Melilite Group

A group of tetragonal sorosilicates with a disilicate anion (Si₂O₇)^6- or an Al or B-bearing derivative thereof and the general formula given above, where M denotes a small- to medium-sized divalent or trivalent cation (mostly Mg and Al, or rarely Fe, B, Zn, Be, Si, etc) and X is Si, Al or rarely Be or B. In general, Al or B replace one Si atom when M is a trivalent ion, but the charge can also be balanced by coupled substitution of Ca²⁺ with a monovalent ion, especially Na, and M³⁺ with M²⁺, such as in Alumoåkermanite, where Al³⁺ is still dominant on the M-site but the mineral is far from end-member composition. In petrology "melilite" usually refers to minerals in the åkermanite-gehlenite series, by far the most abundant members of the group. A large number of synthetic members are known.