foidite

Foidite is an igneous rock that is silica-undersaturated and contains feldspathoids as essential minerals. The defining feature of foidite is the presence of feldspathoids such as nepheline, leucite, or sodalite, in addition to alkali feldspar, with little or no quartz. The name reflects the prominent role of feldspathoid minerals in its composition.

Mineralogy and texture commonly include one or more feldspathoids (nepheline and/or leucite) together with alkali feldspar; mafic minerals such as pyroxenes (for example augite or aegirine) and sometimes olivine can be present. The rock generally has a crystalloblastic to porphyritic texture, with a glassy or fine-grained groundmass in volcanic varieties. Accessory minerals may include zircon, apatite, magnetite, and rare aluminous phases.

Formation and setting: Foidites form from silica-undersaturated, alkaline magmas. They are typically produced in intraplate tectonic environments or continental rifts and can be related to nephelinite, phonolite, or related alkaline magmas. The absence or very low abundance of quartz reflects the silica undersaturation of the melt, and the crystallization of feldspathoids helps stabilize the composition during cooling.

Relationship to other rocks: Foidite sits among feldspathoid-bearing rocks and contrasts with quartz-rich rocks such as granites and rhyolites. Variants may be described as leucite-rich, nepheline-rich, or sodalite-bearing, depending on which feldspathoid dominates the mineral assemblage. This group is used to characterize alkaline, low-silica magmatic suites and their evolutionary trends.