Characterization of the expandable clays formed from kaolinite at 200ºC

Abstract

The hydrothermal reaction of kaolinite in the system Na2O-K2O-MgO-Al2O3-SiO2- H2O-HCl, at near-neutral pH conditions, has been investigated at 200ºC, 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 Å (smectite) or 14 Å (chlorite) are dominant. In addition, packets with periodicities of 12 –13 Å are also observed. Only rarely, randomly to ordered sequences of 10 and 14 Å 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

“…In both cases, dissolution of kaolinite and formation of Li-A(BW) zeolite (Norby et al, 1986) and a new phyllosilicate with a basal spacing of~12.5 Å was observed from short reaction times. This behaviour is different from that observed in Li-free reactions (Bentabol et al, 2003c), where the formation of new phases is only detected by XRD in the products obtained after 15 days, and which may be related to the higher pH of the Li-bearing reactions.…”

“…nature of the stable phases. The progress of the reactions has been greater than that observed in Lifree systems (Bentabol et al, 2003c). Indeed, the formation of the new phyllosilicates is observed at shorter reaction times, indicating that both the high pH and the chemical modifications induced in the initial system by the zeolite precipitation favour the formation of 2:1 and 2:1:1 structures from kaolinite.…”

“…Indeed, the formation of the new phyllosilicates is observed at shorter reaction times, indicating that both the high pH and the chemical modifications induced in the initial system by the zeolite precipitation favour the formation of 2:1 and 2:1:1 structures from kaolinite. In addition, zeolite precipitation causes the enrichment of the systems in Mg, and all the phases formed show a larger Mg content than those obtained in Li-free systems (Bentabol et al, 2003c). The presence of either K or Na in the initial system also influences the structure and chemical composition of the neoformed phyllosilicates.…”

“…Most frequently, the phases formed during hydrothermal experiments are crypto-crystalline and therefore more soluble than natural minerals. Tosudite solubility was estimated from the solution analyses (Bentabol et al, 2003c), assuming that solutions fall on the equilibrium tosudite-montmorillonite, obtaining a solubility constant of log K Tos = 17.214. This value was then used to estimate the clinochlore solubility constant, which is log K Chl = 41.277.…”

“…Formation of illite from kaolinite in the presence of KOH solutions was also investigated by Huang (1993) and Bauer et al (1998) among others. On the contrary, higher silica contents produced partially expandable phases, at pH of the order of 7À8 (Bentabol et al, 2003c). In the first case, the presence of Mg in the system led to the formation of 14 Å chlorite-like layers that cover the illitic packets.…”