silicaoversaturated

Silicaoversaturated is a term used in igneous and metamorphic petrology to describe a state in which the activity of silicon dioxide (SiO2) is high enough that free silica (quartz) is stable in the mineral assemblage and feldspathoid minerals are not required to balance silica. In practical terms, rocks that are silicaoversaturated commonly crystallize quartz together with feldspars (and other silica-bearing minerals), whereas rocks that are silica-undersaturated may form feldspathoids such as nepheline or leucite when silica activity is too low to support quartz.

The concept is most often applied to magmatic systems and is inferred from chemical composition using normative mineralogy, for example via CIPW norms, or by calculating a silica saturation index. Silicaoversaturation depends on factors such as temperature, pressure, and the activity of volatiles in the melt or-fluid, and it reflects the balance between silica supply and the availability of alumina and alkalis to form silicate frameworks.

Implications of silicaoversaturation include the typical mineral assemblages and crystallization paths of the rocks. Silicaoversaturated magmas tend to produce quartz-rich rocks such as granites and rhyolites, with minerals like quartz and feldspars dominating the texture. Silica-undersaturated melts, in contrast, can accommodate feldspathoids and lack free quartz. This state influences crystallization sequence, melt viscosity, and magmatic evolution, as well as hydrothermal and metamorphic processes that alter silica activity.

Examples commonly associated with silicaoversaturation include many granitic and rhyolitic suites, which contrast with nephelinite- or leucite-bearing, silica-undersaturated rocks.