the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In jadeite and aegirine this is added by the inclusion of a +3 cation (aluminium and iron(III) respectively) on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron(II) components are known as omphacite and aegirine-augite. With 80% or more of these components the pyroxene is classified using the quadrilateral diagram. A wide range of other cations that can be accommodated in the different sites of pyroxene structures. In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and then filling the site with the remaining aluminium and finally iron(III); extra aluminium or iron can be accommodated in the Y site and bulkier ions on the X site. Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, and there are several alternative schemes: Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively. For example, Na and Al give the jadeite ) composition. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site. This leads to e.g., . The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g., . In nature, more than one substitution may be found in the same mineral. Pyroxene minerals Clinopyroxenes (monoclinic) Aegirine, Augite, Clinoenstatite, Diopside, Esseneite, Hedenbergite, Jadeite, Jervisite, Johannsenite, Kanoite, Kosmochlor, Namansilite, Natalyite, Omphacite, Petedunnite, Pigeonite, Spodumene, Orthopyroxenes (orthorhombic) Enstatite, Bronzite, intermediate between enstatite and hypersthene Hypersthene, Eulite, intermediate between hypersthene and ferrosilite Ferrosilite, Donpeacorite, Nchwaningite, See also Clinopyroxene thermobarometry References C. Michael Hogan (2010). Calcium. eds. A. Jorgensen, C. Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. External links Mineral Galleries Video Section: Lunar Explorers (link to youtube: The Lunar Crust) Inosilicates Metasilicates