Textural and structural transformations of kaolinites in aqueous solutions at 200°C

Abstract

We have studied the processes affecting disordered kaolinites during hydrothermal treatment at 200°C, pH=4 and increasing time of reaction. The starting materials include untreated (natural), poorly ordered Georgia kaolinite (KGa-2), and this sample after intense grinding. Chemical analyses of equilibrium solutions show a higher dissolution rate in the case of the ground kaolinite. In both cases, Si contents are higher than Al contents, indicating a non-stoichiometric dissolution. The solid products of the reactions with natural kaolinite show scarce differences with the starting sample: The XRD patterns reveal a slight sharpening of the basal reflections and a moderate increase of the Hinckley index. The solid products of the reactions with ground kaolinite show, on the contrary, remarkable textural and structural differences with the starting material: Most of the observed particles are spherical and the granulometric analyses indicate a notable increase of the mean particle size. The XRD patterns show an increase in both intensity and resolution of the non-basal reflections, which reflect the decrease of translation defects at the same time that the 13̄1 reflection indicates an increase of the W_C-type defects. The evolution observed on the IR spectra, at increasing run times, also indicate a notable increase in crystallinity. The formation of spherical aggregates of kaolinite crystals and the increase in structural order indicate that the hydrothermal reactions caused the recrystallization of the ground kaolinite. These results suggest a dissolution–precipitation process, which is notably less developed in the case of natural kaolinite.

Introduction

The transformations among the several polytypes of the kaolin group minerals are characteristic phenomena during weathering, diagenesis and hydrothermal alteration. The transformations of allophane to halloysite and of halloysite to kaolinite have been observed under weathering conditions (e.g., Siefferman and Millot, 1969, Keller, 1977, Nagasawa, 1978, Churchman and Gilkes, 1989). The transformations of kaolinite to dickite and occasionally to nacrite have been described in sedimentary basins with increasing temperature and pressure (e.g., Shutov et al., 1970, Ehrenberg et al., 1993, Ruiz Cruz, 1996, Ruiz Cruz and Moreno Real, 1993). Dickite-rich zones has also been observed in hydrothermally altered rocks, where dickite forms at temperatures >150°C (Hanson and Zamora, 1981).

Experiments relevant to these transformations have been also reported. Room temperature experiments showed that the halloysite to kaolinite transformation occurs in aqueous solutions of oxalic acid and EDTA (La Iglesia and Galán, 1975). De Kimpe (1976) reported the synthesis of kaolin minerals from aluminosilicate gels under low temperature conditions. Dissolution of kaolinite, halloysite and allophane-like amorphous hydrous aluminosilicate in aqueous solutions at 47°C was investigated by Tsuzuki and Kawabe (1983), who suggest that these polymorphic transformations proceed by a dissolution–precipitation process.

Dissolution of kaolinite and dickite at the temperature range 150–300°C and pH range 1–2.5 has also been investigated to study the kaolinite–dickite relative stability (Zotov et al., 1998). From solubility measurements, these authors conclude that dickite is the stable polymorph to at least 350°C, in contrast to the thermodynamic parameters, which suggest that dickite is unstable relative to kaolinite Naumov et al., 1974, Robie et al., 1979, Haas et al., 1981, Robinson et al., 1982. The pervasive presence of kaolinite in subsurface environments is explained on the basis of the lower rates and higher activation energies necessary for the nucleation and crystal growth of dickite relative to kaolinite. These authors estimate that an increase in temperature to about 120–150°C is required for the precipitation of dickite. Nevertheless, experiments of synthesis of kaolin minerals from gel in this temperature range and even at higher temperatures, lead to the formation of kaolinite (e.g., Eberl and Hower, 1975, De Kimpe and Kodama, 1984, De Kimpe et al., 1981, Huertas et al., 1993, Huertas et al., 1999).

In this context, the present study investigates the processes affecting to kaolinite in aqueous solutions, at 200°C and pH≈4. Taking into account the differences in solubility between ordered kaolinite, halloysite and amorphous phases (e.g., Tsuzuki and Kawabe, 1983, Carroll and Walther, 1988, Huertas et al., 1999), we have used the same kaolinite in natural and artificially disordered states, this later with the aim of facilitate the transformations. The results may be useful for interpretation of the occurrences and transformations of the kaolin polymorphs in sedimentary basins and hydrothermal systems.