Abstract— In situ high-temperature powder X-ray diffraction (HTPXRD), differential scanning calorimetry (DSC), and thermogravimetry (TG) of three natural borosilicates have been performed in the temperature range of 25–1200°С. Axinite-(Mn) melts incongruently at 900°С with the formation of anorthite and bustamite. Leucosphenite decomposes at 850°С to form fresnoite and cristobalite. According to DSC data, kornerupine decomposes at 1177°С to form sapphirine, indialite, and spinel as the products of heating kornerupine. The values of the thermal expansion tensor and its orientation were determined using the HTPXRD data. The study showed that the three borosilicates expand weakly and almost isotropically. The average bulk thermal expansion coefficients are 21.3, 22.7, and 32.9 × 10^–6°С^–1 for axinite-(Mn), kornerupine, and leucosphenite, respectively. Leucosphenite has the maximum volumetric expansion, most likely due to the pronounced layered character of its crystal structure. The least symmetric structure of axinite-(Mn) has the maximum anisotropy of thermal expansion in the temperature range of 600–900°С.