Alkali metal 1:1:x borates represent a series of common chemical-genetic compounds that are formed in the triple systems at the highest pH values. A new type of coordination (5th class) of water molecules (according to classification of Ferraris and Franchini-Angela of water molecules in crystalline hydrates) is found in the structures of potassium (rubidium)1:1:2 2/3 borate hydrates.