Feldspathoid

Feldspathoids are a group of tectosilicate minerals that form in silica-undersaturated, alkali-rich igneous rocks. They are chemically related to feldspars but differ in composition and structure, notably their lower silica content and the presence of cavities within their framework that can host large anions and extra-framework cations.

Structurally, feldspathoids are aluminosilicates with a three-dimensional framework built from silicon-oxygen and aluminum-oxygen tetrahedra. The framework is less silica-rich than that of feldspars, leading to the formation of voids or channels that accommodate charge-balancing anions and cations. This arrangement prevents the co-crystallization of quartz with feldspathoids in the same rock.

They occur in rocks that are rich in alkalis and relatively poor in silica, such as nephelinites, phonolites, tephrites, and related alkaline rocks. Because quartz stability requires higher silica activity, feldspathoids are restricted to silica-undersaturated environments and are rarely found in silica-saturated rocks.

Common feldspathoid minerals include nepheline, leucite, sodalite, cancrinite, nosean, and hauyne. These minerals often accompany alkali feldspars in the same rock, but their presence signals a different silicate chemistry than that of ordinary feldspars.

In geology, the presence of feldspathoids is a diagnostic indicator of silica undersaturation and alkali-rich magmatism. They help classify igneous rocks and inform petrogenetic interpretations of magmatic differentiation and crystallization conditions.