MINERALS/Feldspathoids 539 Figure Frequency distribution of plagioclase compositions from 1883 microprobe results from 147 metamorphic calc silicate rocks in the central Alps (after Wenk et al (1991)) conditions to which they have been subjected In meteorites, a whole range of plagioclase compositions occur, but there are two main clusters at around An90 and An15 In lunar rocks the great majority of feldspars are calcic plagioclase An90 to An95 representing the lunar anorthosites See Also Igneous Processes Igneous Rocks: Granite Minerals: Definition and Classification; Other Silicates Rocks and Their Classification Sedimentary Rocks: Mineralogy and Classification; Sandstones, Diagenesis and Porosity Evolution Solar System: Meteorites; Moon Further reading Deer WA, Howie RA, and Zussman J (1992) An Introduc tion to the Rock Forming Minerals, 2nd edn London: Longman Wenk E, Schwander H, and Wenk H R (1991) Microprobe analyses of plagioclases from metamorphic carbonate rocks of the central Alps European Journal of Minerals 3: 181 191 Feldspathoids M D Welch, The Natural History Museum, London, UK Copyright 2005, Natural History Museum All Rights Reserved Structure Feldspathoid structures consist of rings of AlO4 and SiO4 tetrahedra that are connected to form a fully polymerized three-dimensional framework enclosing cavities occupied by large interstitial monovalent or divalent cations (sodium, potassium, calcium, and caesium) and, in some cases, anions (chloride, sulphide, and sulphate) These cavities are aligned to form channels that pass through the structure Compared with feldspars (see Minerals: Feldspars), the feldspathoids are silica poor Three main tetrahedral-framework topologies are found in the feldspathoid group, and these are represented by the minerals nepheline, leucite, and sodalite They are distinguished primarily by their different ring types and by how these rings are connected to each other The nepheline structure consists of rings of six tetrahedra, whereas the structures of leucite and sodalite consist of both four-membered and sixmembered rings Nepheline Subgroup Minerals in the nepheline subgroup have the chemical composition ABO4, where A is Na, K, Ca, or vacant,