Crystal chemistry of silicofluorides

The family of silicofluorides includes compounds of alkaline metals or ammonium with silicium and fluorine. Their common structural characteristic is the presence of isolated [SiF6] octahedra. There is a major difference between the coordination of sodium compared with that of potassium and ammonium. As a result, the crystal structure of malladrite (Na2SiF6) [1][2] is significantly different from those of potassium or ammonium compounds. In malladrite fluorine at oms form a hexagonal eutaxy. In this arrangement half of vacant octahedrally-coordinated XF6 sites are occupied in ratio 1SiF6:2NaF6. On the contrary, the structures of potassium or ammonium silicofluorides are based on eutactic arrangements where both fluorine and potassium/ammonium partake and silicon atoms fill isolated octahedral holes surrounded exclusively by fluorine atoms. Structures with both cubic-eutaxy and hexagonal-eutaxy stackings can be found in this part of the family, as well as with a special ...ABCBABCB... stacking of layers. In the structure of the new mineral heklaite (KNaSiF6) [3][4] the eutactic arrangement of fluorine and Na/K atoms is not present, due to distortion of K coordination. It is a peculiarity of this structure that the cation with shorter bond lengths (Na) has a larger coordination number (10) than the cation with the longer bond lengths (K; CN 9) [3]. The fundamental differences between the Na and K coordinations in heklaite, plus the difference between its structure and those of Na2SiF6 and K2SiF6 polymorphs suggest absence of solid solution in the pseudo-binary phase field Na2SiF6-K2SiF6 which is confirmed by the field observations on the type locality for this mineral, Hekla volcano on Iceland [4]. (18) (PDF) Crystal chemistry of silicofluorides. Available from: https://www.researchgate.net/publication/276859488_Crystal_chemistry_of_silicofluorides [accessed Oct 05 2021].