feldispatoidit

Feldispatoidit is a group of silicate minerals that are structurally similar to feldspars but contain a higher proportion of alkali metals and aluminum, and a lower proportion of silica. This difference in composition leads to a lower silica saturation and a tendency to crystallize in the cubic or hexagonal systems, unlike the triclinic or monoclinic systems of feldspars.

Key members of the feldispatoid group include nepheline, leucite, sodalite, nosean, haüyne, and lazurite. These minerals are typically found in silica-undersaturated igneous rocks such as alkali basalts, phonolites, and tephrites, which form in environments where the magma has a deficiency in silica. They are generally considered to be rare in continental crustal rocks, which are typically silica-rich.

Feldispatoids can often be distinguished from feldspars by their optical properties and sometimes by their crystal habit. For example, nepheline often occurs as euhedral hexagonal crystals, while leucite typically forms trapezohedral crystals. Sodalite is known for its distinctive blue color and isometric cubic crystal system.

The presence of feldispatoids in a rock is a significant indicator of its origin and geochemical evolution, suggesting formation from a silica-poor magmatic source. They can also be associated with certain types of economic mineral deposits, particularly those containing rare earth elements or gemstones like lapis lazuli, which is primarily composed of lazurite.