Foidolites are a group of very rare silica-undersaturated plutonic igneous rock dominated by feldspathoids which constitute more than 60% of the felsic minerals present. Other constituents of foidolites include alkali feldspar and plagioclase, mafic minerals like biotite, hornblende, pyroxene, and olivine, and accessory minerals. Foidolites (not to be confused with foidites, one of their volcanic counterparts) are named according to the most abundant feldspathoid present (e.g. hauyinolite, nephelinolite, leucitolite). Nephelinolites and leucitolites are by far the most abundant rock types of foidolites and are further subdivided based on the content of mafic minerals (or color index), as follows:
Urtite (M < 30%): consisting of nepheline (more than 70%), aegirine-augite, and no feldspar. Named after Lujavr-Urt, Lovozero Complex, Kola Peninsula, Russia.
Ijolite (M = 30 – 60%): consisting of pyroxene and nepheline (30 – 70% in the original definition). Named after Iijoki, Kuusamo, Finland.
Melteigite (M > 60%): melanocratic nephelinolite, containing about 10 to 30% nepheline. Named after Melteig, Fen Complex, Telemark, Norway.
Italite (M< 30%): consisting almost entirely of leucite and containing some glass. Named after Italy, where the type locality is (Alban Hills, near Rome).
Fergusite (M = 30 – 60%): consisting of leucite (up to 70% in the original definition) and pyroxene. Named after Fergus County, it is exposed in the Highwood Mts. of Montana, USA.
Missourite (M > 60%): melanocratic leucitolite with clinopyroxene and olivine. Named after the Missouri River, its type locality is in the Highwood Mts. of Montana, USA.
Plutonic igneous rock
• alkali feldspar
F > 60%
• urtite (M < 30%)
• ijolite (M = 30 – 60%)
• melteigite (M > 60%)
• italite (M < 30%)
• fergusite (M = 30 – 60%)
• missourite (M > 60%)
Extrusive equivalent: foidite, phonolitic foidite, tephritic foidite