Is apophyllite a zeolite
No. A zeolite has an open framework of linked tetrahedra (IMA, 1997). Apophyllite is a group of six species that grows beside zeolites in basalt cavities.
No. Apophyllite is not a zeolite. The 1997 IMA zeolite report defines a zeolite by a framework of linked tetrahedra with open channels. It does not list apophyllite, and the fluorapophyllite sheet cites the sheet-silicates volume of Rock-forming minerals. Apophyllite is a group of six IMA species, hardness 4.5 to 5, density 2.33 to 2.37, that grows with zeolites in basalt.
In short
- A zeolite is defined by its framework. The IMA zeolite report of 1997 requires a framework of linked tetrahedra containing open channels and cages, usually holding water and exchangeable cations. The report does not mention apophyllite, and the site's which minerals are zeolites page lists the nine zeolite sheets in the set.
- The framework test, from the sheets' own references. Of the 9 zeolite-group sheets in this site's 176-sheet reference set, 6 cite volume 4 of Deer, Howie and Zussman's Rock-forming minerals, framework silicates, and none cites volume 3, sheet silicates. The fluorapophyllite sheet cites volume 3.
- Apophyllite is a group, not a species. The IMA list carries six species: fluorapophyllite-(K), -(Na), -(Cs) and -(NH4), and hydroxyapophyllite-(K) and -(NH4). The fluorapophyllite sheet says distinguishing it from hydroxyapophyllite requires chemical analysis. The apophyllite species page covers the group.
- They are filed together because they grow together. Both apophyllite sheets list zeolites first among their associates, and both place apophyllite in amygdules or druses in basalts. Of the 9 zeolite sheets, heulandite is the one that names apophyllite.
- Hardness does not separate them; density usually does. Five of the seven zeolite sheets that print a hardness overlap apophyllite's 4.5 to 5. Eight of the nine print a measured density below 2.33; only gonnardite, at 2.25 to 2.36, overlaps.
| Sheet | Group on the sheet | Rock-forming minerals volume cited | Hardness | Density, measured | Names the other in Association? |
|---|---|---|---|---|---|
| Fluorapophyllite | No group line; series with hydroxyapophyllite | v. 3, sheet silicates | 4.5 to 5 | 2.33 to 2.37 | Yes: zeolites |
| Hydroxyapophyllite | No group line; series with fluorapophyllite | None cited | 4.5 to 5 | 2.37 | Yes: zeolites |
| Analcime | Zeolite group | v. 4, framework silicates | 5 to 5.5 | 2.24 to 2.29 | No |
| Barrerite | Zeolite group | None cited | Not determined | 2.13 | No |
| Gonnardite | Zeolite group | None cited | 5 | 2.25 to 2.36 | No |
| Heulandite | Zeolite group | v. 4, framework silicates | 3.5 to 4 | 2.10 to 2.20 | Yes: apophyllite |
| Mesolite | Zeolite group | v. 4, framework silicates | 5 | 2.26 | No |
| Natrolite | Zeolite group | v. 4, framework silicates | 5 to 5.5 | 2.20 to 2.26 | No |
| Scolecite | Zeolite group | v. 4, framework silicates | 5 to 5.5 | 2.25 to 2.29 | No |
| Stellerite | Zeolite group | None cited | Not determined | 2.13 | No |
| Stilbite-Ca | Zeolite group | v. 4, framework silicates | 3.5 to 4 | 2.19 | No |
Is apophyllite a zeolite? What a zeolite is, in one sentence
A zeolite is a mineral built on a framework of linked tetrahedra with open channels and cages, and apophyllite is not one. The IMA zeolite report (Coombs et al., Canadian Mineralogist 35, 1997) defines a zeolite mineral as a crystalline substance whose structure is characterised by a framework of linked tetrahedra, each of four oxygen atoms around a cation. The framework contains open cavities, channels and cages, usually occupied by water and extra-framework cations that are commonly exchangeable. Dehydration occurs mostly below about 400°C and is largely reversible. The report does not mention apophyllite anywhere in its text.
The framework test, stated once: if the structure is not a framework with open channels, the mineral is not a zeolite, however much water it holds and whatever it grows on. Water content is not the test; that argument, with the numbers, is on which minerals are zeolites.
Where the Handbook sheets file apophyllite: 6 of 9 against 1
The Handbook sheets file the zeolites with the framework silicates and fluorapophyllite with the sheet silicates. Each sheet's reference list cites the standard text, Deer, Howie and Zussman's Rock-forming minerals, by volume, and the volume title is the structural class. Method: the 176 sheets were converted with pdftotext -layout and searched by script for the citation and its volume title.
- 14 of 176 cite framework silicates: six of the nine zeolites (analcime, heulandite, mesolite, natrolite, scolecite, stilbite-Ca), the six feldspars, and marialite and meionite.
- 13 of 176 cite sheet silicates, among them fluorapophyllite, prehnite, the micas, talc and vermiculite. None of the nine zeolites is among them.
- Barrerite, gonnardite, stellerite and the hydroxyapophyllite sheet cite neither volume.
The fluorapophyllite sheet's name field adds a structural hint of its own: the name is from the Greek for away from and leaf, for the way it exfoliates on heating. Three of the 176 sheets use the word exfoliat, and the other two are pyrophyllite and vermiculite, both sheet silicates. A look-alike, gyrolite, is compared on its own page.
Why apophyllite gets called a zeolite: it grows with them
Apophyllite and the zeolites share cavities, not structure. The fluorapophyllite sheet gives its occurrence as a secondary mineral in amygdules or druses in basalts, and both apophyllite sheets list zeolites first among their associates. Of the 9 zeolite sheets in the set, 1 names apophyllite among its associates: heulandite. A basalt-cavity plate with apophyllite on stilbite or heulandite genuinely carries zeolites, and a dealer who files it under zeolites is describing the plate, not the apophyllite.
Separating the two on a plate. Hardness will not do it: five of the seven zeolite sheets that print a hardness overlap apophyllite's 4.5 to 5. Density usually will on a clean crystal. Eight of the nine zeolite sheets print a measured density below 2.33. A 20.00 g apophyllite weighs 11.42 to 11.56 g in water, against 10.87 g for stilbite-Ca at 2.19 and 10.48 g for heulandite at its 2.10 low end. Gonnardite, at 2.25 to 2.36, overlaps and cannot be separated this way. Shape helps: apophyllite is tetragonal, commonly pseudocubic, with a perfect {001} cleavage and a pearly lustre on {001}. The method is on how to measure specific gravity at home.
When the answer changes: old labels, group names and industry
- An old label reading just apophyllite. Both sheets cite the 1978 nomenclature revisions in the apophyllite group, and the IMA-CNMNC list of minerals now lists six species. The fluorapophyllite sheet says distinguishing it from hydroxyapophyllite requires chemical analysis, so apophyllite on a label is correct as a group name and a species name is a presumption without analysis.
- A label reading zeolite on a mixed plate. Correct for the stilbite, heulandite or scolecite on it; not for the apophyllite.
- Industrial and synthetic use. The 1997 report says its definition concerns naturally occurring minerals, irrespective of how the term may be applied to synthetic materials and in industry. A product sold as zeolite is outside this question.
More classification questions are indexed at the collecting guides and the species in the species index. Nothing here is offered for sale; the wanted list is the only commercial route.