Effect of Different Evaporation Rates on Gypsum Habit: Mineralogical Implications for Natural Gypsum Deposits

When a solution saturated in CaSO4·2H2O evaporates at room temperature, gypsum crystals precipitate with both single and twinning habits. However, the twinning laws of gypsum involved in this process have long been debated. Recently, easy steps have been described to univocally recognize the twinning laws of gypsum. Therefore, in this study, we have replicated evaporation experiments and focused on identifying these twinning laws. Different precipitation frequencies of gypsum crystals with (i) curved habits and (ii) twins, according to the 100 and 101 penetration twinning laws, have been observed as the evaporation rate increased. The crystal aspect ratio (length/width crystal ratio) might serve as a quick measure for distinguishing between 100 and 101 penetration twinning laws, and the occurrence of these twinning laws in sedimentary environments is suggested. Moreover, high evaporation rates promote curved crystals with both symmetric and asymmetric habits. Based on crystallographic considerations, the asymmetric habit might be explained by a homoepitaxial mechanism, where systematic rotations of the common 2D coincidence cells occur. These results describe which of the twinning laws of gypsum are possible in a pure solution and, for the first time, establish a correlation between different gypsum habits and different evaporation rates, contributing to a better understanding of gypsum habit in evaporitic environments.