Hydrothermal Crystallization in the KF-ZrO_2-SiO_2-H_2O System at 400 ℃: Phase Equilibria and Modeling of ZrSiO_4, K_2ZrSi_6O_(15), K_2ZrSi_3O_9, K_3ZrF_7 , and ZrO_2 Crystal Structures

摘要

The phase equilibria in the KF-ZrO_2 (nanocrystals)-SiO_2-H_2O system are studied for the ZrO_2:SiO_2 molar ratio ranging from 2:1 to 1:6 and the KF concentration CKF ranging from 3 to 36 wt % under 0.1 GPa at 400 ℃. We established crystallization of three silicates—ZrSiO_4 (zircon), K_2ZrSi_6O_(15) (deleite), K_2ZrSi_3O_9 (wadeite)—and K_3ZrF_7 fluoride and ZrO_2 and SiO_2 oxides. The structures of K- and Zr-based silicates obtained can be represented as an open framework formed by M octahedra (ZrO_6) that share vertices with T tetrahedra (SiO_4). With the increase of KF concentration, silicates "substitute" each other in the following sequence: ZrSiO_4 => K_2ZrSi_6O_(15) => K_2ZrSi_3O_9. Depending on the ZrO_2:SiO_2 ratio and CKF concentration, K_3ZrF_7 fluoride is formed simultaneously with K_2ZrSi_3O_9 and K_2ZrSi_3O_9 + K_2ZrSi_6O_(15). The characteristic features of formation of Zr-containing phases are discussed in the framework of the model of the matrix assembly of crystal structures from subpolyhedral structural units—clusters of cyclic type. The features of the phase formation in the system are compared with the characteristics determined earlier for the KOH-ZrO_2 (nanocrystals)-SiO_2-H_2O system at 400 ℃.