Abstract Well-ordered crystals of the magnesium manganese oxyborates ludwigite, orthopinakiolite, and takeuchiite were heated in air at temperatures of between 800 and 1200°C for periods of time varying from 2 hr at the higher temperature to 3 days at the lower temperature. At 1100°C and below neither takeuchiite nor orthopinakiolite reacted, but ludwigite transformed to takeuchiite at 1100°C, to orthopinakiolite at 900°C, and to pinakiolite at 800°C. At 1200°C takeuchiite transformed to ludwigite, orthopinakiolite transformed to takeuchiite, and ludwigite did not react. Mechanisms for these transformations are suggested.