Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Variable
    Chemistry : Variable
    Rarity : Quite common

Feldspathoids are a group similar in structure to feldspars (hence their name). They differ in their lower SiO₂ content : they contain around 1/3 less silica and are therefore essentially formed from magmas rich in alkalis (K, Na) and poor in SiO₂. They are essential minerals of magmatic rocks undersaturated in silica (nepheline syenites, phonolites, trachytes...). Their structure frequently includes fairly large structural gaps, which results in a drop in density (2.15 to 2.5 for feldspathoids against 2.54 to 2.75 for feldspars), and integration into these gaps of large ions (K, Na, Cl) or anionic groups (CO₃, SO₄...). The most common minerals in the group are nepheline and leucite. There are also less common species, such as sodalite, which has the particularity of containing chlorine, cancrinite and nosean, which incorporate CO₃ and SO₄ respectively, and lazurite which retains SO₄, sulfur and chlorine in its gaps. structural. Finally, let us note that analcime is classified in the feldspathoids by certain authors and in the zeolites by others.