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 SO₄ ^2--rich seawater from the Red Sea with CaCl₂·2H₂O and Na₂SO₄ added to control oversaturation (ω_gypsum ≈ 1.7-7) and Ca²⁺/SO₄ ^2- molar ratios (r). At elevated oversaturation (ω_gypsum ≈ 7) crystal morphology was found to be predominantly stellate. Closer to equilibrium (ω_gypsum ≤ 2) crystals become acicular or tabular. Morphology of the crystals at lower ω_gypsum was determined by r. Crystals precipitated from solutions with similar ω_gypsum 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 ω_gypsum 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.