The high-temperature behaviour of three Ca borosilicates has been studied by in situ powder high-temperature X-ray diffraction (HTXRD), differential scanning calorimetry and thermogravimetry in the temperature range 30–900 °C for natural samples of datolite, CaBSiO₄(OH), and ‘bakerite’, Ca₄B₅Si₃O₁₅(OH)₅, and a synthetic analogue of okayamalite, Ca₂B₂SiO₇. The latter was obtained by heating datolite at 800 °C for 5 h. Datolite and bakerite start to dehydroxylate above 700 and 500 °C, respectively, and decompose fully to form a high-temperature modification of okayamalite, HT-Ca₂B₂SiO₇, and wollastonite, CaSiO₃ at about 730 °С. Above 900 °C, HT-okayamalite decomposes with the formation of wollastonite, CaSiO₃, and metaborate CaB₂O₄. The latter melts at about 990 °C. Above 1000 °C, only the existence of wollastonite, CaSiO₃ and cristobalite, SiO₂ was observed. According to the HTXRD data, in the temperature range 30–500 °C, datolite and ‘bakerite’ demonstrate very similar and relatively low volumetric thermal expansion: α_v = 29 and 27 × 10⁻⁶ °C⁻¹, respectively. A high thermal expansion anisotropy (α_max/α_min ~ 3) is caused by both the layered character of the crystal structures and the shear deformations of their monoclinic unit cells. The direction of maximum expansion is intermediate between the normal direction to the layers and the (a + c) vector. A possible transformation mechanism from the datolite to the okayamalite structure topology is proposed from geometrical considerations. The synthetic analogue of okayamalite, Ca₂B₂SiO₇, undergoes a reversible polymorphic transition at about 550 °C with a decrease in symmetry from tetragonal to orthorhombic. The crystal structure of the high-temperature (HT) modification of okayamalite was solved from the powder-diffraction data [900 °C: P2₁2₁2, a = 7.3361(4), b = 7.1987(4), c = 4.8619(4) Å, V = 256.76(3) ų, R_wp = 6.61, R_Bragg = 2.68%].