subalkali

Subalkali is a term used in geology and soil science to describe rocks and soils that have a lower content of alkali elements, specifically sodium and potassium, than a true alkali rock or soil. Alkali rocks are characterized by their high proportion of alkali feldspars and mafic minerals rich in sodium and potassium. Subalkali rocks, conversely, contain a greater abundance of plagioclase feldspars, which have a lower alkali content than alkali feldspars. This distinction is important in classifying igneous rocks and understanding their formation processes.

The chemical composition of subalkali rocks typically falls within the subalkaline field on TAS (Total Alkali-Silica) diagrams, a common graphical method for classifying volcanic rocks. This classification is based on the total amount of alkali oxides (Na2O and K2O) relative to silica content. Subalkali rocks include common rock types such as basalts, andesites, and dacites, which are prevalent in many geological settings, including island arcs and continental margins.

In soils, the term subalkali is less commonly used as a formal classification but can describe soils developed from parent materials that are relatively low in alkali elements. Such soils may have different weathering characteristics and nutrient availability compared to soils derived from alkali-rich parent materials. The differentiation between alkali and subalkali is primarily a geochemical distinction reflecting the source magma's composition and its subsequent crystallization history.