Stilbite vs Heulandite: Telling Two Peach Zeolites Apart

Short answer: shape decides it. Stilbite almost never grows as clean single crystals; it bundles into sheaves, bow ties and fans that pinch in the middle. Heulandite grows as separate, flat crystals shaped like an old-fashioned coffin: wide in the center, tapering to the ends, with a flat top. Color, luster and hardness overlap so much that labels are often wrong, and a mislabeled peach zeolite is the most common error in Indian specimen listings.

Why are these two so easy to mix up?

Put a peach stilbite and a peach heulandite side by side and the list of shared traits is long. Both are calcium-sodium aluminosilicate zeolites with water locked in open channels. Both are monoclinic. Both have one perfect cleavage, and on both that cleavage has a pearly shine. Both come in white, cream, salmon, peach, pink and brick red, and both sit at roughly Mohs 3.5 to 4. Minerals.net's identification notes for each mineral say the same thing about the other: different crystal shape, otherwise difficult to distinguish (Minerals.net, stilbite).

They also grow together. Heulandite, stilbite and apophyllite are standard companions in Indian basalt pockets, and a single specimen can carry two generations of peach crystals that came from different minerals. The Deccan Traps guide explains how those pockets fill; this page sticks to separating the two.

Stilbite and heulandite side by side

Data from Webmineral, stilbite-Ca, Webmineral, heulandite-Ca and Minerals.net, heulandite.

What tests separate them?

None of these need a lab. Work through them in order and stop when one gives a clear answer.

The silhouette test

Find a crystal that sticks out from the group and look at its outline edge-on. A stilbite "crystal" is usually a bundle: dozens of thin blades stacked and fanned, so the outline flares at both ends and narrows at the waist, which is where the bow tie and wheat sheaf names come from. Webmineral lists the wheat sheaf habit as bundle-shaped aggregates resembling hand-reaped wheat. Heulandite outlines are single plates that bulge in the middle and taper toward each end, the coffin shape Minerals.net describes as a wider center with narrower edges and flat top and bottom.

The waist test

Look at the middle of each crystal from the side. A pinched waist with splayed ends means stilbite. A thicker middle with thinner ends means heulandite. This works even when the color is identical and the crystals are only a few millimeters long.

The cleavage-face test

Both minerals have a pearly cleavage face, but where it sits differs. On heulandite the large, flat face of each coffin is the pearly cleavage, so whole crystals flash like mica plates as you rock the specimen. Stilbite sheaves also show their sheen on the broad face, but it is rippled and broken into strips, because the bundle is made of many slightly splayed blades rather than one plate.

The single-light test

In a dark room, shine one small flashlight across the specimen at a low angle. Stilbite sheaves light up as a fibrous, silky band that moves as you turn the piece. Heulandite plates light up one at a time, each as a single bright mirror.

UV, if you have it

Minerals.net notes that stilbite is occasionally fluorescent yellow in UV light. A yellow response under a UV lamp leans toward stilbite, but no response proves nothing, because many specimens of both are inert.

Optics, for the curious

A polarizing microscope or a thin cleavage flake on a refractometer will show stilbite as biaxial negative and heulandite as biaxial positive, per Webmineral data. Few collectors go this far, but it is the definitive amateur test.

Where does each come from?

Minerals.net calls Maharashtra, India the source of the largest and most significant stilbite finds, naming Pune, Jalgaon, Nasik, Aurangabad, Mumbai and Ahmadnagar. Heulandite comes from the same districts, and the Shakur Quarry in Nasik is known for dark green heulandite colored by inclusions of celadonite. Green is a useful clue: green stilbite is uncommon, while green heulandite from Nasik is a recognized trade item.

Outside India the classic localities overlap almost completely. Teigarhorn in Iceland, Paterson and Prospect Park in New Jersey, and the Bay of Fundy shore of Nova Scotia produced fine examples of both. A few spots favor one species: Malmberget in Sweden is known for brownish-yellow stilbite plates, and the Rats Nest claim near Challis, Idaho, for pink to peach heulandite with mordenite.

Both are common zeolites. The USGS lists heulandite among the most common of the roughly 40 natural zeolites identified (USGS, zeolites), alongside mordenite and others; stilbite is equally widespread in basalt.

Do the names stilbite-Ca and heulandite-Ca matter?

In 1997 the zeolite subcommittee of the International Mineralogical Association split each mineral into a series by dominant cation. Stilbite became stilbite-Ca and stilbite-Na; heulandite became heulandite-Ca, -Na, -K and -Sr, with heulandite-Ba added in 2002. Minerals.net notes that almost all specimens in collections are the calcium members, and that the split is rarely made on labels. A label reading simply "stilbite" or "heulandite" is normal and acceptable; a label that adds "-Na" without analysis is guessing.

Heulandite also shares its framework with clinoptilolite, a higher-silica relative. Clinoptilolite is the zeolite mined in bulk for industry, but collector crystals labeled heulandite are not industrial material.

Which one is worth more?

Neither species is rare, so price follows aesthetics. For stilbite, buyers pay for large, sharp bow ties with strong peach or salmon color and for contrast with a companion mineral such as green apophyllite. For heulandite, the premium pieces are large, lustrous single coffins, red crystals and the green Nasik material. Combination pieces, such as apophyllite cubes on stilbite on a heulandite base, usually sell for more than any one of their parts.

Mislabeling cuts both ways. A specimen sold as "pink stilbite" that is really heulandite is not worth less, but it is a sign that the seller did not look closely, and that matters for every other claim on the label. In crystal traditions stilbite is associated with creativity and heulandite with calm; neither meaning should drive a price.

How should you handle either one?

Both are soft, both cleave in one direction, and both hold water in their structures. Webmineral lists a synchrotron study of stilbite dehydration, a reminder that heat changes these minerals. Keep them out of direct sun and away from heat sources, and dust them with a soft brush or a blower. The apophyllite and zeolite care guide covers cleaning and transport.

Choosing stilbite or heulandite specimens

Before buying, look for one photo that shows a crystal in profile; without it, nobody can confirm which mineral it is. Check that the label names a locality, and look at the tips of sheaves and the edges of coffins, because both chip easily and damage there is the most common hidden flaw.

• Yellow Orange Heulandite on Mordenite, 4 kg: a cabinet piece where the heulandite coffins are large enough to run every test above.
• Green Apophyllite Cubes with Peach Stilbite on Heulandite Specimen 556g: both zeolites on one base, ideal for learning the bow tie versus coffin difference.
• Green Apophyllite Cubes with Peach Stilbite Specimen 322g: a smaller, lower-priced example of the classic peach stilbite and green apophyllite contrast.
• Marshy Stilbite Cluster from India: stilbite on its own, useful for seeing the sheaf habit without companion minerals.
• Red Mordenite with Stilbite Specimen 355g: stilbite paired with tufted red mordenite, a third zeolite that is easy to tell apart by its fibers. The International Gem Society notes that mordenite's color is due to staining, so treat the red as surface color.

Frequently asked questions

Is heulandite just a type of stilbite?

No. They are separate zeolite species with different frameworks and formulas, although both are calcium-sodium aluminosilicates with water in their structures. They share cleavage, luster, color range and hardness, which is why they get confused, but heulandite's coffin-shaped crystals and stilbite's bow tie bundles reflect real structural differences.

Can you tell stilbite from heulandite by color?

Not reliably. Both run from white to peach, salmon, pink and red. Green is the one useful color clue, since dark green heulandite colored by celadonite comes from the Nasik district of India, while green stilbite is uncommon. For everything else, use crystal shape.

What does a stilbite bow tie look like?

A bundle of thin, bladed crystals stacked side by side, pinched in the middle and flared at both ends, like a bow tie or a sheaf of cut wheat tied at the waist. Single bow ties can reach several centimeters on Indian specimens, and they often grow scattered across green or clear apophyllite.

Is stilbite or heulandite fragile?

Both are fragile. Each sits around Mohs 3.5 to 4 with one perfect cleavage, so edges chip and plates split if knocked. Lift specimens by the base, never by the crystals, and keep them away from heat, which can drive water out of their structures and dull or crack them.

Why do some labels say stilbite-Ca?

Since 1997 the official names include the dominant cation, so stilbite-Ca and heulandite-Ca are the calcium-rich members. Nearly all specimens in collections are the calcium members, so most dealers and collectors simply write stilbite or heulandite. Both forms of the label are correct.

Sources

• Minerals.net, Stilbite: https://www.minerals.net/mineral/stilbite.aspx
• Minerals.net, Heulandite: https://www.minerals.net/mineral/heulandite.aspx
• Webmineral, Stilbite-Ca Mineral Data: http://webmineral.com/data/Stilbite-Ca.shtml
• Webmineral, Heulandite-Ca Mineral Data: http://webmineral.com/data/Heulandite-Ca.shtml
• U.S. Geological Survey, Zeolites Statistics and Information: https://www.usgs.gov/centers/national-minerals-information-center/zeolites-statistics-and-information
• International Gem Society, Mordenite Value, Price, and Jewelry Information: https://www.gemsociety.org/article/mordenite-jewelry-and-gemstone-information/