Szomolnokite (Fe2+SO4·H2O) is a monoclinic iron sulfate mineral forming a complete solid solution with magnesium end-member kieserite (MgSO4·H2O). In 1877 szomolnokite's name was derived by Joseph Krenner from its type locality of oxidized sulfide ore containing iron in Szomolnok, Slovakia (Hungary at the time). As of mid-January 2020 the only continent on which szomolnokite has not been found and reported is Antarctica. At room temperature szomolnokite is stable up to a pressure of 6.2 GPa, and then transforms into triclinic crystal structure.