Silicate carbonate

The silicate carbonates are double salts that contain both silicate and carbonate in their formula. Most compounds are natural minerals containing calcium or rare earth elements. However, some have been made experimentally. Silicate carbonate minerals can be formed in limestone metamorphosed by heating from igneous intrusions. Scawtite forms where the activity of calcium is high compared to H+. Spurrite forms in a limited range of calcium activity and high silica activity. In magma, a carbonate rich melt is imiscible with a silicate melt.

Structures
Silicate carbonates contain carbonate triangles, and silicate tetrahedrons, SiO4. Tillyite contains disilicate Si2O7 units.