. The apparent crystallite sizes determined via XRD are in agreement with observations on FESEM images. The sepiolite aggregates consist of lath-shaped agglutinations of prisms and pinacoids elongated along [001], each lath including several crystallites in that direction. The surface area magnitudes are in the range of previous experimental measurements of other sepiolites. The results obtained show the effectiveness of vibro-milling as the procedure to use for the comminution of sepiolite.

Weathering evolution in lutites of the K/Pg transition red beds of the Tremp Group (Tremp-Isona Basin, south Pyrenees)

Abstract The Tremp-Isona basin (south-central Pyrenees, Lleida, Spain) shows maximum development of the Tremp Group (early Maastrichtian to late Paleocene) covering a wide geological record across the Cretaceous-Paleogene (K/Pg) boundary in continental facies. The mineralogy and geochemistry of lutites were used to assess the evolution of weathering from the Maastrichtian to the Eocene, and particularly for the red beds of the Lower Red and Upper Red Units (pre- and post-K/Pg, respectively). Chemical weathering decreased initially in the Maastrichtian (Gray Unit to Lower Red Unit), increasing subsequently from the Paleocene (Upper Red Unit) to Eocene units. ANOVA analysis of mineralogical compositions and cluster hierarchical analysis have been useful tools to select convenient lutites for assessment of weathering evolution by the chemical alteration index and by broadening of the 001 X-ray diffraction line of illite after ethylene glycol solvation. The lesser chemical weathering found in the Upper Red Unit (Danian) is interpreted in the context of relative warming and aridification, in climates of contrasting seasons and pronounced dry seasons.