feldspatoizi

Feldspatoizi, also known as feldspathoids in English, are a group of silica-undersaturated tectosilicate minerals that resemble feldspars in their framework structure but differ chemically. They form in magmatic systems where silica is limited, and their presence signals rocks with low silica activity.

Chemically, feldspatoizi share the feldspar-type three-dimensional framework of silicon and aluminum tetrahedra, but they contain less silica and accommodate different cations and anions. Because of their lower silica content, they cannot crystallize together with quartz in the same rock; rocks hosting feldspatoizi are typically silica-undersaturated and often alkaline.

Geological occurrence and formation occur mainly in igneous environments such as nephelinite, phonolite, and other alkaline or silica-poor magmas. Feldspatoizi crystallize late in the magmatic sequence and are commonly associated with minerals like nepheline, leucite, sodalite, cancrinite, and nosean. They are relatively rare in most crustal rocks but are important petrological indicators in the study of alkaline igneous rocks and planetary differentiation.

Common minerals within the group include nepheline and leucite, which are among the more widely recognized feldspatoizi. Other members, such as sodalite and cancrinite, broaden the range of compositions observed in silica-undersaturated systems. The term feldspatoizi emphasizes their feldspar-like structure while acknowledging their distinct silica-lean chemistry.

In petrology, feldspatoizi help define rock classifications and provide clues about magma composition, evolution, and tectonic setting. They are typically discussed in the context of alkaline rocks and silica-undersaturated igneous processes. See also feldspar, quartz, and nephelinite.