Heulandite
Heulandite is a zeolite series, hardness 3.5 to 4, density 2.10 to 2.20, coffin-shaped crystals to 12 cm. Most labels omit the suffix the species name requires.
Heulandite is a calcium-sodium zeolite, monoclinic, hardness 3.5 to 4, with a density of only 2.10 to 2.20 — lighter than quartz. Crystals are tabular on {010} and widest at the centre, the form called coffin-shaped, reaching 12 cm. It is a series, so a complete name carries a suffix.
In short
- “Heulandite” on its own is an incomplete name. It is a series, and the valid species carry a chemical suffix — heulandite-Ca, heulandite-Na, heulandite-K, heulandite-Sr and heulandite-Ba — after the dominant extra-framework cation. Almost no dealer label carries one.
- The boundary with clinoptilolite is a silicon-to-aluminium ratio, not an appearance. The two are structurally the same and the division is drawn at Si/Al = 4. You cannot see it, and a great deal of material sold as heulandite has never been analysed.
- Density 2.10 to 2.20 is the field check. That is lighter than quartz at 2.65 and lighter than calcite at 2.71. A large zeolite specimen that feels too light for its size is behaving correctly.
- Perfect cleavage on {010}, and the pearly lustre sits on that same face. The combination of a perfect cleavage and hardness 3.5 to 4 makes the tabular crystals easy to split, and a split crystal shows a bright pearly plane where a face should be.
- The classic localities are Teigarhorn at Berufjördur in Iceland, the Faroe Islands, the Deccan basalt quarries of Maharashtra in India, Nova Scotia, the New Jersey traprock, and around Glasgow in Scotland.
| Species | Habit | Hardness | Density | What separates it |
|---|---|---|---|---|
| Heulandite series | Tabular on {010}, coffin-shaped, to 12 cm | 3.5–4 | 2.10–2.20 | Si/Al below 4. Needs analysis to separate from clinoptilolite |
| Clinoptilolite series | Structurally identical; often smaller, platy | 3.5–4 | 2.1–2.2 | Si/Al of 4 or above. Same structure, same look |
| Stilbite series | Sheaf-like and bow-tie bundles | 3.5–4 | 2.1–2.2 | Habit. The sheaf is diagnostic where heulandite is a single tablet |
| Apophyllite | Tabular to prismatic, square cross-section | 4.5–5 | 2.3–2.4 | Harder, tetragonal, and the {001} cleavage is at right angles to the prism |
| Barite | Tabular, often bladed | 3–3.5 | 4.48 | Twice the density. Never confusable once lifted |
| Calcite | Rhombs, scalenohedra | 3 | 2.71 | Effervesces in acid; heulandite does not |
Why your label is probably incomplete, and whether it matters
Zeolite nomenclature was reformed in 1997, and the reform split several long-standing mineral names into series. Heulandite was one of them. What had been a single species became a series of species distinguished by the dominant extra-framework cation, written with a chemical suffix: heulandite-Ca, heulandite-Na, heulandite-K, heulandite-Sr and heulandite-Ba. Heulandite-Ca is the common one.
Under the current rules, a name without a suffix is a series name rather than a species name. The nomenclature is maintained by the IMA Commission on New Minerals, Nomenclature and Classification, which is the body that approves mineral names and the changes to them.
Does it matter for a collector? Honestly, less than the pedantry suggests, but more than nothing. If you are cataloguing a display collection, “heulandite” is a perfectly serviceable entry and everybody knows what you mean. If you are building a species collection where the count matters, the suffix is the species and an unsuffixed label does not tell you which one you have.
The defensible practice is the one that does not destroy information: write what the label says, and note separately that the suffix is undetermined. Do not invent heulandite-Ca on the grounds that it is the commonest. That converts a known unknown into a false certainty, which is the worse of the two states to leave a specimen in.
Heulandite or clinoptilolite: a boundary you cannot see
The harder problem is not the suffix, it is the neighbouring series. Clinoptilolite has the same structure as heulandite. Same framework, same topology, overlapping appearance, overlapping habit. The division between them is drawn on the silicon-to-aluminium ratio, at Si/Al = 4 — below that it is heulandite, at or above it is clinoptilolite.
There is no way to determine a Si/Al ratio by looking. There is no hardness difference, no density difference worth using, no streak difference, no reliable habit difference. The Handbook of Mineralogy's two published analyses of heulandite illustrate the compositional spread within the series alone: material from Trentino-Alto Adige in Italy running Si7.05 against Faroe Islands material at Si6.68, with quite different calcium, sodium, strontium and barium contents.
The practical position: a tabular, coffin-shaped, centimetre-scale crystal from a classic basalt locality is being reasonably called heulandite, and a fine-grained sedimentary or tuffaceous zeolite is more likely to be clinoptilolite. That is a probability derived from geology, not an identification. Clinoptilolite dominates in altered volcanic tuffs and sediments; the big display crystals from basalt cavities are usually heulandite. Neither statement is a determination for any individual specimen.
This is the same situation as the copper sulphides, and the same discipline applies — see our page on identifying a mineral specimen for where the useful tests stop.
The coffin-shaped habit, and the cleavage that ruins it
Heulandite's crystal form is genuinely distinctive and it is the reason the species is collected. Crystals are tabular parallel to {010} and elongated, widest at the centre — which is what produces the outline the literature has called coffin-shaped since the nineteenth century. They reach 12 cm. Twinning occurs with {100} as the twin and contact plane.
The problem is that the perfect cleavage is on {010}: the same plane as the broad tabular face. A crystal that takes a knock splits parallel to its own best face, and the result is a bright, pearly, perfectly flat surface where a slightly duller, textured crystal face used to be.
This is worth knowing when buying, because a cleaved heulandite can look better than an intact one in a photograph. The pearly cleavage plane is brilliantly reflective. The tells are that a cleavage surface has no growth texture at all, that it will not have the very slight convexity of a real {010} face, and that the crystal outline will be truncated somewhere it should not be. Raking light across the face at a low angle shows growth steps on a genuine face and nothing on a cleavage. Our guide to spotting repairs and damage covers the technique.
The other consequence is for cleaning: the cleavage plus hardness 3.5 to 4 rules out ultrasonic cleaning entirely. Zeolite specimens from the Indian quarries frequently carry a clay or celadonite film, and removing it is a patient job with water and a soft brush. See how to clean mineral specimens.
Where it forms, and the localities on the labels
Heulandite forms in cavities in basalts, in highly weathered andesites and diabases, and as a devitrification product of volcanic glasses and tuffs. Its associates are the other zeolites, plus datolite, apophyllite and calcite.
The Handbook singles out exceptional crystals from Teigarhorn at Berufjördur in Iceland — the classic locality, and one of the great zeolite occurrences. Haldarsvík on Streymoy in the Faroe Islands is the second North Atlantic basalt locality. Britain's occurrence is around Glasgow in Dumbartonshire, in the Clyde Plateau lavas. Switzerland gives Giebelsbach near Fiesch in Valais, which is where the Handbook's reference X-ray powder pattern was taken.
In the United States: Paterson in Passaic County and Bergen Hill in Hudson County, New Jersey — the traprock quarries — plus Goble in Columbia County, Oregon, and the Skookumchuck Dam near Bucoda in Washington. Canada gives Cape Blomidon in Nova Scotia. Brazil has occurrences near Bento Gonçalves in Rio Grande do Sul, and Australia has Tambar Springs near Gunnedah in New South Wales.
Large crystals from the Nashik, Pune and Mumbai districts of Maharashtra, India are what most collectors will actually encounter. The Deccan trap quarries have supplied the world's zeolite specimens for decades, they are worked commercially for aggregate rather than for minerals, and the specimens are recovered by quarry workers. Attribution is often to a district rather than to a quarry as a result, and that is a real limitation on Indian zeolite labels rather than carelessness. If quarry-level provenance or an analysed suffix is what you actually need, that is worth stating on a wanted list rather than hoping to find it in a listing, because it narrows the field to a handful of specimens.
The species is named for John Henry Heuland (1778–1856), the English mineral collector and dealer — one of the few species named for a dealer rather than a mineralogist, which is a piece of trade history worth knowing.