Perusing my CRC the other day, I noticed that sodium sulphate heptahydrate has a solubility of 19.5g/100cc. The anhydrite has asolubility of 4.76g/100cc. At a concentration of say 18 g/100cc why doesn't the heptahydrate crystallise out as the anhydrite then reabsorb water into its structure to become the heptahydrate and then redissolve? The respective MWs of 142 and 268 don't account for the discrepancy. Any ideas?