Felspathoidit

Felspathoidit is a group of tectosilicate minerals that are chemically similar to feldspars but contain less silica. This silica deficiency means that felspathoids are stable only in undersaturated magmas, which are magmas with a lower silica content. In contrast, feldspars are stable in more silica-rich magmas. Felspathoids are often found in association with alkali-rich igneous rocks, such as nephelinites and phonolites, which are typically formed in environments with limited silica availability.

The most common felspathoid minerals include nepheline, leucite, and sodalite. Nepheline has the chemical formula Na3KAl4Si4O16, while leucite is KAlSi2O6, and sodalite is Na8(Al6Si6O24)Cl2. These minerals have distinct crystal structures and physical properties, although they share the characteristic aluminum-silicate framework with feldspars, but with a different ratio of silica to aluminum and the presence of other cations like sodium, potassium, and calcium, as well as anions like chloride.

Due to their presence in silica-undersaturated magmas, felspathoids are important indicators of the geological conditions under which certain igneous rocks formed. Their formation is often linked to magmatic differentiation processes occurring in continental rift zones or within intraplate volcanic settings where the source mantle rocks are relatively depleted in silica. The abundance and specific types of felspathoids present in a rock can provide valuable clues about the magma's origin and evolution.