Tilleyite

Tilleyite is a rarely occurring calcium sorosilicate mineral with formula Ca3[Si2O7]*2CaCO3 (sometimes represented as Ca5(CaCO3)2[Si2O7]). It is chemically a calcium silicate with additional carbonate ions. Tilleyite crystallizes in the monoclinic crystal system and forms only poorly developed, irregularly defined, tabular crystals and spherical grains. In its pure form it is colorless and transparent, however due to multiple refractions of light from lattice defects or polycrystalline formation, it can also appear white, with the transparency decreasing accordingly.

History
Tilleyite was first described from an occurrence at the Crestmore Quarry in Riverside County, California in 1932 by Esper Larsen and Kingsley Dunham, and named after Cecil Edgar Tilley (1894-1973), a professor of geology at Cambridge University, in recognition of his contributions to the study of metamorphism. Its type material is held at Harvard University, and the National Museum of Natural History.

Crystal Structure
Tilleyite crystallizes monoclinically in the space group P21/a (space group no. 14, position 3) with the lattice parameters a = 15.11  Å; b  = 10.24Å; c  = 7.58 Å,and β = 105.2°, with 4 formula units per unit cell.

Formation and occurrence
Tilleyite is formed by contact metamorphism in the zone between volcanics and limestones at low pressure and high temperatures. Associated minerals include calcite, fluorite, gehlenite, grossular, vesuvianite and wollastonite.

More generally, it occurs at gabbro-limestone contacts, such as at Carlingford, Ireland, and on the island of Muck, Scotland.