Gmelinite
Gmelinite: a zeolite series, gmelinite-Ca, -Na and -K on the IMA list: hexagonal crystals, hardness 4 to 4.5, density 2.00 to 2.17, easily taken for chabazite.
Gmelinite is a zeolite series, not one species: the IMA list has gmelinite-Ca, gmelinite-Na and gmelinite-K, each (Si8Al4)O24·11H2O. All three Handbook sheets give hexagonal crystals, hardness 4 to 4.5 and density 2.00 to 2.17. Chabazite overlaps it in hardness, density, index and habit; the cell’s c length, about 10 Å against 15 Å, separates them.
In short
- Three rows, one name. A script over the September 2026 IMA-CNMNC Master List finds Gmelinite-Ca (A, 1997 s.p., Italy), Gmelinite-Na (Rn, renamed, 1997 s.p., United Kingdom / Italy) and Gmelinite-K (A, 1999-039, Russia / Italy), and no plain gmelinite. The IMA zeolite report (Coombs et al. 1997) calls gmelinite-Na the most common species of the series and names each member by its most abundant extra-framework cation.
- What the three sheets share. Hexagonal, P63/mmc, zeolite group. The Handbook of Mineralogy -Ca and -Na sheets give pyramidal, tabular or rhombohedral crystals to 4 cm, hardness 4.5; the -K sheet radial aggregates of columnar to acicular crystals to 3 mm, hardness 4. Densities 2.00 to 2.17: a 20.00 g piece reads 10.00 to 10.78 g in water.
- The c-axis rule. Against the five chabazite sheets, every bench number overlaps: hardness 4 to 4.5 against about 4 to 5, density 2.00 to 2.17 against 1.98 to 2.20, indices 1.471 to 1.494 against 1.465 to 1.507, and both sheets allow rhombohedral crystals. The cell does not: a is 13.715 to 13.80 Å for gmelinite and 13.715 to 13.863 Å for chabazite, but c is 9.964 to 10.256 Å for gmelinite and 14.8724 to 15.165 Å for chabazite. That takes an X-ray pattern.
- Documented failures. The -K sheet heads itself Na4(Si8Al4)O24·11H2O, though its analysis is K 1.62 to Na 0.86. The -Na sheet prints Z = 4 on the cell for which the others print Z = 2; at Z = 4 its ideal formula gives 4.02 to 4.10, and at Z = 2 it gives 2.01 to 2.05, so its printed D(calc.) 2.098 does not reproduce. The -Ca sheet’s 2.095 comes from its strontium-rich Montecchio analysis (2.099), not the ideal formula (2.011).
- Where. The -Ca and -Na sheets list Montecchio Maggiore, Italy; Glenarm and elsewhere in Co. Antrim; the Isle of Skye; Pyrgos, Cyprus; Bergen Hill and Great Notch, New Jersey; Five Islands, Nova Scotia; the Ilímaussaq intrusion, Greenland; and large crystals from Bekiady, Madagascar. The -K sheet marks Mount Alluaiv, Lovozero, Kola Peninsula, as type locality, and adds Fara Vicentina, Italy.
| Criterion | Gmelinite (-Ca, -Na, -K) | Chabazite (five sheets) | Does it separate them? |
|---|---|---|---|
| IMA rows | 3, suffix -Ca, -Na, -K | 5, suffix -Ca, -K, -Mg, -Na, -Sr | Only with an analysis |
| Crystal system (sheet) | Hexagonal, P63/mmc | Pseudohexagonal or hexagonal, point group 3 2/m | Not in the hand |
| Habit | Pyramidal, tabular or rhombohedral, to 4 cm (-K columnar to acicular, to 3 mm) | Rhombohedral, nearly equant, to 4.5 cm (-Ca, -K, -Na); rhombohedra to 0.4 mm (-Mg) | No: both may be rhombohedral |
| Hardness | 4.5 (-Ca, -Na); 4 (-K) | 4–5; about 4 (-Mg); 4–4.5 (-Sr) | No |
| Density | 2.00–2.17 | 1.98–2.20 | No |
| 20.00 g in water | 10.00–10.78 g | 9.90–10.91 g | No |
| Refractive indices | ω 1.476–1.494, ε 1.474–1.480; -K 1.477 and 1.471 | 1.478–1.493; -Mg 1.465–1.469; -Sr 1.503–1.507 | No |
| Cell a | 13.715–13.80 Å | 13.715–13.863 Å | No |
| Cell c | 9.964–10.256 Å | 14.8724–15.165 Å | Yes, by X-ray: a 4.6 Å gap |
| IZA framework code (Coombs et al. 1997) | GME | CHA | Yes, on paper |
What is gmelinite? A zeolite series of three species
The IMA rows. The September 2026 list has Gmelinite-Ca, Ca2(Si8Al4)O24·11H2O, A, 1997 s.p., Italy; Gmelinite-Na, Na4(Si8Al4)O24·11H2O, Rn (renamed), 1997 s.p., with United Kingdom / Italy in the country column and the Edinburgh Journal of Science (1825) as first reference; and Gmelinite-K, K4(Si8Al4)O24·11H2O, A, 1999-039, Russia / Italy. A script finds no plain gmelinite row. The IMA zeolite report (Coombs et al. 1997) gives the series as (Na2,Ca,K2)4[Al8Si16O48]·22H2O, framework code GME, with Brewster (1825) as the original reference, and says Na-dominant members are the most common.
How a label should read. The suffix names the most abundant extra-framework cation in atomic proportions, so it takes an analysis; a specimen without one is honestly labelled gmelinite. The same rule names the members of chabazite and phillipsite; the worked cation-share method is on those pages and is not repeated here. Older names the 1997 report equates with gmelinite include natrochabazite, soda-chabazite, ledererite, hydrolite (of Leman) and sarcolite (of Vauquelin). Other zeolites are on which minerals are zeolites and in the species index.
The three Handbook sheets, side by side
Gmelinite-Ca and gmelinite-Na. The two sheets print the same crystal data (the -Ca sheet in brackets): euhedral crystals, pyramidal, tabular or rhombohedral, striated, to 4 cm, rarely radiating or granular; penetration twins common; one distinct cleavage and a parting; uneven fracture; brittle; hardness 4.5; piezoelectric; colourless, white, reddish white, salmon-red, yellowish or greenish white; uniaxial (−) or (+), possibly anomalously biaxial, ω 1.476 to 1.494, ε 1.474 to 1.480. Density: -Ca 2.11, -Na 2.02 to 2.17. The -Ca sheet’s analyses are a strontium-rich Montecchio Maggiore microprobe analysis, (Ca1.03Sr0.68Na0.39K0.06), and Great Notch, New Jersey, (Ca1.12Na0.98Sr0.12K0.06); the -Na sheet’s are Montecchio Maggiore, (Na2.32Ca0.52Sr0.06K0.02), and White Head, Co. Antrim, (Na1.42Ca1.28Sr0.06K0.02). Recast on 24 oxygens by script, the cation figures of all four reproduce to within 0.01, except at White Head, where the printed Sr 0.06 and K 0.02 come out the other way round (K 0.06, Sr 0.02 from K2O 0.27 and SrO 0.25).
Gmelinite-K. A different look: granular or radial aggregates of columnar to acicular crystals to 3 mm, commonly with a hexagonal outline; conchoidal fracture; hardness 4; density 2.00; colourless to brownish with a white streak; uniaxial (−), ω 1.477, ε 1.471. Its one analysis, from Mount Alluaiv, Lovozero, by microprobe and TGA, is (K1.62Na0.86Ca0.28), and its setting is drusy aggregates in cavities in brecciated pegmatite with K-feldspar, nepheline, sodalite and cancrinite: a pegmatite setting, where the -Ca and -Na sheets lead with basalts.
Gmelinite or chabazite? The c-axis rule
Why they are confused. The 1997 report says gmelinite’s structure is similar to chabazite’s and that the two are commonly intergrown (Strunz 1956), gmelinite having a different stacking of the double 6-membered rings. The Gmelinite-Ca sheet lists chabazite among its associates. On the sheets, hardness, density, refractive index and the rhombohedral habit all overlap, as the table shows; the chabazite page records gmelinite-Na’s 10.10 to 10.78 g inside chabazite’s window, and this page agrees.
The rule. Both are written on hexagonal axes with a near 13.7 to 13.9 Å, but gmelinite’s c is 9.964 to 10.256 Å on its three sheets (the report: 9.97 to 10.25) and chabazite’s is 14.8724 to 15.165 Å on its five. A powder pattern or single-crystal measurement settles it; nothing on a bench does. A label reading gmelinite on rhombs from basalt is a determination to query. Whether a piece is a zeolite at all is on is chabazite a zeolite; reading a balance is on how to measure specific gravity at home.
When the sheets disagree with themselves
The heading. The Gmelinite-K sheet’s heading prints Na4(Si8Al4)O24·11H2O, the gmelinite-Na formula. Its name note says the suffix indicates the predominance of K, its analysis gives K 1.62 against Na 0.86, and the IMA list writes K4(Si8Al4)O24·11H2O, so the heading reads as a slip.
Z and density. Worked as Z × formula mass × 1.66054 ÷ (√3/2 · a² · c): the -Ca sheet’s cell (a 13.800, c 9.964 Å, Z = 2) gives 2.011 for the ideal formula and 2.099 for the printed Montecchio formula, against D(calc.) 2.095. The -K sheet’s cell gives 2.007 for its analysis, against D(calc.) 2.01, and 2.129 for ideal K4. The -Na sheet prints Z = 4 on a cell of the same size; at Z = 4 the ideal formula gives 4.02 to 4.10, twice the measured 2.02 to 2.17, and at Z = 2 the ideal formula and both analyses give 2.00 to 2.06, so its printed 2.098 is not reproduced from its own data. This page shows the arithmetic and does not correct the sheets.
Where gmelinite is found
-Ca and -Na. Formed from calcium-rich or sodium-rich fluids, in basalts and related igneous rocks, also pegmatites; associates analcime, chabazite, calcite, aragonite and quartz. Both Distribution fields list Montecchio Maggiore, Vicenza, Italy; Glenarm and elsewhere in Co. Antrim, Ireland; the Isle of Skye, Scotland; Pyrgos, Cyprus; Bergen Hill, Great Notch, Paterson and Prospect Park, New Jersey; Springfield, Oregon; Pinnacle Rock, Five Islands and Two Islands, Nova Scotia; the Ilímaussaq intrusion, Greenland; Sarbay-Sokolov, Kazakhstan; Flinders, Victoria; and large crystals from Bekiady, Madagascar. The -Na sheet adds that the mineral is widespread in small amounts. Analcime, the first associate the -Ca sheet names, has its own page: analcime.
-K. Mount Alluaiv, Lovozero alkaline massif, Kola Peninsula, Russia, marked [TL] on the sheet, and Fara Vicentina, Vicenza, Italy. More zeolites: mordenite, harmotome and natrolite. Hazard questions about any zeolite are routed to toxic minerals in a collection; that is not advice. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.