Size Distribution and Morphology of Gypsum Crystals Precipitating from Hypersaline Solutions

Morphology and crystal size distribution of gypsum precipitated from a wide range of hypersaline compositions (ionic strength = 5.5–10 m) were studied in batch experiments containing mixtures of Ca2+-rich Dead Sea brine and SO42–-rich seawater from the Red Sea with CaCl2·2H2O and Na2SO4 added to control oversaturation (Ωgypsum ≈ 1.7–7) and Ca2+/SO42– molar ratios (r). At elevated oversaturation (Ωgypsum ≈ 7) crystal morphology was found to be predominantly stellate. Closer to equilibrium (Ωgypsum ≤ 2) crystals become acicular or tabular. Morphology of the crystals at lower Ωgypsum was determined by r. Crystals precipitated from solutions with similar Ωgypsum had a larger aspect ratio at lower r (i.e., acicular morphology at lower r). Single crystals were found to keep their aspect ratio during growth as crystal size increased linearly with time. Crystal size distribution (CSD) of the crystal populations precipitated at lower Ωgypsum were log-normal. Such CSD cannot develop solely by a combined mechanism of nucleation and growth as perceived in classical theories. We therefore interpret the log-normal CSD as being the result of agglomeration during nucleation and/or early stages of growth.