M. Bentabol

Hydrothermal synthesis of Mg-rich and Mg-Ni-rich kaolinite

Mg-rich kaolinite and Mg+Ni-rich kaolinite have been synthesized in hydrothermal experiments (200 and 400°C) from poorly crystalline kaolinite and Mg- and Mg+Ni-bearing solutions. Al-rich serpentine and Al-rich chlorite were obtained as sub-products of the reactions. The formation of these phases occurred through a dissolution-precipitation mechanism that led to spherical kaolinite after short reaction times. A morphological evolution towards platy particles and stacks occurred at increasing run times.Identification of the several phases was carried out using a combination of X-ray diffraction and transmission/analytical electron microscopy. Analytical data indicate that the Mg content in kaolinite increased as a function of the reaction time and temperature, reaching up to 0.46 atoms per half formula unit (a.p.h.f.u.). The measured (Mg+Ni) content reached up to 0.56 a.p.h.f.u.. Both the gradual increase of the b-cell parameter of kaolinite at increasing Mg contents and the presence of new bands on the FTIR spectra of the synthesized kaolinite point to a Mg-for-Al replacement in the octahedral sheet rather than to the presence of serpentine-like layers interstratified in the kaolinite structure.

Hydrothermal transformation of kaolinite to illite at 200 and 300°C

Abstract The hydrothermal reaction of kaolinite in the system Na2O-K2O-(MgO)-Al2O3-SiO2- H2O has been investigated at 200 and 300ºC. The study of the solid products indicates that, at 200ºC, disordered illite was the first phase formed, which evolved at increasing run times towards well ordered illite and analcime. After longer reaction times illite and kaolinite were the sole phases identified by XRD. The b parameter of the illites increased at longer reaction times, from 9.05 to 9.08 Å . Study by TEM/AEM revealed the presence of surface chlorite-like layers, covering the illite packets, however. The trend observed in the products of the reaction at 300ºC was similar, except for the presence of boehmite or pseudoboehmite at the higher temperature. The values of the b parameter of illite, of the order of 9.12 - 9.16 Å , indicate that illites formed at 300ºC have a more trioctahedral character. The results obtained also indicate that Mg determines which product will be formed, Kand Na-zeolites and Al-rich illites being the phases formed in the absence of Mg.

SYNTHESIS OF Ni-RICH 1:1 PHYLLOSILICATES

Rapid dissolution of partly amorphized kaolinite in the systems kaolinite + NiCl2, kaolinite + Ni(OH)2, and kaolinite + NiCl2 + Ni(OH)2, at a temperature of 200°C and at pH between 5.3 and 7.4, leads to the precipitation of Ni-poor kaolinite, Ni-rich kaolinite and Al-Ni-serpentine. Identification of the phases was carried out using a combination of X-ray diffraction and transmission/analytical electron microscopy. Ni-bearing kaolinite shows variable morphologies in the systems studied: stacks of kaolinite with relatively small Ni contents and fine-grained curved particles of Ni-rich kaolinite dominate in the Cl-bearing system; spherical particles with a disordered structure and relatively uniform Ni contents (in the order of 0.15 atoms per formula unit (a.p.f.u.)) and platy particles of Al-Ni-serpentine characterize the products formed in the Ni(OH)2-richest systems. The presence of Ni(OH)2 in the systems (with and without Cl) favors the dissolution process as well as rapid precipitation of spherical particles, and the formation of serpentine. A difference from Mg systems studied previously is a well defined phase intermediate in composition between kaolinite and serpentine which originated in the Ni-bearing systems. Increasing Ni content is clearly reflected in the parallel increase in the b cell parameter of kaolinite. The average composition of the coexisting Al-Ni-serpentine is: (Al1.24Ti0.01Fe0.02Ni1.31) (Si1.58Al0.42)O5(OH,Cl)2.

Chemistry, morphology and structural characteristics of synthetic Al-lizardite

Abstract Al-lizardite has been synthesized under hydrothermal conditions (200ºC). Morphologically, Al-lizardite consists of thin platy particles ~400 Ǻ wide and ~150 Ǻ thick. Structurally, the X-ray diffraction patterns indicate that the 2H2 polytype is dominant, with cell parameters: a = 5.311(0.006) Ǻ; c = 14.333 (0.013) Ǻ and space group P63. High-resolution transmission electron microscopy images revealed, however, the presence of other polytypes and abundant stacking disorder. Chemically, Al-lizardite consists of a single population with average tetrahedral composition Si1.74Al0.26. In contrast to previously described Al-rich serpentines (amesite and Al-lizardite), this Al-lizardite is characterized by an asymmetrical Al distribution, with VIAl ≈0.70 and IVAl ≈0.25 atoms per formula unit.

Characterization of the expandable clays formed from kaolinite at 200°c

The hydrothermal reaction of kaolinite in the system Na2O-K2O-MgO-Al2O3-SiO2-H2O-HCl, at near-neutral pH conditions, has been investigated at 200°C, for times from 12 h to 180 days. X-ray diffraction (XRD) study of the solid products indicates that the phases formed from 15 days’ reaction time are randomly ordered mixed-layer chlorite-smectite, dioctahedral on average, with a chlorite:smectite ratio of ~1:2. No structural evolution was observed with increasing reaction time. Investigation by transmission electron microscopy revealed, on the contrary, that very thin packets with basal spacing of either 10 A (smectite) or 14 A (chlorite) are dominant. In addition, packets with periodicities of 12–13 A are also observed. Only rarely, randomly to ordered sequences of 10 and 14 A are observed in the packets. These observations indicate that the mixture of these phases behaves in XRD as randomly ordered mixed-layer chlorite-smectite. Chemical analysis of the solutions suggests that the neo-formed phases are metastable and would evolve, at longer reaction times, toward ordered mixed-layer chlorite-smectite or to clinochlore.

Hydrothermal transformations of kaolinite at 200 and 250°C in the systems Li2O–Na2O–MgO–Al2O3–SiO2–H2O–HCl and Li2O–K2O–MgO–Al2O3–SiO2–H2O–HCl

Abstract The hydrothermal reaction of kaolinite in the systems Li2O-K2O-MgO-Al2O3- SiO2-H2O-HCl and Li2O-Na2O-MgO-Al2O3-SiO2-H2O-HCl, at pH between 9 and 11, has been investigated at 200-250ºC from 1 to 180 days. The X-ray diffraction study of the K-bearing system indicates that the solid products formed were Li-A(BW) zeolite, crystalline silica, and randomly ordered chlorite-smectite mixed layers, dioctahedral on average, with a chlorite: smectite ratio of ~1:4. Study by electron microscopy reveals, however, the presence of mixtures of very thin particles of smectite, di, trioctahedral chlorite and mixed-layer phases. The X-ray study of the solid products formed in the Na-bearing system reveals that the neo-formed phases were Li-A(BW) zeolite, analcime and partially ordered chlorite-vermiculite mixed-layers with high chlorite: vermiculite ratio. The transmission microscopic study of these phases shows the presence of dioctahedral ‘vermiculitic’ curved particles and di, trioctahedral chlorite-like particles. In both cases, long reaction times cause formation of a 7 Å phase with composition intermediate between kaolinite and serpentine. The composition of the solutions suggests that the stable phases would be chlorite/smectite mixed-layers, with compositions near tosudite in the case of the K-bearing reaction and both a dioctahedral phase (tosudite?) and a trioctahedral one (clinochlore) in the case of the Na-bearing reaction.