Identifying the impurities that are able to promote the selection of specific gypsum twin laws has relevance for geological studies aimed at interpreting the gypsum depositional environments in ancient and modern deposits. The results of this study provide new insights into the mineralogical implications of twinned gypsum crystals and should help future research to make a better use of the twin laws observed in gypsum in ancient sedimentary successions as a proxy for the chemistry of the original brine.