About the Features of Exsolution Phenomena of NaCl-KCl Solid Solutions from Arsenatnaya Fumarole (Tolbachik Volcano, Kamchatka)

Abstract

The fumarolic formations of GTFE (The Great Tolbachik Fissure Eruption of 1975–76) slag cones are excellent polygons for studying the peculiarities of mineral formation processes at high temperatures (up to 500 °C) and atmospheric pressure. Halite and sylvine are the most common occurring minerals formed from the gas phase. Their aggregates from Tolbachik fumaroles are characterised by structural heterogeneity. The samples from different sections of Arsenatnaya fumarole located on the Second cone of the Northern Breakthrough of the GTFE varying in temperature and in mineral associations were selected for the study. Their investigation by scanning electron microscopy (SEM) was carried out after four months—the period, after which the final exsolution of the NaCl-KCl solid solution took place. For the NaCl-KCl system from Arsenatnaya fumarole, NaCl:KCl ratio increases while the temperature decreases. The most “pure” halite is observed in the secondary and, probably, redeposited crusts from the low-temperature parts of the fumarole. A mixture of KCl and NaCl is observed in the intermediate zone between the hottest and the coldest parts of the fumarole. For sylvine from the mid-temperature zones of Arsenatnaya fumarole, a rubidium impurity is characteristic (up to 0.2%wt).