The thermal behavior of natural borosilicate howlite, Ca2B5SiO9(OH)5, is studied by the methods of high-temperature X-ray diffraction and differential thermal analysis in the temperature range of 30-1100°C. The thermal expansion is anisotropic (αmax/αmin = 3.6); the degree of anisotropy increases (αmax/αmin = 4.3) with a temperature increase up to 480°C (360-480°C).