feldspathoidtectosilicate

Feldspathoidtectosilicate is not a standard term in mineralogy. In most classifications, feldspathoids are a subset of tectosilicates, so the phrase is usually redundant or used informally to emphasize that these minerals have a tectosilicate framework while being silica-undersaturated. There is no universally recognized category by this exact name.

In standard practice, feldspathoids are framework silicates (tectosilicates) with SiO4 tetrahedra linked into a three-dimensional network, but with lower overall silica content than feldspars. To maintain charge balance in the framework, feldspathoids incorporate alkali metals (such as sodium and potassium) and aluminum, and they often include other interstitial species. Because they require low silica activity, feldspathoids do not form with free quartz in the same rock.

Common feldspathoid minerals include nepheline, leucite, analcime, cancrinite, sodalite, and lazurite. These minerals share the tectosilicate framework but differ in chemical composition and crystal structure, giving rise to a variety of crystal habits from tabular to cubic forms.

Geologically, the presence of feldspathoids is diagnostic of silica-undersaturated conditions and alkali-rich magmas. They typically occur in silica-undersaturated igneous rocks such as nephelinite, phonolite, and various syenites or other alkaline rocks. Their occurrence helps petrologists classify rocks and infer crystallization conditions, especially the silica activity during magma evolution.