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Dear Colleagues,

The most commonly studied materials (minerals, rocks, sediments, soils, and different products) in applied mineralogy, materials sciences and engineering are monophasic or polyphasic and largely (but not exclusively) crystalline. Their microstructure (which can be referred to different names in different fields, such as textures, micro-textures or fabrics) mainly concern particle size, orientation, distribution of sizes and orientations of the constitutive phases of the material at microscopic or sub-microscopic level.

For more than 60 years, microstructural aspects at a sub-microscopic level have mainly been studied by X-ray powder diffraction microstructural analysis and X-ray texture analysis. Different techniques have subsequently been used to image materials from microscopic to atomic level imaging: conventional transmission electron microscopy (TEM), scanning electron microscopy (SEM), emission electron microscopy (EEM), high-resolution transmission EM (HREM), low-energy EM (LEEM), X-ray micro-tomography (micro-CT, high-resolution micro-CT, etc.), scan tunnelling microscopy (STM), atomic force microscopy (AFM), etc.

These methods of microstructure analysis can better explain the properties of mineral raw materials and products in different fields (earth sciences, engineering geology, economic geology, materials sciences, etc.). The understanding of microstructures will continue to improve the optimization of mineral processing and interpretation of the formation conditions and behaviour of mineral assemblages.

Prof. Dr. Joaquín Bastida Cuairán
Dr. Pablo Rafael Pardo Ibáñez
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Keywords

mineral microstructure
industrial minerals and rocks
sediment microstructure
soil microstructure
cement microstructure
ceramic microstructures
microstructure analysis
microstructure imaging

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19 pages, 8784 KiB

Open AccessArticle

Dehydroxylation of Kaolinite and Halloysite-Rich Samples: An In Situ Study of the Texture and Structural Evolutions

by Imane Daou, Cristian Mocuta, Gisèle Laure Lecomte-Nana, Nicolas Tessier-Doyen, Claire Peyratout, René Guinebretière and Dominique Thiaudière

Minerals 2023, 13(11), 1418; https://doi.org/10.3390/min13111418 - 07 Nov 2023

Viewed by 813

Abstract

Halloysite and kaolinite are dioctahedral TO phyllosilicates that drive the interest of scientists for formulating environmentally friendly materials, and consequently in the field of ceramics. The main scope of this study was the understanding of the texture evolution upon the dehydroxylation reaction and the influence of the presence of halloysite. In situ synchrotron (002) and (111) poles figures were recorded on the DiffAbs beamline at SOLEIL Synchrotron, from room temperature to 1000 °C, on kaolinite and/or halloysite-rich samples shaped by tape casting. Commercial kaolins and halloysite provided by Imerys company were used. The samples were labeled KRG100, KCS100, H100, KRG50H50 and KRG59H50 in relation with the wt. % of kaolin (KRG, KCS) or halloysite (H) clays. In samples KCS100 and KRG100, a strong texture was observed until in situ annealing at 700 °C, with respect to the c-axis of kaolinite. On the contrary, the texture with respect to the c-axis of halloysite for the sample H100 was weak whatever the temperature was. Moreover, this weak texture disappeared before the complete dehydroxylation of halloysite. This is due to the opening of some halloysite tubes. When considering the samples KRG50H50 and KCS50H50, a significant texture was observed with the c-axis preferentially oriented perpendicular to the sample surface. The presence of kaolinite platelets predominated onto the alignment of halloysites tubes. Furthermore, it was noted that the halloysite influenced the (002) diffracted intensity into the temperature range 20 °C to 400 °C. Above 400 °C, the behavior obtained for the (002) reflection in samples KRG50H50 and KCS50H50 was similar to the behavior noticed for pure kaolins KRG100 and KCS100, respectively. The dehydroxylation temperature range appeared to be relevant with combined effect of kaolinite and halloysite transformations arising from KRG100 or KCS100 and H100 samples. Therefore, the onset point of dehydroxylation is 550 °C ± 25 °C for KRG100, KCS100, KRG50H50 and KCS50H50. For the pure halloysite H100 sample, the dehydroxylation starts at the lower temperature 475 °C. It was also noted that during the dehydroxylation of kaolinite, the characteristic portion of ring related to the diffracted intensity of the (111) reflection located at χ = 45° tended to disappear above 550 °C and led to the formation of a new transitory phase with a (111) reflection with perpendicular alignment to the c-axis. Indeed, an epitaxial relationship with the (111) kaolinite reflection could be assumed. Further X-ray scattering experiments allowed highlighting the effective offset temperature of the dehydroxylation, which was identified as close to 720 °C. The metakaolinite achieved structural transformation to another transitory phase at 1000 °C. Full article

(This article belongs to the Special Issue Microstructural Characterization in Applied Mineralogy)

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