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 (α_max/α_min = 3.6); the degree of anisotropy increases (α_max/α_min = 4.3) with a temperature increase up to 480°C (360–480°C).