New solid solutions Ca3B2 + x Si1–x O8–x/2

New solid solutions Ca3B2 + x Si1–x O8–x/2 based on monoclinic borosilicate β-Ca3B2SiO8 are prepared using crystallization from a melt. The homogeneity range of solid solutions (0 < х ≤ 0.86) is determined and the thermal behavior of solid solution Ca3B2.4Si0.6O7.8 is studied using high-temperature X-ray powder diffraction: a reversible β ↔ α polymorphic transition is discovered in the solid solution (х = 0.4) at 550°C, decomposition begins in the solid state above 940°C with the formation of CaSiO3. The thermal expansion of both modifications is anisotropic: 20–540°C (α11 = 17, α22 = 16, α33 = 0, α V = 32 × 10−6°C−1) and 560–900°C (α11 = 15, α22 = 24, α33 = 3, and α V = 42 × 10−6°C−1).