Potassium Strontium Tetraborate Decahydrate and Ammonium Calcium Tetraborate Octahydrate

The crystal structures of potassium strontium tetraborate decahydrate, K2Sr[H4B4O9]2.10H2O, and ammonium calcium tetraborate octahydrate, (NH4)2Ca[H4B4O9]2.8H2O, have been determined. The two structures have similar cell parameters but different space groups. The first compound is homeotypic and the second is isotypic with potassium calcium tetraborate octahydrate and rubidium strontium tetraborate octahydrate. The structures contain alternate layers of tetraborate ions and two of the water molecules, along with the cations and the remaining water molecules. The cation size and the hydrolysis constant of each cation define the space group and the number of water molecules of hydration.