Hydrothermal synthesis (200 °C) of Co–kaolinite and Al–Co–serpentine

In the systems CoO–Al2O3–SiO2–H2O and CoO–Al2O3–SiO2–HCl–H2O, at initial pH between 5.5 and 8.1 and temperature of 200 °C, kaolinite is unstable and the following phases form through a dissolution-precipitation process: a) kaolinite and Co-bearing kaolinite; b) Al–Co–serpentine; and c) poorly crystalline phases. Identification of the several phases was carried out from a combination of X-ray diffraction and transmission/analytical electron microscopy. linite shows variable morphologies: a) Platy lath-shaped particles with very low Co content; b) Spherical particles, with relatively constant Co contents (in the order of 0.10 apfu); c) Kaolinite stacks with very variable Co contents (up to 0.25 apfu). Analytical data indicate that the presence of Co(OH)2 in the system favors the dissolution process as well as serpentine formation but it leads to the parallel formation of abundant poorly crystalline phases. The Co-content in kaolinite increased as a function of the Co(OH)2/CoCl2 ratio in the initial systems, and it is reflected by a parallel increase of the b-cell parameter of kaolinite. The average composition of the coexisting Al–Co–serpentine is: (Al1.20Fe0.11Co1.27)(Si1.64Al0.36)O5(OH,Cl)2, with Cl contents in the order of 0.14 apfu. semblage Co–kaolinite + Al–Co–serpentine, which appears to be stable at 200 °C, has not been described in natural environments, probably because it requires unusual Al- and Co-rich chemical systems.