feldespatoides

Feldespatoides is a term used in mineralogy to refer to a group of aluminosilicate minerals that resemble feldspars but belong to silica-undersaturated systems. The concept is employed to discuss minerals that are structurally intermediate between the classic tectosilicates (feldspars) and the feldspathoids, typically forming under magmatic conditions that limit silica activity.

In terms of structure and chemistry, feldespatoides are thought to possess a framework of corner-sharing tetrahedra similar to feldspars and feldspathoids. However, their low silica content and the presence of alkali or alkaline-earth cations in interstitial sites lead to variations in framework connectivity and stability. This combination yields minerals that can accommodate substantial Al and large cations while avoiding the quartz saturation characteristic of silica-rich rocks.

Occurrence of proposed feldespatoides is associated with silica-undersaturated, alkaline igneous settings, such as nephelinite- or phonolite-related systems. They are expected to occur together with other feldspathoid minerals and in rocks lacking free quartz, reflecting a petrogenetic environment distinct from that of ordinary feldspars.

Taxonomic status is informal and not widely formalized in IMA nomenclature. The term feldespatoides serves mainly as a useful shorthand in petrogenetic discussions for materials that do not fit neatly into traditional feldspar or feldspathoid categories. As such, specific mineral identifications labeled as feldespatoides would require detailed crystallographic and chemical characterization to determine their precise relationship to established groups.

See also: feldspar, feldspathoid, nephelinite, phonolite.