Influence of Synthesis pH on Kaolinite “Crystallinity” and Surface Properties

Abstract

Hydrothermal syntheses were performed at various pH values and temperatures to induce variability in kaolinite defect density. Temperature of synthesis ranged from 200 to 240~ for 21 d. Initial pH at room temperature ranged from 0.5 to 14. The starting material was a hyd~othermally treated gel, with an atomic Si/A1 ratio of 0.93, partly transformed into kaolinite.Kaolinite was obtained for a wide range of pH. Although no influence of temperature on "crystallinity" (i.e., defect density) was observed, the effect of pH was important. A continuous series was obtained from a low-defect kaolinite, with high thermal stability and a hexagonal morphology for the most acidic final pH, to a high-defect kaolinite, with low thermal stability and lath shape for the most basic final pH. These variations of kaolinite properties appear related to the pH dependence of kaolinite surface speciation. Increasing pH value results in increased cation adsorption on the kaolinite external surfaces and increases in the elongation of particles.

The temporal evolution of pedogenic Fe–smectite to Fe–kaolin via interstratified kaolin–smectite in a moist tropical soil chronosequence

“…7) which is similar to dehydroxylation of disordered Fe-rich kaolin observed in soils (Varajāo et al, 2001;Hart et al, 2002) and created in synthesis experiments (Petit and Decarreau, 1990;Iriarte et al, 2005), and also to Fe-poor synthesized kaolins (Fialips et al, 2000). Two other notable endothermic peaks occur between the dehydration range and the silicate dehydroxylation range.…”

Visible‐near‐infrared reflectance spectroscopy of volcanic acid‐sulfate alteration in Nicaragua: Analogs for early Mars

“…Furthermore, experimental and field studies show that Al phyllosilicates, such as kaolinite and halloysite, frequently form in acidic and/or hydrothermal areas [e.g., Fialips et al, 2000;Ece et al, 2008;Fernández-Remolar et al, 2011]. This is consistent with the bulk of Nicaragua phyllosilicates we identified being Al-rich kaolinite or halloysite.…”

Kaolinite: Defect defined material properties – A soft X-ray and first principles study of the band gap

“…The kaolinite synthesis followed the hydrothermal route under acidic conditions described in detail in Ref. [2], in order to achieve a minimum defect density.…”

“…Natural kaolinite is a residual weathering product, deviating from the ideal structure. Kaolinite may also be synthetically obtained from hydrothermal processing, allowing for pure and well crystallized kaolinite products [2]. The atomic structure of pure kaolinite has previously been investigated experimentally [3][4][5] as well as theoretically [6,7].…”