The crystal structure of braitschite, Ca6.15Na0.85RE2.08[B6O7(OH)3(O,OH)3]4(H2O), is reported here in space group P6/m with unit-cell parameters a = 12.1506(6), c = 7.3678(4) Å, and V = 942.03(8) Å3. Data were collected from a single crystal using a MoKα source and a CCD detector, solved by direct methods, and refined to an R factor of 2.81%. The mineral structure consists of hexaborate fundamental building blocks that polymerize along  and are bound by Ca2+ and REE3+ counter-ions. The framework forms hexagonal channels, which are occupied alternately by Ca2+ cations and water molecules. In an investigation of its thermal stability, braitschite maintains its crystallinity to a temperature of 400 °C, after which it undergoes decomposition. Using Rietveld refinements against powder X-ray diffraction data, we were able to track the loss of water molecules in channels and hydroxyl groups in the covalent B-O network with increasing temperature.