Laumontite
Laumontite, CaAl2Si4O12·4H2O: a white to pink monoclinic calcium zeolite, hardness 3 to 4, density 2.23 to 2.41, and, partly dehydrated, “leonhardite”.
Laumontite is a calcium zeolite, CaAl2Si4O12·4H2O on the IMA list (A, 1997 s.p., France). Its Handbook sheet gives white to pink monoclinic prisms to 20 cm, hardness 3 to 4, density 2.23 to 2.41, perfect {010} and {110} cleavage, and a partial loss of water on exposure to air. “Leonhardite”, the partly dehydrated form, is not a separate species.
In short
- Formula and status. The September 2026 IMA-CNMNC Master List gives CaAl2Si4O12·4H2O, status A, 1997 s.p., France; the Handbook of Mineralogy sheet writes the same and puts it in the zeolite group. Worked on standard atomic weights: formula mass 470.43, ideal SiO2 51.09, Al2O3 21.67, CaO 11.92 and H2O 15.32%.
- Leonhardite is laumontite. No IMA row and no Handbook sheet (404 on 10 November 2026). The IMA zeolite report (Coombs et al. 1997) calls leonhardite a partially and in most cases reversibly dehydrated form of laumontite, not accepted as a separate species, and says laumontite, except where unusually rich in K and Na, reversibly loses ca. 4H2O from its Ca4 cell content at low humidity to form it.
- The water ladder. Per formula unit the sources give 4.5 H2O (the report’s 18 per Ca4), 4 (IMA) and about 3.5 (the report’s leonhardite, 4 fewer per Ca4). The sheet’s Halle analysis works out at 3.11 H2O from H2O+ alone, the printed 3.12, and 3.74 counting H2O− too. On the sheet’s own cell those contents give densities from 2.237 to 2.360.
- Documented failure: the balance cannot see the water. A 20.00 g piece reads 11.36 g in water at four waters and 11.20 g at three and a half, 0.16 g apart; the sheet’s measured range, 2.23 to 2.41, spans 11.03 to 11.70 g. Weighing does not tell laumontite from leonhardite, and the sheet’s D(calc.) 2.25 fits about 3.3 H2O, not 4 (2.316).
- Where. The sheet calls it relatively common and names well-crystallised specimens from Huelgoat, Finistère, France; Săcărâmb (Nagyág), Romania; St. Gotthard, Switzerland; Tirol, Austria; Baveno, Italy; Poona and Khandivali quarry near Bombay, India (large crystals); and, in the USA, the Pine Creek tungsten mine, California (exceptional crystals), New Jersey, Virginia, Oregon and the Keweenaw Peninsula, Michigan.
| Source and form | H2O per formula | H2O by mass | Density on the sheet’s cell | 20.00 g in water | What the source says |
|---|---|---|---|---|---|
| IMA zeolite report (Coombs et al. 1997), Ca4[Al8Si16O48]·18H2O | 4.5 | 16.91% | 2.360 | 11.53 g | Cell content as written; its own Nasik cell gives 2.304 |
| IMA list and Handbook sheet, CaAl2Si4O12·4H2O | 4 | 15.32% | 2.316 | 11.36 g | Undergoes a partial loss of H2O on exposure to air (sheet) |
| Halle, Germany, H2O+ plus H2O− (sheet analysis) | 3.74 | 14.41% printed | 2.293 | 11.28 g | Total 99.92, reproduced |
| “Leonhardite” (Coombs et al. 1997): ca. 4 H2O fewer per Ca4 | about 3.5 | 13.66% | 2.272 | 11.20 g | Not accepted as a separate species |
| Sheet D(calc.) 2.25, solved for water | 3.26 | — | 2.25 (printed) | 11.11 g | The sheet does not state its basis |
| Halle, H2O+ alone | 3.11 | 12.00% printed | 2.237 | 11.06 g | Sheet prints “3.12H2O”: reproduced |
| Sheet, measured | — | — | 2.23–2.41 | 11.03–11.70 g | Spans every row above |
| Wairakite (IMA list only), Ca(Si4Al2)O12·2H2O | 2 | 8.29% | — | — | Same Ca:Al:Si as laumontite, half the water, by formula |
What is laumontite? A calcium zeolite with four waters
The formula. The September 2026 IMA list gives CaAl2Si4O12·4H2O, status A (approved), 1997 s.p. (special procedure; the IMA zeolite subcommittee report is dated 1997), and France as the country of first description. The Handbook of Mineralogy laumontite sheet writes the same formula, Mineral Group: zeolite group. The IMA zeolite report (Coombs et al. 1997) gives the cell content as Ca4[Al8Si16O48]·18H2O, Z = 1, says laumontite has a topologically distinctive framework and that Ca is always the dominant extra-framework cation, and so treats it as a single species with no suffix. Its naming note: as lomonite, Jameson (1805); laumonite, Haüy (1809); laumontite, von Leonhard (1821).
What the sheet describes. Square prisms with steep oblique terminations, stout, commonly elongated, to 20 cm; also radiating, columnar and fibrous, in interlocking aggregates, and massive; twinned on {100}, typically with terminal re-entrants. Cleavage {010} and {110} perfect, fracture uneven, brittle, hardness 3 to 4. White to grey, pink, yellowish, brownish or golden brown, with a white streak and a vitreous lustre, pearly on cleavages. Reading the two cleavages on a broken prism is covered on cleavage or fracture: how to tell. Other zeolites are on which minerals are zeolites and in the species index.
Does laumontite lose water? What leonhardite means
The sheet’s sentence. Under Physical Properties: “Undergoes a partial loss of H2O on exposure to air.” The sheet does not use the word leonhardite and gives no rate or humidity. The IMA list has no leonhardite row, and the Handbook has no leonhardite sheet. The 1997 report discredits the name: leonhardite, a partially and in most cases reversibly dehydrated form of laumontite, is not accepted as a separate mineral species. In its laumontite entry it adds that, except where unusually rich in (K,Na), laumontite reversibly loses ca. 4H2O at low humidity at room temperature and pressure to form the variety termed leonhardite, and that the “primary leonhardite” of Fersman (1908) is laumontite with about 1.5 Ca replaced by 3(K,Na) and reduced water.
How to read an old label. A specimen labelled leonhardite is laumontite by current nomenclature; the label records a state of hydration, not a species. This page reports what the sources say about the water and gives no storage or humidity recipe: none of the sources here prints one as tested.
The water ladder, worked
Method. Each oxide of the sheet’s Halle, Germany, analysis (SiO2 50.70, Al2O3 22.53, Fe2O3 0.04, CaO 11.54, Na2O 0.40, K2O 0.30, H2O+ 12.00, H2O− 2.41, total 99.92) is divided by its molar mass and scaled to 12 oxygens. That gives Ca 0.962, Na 0.060, K 0.030, Al 2.066, Si 3.945, which reproduce the printed (Ca0.96Na0.06K0.03)Σ=1.05Al2.07Si3.94O12, and 3.11 H2O from H2O+ alone, the printed 3.12. Counting H2O− as well gives 3.74. The sheet’s analysis is therefore written short of the formula’s four waters, nearer the report’s leonhardite.
Documented failures. First, density: on the sheet’s cell, 1349.17 ų with Z = 4, the IMA formula gives 2.316, not the printed D(calc.) 2.25; solving for water, 2.25 needs about 3.26 H2O, and 3.3 H2O, the water in the formula in the title of the sheet’s reference (4), Yakubovich and Simonov 1985, gives 2.254. The sheet does not say which it used. Second, the balance: four waters against three and a half is 0.16 g on a 20.00 g piece, while the measured range runs from 11.03 to 11.70 g; method on how to measure specific gravity at home. The cell is held fixed in this arithmetic, so it shows the mass effect only, not the structure’s.
Laumontite in thin section and by X-ray
Thin section. Colourless in thin section; biaxial (−), α 1.502 to 1.514, β 1.512 to 1.522, γ 1.514 to 1.525, 2V(meas.) 26° to 47°, with Y = b, X∧a 10° to 26° and Z∧c 8° to 33°, and distinct, weakly inclined dispersion, r < v. The sheet prints ranges, so a single grain sits somewhere inside them, and this page does not derive a single birefringence from range ends.
X-ray powder pattern. The sheet’s pattern is from synthetic material: 4.16 (100), 3.51 (80), 9.5 (60), 3.27 (55), 6.84 (40), 3.66 (35), 3.03 (35). The cell it lists is monoclinic C2/m, a 14.724, b 13.075, c 7.559 Å, β 112.01°; the 1997 report’s Nasik, India cell is a 14.845, b 13.167, c 7.5414 Å, β 110.34°. Other zeolite questions are answered on is chabazite a zeolite, is thomsonite a zeolite and is okenite a zeolite; their worked checks are not repeated here.
How laumontite forms and where it is found
Formation, as the sheet prints it. Of hydrothermal origin, lining cavities in igneous rocks. Thick sedimentary beds rich in laumontite may form by decomposition of analcime or metamorphism of plagioclase; it may be authigenic, cementing sandstones. Associates: zeolites, apophyllite, datolite, calcite and chlorite. The sheet cites a 1971 study of the pressure–temperature stabilities of laumontite, wairakite and lawsonite; this page draws no pressure or temperature conclusion from a title. By IMA formula, wairakite, Ca(Si4Al2)O12·2H2O, has laumontite’s calcium, aluminium and silicon with half the water; analcime is the zeolite whose decomposition the sheet names.
Localities, from the Distribution field. Relatively common. Well-crystallised specimens from Huelgoat, Finistère, France; Săcărâmb (Nagyág), Romania; St. Gotthard, Ticino, Switzerland; Floitental, Zillertal and other places in Tirol, Austria; Baveno, Piedmont, Italy; Poona and the Khandivali quarry near Bombay, Maharashtra, India, large crystals; in the USA, exceptional crystals from the Pine Creek tungsten mine, Bishop, Inyo Co., California, and Drain, Oregon, Bergen Hill and Paterson, New Jersey, the Goose Creek quarry, Leesburg, Virginia, and the Keweenaw Peninsula, Michigan. The sheet marks no type locality; see what is a type locality. No source fetched for this page states a use for laumontite, so none is given. Any hazard from fibrous zeolite material is 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.