Hydrothermal Synthesis (200 ℃) Of Co-kaolinite And Al-co-serpentine

摘要

In the systems CoO-Al_2O_3-SiO_2-H_2O and CoO-Al_2O_3-SiO_2-HCl-H_2O, at initial pH between 5.5 and 8.1 and temperature of 200 ℃, 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. Co-kaolinite 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/CoCl_2 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: (Al_(1.20)Fe_(0.11)Co_(1.27))(Si_(1.64)Al_(0.36))O_5(OH,Cl)_2, with Cl contents in the order of 0.14 apfu. The assemblage Co-kaolinite+Al-Co-serpentine, which appears to be stable at 200 ℃, has not been described in natural environments, probably because it requires unusual Al- and Co-rich chemical systems.