540 MINERALS/Feldspathoids Figure The different tetrahedral rings in feldspathoids (A) The undistorted six membered rings exhibited by the high temperature form of tridymite (B) The highly distorted six membered rings exhibited by nepheline and the low temperature form of tridymite (C) The four membered rings that occur in leucite and sodalite Figure A sheet of undistorted hexagonal rings, as occur in the high temperature form of tridymite and B is Al or Si The tetrahedral framework of nepheline-type structures is related to that of the mineral tridymite (see Minerals: Quartz), which has two polymorphs: a high-temperature largely undistorted hexagonal framework collapses upon cooling to a low-temperature structure in which all the six-membered rings are highly distorted (Figure 1B) A sheet of undistorted hexagonal rings in the high-temperature form of tridymite is shown in Figure Half the tetrahedra point up; the other half point down In the nepheline structure these sheets are connected to each other by apical oxygen ions to form cavities that are aligned into channels running parallel to the crystallographic c-axis Insertion of cations into the channels causes distortions of the framework that are similar to those associated with the thermally induced collapse of high-temperature tridymite to low-temperature tridymite For this reason, minerals of the nepheline group are often referred to as having ‘stuffed tridymite’ structures The common stoichiometry of natural nephelines is Na3KAl4Si4O16, which arises from the ordering of potassium and sodium in the two different channels (Figure 3) Figure (A) The characteristic sheet of distorted and undis torted six membered rings of AlO4 and SiO4 tetrahedra in the nepheline type structure Blue circles are sodium ions occupying smaller channel sites above and below the sheet, and black circles are potassium ions occupying the larger regular chan nels The projection of the unit cell is shown in red (B) A complete double sheet of the nepheline type structure, showing the two kinds of channel Leucite and Sodalite Groups The leucite and sodalite structures differ from the nepheline structure in that they also contain squares of four tetrahedra (Figure 1C) However, the ways in