Observations on growth of NaCl crystals out of the surface of sodalite [Na₈(Al₆Si₆O₂₄)Cl₂]

When the surface of sodalite [Na₈(Si₆Al₆O₆)Cl₂] crystals was treated with an etching solution, growth of NaCl crystals could be observed. The etchant used is a dilute nitric acid solution. The acid attacks the surface and breaks it down into an Si-Al gel in which the crystals grow rapidly. Optical photography and electron-microprobe images show crystals with steplike patterns and hollow depressions at the centres of crystal faces. Preferential growth and aggregation of crystallites at grain boundaries is distinctly observed.