Origin and evolution of peralkaline rhyolite - Research Paper Example

Rhyolites in most cases are porphyritic, glassy groundmass or fine-grained. Often, it is not easy to recognize rhyolites without doing a chemical analysis because they have a glassy groundmass nature. Most rhyolites principally consist of glass, and are referred to as obsidian or are partly devitrified and are called pitchstones. In addition, most are lined because of the small disparities in composition of the glass, thereby resulting to differences in crystallites content. Alkali rhyolites are considered alkali feldspars if their content is >90% feldspars.

They are peralkaline and in most instances contain alkali pyroxenes and/or amphiboles. The alkali rhyolites can either be pantellerite or commendite, and peralkaline rhyolites are grouped under pantellerite. Peralkaline rhyolites are mostly widespread in continental rift settings. They contain alkali quartz and feldspar as their key phenocryst phase. Peralkaline rhyolites are distinguished from metaluminous rhyolites because of the presence of amphiboles and sodic pyroxenes in them. They may at times be found in oceanic island settings.

In addition, they are commonly associated with, trachytes, hawaiites, mugearites, and basalts that are slightly alkaline and silica-saturated. Additionally, peralkaline rhyolites have high iron levels and low aluminum levels compared to comendite like the peralkaline igneous rock. An example of peralkaline rhyolite is the Pantelleria, which is usually found occurring as a vitrophyre that comprises phenocrysts of sanidine or anorthoclase. Furthermore, quartz occurs in peralkaline rocks that are strong.

Mafic minerals can include aenigmatite, aegirine, ilmenite, fayalite, and sodic amphibole (normally ferrorichterite or arfvedsonite) (Ray, Gautam, and Biswajit, p.47). White et al. (p.133) asserts that often, the formation of peralkaline magmas comes as a result of the sodium-rich clinopyroxene (i.e., aegirine and