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 (fĕld-spăth′oid′, fĕl-)
Any of a group of relatively rare minerals that are chemically very similar to feldspars, but have too little silica to qualify as feldspars.

[feldspath (obsolete variant of feldspar, from obsolete German Feldspath, feldspar; see feldspar) + -oid.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(ˈfɛldspəˌθɔɪd) or


(Geological Science) any of a group of rock-forming minerals, such as leucite and sodalite, that are similar to feldspars but contain less silica
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(ˈfɛld spæˌθɔɪd, ˈfɛl-)

1. Also, feld`spath•oi′dal. of or pertaining to a group of minerals similar in chemical composition to certain feldspars except for a lower silica content.
2. a mineral of this group, as nepheline or leucite.
[1895–1900; < German Feldspat(h) (see feldspar) + -oid]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
References in periodicals archive ?
(Web2) FGH afghanite a hydrous hexagonal feldspathoid mineral of the cancrinite group.
Although microcline and calcium feldspar were feldspathoid and contained the same crystal morphology, the material transformation resulted in changes to the macromechanical properties of the sandstone.
Frost and Frost (2008) devised a means of classifying alkaline igneous rocks and comparing their geochemistry with "normal" granitic rocks based a plot of alkalinity index (AI) versus feldspathoid silica saturation index (FSSI) (refer to Fig.
Group 2 comprises quartz syenites, alkali quartz syenites, syenites (315 [+ or -] 15 Ma), feldspathoid syenite, and ijolite (297 [+ or -] 4 Ma; [3]), carbonatite, lamprophyre, and associated pegmatite and fenites.
Topaz is a silicate mineral most often found in igneous rocks of felsic (relating to an igneous rock that contains a group of light-colored silicate minerals, including feldspar, feldspathoid, quartz, and muscovite) composition.
Both the studied rock samples are porphyritic, with a small amount of phenocrysts formed by ferrifassaite passing to sodic ferrifassaite with kaersutite rims, oligoclase-andesine, and minor zeolitized feldspathoid. Andesine to oligoclase ([An.sub.27-30]) predominate over sodic sanidine and scarce clinopyroxene and titanomagnetite in the matrix of fresh trachyandesite facies.
In its thin sections, the rock has a hypo-crystalline matrix which is composed of porphyritic texture and is rich in a sprinkled opaque mineral, plagioclase, feldspathoid mineral and obsidian.
The change from incompatible to compatible behavior heralds the onset of significant plagioclase (or feldspathoid) precipitation/removal.
It is a partially decomposed granite feldspathoid having a greater diversity of alkaloids causing it to go into a gradual melt with less surface tension than true feldspars.
Afghanite [[(Na,Ca,K).sub.8][(Si,Al).sub.12][O.sub.24][(S[O.sub.4],Cl,C[O.s ub.3],[H.sub.2]O).sub.3]] is a feldspathoid of the cancrinite group.