A metamorphic silicate mineral related chemically and structurally to the epidote group of minerals. Its composition is Ca(H₂O)Al₂(OH)₂(Si₂O₇), and it is a sorosilicate based on the dimeric (Si₂O₇)⁶⁻ radical, where two (SiO₄) tetrahedra link at a common vertex to form a bow-tie arrangement. The water molecule is bonded directly to the calcium ion and is tightly held. Although the calcium (Ca²⁺) and the aluminium (Al³⁺) cations are octahedrally coordinated by anions (and the water molecule), and the silicon cation (Si⁴⁺) is tetrahedrally coordinated by the anions, the structure is not limitingly closest-packed.