Hydrothermal Crystallization in the CsOH-ZrO_2-SiO_2-H_2O and CsCl-ZrO_2-SiO_2-H_2O Systems at 400 deg C and 0.1 GPa: The Structure Assembly Mechanism from Subpolyhedral Clusters for Cs_2ZrSi_6O_(15) (C2/m) and Cs_2TiSi_6O_(15) (C2/c)

摘要

ZrSiO_4 (zircon) and Cs_2ZrSi_3O_9 (the wadeite type) were found to form in the CsOH-ZrO_2-SiO_2-H_2O system under temperature gradient conditions at 400 deg C and 0.1 GPa in the region c_(CsOH) = 5-20 mol percent. In the CsCl-ZrO_2-SiO_2-H_2O system in the region c_(CsCl) =5-20 mol percent, SiO_2 (quartz), ZrSiO_4, and Cs_2ZrSi_6O_(15) crystallize. Using local crystal-structural intersection of space groups, invariant substructural units (precursor SSUs) were identified in the structures of Cs_2ZrSi_6O_(15) (C2/m), Cs_2TiSi_6O_(15) (C2/c), and K_2ZrSi_6O_(15) (isostructural to K_2TiSi_6O_(15), P1-bar) in the form of 12-polyhedron ring clusters of composition A_2M_2T_(10) (A=K, Cs). Modeling of the structure assembly from SSUs was based on the idea of achieving the maximum SSU connectivity upon going to a higher self-organization level. The type of framework MT structure was determined to be 1L (L means a layer). This type shows that SSUs along axis b = 7.506 A (for Cs_2ZrSi_6O_(15)) and b= 7.423 A (for Cs_2TiSi_6O_(15)) fully recover the topological structure of a 3D MT-framework.