Dendritic structures in gel grown Gypsum

Sparingly water soluble Calcium Sulphate dihydrate (Gypsum) single crystals are grown by gel method, derived from the highly soluble reactants, Calcium Chloride and ammonium sulphate. By the diffusion of 1M Calcium Chloride solution into Sodium metasilicate gel (Sp.gr: 1.06 gm Cm_–3, pH ∼ 7.5), imbedded before setting, with varied amounts of weak acid (acetic acid) and 20 ml of 1M ammonium sulphate solution, dendritic structures are commonly observed in the gel at the interface, at a particular concentration of the acid. The needle shaped tabular crystals are also observed to grow at the greater depths in the gel. Dendrites are studied by optical, X-ray diffraction and Infra-red studies. It has been established that the dendrites are Calcium Sulphate dihydrate crystals formed probably due to the influence of soluble impurities (like calcium acetate, in the present case), when the rate of diffusion is fairly high and the twinning mechanism favoured. The implications are discussed.