Abstract
Zykaite, nominally Fe-4(AsO4)(3)(SO4)(OH) 15H(2)O, is a common ferric arsenate in damp, moist environments, especially in underground spaces where the humidity does not decline during the year and the temperature fluctuations are minimal. Thermodynamic modeling with our data show that the transformation of zykaite to other ferric arsenates and sulfoarsenates (scorodite, parascorodite, kankite, bukovskyite, hydrous ferric arsenates) is thermodynamically driven.