Is Leucite a Feldspathoid?
Leucite, K(AlSi2O6), is a feldspathoid in rock classification and a zeolite on the 1997 IMA list. Hardness 5.5 to 6; its crystals told from analcime by weight.
Yes, in rock classification, with a qualification. BGS Research Report RR 99-06 lists leucite among the feldspathoids, the F group of the QAPF scheme, and the Strekeisen atlas calls it a feldspathoid. But Coombs et al. 1997, the IMA zeolite report, includes leucite in its zeolite list. Leucite is K(AlSi2O6), hardness 5.5 to 6, density 2.45 to 2.50.
In short
- The two-list rule. In rock classification, yes: BGS Research Report RR 99-06 defines F as feldspathoids (foids) including nepheline, leucite, pseudoleucite, kalsilite, sodalite, nosean, haüyne, cancrinite and analcime, and says minerals in Q and F are mutually exclusive. In zeolite nomenclature, also yes to a different question: Coombs et al. 1997, Canadian Mineralogist 35 says leucite has the same framework as analcime, conforms to the adopted definition, and includes it in the list of zeolites.
- What the second source says. The Alex Strekeisen petrography atlas defines feldspathoids as aluminosilicates of sodium, potassium or calcium with too little silica to form feldspar, names leucite among them, and gives leucite plus quartz equals K-feldspar. Its leucite page says leucite is usually included as a feldspathoid by petrologists and, structurally related to analcime, typically among the zeolites too.
- What the sheet says about its symmetry. The Handbook of Mineralogy leucite sheet: tetragonal, pseudocubic; uniaxial (+), may be anomalously biaxial; ω 1.508, ε 1.509, a birefringence of 0.001 worked; twinning common and repeated, from two complex displacive phase transformations during cooling. It does not call leucite isotropic.
- The trapezohedron triage. Leucite, analcime and the garnets all grow as many-faced, ball-like crystals. A 20.00 g piece weighed in water reads 11.84 to 12.00 g as leucite, 11.07 to 11.27 g as analcime, and 14.42 g or more for any of five garnet sheets. Documented failure: in thin section leucite’s 0.001 birefringence and analcime’s anomalous biaxiality both look nearly dark.
- Not a feldspar. The September 2026 IMA-CNMNC Master List lists leucite as K(AlSi2O6); no line contains feldspathoid. The feldspar question for nepheline is on is nepheline a feldspar and not repeated here.
| Mineral | Formula (IMA) | Habit (sheet) | Optic class (sheet) | Refractive index | Hardness | Density | 20.00 g in water | BGS F list / Coombs zeolite list |
|---|---|---|---|---|---|---|---|---|
| Leucite | K(AlSi2O6) | Pseudocubic crystals showing {112}, to 9 cm | Uniaxial (+); may be anomalously biaxial | ω 1.508, ε 1.509 | 5.5–6 | 2.45–2.50 | 11.84–12.00 g | Yes / Yes |
| Analcime | Na(AlSi2O6)·H2O | Commonly trapezohedra {211}, to 25 cm | Isotropic; anomalously biaxial (−) | 1.479–1.493 | 5–5.5 | 2.24–2.29 | 11.07–11.27 g | Yes / Yes |
| Grossular | Ca3Al2(SiO4)3 | Dodecahedra or trapezohedra, to 15 cm | Isotropic; weak strain birefringence | 1.734 | 6.5–7 | 3.594 | 14.44 g | No (M) / No |
| Pyrope | Mg3Al2(SiO4)3 | Dodecahedron or trapezohedral forms, to 20 cm | Isotropic | 1.714 | 7–7.5 | 3.582 | 14.42 g | No (M) / No |
| Andradite | Ca3Fe3+2(SiO4)3 | Dodecahedra, trapezohedra, to 5 cm | Isotropic; typically weakly anisotropic | 1.887 | 6.5–7 | 3.8–3.9 | 14.74–14.87 g | No (M) / No |
| Spessartine | Mn2+3Al2(SiO4)3 | Dodecahedra or trapezohedra, to 10 cm | Isotropic; may show weak anisotropism | 1.800 | 7–7.5 | 4.190 | 15.23 g | No (M) / No |
| Almandine | Fe2+3Al2(SiO4)3 | Dodecahedra or trapezohedra, to 1 m | Isotropic; anomalously biaxial | 1.830 | 7–7.5 | 4.318 | 15.37 g | No (M) / No |
Is leucite a feldspathoid? What the two sources say
Yes, as rock classification uses the word: both a British Geological Survey report and a petrography atlas place leucite among the feldspathoids. BGS Research Report RR 99-06, the BGS Rock Classification Scheme volume on igneous rocks (Gillespie and Styles), sets out the modal parameters for the QAPF diagrams: Q is quartz, tridymite and cristobalite; A the alkali feldspars; P plagioclase and scapolite; and F the feldspathoids (foids), including nepheline, leucite, pseudoleucite, kalsilite, sodalite, nosean, haüyne, cancrinite and analcime. It adds that minerals in Q and F are mutually exclusive.
The atlas definition. The Alex Strekeisen petrography atlas defines the feldspathoids as a family of rock-forming aluminosilicates of sodium, potassium or calcium having too little silica to form feldspar, not a true group because of considerable structural variation, and says they never occur with quartz. It names nepheline, leucite, sodalite and cancrinite, writes leucite KAlSi2O6, and gives the reaction leucite plus quartz equals K-feldspar. The same atlas is the source on nepheline; the BGS report is the second, independent source this page adds.
What the mineral lists say. The September 2026 IMA list has no class column; a whole-line scan finds 0 lines containing feldspathoid and 2 containing leucite, leucite itself, K(AlSi2O6), A, 1997 s.p., Italy, and ammonioleucite, (NH4)(AlSi2O6). The Handbook leucite sheet prints no group line. So the word is a petrological family name, sourced here to BGS and the atlas, not an IMA category.
Is leucite a zeolite? The two-list rule
The two-list rule: in a rock, leucite is counted with the feldspathoids; in the 1997 IMA zeolite report, it is on the list of zeolites. Coombs et al. 1997, Canadian Mineralogist 35, the report of the IMA subcommittee on zeolites, says leucite has seldom been regarded as a zeolite, as it does not display a full range of zeolitic behaviour, but that it has the same framework structure as analcime and conforms to the adopted definition. Leucite and ammonioleucite are included in its list of zeolites.
Why both answers stand. The two documents answer different questions. BGS sorts minerals in a rock to name the rock, and its F list contains analcime as well as leucite. Coombs et al. define a zeolite by framework and behaviour, and include analcime and leucite. The Strekeisen atlas leucite page says the same in one sentence: leucite is usually included as a feldspathoid by petrologists and, through its structural relation to analcime, typically among the zeolites too. The zeolite pages are which minerals are zeolites and analcime.
What changes on a label. Nothing in the name: leucite is the IMA name. A label that adds feldspathoid follows BGS and the atlas; one that adds zeolite follows Coombs et al. 1997. Neither is wrong on these sources. This page withholds the transformation temperatures the atlas gives, because the sheet gives none.
Leucite or analcime or garnet? The trapezohedron triage
The trapezohedron triage: a 20.00 g ball-shaped crystal reading under 12.00 g in water is leucite or analcime, never one of the five garnet sheets. The Strekeisen leucite page describes trapezohedral crystals; the leucite sheet gives pseudocubic crystals showing {112}, to 9 cm. The analcime sheet gives trapezohedra {211} to 25 cm, and the grossular, pyrope, andradite, spessartine and almandine sheets dodecahedra or trapezohedra. Worked as 20 − 20/D: leucite 11.84 to 12.00 g, analcime 11.07 to 11.27 g, the garnets 14.42 to 15.37 g. Method: how to measure specific gravity at home.
Leucite from analcime. The weights are 0.57 g apart at their nearest. Hardness 5.5 to 6 against 5 to 5.5; index 1.508 to 1.509 against 1.479 to 1.493; and the settings differ on the sheets: leucite with potassium-rich mafic and ultramafic lavas, which it may almost entirely compose, analcime, on its own sheet, in silica-poor igneous rocks such as basalts and phonolites, in lake beds, and authigenic in sandstones and siltstones. The garnet side is on grossular, almandine and how to tell if garnet is real.
Documented failures. Under crossed polars leucite’s birefringence, 0.001 worked from the sheet, and analcime’s anomalous biaxiality both leave the grains nearly dark, so on these sheets the polarising microscope, which separates nepheline from feldspar, is not a reliable way to separate these two. The weight does not separate leucite from cancrinite either, 11.74 to 12.03 g, as noted on is petalite a feldspar. And a crystal on lava matrix weighs as a mixture.
Where leucite is found, and its feldspathoid company
The sheet says leucite occurs with potassium-rich mafic and ultramafic lavas and hypabyssal rocks, and may almost entirely compose them. Its association line is potassic feldspar, nepheline, analcime, natrolite and kalsilite, so three of its five associates are on the BGS F list. The Distribution field: Vesuvius, Monte Somma and Roccamonfina, Campania; Villa Senni, Frascati and Ariccia, Lazio; Vulcano, Lipari Islands; around the Laacher See, Eifel, Germany; the Leucite Hills, Wyoming; the Bear Paw Mountains, Montana; Magnet Cove, Arkansas; the West Kimberley district, Western Australia; Kilimanjaro, Tanzania; around Ruwenzori and Kariya, Uganda; and Mt. Nyiragongo, Congo (Zaire).
Composition, worked. KAlSi2O6 is 55.06% SiO2, 23.36% Al2O3 and 21.58% K2O, the figures in the sheet’s ideal column; the Villa Senni analysis prints 21.45% K2O. The other F-list minerals with live pages are sodalite and haüyne; the feldspars are on how to identify feldspar and orthoclase. The name is from the Greek leucos, white, for its colour, per the sheet.
Where to go next. The classification guides are at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.