THE GYPSUM-BRUSHITE SYSTEM - CRYSTALLIZATION FROM SOLUTIONS POISONED BY PHOSPHATE IONS

Gypsum and a non-stoichiometric calcium phosphate sulphate hydrate (CP SH) were grown from solutions poisoned by phosphate ions. The pH range d from 4.7 to 5.6 and the sulphate over phosphate ratios from 1.5 to 9 . In some cases, when the phosphate concentration was high (30%-40%) C PSH was stable. When the phosphate concentration was lower (10%-20%), CPSH formed as first phase in association with gypsum, but dissolved a s soon as gypsum crystallized and grew at its expense. In that case, g ypsum contained up to 10% phosphate. On the other hand, when gypsum cr ystallized alone, as first phase, it did not contain detectable amount s of phosphate.