feldspathoids

Often, feldspathoids and rare calc-silicates such as scapolite are found in more marginal areas.

Examples are quartz, the feldspars, feldspathoids, and the zeolites.

The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content.

Because of the presence of feldspathoids, nepheline syenite is classified to be a typical alkaline rock.

Alteration of feldspars and feldspathoids releases silica colloid.

It is silica-undersaturated and may contain feldspathoids, alkali feldspar and phlogopite.

Analcime is usually classified as a zeolite mineral, but structurally and chemically it is more similar to the feldspathoids.

Tridymite or quartz may be present in the fine-grained groundmass of tholeiitic basalt, and feldspathoids are absent.

Mineral assembly is usually abundant feldspathoids (leucite or nepheline), plagioclase, and lesser alkali feldspar.

Phonolites, as they are products of low degree partial melts, are silica undersaturated, and have feldspathoids in their normative mineralogy.

Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz.

Silicate minerals associated with such compositions are pyroxene, olivine, and silica-undersaturated minerals such as nepheline and other feldspathoids.

Alkalies were already linked with feldspars and feldspathoids both in alkali syenites and in metasomatic fenites to be contained by residual carbonate.

Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks.

Mineral assemblages in phonolite occurrences are usually abundant feldspathoids (nepheline, sodalite, hauyne, leucite and analcite) and alkali feldspar (sanidine, anorthoclase or orthoclase), and rare sodic plagioclase.

The mineral assembly is usually abundant feldspathoids (nepheline or leucite), plagioclase, and augite, together with olivine and lesser iron-titanium oxides such as ilmenite and magnetite-ulvospinel; minor alkali feldspar may be present, as illustrated by the position of the field for basanite in the QAPF diagram.

From the remaining chemical constituents, AlO and KO are allocated with silica for orthoclase; sodium, aluminium and potassium for albite, and so on until either there is no silica left (in which case feldspathoids are calculated) or excess, in which case the rock contains normative quartz.