feldspathoide

Feldspathoids are a group of tectosilicate minerals that are chemically and structurally related to feldspars but differ in silica saturation. They crystallize in silica-undersaturated, alkali-rich igneous rocks, where there is not enough silica to form feldspar and quartz. In such rocks, the feldspathoid framework can accommodate relatively large anions and complex species, producing a distinctive set of minerals.

The group includes nepheline, leucite, sodalite, cancrinite, nosean, lazurite, hauyne, tugtupite and related minerals. These often occur together with alkali feldspars and mafic minerals in rocks such as nephelinite, phonolite, tephrite, certain syenites, and carbonatites. Many feldspathoids display striking colors and forms and are of interest in petrography and mineralogy.

Chemically, feldspathoids are aluminosilicates with a framework of SiO4 and AlO4 tetrahedra, but with a lower overall silica content than feldspars. The reduced silica saturation leads to open frameworks that can host extra-framework anions or molecules (for example carbonate, chloride or sulfide units). This feature contributes to their distinctive crystallography and sometimes unusual optical properties, as seen in lazurite of lapis lazuli.

In petrography and geology, feldspathoids are distinguished from feldspars by their absence of quartz in the same rock and by their characteristic chemistry and crystal structures. They are most commonly associated with alkaline magmas and are not typical of ordinary crustal rocks, reflecting specific magmatic conditions that favor silica undersaturation.