Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation

Abstract

The <1 µm fraction of a trans-vacant 1M illite (RM30) was studied by conventional and synchrotron X-ray diffraction (XRD) techniques, combined with thermogravimetric (TG, DTG) methods to investigate the structural transformation of illite at different temperatures and degrees of dehydroxylation (D T ). The oriented specimens preheated at 300 and 680 °C correspond to the non-dehydroxylated (D T = 0) and completely dehydroxylated (D T = 100%) 1 M illite structures. Deviation of the basal reflection positions from rationality, expressed by the coefficient of variation of d(00l) values, progressively increase from 0.05 at D T = 4%, to 0.14 at D T = 51%, and then decrease to 0.06 at D T = 95%. Similarly, for each 00l reflection the full width at half-maximum (FWHM) shows a bell-like evolution with increasing preheating temperature. Both of these features are characteristic for mixed-layered structures.The experimental profiles of 00l reflections from the oriented partially dehydroxylated specimens perfectly matched the profiles from XRD pattern simulations calculated in terms of a mixed-layered structure in which the non-dehydroxylated (ND) and completely dehydroxylated (CD) illite layers are interstratified with a strong tendency to segregation. The content of the CD layers in the modeled mixed-layered structures of the preheated specimens show a significant linear correlation with the corresponding D T values (R 2 = 0.99).In random powder XRD patterns collected with synchrotron radiation, the preheated specimens show a distinctive trend in the unit-cell parameters. However, the accuracy in determination of the unit-cell parameters at first decreases up to D T = 61% and then increases with a further increase in D T . The evolution of FWHM values of individual hkl reflections is also similar to that observed for each 00l peak from oriented sample preparations. The unexpected evolution of the unit-cell parameters during progressive dehydroxylation is explained by the interstratification of ND and CD layers in illite.The formation of the mixed-layered structures during Al-rich 1M illite dehydroxylation is in agreement with the prediction from the kinetic model of partially dehydroxylated dioctahedral 2:1 clay structures where dehydroxylation of each portion of the initial OH groups corresponds to a zero-order reaction that is independent of the structural and chemical composition. The reaction is homogeneous and during partial dehydroxylation of the illite structure, ND layers transform into the CD layers without formation of an intermediate phase. A layer-by-layer dehydroxylation mechanism is suggested for thermally induced illite structural transformation.

Determination of the phase composition of partially dehydroxylated kaolinites by modelling their X-ray diffraction patterns

“…This conclusion agrees with the results obtained by Bergaya et al (1996), who studied the ‘Gabriel’ (UK) kaolinite sample by transmission electron microscopy and selected area electron diffraction using controlled-rate thermal analysis and concluded that only two phases – initial non-dehydroxylated kaolinite and metakaolinite – coexist during the sample dehydroxylation. Such coexistence probably means that the dehydroxylation process occurs as a zero-order reaction, which is also typical of partially dehydroxylated dioctahedral 2:1 layer silicates including clay minerals (Drits et al , 2011a, 2011b, 2012a, 2012b). This implies that, during partial dehydroxylation, a non-dehydroxylated layer transforms into a completely dehydroxylated layer without the formation of any intermediate structure.…”

“…• At each stage of partial dehydroxylation, the heated specimens consist of two phases: initial non-dehydroxylated kaolinite and metakaolinite. • The direct transformation of non-dehydroxylated kaolinite crystallites into metakaolinite means that the process corresponds to a zero-order reaction (Drits et al, 2011a(Drits et al, , 2011b(Drits et al, , 2012a(Drits et al, , 2012bDrits & Derkowski, 2015), according to which non-dehydroxylated kaolinite crystallites transform into hydroxylated metakaolinite. • The XRD pattern of metakaolinite and thus its structure remain the same regardless of the degree of partial dehydroxylation.…”

“…The direct transformation of non-dehydroxylated kaolinite crystallites into metakaolinite means that the process corresponds to a zero-order reaction (Drits et al , 2011a, 2011b, 2012a, 2012b; Drits & Derkowski, 2015), according to which non-dehydroxylated kaolinite crystallites transform into hydroxylated metakaolinite.…”

“…For the Imerys sample: At each stage of partial dehydroxylation, the heated specimens consist of two phases: initial non-dehydroxylated kaolinite and metakaolinite.The direct transformation of non-dehydroxylated kaolinite crystallites into metakaolinite means that the process corresponds to a zero-order reaction (Drits et al , 2011a, 2011b, 2012a, 2012b; Drits & Derkowski, 2015), according to which non-dehydroxylated kaolinite crystallites transform into hydroxylated metakaolinite.The XRD pattern of metakaolinite and thus its structure remain the same regardless of the degree of partial dehydroxylation. Owing to this, the values of the contents of metakaolinite in the specimens heated at different temperatures were determined with high accuracy.The profile and intensity distribution of the XRD pattern of metakaolinite formed after heating of the Imerys sample at 550°C are not typical of metakaolinites obtained for other kaolinites and are characterized by an intense and strongly asymmetrical shape of the observed diffraction maximum (at d = 4.492 Å) (Fig.…”

Significance of H2 and CO release during thermal treatment of natural phyllosilicate-rich rocks

“…8), from 400 to 850 ˚C for chlorites (Lempart et al, 2018) and from 750 to 1100 ˚C for talc (Zhang et al, 2006). This process consists of a reaction between hydroxyl groups (OH) forming water molecules (Drits et al, 1995;Drits et al, 2012a;Drits et al, 2012b;Guggenheim et al, 1987;Vedder and Wilkins, 1969):…”