Crystal structures of leucites – past, present, and future?

The synthetic anhydrous analogue of the mineral leucite has the chemical formula KAlSi<inf>2</inf>O<inf>6</inf>, it has a silicate framework crystal structure with the same topology as the zeolite analcime. In this crystal structure Al partially replaces Si in the framework, a K extraframework cation is incorporated to balance the charges. Synthetic analogues of leucite are known with the general formulae ACX<inf>2</inf>O<inf>6</inf> and A<inf>2</inf>BX<inf>5</inf>O<inf>12</inf>, where A is a monovalent cation, B is a divalent cation, C is a trivalent cation and X is Si or Ge. The crystal structures of these analogues have the same topology but otherwise show differences in crystal structure symmetry and framework cation ordering. This paper reviews the work done on leucite analogue crystal structures over the years and looks forward to work that could be done on these fascinating materials in the future.