Which minerals are zeolites
Zeolites are open tetrahedral frameworks holding loose water; the IMA accepted 82 in 1997. 9 of 176 Handbook sheets say Zeolite group. Prehnite is not one.
Which minerals are zeolites? Those with an open framework of linked tetrahedra whose channels usually hold water, lost mostly below about 400 C and largely regained: the IMA definition of 1997, which accepted 82 species. Nine of the 176 Handbook sheets in this site's reference set say Zeolite group, including scolecite, hardness 5 to 5.5. Prehnite, apophyllite and okenite are not zeolites.
In short
- A zeolite is defined by structure, not by where it grows or how much water it holds. The IMA zeolite report of 1997 defines one as a crystalline substance with a framework of linked tetrahedra containing open channels and cages, usually occupied by water and exchangeable cations, with dehydration mostly below about 400 C and largely reversible. It accepted 82 species.
- Nine of the 176 Handbook sheets in this site's reference set carry the line Mineral Group: Zeolite group: analcime, barrerite, gonnardite, heulandite, mesolite, natrolite, scolecite, stellerite and stilbite. All nine are accepted in the 1997 report. Their measured densities run from 2.10 to 2.36.
- Is prehnite a zeolite? No. Its sheet has no zeolite group line, the 1997 report does not list it, and it sits outside the zeolites' range on both numbers a collector can measure: hardness 6 to 6.5 and density 2.80 to 2.95.
- Is apophyllite a zeolite? Is okenite? No, and water is not the test. Fluorapophyllite is 15.89% water by mass and okenite 16.49%, more than analcime at 8.18% or natrolite at 9.48%. Neither is in the 1997 report, and the reference cited on okenite's sheet calls it a chain and sheet silicate.
- The confusion is about company. Six sheets that are not zeolites list zeolites among their associates, prehnite and both apophyllites among them. Their sheets and the zeolite sheets alike place them in cavities in basalts, which is how they end up filed together.
| Species | Formula as the sheet prints it | Zeolite? | Water by mass, calculated | Hardness | Density, measured | IMA list entry |
|---|---|---|---|---|---|---|
| Analcime | NaAlSi2O6·H2O | Yes: Zeolite group | 8.18% | 5 to 5.5 | 2.24 to 2.29 | Analcime, 1997 s.p. |
| Natrolite | Na2Al2Si3O10·2H2O | Yes: Zeolite group | 9.48% | 5 to 5.5 | 2.20 to 2.26 | Natrolite, 1997 s.p. |
| Mesolite | Na2Ca2Al6Si9O30·8H2O | Yes: Zeolite group | 12.37% | 5 | 2.26 | Mesolite, 1997 s.p. |
| Scolecite | CaAl2Si3O10·3H2O | Yes: Zeolite group | 13.78% | 5 to 5.5 | 2.25 to 2.29 | Scolecite, 1997 s.p. |
| Gonnardite | Na2CaAl4Si6O20·7H2O | Yes: Zeolite group | 15.60% | 5 | 2.25 to 2.36 | Gonnardite, redefined 1997 s.p. |
| Heulandite | (Ca,Na2)Al2Si7O18·6H2O | Yes: Zeolite group | 15.74% (Ca end) | 3.5 to 4 | 2.10 to 2.20 | A series: heulandite-Ca, renamed 1997 s.p., and others |
| Stilbite | NaCa2Al5Si13O36·14H2O | Yes: Zeolite group | 17.62% | 3.5 to 4 | 2.19 | Stilbite-Ca, 1997 s.p. |
| Barrerite | (Na,K,Ca)2Al2Si7O18·7H2O | Yes: Zeolite group | 17.75% (Na end) | Not determined | 2.13 | Barrerite, IMA 1974-017 |
| Stellerite | CaAl2Si7O18·7H2O | Yes: Zeolite group | 17.89% | Not determined | 2.13 | Stellerite, 1997 s.p. |
| Prehnite | Ca2Al2Si3O10(OH)2 | No | 4.37%, as OH only | 6 to 6.5 | 2.80 to 2.95 | Prehnite, grandfathered, 1788 |
| Fluorapophyllite | KCa4Si8O20(F,OH)·8H2O | No | 15.89% (F end) | 4.5 to 5 | 2.33 to 2.37 | Fluorapophyllite-(K), renamed 1978 s.p. |
| Okenite (sheet outside the set) | Ca5Si9O23·9H2O | No | 16.49% | Not quoted | 2.28 to 2.33 | Okenite, grandfathered, 1828 |
Which minerals are zeolites? The definition, and the nine sheets that meet it
The definition is the IMA's, adopted in 1997. A zeolite mineral is a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation, containing open channels and cages that are usually occupied by water and exchangeable cations; the channels are large enough to let guest species pass, and dehydration occurs mostly below about 400 C and is largely reversible. The report explicitly excludes frameworks whose channels are too tight for that behaviour, naming feldspars, nepheline, sodalites and scapolites. It lists 82 accepted zeolite species in 13 compositional series.
Method for this site's count. The 176 Handbook of Mineralogy sheets in the reference set were converted with pdftotext -layout and searched for the Handbook's own Mineral Group line. Nine read Zeolite group: analcime, barrerite, gonnardite, heulandite, mesolite, natrolite, scolecite, stellerite and stilbite. All nine are accepted in the 1997 report, heulandite and stilbite as series whose members carry a suffix, so the two sources agree on the set. On the September 2026 IMA-CNMNC list of minerals, eight carry the date 1997 s.p., the special-procedure year of that report's redefinitions and renamings; barrerite carries its approval number, 1974-017.
What the nine share that a collector can measure: low density. Their measured densities run from 2.10, heulandite, to 2.36, gonnardite. That is a range across all nine, not a sample, and it is the first number to check on a white crystal from a basalt cavity. Several names on labels are now series, so a complete name carries a suffix: stilbite-Ca, heulandite-Ca. The heulandite page explains why most labels omit it.
Is prehnite a zeolite? No, and two measurements show it
Prehnite is not a zeolite: hardness 6 to 6.5 and density 2.80 to 2.95, against 2.10 to 2.36 for the nine zeolite sheets. Its sheet has no zeolite group line, and the 1997 report does not list it among the 82; the word prehnite appears in that report only inside the title of a reference on metamorphism. Its formula, Ca2Al2Si3O10(OH)2, carries its hydrogen as hydroxyl rather than molecular water, 4.37% by mass expressed as H2O.
Why the question gets asked at all: prehnite's sheet lists its associates as Zeolites, datolite, pectolite, calcite, epidote, copper, and its occurrence as veins and cavities in mafic volcanic rocks. It grows beside zeolites in the same cavities. The decision rule, stated once: a green or white cavity mineral that resists a steel point and measures above 2.8 is not one of these nine zeolites. The density method is on how to measure specific gravity at home, the hardness method on how to test mineral hardness properly, and the species on the prehnite page.
Is apophyllite a zeolite, or okenite? Water content is not the test
Apophyllite and okenite hold as much water as a zeolite and are still not zeolites. Calculated from the sheets' formulas with standard atomic weights: fluorapophyllite, KCa4Si8O20F·8H2O, is 15.89% water by mass; okenite, Ca5Si9O23·9H2O, is 16.49%. Both figures match the pure-formula columns printed on their sheets. Analcime is 8.18% and natrolite 9.48%. If water decided it, apophyllite would be more of a zeolite than analcime; the IMA definition decides it on structure instead.
Neither name appears anywhere in the 1997 report. Okenite's Handbook sheet is outside this site's 176-sheet set, fetched as a comparator; among its references is Merlino's 1983 paper titled Okenite, Ca10Si18O46·18H2O: the first example of a chain and sheet silicate. Fluorapophyllite and hydroxyapophyllite are in the set, with no zeolite group line, and both list Zeolites first among their associates. On the IMA list the apophyllites carry a 1978 renaming date and okenite is grandfathered from 1828.
In the hand, apophyllite is tetragonal, often pseudocubic, with a perfect {001} cleavage and a pearly lustre on it; okenite is recorded as bladed to typically fibrous crystals, elastic, and as compact radial fibrous balls to 4 cm. The species page for apophyllite covers its naming. For fibrous white material, a confident label needs more than a hand lens, and a label reading zeolite without a species name is not a wrong label, just an incomplete one. The group pages are indexed at the collecting guides; nothing on this site is offered for sale, and the wanted list is the only commercial route.