The CaBAsO 5 (2CaO·As 2 O 5 ·B 2 O 3 ) quaternary compound, which incongruently melts at 975°C decomposing into Ca2As2O7 and molten B2O3 and crystallizes in the hexagonal structure with the lattice parameters a = 685.5 and c = 1356.1 pm and an experimental density of 3.70 g·cm–3, is formed in the B–Ca–O–As system (Bauer 1965, 1966). To prepare the title compound, CaHAsO4·2H2O and crystalline H3BO3 were mixed (molar ratio 1:1.1), and the mixture was heated in a Pt crucible to 600°C (Bauer 1965). This temperature was maintained until the complete elimination of H2O (about 2 h), and the sintered product was heated in a Pt crucible in an O2 stream at 900°C for 3 days.