Unusually twofold coordinated boron in the form of an isolated metaborate anion has been obtained via thermal dehydration of a B(OH) − 4 containing sodalite, grown from kaolinite and sodium perborate in 4 M NaOH under hydrothermal conditions. The kinetics of this metaborate formation inside the aluminosilicate [4 6 6 8 ] polyhedral cages of the sodalite host matrix have been studied in dependence of heating time and temperature. Powder XRD and IR spectroscopy as well as MAS NMR of the nuclei 11 B, 23 Na, and 29 Si have been applied to reveal the extent of this metaborate formation inside the aluminosilicate sodalite matrix.