sienita

Sienita is a coarse‑grained intrusive igneous rock term used in several languages (notably Spanish) for what is commonly called syenite in English. It is characterized by a predominance of alkali feldspar with lesser plagioclase and typically little or no quartz. Depending on the magma composition, sienite may also contain feldspathoids such as nepheline.

In terms of mineralogy and texture, sienite is usually phaneritic, meaning its crystals are large enough to be seen without a microscope. The principal mineral is alkali feldspar (such as orthoclase or microcline), accompanied by plagioclase. Quartz is generally absent or present only in trace amounts. Mafic minerals, when present, can include hornblende, biotite, or pyroxene, and accessory minerals may include magnetite, zircon, and apatite. Varieties of sienite can include nepheline-syenite when silicate excess allows feldspathoids to crystallize.

Classification within the broader syenite family distinguishes alkali feldspar–dominant sienites from silica-undersaturated varieties that harbor feldspathoids (such as nepheline). In many contexts, sienite (syenite) denotes rocks with little quartz, contrasting with granitic rocks that contain more quartz.

Formation and distribution: Sienite forms from alkali-rich magmas that differentiate in the crust, typically in continental crustal settings such as late-stage plutonic complexes, continental rifts, or regions of alkaline magmatism. It occurs in many parts of the world, often in association with related alkaline intrusions and other felsic to intermediate rocks. The study of sienite helps illuminate alkaline magmatism and crustal differentiation processes.