Stilbite
Stilbite is a zeolite, hardness 3.5 to 4, density 2.19, sheaf-like crystals to 14 cm. The IMA list holds stilbite-Ca and stilbite-Na, and no plain stilbite.
Stilbite is a low-temperature hydrothermal zeolite, hardness 3.5 to 4, measured density 2.19, forming thin tabular crystals to 14 cm that gather into sheaf-like aggregates, twinned ubiquitously on {001}, with a pearly lustre on the perfect {010} cleavage. The September 2026 IMA list carries stilbite-Ca and stilbite-Na and no unsuffixed stilbite at all.
In short
- There is no species called stilbite. The IMA-CNMNC master list for September 2026 holds stilbite-Ca and stilbite-Na, both approved in 1997, and nothing in between. A specimen labelled plain stilbite has been identified to a series, not to a species.
- Hardness 3.5 to 4 and a density of 2.19 make it one of the lightest minerals a collection is likely to hold. It is also one of the most easily damaged, because the {010} cleavage is perfect and the crystals are thin.
- Crystals reach 14 cm, flattened on {010}, and the diagnostic form is the sheaf: a bundle that splays at both ends. Twinning on {001} is described by the Handbook of Mineralogy as ubiquitous.
- Heulandite is the look-alike and the physical properties do not separate them. Both run 3.5 to 4 in hardness, both sit near 2.1 to 2.2 in density, both cleave perfectly on {010}. Habit does the work: heulandite crystals are coffin-shaped, stilbite builds sheaves.
- The great crystals are Indian. Maharashtra — the Poona, Nasik and Bombay districts — supplies the material most collectors own, but there is a long-standing Scottish occurrence in the Glasgow district of Dumbartonshire.
| Property | Stilbite-Ca | Heulandite-Ca | Does it separate them? |
|---|---|---|---|
| Hardness | 3.5 to 4 | 3.5 to 4 | No |
| Measured density | 2.19 | 2.10 to 2.20 | No — the ranges overlap |
| Cleavage | {010}, perfect | {010}, perfect | No |
| Lustre | Vitreous, pearly on cleavage | Vitreous, pearly on {010} | No |
| Habit | Thin tabular on {010}, to 14 cm, in sheaf-like, fascicled or globular aggregates | Tabular on {010}, widest at the centre, to 12 cm — the classic coffin shape | Yes. This is the only bench test that works |
| Twinning | Ubiquitous on {001}, cruciform and penetration twins | On {100}, as twin and contact plane | Sometimes, if the twin is visible |
Why the IMA list has no species called stilbite
The IMA-CNMNC master list for September 2026 contains two entries beginning with the word stilbite. Stilbite-Ca, NaCa4(Si27Al9)O72 · 28H2O, approved in 1997 with Germany as its country of first description. Stilbite-Na, Na9(Si27Al9)O72 · 28H2O, approved in the same 1997 procedure, from Italy. There is no row for stilbite on its own.
This is the third time this site has run into the same thing. Apophyllite is now fluorapophyllite-(K), hydroxyapophyllite-(K) and fluorapophyllite-(Na); bare tetrahedrite has been replaced by seven suffixed species. The pattern is the same in every case: a name that once covered a compositional range has been split into the end-members, and the old name survives only as the name of the series.
What this means in practice is narrow and worth being precise about. An old label reading stilbite is not wrong and is not a fake. It records a determination made to the standard of its day, and for the overwhelming majority of material the answer would come out stilbite-Ca. But it is a group determination, and if you are cataloguing to species you should write it as such. Our note on how mineral names are formed covers the wider nomenclature question, and labelling and cataloguing covers recording the date of a determination, which is the part people skip.
Stilbite against heulandite, and why the comparison table is mostly negative
Look again at the table above. Five of the six rows say the two species cannot be told apart on that property. Hardness is 3.5 to 4 for both. Density is 2.19 against 2.10 to 2.20. Both cleave perfectly on {010} and both go pearly on the cleavage. None of the standard bench tests does anything at all.
What works is shape. Heulandite crystals are tabular, elongated, and widest across the middle, which is why the Handbook of Mineralogy calls them coffin-shaped — a single crystal looks like a flattened lozenge. Stilbite is also tabular but grows in bundles, and the bundles splay: the classic form is a sheaf of wheat pinched at the waist, or a bow tie. If you are looking at a splaying bundle rather than a single flat plate, you are almost certainly looking at stilbite.
The honest caveat is that neither habit is compulsory. Stilbite occurs granular and globular; heulandite occurs massive. A crust with no crystal form on it is not identifiable by eye and should be recorded as a zeolite until someone runs it. That is not a failure of technique, it is what the material is like.
Where stilbite comes from, including the British occurrence people forget
Stilbite is a common zeolite of amygdules and cavities in basalt and andesite, and it also turns up in hot-spring deposits and as a cement in some sandstones. The Handbook of Mineralogy lists many occurrences of large and fine crystals, and a few are worth naming.
Maharashtra, India — the Bombay, Poona and Nasik districts — produces the exceptional crystals, and that Deccan basalt material is what most collections actually hold. Iceland has the Helgustadir mine at Reydarfjord and Teigarhorn at Berufjord; several of the Faeroe Islands are productive; there is fine material along the Bay of Fundy in Nova Scotia and at Great Notch and Paterson in New Jersey.
And there is a British locality that rarely gets mentioned: the Glasgow district of Dumbartonshire, Scotland. It sits in the Handbook's distribution list alongside Iceland and the Faeroes. It is not a source of display material and never was, but if you are building a British suite rather than a world suite it is the entry that belongs on the shelf. Our locality notes cover the classic British orefields in more depth.
Handling: soft, perfectly cleaved, and easily ruined by cleaning
At hardness 3.5 to 4 with a perfect {010} cleavage on thin tabular crystals, stilbite is near the top of the list of species that get destroyed by well-meant cleaning. Never put a zeolite in an ultrasonic tank. The mechanism that lifts clay out of a vug is the same mechanism that shakes a splayed sheaf apart at its cleavages, and there is no way to undo it.
Zeolites are also hydrated. Stilbite-Ca carries 28 molecules of water per formula unit, and the two published analyses split that between water driven off above and below 100 degrees C. Material stored in a hot, dry room can lose the loosely held fraction. That is not the dramatic failure you get with chalcanthite, but it is a reason not to put zeolites on a sunny windowsill or above a radiator. Our guidance on cleaning mineral specimens and on storing a collection where humidity swings both apply here.
If you are looking for stilbite of a particular locality — Scottish material especially, which almost never appears on the open market — that is the kind of brief worth putting on a wanted list, because it is defined by where it came from rather than by what it looks like.