Mineralogy of the soils altered by fumarolic activity at Nisyros volcano, Greece

Nisyros is the easternmost island of the South Aegean Active Volcanic arc in Greece. It is an active stratovolcanoknown for its intense hydrothermal activity. In this study we present the results of the mineralogical analyses of 20 soilsamples collected in the caldera area. Samples were analysed through X-ray diffraction and the results allow us todivide them in two groups: Lakki Plain and Stefanos Crater. In their majority the soils of Lakki Plain have a mainmineralogical assemblage that consists of quartz, feldspar and gismondine (Ca2Al4Si4O16·9H2O). Gismondine as well asstellerite (CaAl2Si7O18·7H2O), which is appearing in some samples, derive from hydrothermal alteration, whilstwollastonite (CaSiO3) is also found as a product of thermally metamorphosed siliceous carbonates. In the StefanosCrater soils, due to the acid environment and the relatively high temperatures of the fumaroles (about 100°C), the mainassemblages mostly comprises hydrothermal alteration minerals like quartz, sulfur, wollastonite, gypsum andgismondine. The lower amount of feldspars with respect to the Lakki Plain soils can be justified by the high percentageof gismondine, their alteration products. Voltaite (K2Fe(II)5Fe(III)3Al(SO4)12·18H2O) was also found in some of thesamples as a product of hydrothermal alteration of Si deposits. Close to the fumaroles, the occurrence of alunogen(Al2(SO4)3·17H2O), a sulphide alteration mineral found in fumarolic environments, is also noticeable.