What does feldspathoid mean?
A feldspathoid is any member of a group of rock-forming minerals that resemble the feldspars in chemistry and crystal habit but contain too little silicon to coexist stably with quartz. Where a melt or fluid is silica-rich, feldspar and quartz form; where it is silica-deficient, feldspathoids such as nepheline, leucite, sodalite, and analcime appear instead. This mutual exclusivity makes feldspathoids important diagnostic markers: finding one in a rock immediately signals a silica-undersaturated composition, a fact central to igneous classification schemes such as that of the International Union of Geological Sciences. Feldspathoids occur chiefly in alkaline igneous rocks like nepheline syenites and certain lavas. The word is technical, used almost entirely within geology and mineralogy, though it also appears in lapidary contexts because some feldspathoids, notably sodalite and lazurite-bearing varieties, are cut as ornamental stones.
nounAny of a group of rock-forming minerals chemically similar to feldspars but containing less silica, so that they cannot coexist stably with free quartz; examples include leucite, nepheline, and sodalite.
- Any of a group of silica-undersaturated rock-forming minerals related to feldspars but unable to form alongside free quartz (e.g. nepheline, leucite, sodalite).
- Collectively, such minerals when used as a class label in petrographic classification (as in 'a feldspathoid-bearing rock').
"The nepheline in this syenite is the most abundant feldspathoid in the entire pluton."
"Because the magma was unusually low in silica, nepheline crystallized as the dominant feldspathoid rather than feldspar."
"Rocks of the feldspathoid family, such as sodalite and leucitite, occur almost exclusively in silica-poor volcanic provinces."
Regular plural; used both for multiple individual crystals/grains and for members of the mineral group generally.
"Feldspathoids such as nosean and hauyne are rare outside highly alkaline rocks."
Find a feldspathoid like leucite or nepheline in a rock and you can be sure of one thing: there's no quartz anywhere nearby — these minerals are geology's way of saying 'too little silica to go around.'
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of feldspathoid
The term derives from German 'Feldspath' (modern Feldspat), the source of English 'feldspar,' itself from 'Feld' (field) and 'Spath' (spar), a name reflecting the mineral's discovery in fieldstone on Swedish farms. Attaching the Greek-derived suffix '-oid' ('resembling') yields 'feldspathoid,' literally 'feldspar-like,' coined by early nineteenth-century mineralogists who recognized these minerals' similarity to, yet chemical distinctness from, true feldspars. The British spelling 'felspathoid' preserves the older German 'Felspath' form. The same root gives English 'feldspathic' and, via the shortened German spelling, the petrological adjective 'felsic.'
Related word forms
How feldspathoid is actually used
Strictly technical vocabulary confined to geology, petrology, and mineralogy; rarely encountered outside those fields. In modern petrological writing the abbreviation 'foid' (from the IUGS classification) is increasingly common for the same group. The spelling variant 'felspathoid' reflects the older British rendering of the root.
Easily confused with feldspathoid
A feldspar is a common aluminosilicate mineral rich in silica, whereas a feldspathoid ('resembling feldspar') is a silica-poor mineral that forms instead of feldspar where quartz is absent.