Fumarolic Minerals: An Overview of Active European Volcanoes

The fumarolic mineralogy of the Icelandic active volcanoes, the Tyrrhenian volcanic belt (Italy) and the Aegean active arc (Greece) is investigated, and literature data surveyed in order to define the characteristics of the European fumarolic systems. They show broad diversity of mineral associations, with Vesuvius and Vulcano being also among the world localities richest in mineral species. Volcanic systems, which show recession over a longer period, show fumarolic development from the high-temperature alkaline halide/sulphate, calcic sulphate or sulphidic parageneses, synchronous with or immediately following the eruptions, through medium-temperature ammonium minerals, metal chlorides, or fluoride associations to the late low-temperature paragenesis dominated by sulphur, gypsum, alunogen, and other hydrous sulphates. The situation can be different in the systems that are not recessing but show fluctuations in activity, illustrated by the example of Vulcano where the high-temperature association appears intermittently. A full survey of the mineral groups and species is given in respect to their importance and appearance in fumarolic associations

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]