| Popis: |
The first synthesis of wadeite, K2ZrSi3O9, was realised by the authors in 1970 using a hydrothermal technique. These new data were the basis for further research works about the formation of this mineral by way of dry and hydrothermal synthesis, making use of various potassic compounds. The conditions for the formation of wadeite were explained as a function of the chemical environment. The pressure—temperature range of formation was investigated from 100 to 900°C and 100 to 2000 bar. The morphology of the wadeite crystals that were obtained is discussed. Starting from wadeite structure, various isomorphic substitutions were determined (Zr/Hf; K/Rb, Cs; Si/Ge); their study resulted in an interesting diagram of crystallographic parameters as a function of the ionic radius of the substituted alkaline elements: cryst. param. = f(R+alk. element); the bands are linear. The infra-red investigations enabled us to establish the spectrum of (Si3O9)6− and (Ge3O9)6− in wadeite structure as well as the substitutions caused by alkalis and transition metals. Finally a comparative experimental investigation of the ZrSiO4-K2ZrSi3O9-K2ZrSi6O15 system brought to light new data regarding the geochemical behaviour of zirconium in igneous rocks. |
