Feldspathoid

Feldspathoids are silica-poor framework silicate minerals in igneous rocks undersaturated in silica. They do not coexist with quartz in equilibrium.

Silicate minerals: Minerals built from silicon–oxygen tetrahedra, alone or linked into larger structures. Feldspathoids are framework silicates, a structural family within this broader mineral group.
Quartz: A crystalline form of silicon dioxide and a common mineral in silica-saturated rocks. Quartz marks silica saturation and is incompatible with feldspathoids at equilibrium.
Nepheline: A sodium- and potassium-bearing feldspathoid common in silica-undersaturated igneous rocks. It is a widespread feldspathoid and a diagnostic mineral in nepheline-bearing rocks.
Nephelinite: A silica-undersaturated igneous rock composed mainly of nepheline and mafic minerals. Its nepheline content makes it a characteristic feldspathoid-bearing rock.
Feldspar: A major group of framework silicate minerals containing aluminum and calcium, sodium, or potassium. Feldspars share framework structures with feldspathoids but occur across a broader silica range.
Tectosilicate: A silicate mineral whose silicon–oxygen tetrahedra form a three-dimensional framework. Feldspathoids belong to this framework-structured class.
Alkali feldspar: A feldspar group containing potassium and sodium in varying proportions. It commonly occurs with feldspathoids in silica-undersaturated igneous rocks.
Leucite: A potassium-rich feldspathoid found chiefly in silica-undersaturated volcanic rocks. Its occurrence identifies potassium-rich, silica-poor magmatic compositions.
Phonolite: A fine-grained volcanic rock rich in alkali feldspar and feldspathoids. Feldspathoids help distinguish phonolite from silica-saturated volcanic rocks.
Plagioclase: A feldspar series ranging from sodium-rich albite to calcium-rich anorthite. Its chemistry and common rock associations contrast with those of many feldspathoid-bearing assemblages.

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TugtupiteNarrower topic: Tugtupite belongs to this mineral group and forms where silica is scarce.