Okenite
Okenite, Ca10Si18O46(H2O)18: white triclinic calcium silicate in elastic fibrous balls to 4 cm, density 2.28 to 2.33, in basalt cavities in Greenland and India.
Okenite is a hydrated calcium silicate, Ca10Si18O46(H2O)18 on the IMA list: white, triclinic, as bladed laths and compact radial fibrous balls to 4 cm, elastic, with a perfect {001} cleavage and density 2.28 to 2.33. It occurs commonly in basalt amygdules with zeolites, apophyllite and prehnite; its sheet names Poona, Bombay, Nasik and Kolhapur in India.
In short
- The formula, checked. The September 2026 IMA-CNMNC Master List writes Ca10Si18O46(H2O)18, grandfathered, 1828, Denmark (Greenland); the Handbook of Mineralogy sheet writes Ca5Si9O23·9H2O, the same ratios halved. On standard atomic weights the formula mass is 1966.53 and the oxides are CaO 28.52, SiO2 54.99, H2O 16.49, within 0.01 of the sheet’s ideal column.
- The water check. The sheet’s Bombay analysis has 18.02% H2O, 1.53 points above the ideal 16.49. Recast on 46 oxygens, as Merlino 1983, American Mineralogist 68, 614–622 does, it gives 17.99 Si, 9.87 Ca and 20.06 H2O. Merlino reports the same pattern in the literature: 19 water molecules from four Indian analyses and 20 from two others, against the structure’s 18.
- Documented failure: the 2.33 check. With the sheet’s triclinic cell, 1418.5 ų, the IMA formula at Z = 1 gives a calculated density of 2.30, not the printed 2.33. The Bombay analysis’s calcium and water give 2.34. Neither reproduces 2.33 exactly, and the measured 2.28 to 2.33 spans both.
- Why okenite looks fuzzy. The sheet records bladed crystals or laths, typically fibrous along [010], in curved aggregates and compact radial fibrous balls to 4 cm, with an elastic tenacity. How that separates it from the zeolites it grows with is the cotton-ball check on is okenite a zeolite; whether the fibres count as asbestos is on is okenite asbestos. Neither is repeated here.
| Composition | CaO, wt% | SiO2, wt% | H2O, wt% | Ca per 46 O | Si per 46 O | H2O per 46 O | Density on the sheet’s cell |
|---|---|---|---|---|---|---|---|
| IMA formula, Ca10Si18O46(H2O)18 | 28.52 | 54.99 | 16.49 | 10 | 18 | 18 | 2.30 |
| Handbook ideal column (2), Ca5Si9O23·9H2O | 28.51 | 55.00 | 16.49 | 10 | 18 | 18 | Printed calc. 2.33 |
| Handbook analysis (1), Bombay, India | 27.61 | 53.88 | 18.02 | 9.87 | 17.99 | 20.06 | 2.34 |
| Four Indian analyses, per Merlino (1983) | — | — | — | 10 | 18 | 19 | — |
| Christie (1925), Bombay, and Sukheswala et al. (1974), Nasik, per Merlino | — | — | — | — | — | 20 | — |
| Measured on the sheet | — | — | — | — | — | — | 2.28 to 2.33 |
What is okenite? A calcium silicate of basalt cavities
Okenite is Ca10Si18O46(H2O)18 on the September 2026 IMA list. The row is status G, grandfathered, year 1828, with Denmark (Greenland) as the country and the Archiv für die Gesammte Naturlehre 14 (1828), 333 as the first reference; the second is American Mineralogist 68 (1983), 614. The Handbook of Mineralogy okenite sheet writes Ca5Si9O23·9H2O, the same ratios halved, and prints no mineral group. It is 20.38% calcium by mass.
What the sheet records. Triclinic, point group 1̄. Bladed crystals or laths, typically fibrous with [010] as the fibre axis; aggregates of curved crystals; compact radial fibrous balls to 4 cm. Twinning about [010], composition plane {102}, lamellar. Cleavage perfect on {001}; tenacity elastic. Density 2.28 to 2.33 measured, 2.33 calculated. Transparent to translucent, white or slightly yellow, vitreous to pearly; biaxial (−), α 1.530, β 1.536, γ 1.541 to 1.553, 2V large.
Its structure in one line. Merlino 1983, American Mineralogist 68, 614–622 calls okenite the first example of a silicate having both tetrahedral chains and sheets. Why that rules out a zeolite, and the 12–6–0 count of its silicon atoms, are on is okenite a zeolite and are not repeated here; which minerals are zeolites has its water by mass, 16.49%, the figure worked above.
Why is okenite fuzzy? The habit as printed
Because it grows as fibres along one axis, gathered into balls. The sheet’s words are bladed crystals or laths, typically fibrous with [010] as fibre axis, in aggregates of curved crystals and compact radial fibrous balls to 4 cm. Merlino reports that Gard and Taylor (1956) studied a Bombay specimen consisting of fibrous aggregates, found the fibre direction to be [010], and saw repeated lamellar twinning across the {001} cleavage plane. Merlino himself worked on a fragment cut from a long lath-shaped crystal from Kolhapur District, Maharashtra.
Elastic. The sheet’s tenacity line is a single word, elastic. How that compares with the tenacity of the zeolites and gyrolite that grow beside it is the cotton-ball check on is okenite a zeolite, not repeated here. Whether a fibrous habit makes okenite asbestos is a different question with its own page, is okenite asbestos; nothing here is health or safety advice, and other hazard questions go to toxic minerals in a collection.
Is okenite soft? What the sheet’s hardness line says
The sheet cannot be quoted on it. Rendered from the PDF, the line reads “Hardness = .5–5”: a figure before the decimal point is apparently missing on the sheet itself, not only in its text layer. No other source fetched for this page prints a hardness for okenite, so this page gives none, as is okenite a zeolite and the gyrolite page also decline to.
What can be said. The sheet records the tenacity as elastic and the cleavage as perfect on {001}. Neither is a hardness, and this page does not turn them into one. A scratch test on a fibrous aggregate measures the aggregate, not the crystal; the method and its traps are on how to test mineral hardness properly.
The water check and the 2.33 check
Okenite analyses carry more water than its formula. The sheet’s analysis (1), from Bombay, India, reads SiO2 53.88, CaO 27.61, H2O 18.02, with traces of Al2O3, Fe2O3, SrO, Na2O and K2O. A script recasts it on 46 anhydrous oxygens, Merlino’s basis, to 17.99 Si, 9.87 Ca and 20.06 H2O. Merlino wrote that Phadke and Kshirsagar (1980) derived Ca10Si18O46·19H2O from four analyses, while twenty water molecules were calculated from Christie’s 1925 Bombay analysis and Sukheswala and others’ 1974 Nasik analysis. His structure has 18.
The density does not settle it either. The sheet’s cell, a 9.69, b 7.28, c 22.02 Å, α 92.7°, β 100.1°, γ 110.9°, has a volume of 1418.5 ų by the triclinic formula. With the IMA formula at Z = 1 (the sheet’s bracketed Z = [2] for its halved formula is the same thing) the calculated density is 2.30. With the Bombay analysis’s calcium and water it is 2.34. The sheet prints 2.33, which neither composition reproduces exactly; its measured range, 2.28 to 2.33, covers both. A 20.00 g piece reads 11.23 to 11.42 g in water, on the method at how to measure specific gravity at home.
Where is okenite found? The sheet’s localities
Setting. Commonly in amygdules in basalts, with zeolites, apophyllite, prehnite, calcite, quartz and “chalcedony”, in the sheet’s words; chalcedony has its own page. The gyrolite sheet lists okenite among gyrolite’s associates, with stilbite, laumontite, thomsonite, tobermorite and xonotlite.
Distribution, as printed. In Greenland, at Qutdligssat (Kudlisat), Ritenbenk district, and on Quqertarsuaq (Disko) Island at Skarvefjeld and Blæsedal, Godhavn district. On Bordoy and Streymoy, Faeroe Islands. At Scawt Hill, near Larne, Co. Antrim, Ireland. On the Isle of Mull and Morven Peak, Grampian Highlands, Scotland. In India, in Maharashtra, from the Syhadree Mountains, the Bombay quarry, Bombay, around Poona, and in the Nasik and Kolhapur districts. At Rio Putagan, Chile. In the Noche Buena mine, Mazapil, Zacatecas, Mexico. In the USA, at Crestmore, Riverside Co., California; the Fairfax quarry, Centreville, Virginia; and Skookumchuck Dam, near Bucoda, Washington. In the Jeffrey mine, Asbestos, Quebec, Canada. Other localities are known.
Type locality. The sheet marks none. The IMA list gives Denmark (Greenland), and Merlino writes that okenite was first described from Disko Island, Greenland. The term is explained on what is a type locality. More species are at the species index; the questions are in the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.