Bredigite has an ideal composition close to Ca₇Mg(SiO₄)₄ and a structural formula indicating co-ordination numbers, X^[12] X ₂ ^[9] Y ₄ ^[10] M^[6] [TO₄]₄ (T = tetrahedrally co-ordinated ion). Experiments show that Ba or Sr can replace Ca in the large X^[12] sites while Mg in the M site may be partially replaced by Mn²⁺. When X^[12] sites are occupied by two different ionic species, partial site ordering occurs. Bredigite-structured phases may occur in periclase refractories and high-Mg cement clinker. Ca-Mg bredigite reacts slowly with water; data on its hydration behaviour at 20 and 200 C are presented.