Abstract: For the first time, the thermal behavior of a new mineral belomarinaite KNaSO₄ from volcanic exhalations (trigonal crystal system, Sp. gr. P3m1, a = 5.6072(3), c = 7.1781(4) Å, V = 195.45(2) ų) was studied on a natural sublimate sample from the 2012–2013 Tolbachik Fissure Eruption and its synthetic analogue, a low temperature (LT) polymorph of (Sp. gr. P3m1) in the range 30–800°C (high-temperature X‑ray diffraction) and 30–1000°C (DSC and TG). The mineral is stable up to a temperature of 470 ± 5°C, at which it has a polymorphic transformation into a high-temperature (HT) hexagonal polymorphic modification (Sp. gr. P6₃/mmc), which is stable until melted at 840°C based on DSC. The thermal expansion of both modifications is sharply anisotropic, and in the case of the HT phase the parameter a has a U-shape dependence with a minimum at T = 620°C. The volumetric expansion of the α_V HT phase is greater than that for the LT phase by factors of 2–2.5 on average.