Sodalite

Sodalite (ˈsoʊ.dəˌlaɪt ) is a tectosilicate mineral with the formula Na8(Al6Si6O24)Cl2, with royal blue varieties widely used as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group with hauyne, nosean, lazurite and tugtupite. The people of the Caral culture traded for sodalite from the Collao altiplano. First discovered by Europeans in 1811 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become widely important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada. The structure of sodalite was first studied by Linus Pauling in 1930. It is a cubic mineral of space group P3n () which consists of an aluminosilicate cage network with Na+ cations and chloride anions in the interframework. (There may be small amounts of other cations and anions instead.) This framework forms a zeolite cage structure. Each unit cell has two cavities, which have almost the same structure as the borate cage (B24O48)24− found in the zinc borate Zn4O(BO2)6, the beryllosilicate cage (Be12Si12O48)24−, and the aluminate cage (Al24O48)24− in Ca8(Al12O24)(WO4)2, and as in the similar mineral tugtupite (Na4AlBeSi4O12Cl) (see Haüyne#Sodalite group). There is one cavity around each chloride ion. One chloride is located at the corners of the unit cell, and the other at the centre. Each cavity has chiral tetrahedral symmetry, and the cavities around these two chloride locations are mirror images one of the other (a glide plane or a four-fold improper rotation takes one into the other). There are four sodium ions around each chloride ion (at one distance, and four more at a greater distance), surrounded by twelve SiO4 tetrahedra and twelve AlO4 tetrahedra. The silicon and aluminum atoms are located at the corners of a truncated octahedron with the chloride and four sodium atoms inside. (A similar structure called "carbon sodalite" may occur as a very high pressure form of carbon — see illustration in reference.