Foidolite

Foidolite is an intrusive igneous rock composed of less than 10% feldspar and at least 60% feldspathoids. Feldspathoids are a group of framework aluminosilicate minerals that are chemically similar to feldspars but have lower silica content. Common feldspathoids in foidolites include nepheline, leucite, sodalite, and cancrinite. Minor constituents may include pyroxenes (aegirine, augite), amphiboles (hornblende), olivine, and biotite. Apatite, magnetite, and perovskite are typical accessory minerals.

Foidolites are relatively rare rocks, generally associated with alkaline igneous provinces. Their formation is linked to partial melting of the mantle under low-silica conditions, often in regions undergoing rifting or continental extension. The low silica content prevents the crystallization of quartz or significant amounts of feldspar, allowing the silica-undersaturated feldspathoids to dominate the rock's mineralogy.

Due to the presence of minerals rich in sodium, potassium, and other incompatible elements, foidolites can be sources of rare earth elements and other industrially important minerals.

The texture of foidolites is typically phaneritic, meaning the individual mineral grains are visible to the naked eye. Grain size can range from fine-grained to coarse-grained. Foidolites are often named according to the dominant feldspathoid present, such as nepheline syenite (when nepheline is abundant and feldspar is present up to 10%) or leucite foidolite. However, the term "foidolite" itself is used when feldspathoids constitute the vast majority of the rock.