Is prehnite a zeolite?
Prehnite is not a zeolite. Its IMA formula groups (Si3Al)O10, 2.5 oxygens per tetrahedral atom, as in muscovite; all 13 zeolite rows tabled here give 2.0.
No. Prehnite is not a zeolite. The 1997 IMA zeolite report defines a zeolite by a framework of linked tetrahedra with open channels, and does not accept prehnite. The IMA list writes prehnite Ca2Al(Si3Al)O10(OH)2: 2.5 oxygens per tetrahedral atom, as in muscovite, where the 13 zeolite rows tabled give 2.0. Prehnite is also hardness 6 to 6.5.
In short
- The definition. The IMA zeolite report (Coombs et al. 1997) defines a zeolite as a crystalline substance with a framework of linked tetrahedra, each of four oxygen atoms around a cation, containing open channels and cages. Prehnite is not among the species it accepts; the word appears in it only inside the title of a cited paper.
- The oxygen-count test. The 1997 report quotes Hey’s 1930 requirement that O : (Si + Al) = 2. Take the bracketed group the September 2026 IMA-CNMNC Master List writes for each mineral and divide its oxygens by its silicon plus aluminium. All 13 zeolite rows in the table give exactly 2.0. Prehnite’s (Si3Al)O10 gives 10 / 4 = 2.5.
- 2.5 is muscovite’s number. The IMA list writes muscovite KAl2(Si3Al)O10(OH)2, the same bracketed group as prehnite. Fluorapophyllite-(K), (Si8O20), and gyrolite, (Si23Al)O60, also give 2.5. In a framework every tetrahedron corner is shared, 4 × ½ = 2 oxygens each; at 2.5, one corner in four is on average unshared.
- Documented failures. Count both of prehnite’s aluminium atoms, as a bare Handbook formula Ca2Al2Si3O10(OH)2 invites, and the ratio is 10 / 5 = 2.0, the zeolite figure. The test needs the IMA’s structural grouping. And albite, Na(AlSi3O8), gives 2.0 but is a feldspar, which the 1997 report excludes: 2.0 is not sufficient on its own.
- In the hand. Prehnite’s sheet gives hardness 6 to 6.5 and density 2.80 to 2.95; a 20.00 g piece reads 12.86 to 13.22 g in water, against 10.91 to 11.15 g for natrolite. The density boundary for all the zeolites is on which minerals are zeolites and is not repeated here.
| Mineral (IMA name) | IMA formula | Bracketed group | Oxygens | Si + Al in group | O : (Si + Al) | IMA status, year | 1997 zeolite report |
|---|---|---|---|---|---|---|---|
| Prehnite | Ca2Al(Si3Al)O10(OH)2 | (Si3Al)O10 | 10 | 4 | 2.5 | G, 1788 | Not accepted; named only in a reference title |
| Muscovite | KAl2(Si3Al)O10(OH)2 | (Si3Al)O10 | 10 | 4 | 2.5 | A, 1998 s.p. | Not mentioned |
| Fluorapophyllite-(K) | KCa4(Si8O20)F(H2O)8 | (Si8O20) | 20 | 8 | 2.5 | Rn, 1978 s.p. | Not mentioned |
| Gyrolite | NaCa16(Si23Al)O60(OH)8(H2O)14 | (Si23Al)O60 | 60 | 24 | 2.5 | G, 1851 | Not mentioned |
| Albite | Na(AlSi3O8) | (AlSi3O8) | 8 | 4 | 2.0 | G, 1815 | Feldspars excluded: channels too restricted |
| Natrolite | Na2(Si3Al2)O10·2H2O | (Si3Al2)O10 | 10 | 5 | 2.0 | A, 1997 s.p. | Zeolite |
| Mesolite | Na2Ca2(Si9Al6)O30·8H2O | (Si9Al6)O30 | 30 | 15 | 2.0 | A, 1997 s.p. | Zeolite |
| Scolecite | Ca(Si3Al2)O10·3H2O | (Si3Al2)O10 | 10 | 5 | 2.0 | A, 1997 s.p. | Zeolite |
| Gonnardite | (Na,Ca)2(Si,Al)5O10·3H2O | (Si,Al)5O10 | 10 | 5 | 2.0 | Rd, 1997 s.p. | Zeolite |
| Paranatrolite | Na2(Si3Al2)O10·3H2O | (Si3Al2)O10 | 10 | 5 | 2.0 | A, 1978-017 | Doubtful status in 1997 |
| Thomsonite-Ca | NaCa2(Al5Si5)O20·6H2O | (Al5Si5)O20 | 20 | 10 | 2.0 | Rn, 1997 s.p. | Zeolite (as thomsonite) |
| Analcime | Na(AlSi2O6)·H2O | (AlSi2O6) | 6 | 3 | 2.0 | A, 1997 s.p. | Zeolite |
| Leucite | K(AlSi2O6) | (AlSi2O6) | 6 | 3 | 2.0 | A, 1997 s.p. | Included in the list of zeolites |
| Stilbite-Ca | NaCa4(Si27Al9)O72·28H2O | (Si27Al9)O72 | 72 | 36 | 2.0 | A, 1997 s.p. | Zeolite |
| Heulandite-Ca | (Ca,Na,K)5(Si27Al9)O72·26H2O | (Si27Al9)O72 | 72 | 36 | 2.0 | Rn, 1997 s.p. | Zeolite |
| Stellerite | Ca4(Si28Al8)O72·28H2O | (Si28Al8)O72 | 72 | 36 | 2.0 | A, 1997 s.p. | Zeolite |
| Barrerite | Na2(Si7Al2)O18·6H2O | (Si7Al2)O18 | 18 | 9 | 2.0 | A, 1974-017 | Zeolite |
| Chabazite-Ca | Ca2[Al4Si8O24]·13H2O | [Al4Si8O24] | 24 | 12 | 2.0 | A, 1997 s.p. | Zeolite |
Is prehnite a zeolite? The definition in one sentence
A zeolite is a mineral built on a framework of linked tetrahedra with open channels and cages, and prehnite is not one. The Coombs et al. 1997, Recommended nomenclature for zeolite minerals (Canadian Mineralogist 35) defines a zeolite mineral as a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation; the framework contains open cavities in the form of channels and cages, usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. Prehnite is not in its list of accepted species. Read by text search, the word occurs once in the report, inside the title of a 1987 paper by Cho and Liou in its reference list.
The species. The September 2026 IMA-CNMNC Master List gives prehnite as Ca2Al(Si3Al)O10(OH)2, grandfathered, 1788, South Africa. The Handbook of Mineralogy prehnite sheet gives Ca2Al2Si3O10(OH)2, orthorhombic, hardness 6 to 6.5, measured density 2.80 to 2.95, good {001} cleavage, light to dark green, white, yellow, grey or pink, and names it for Colonel Hendrik von Prehn (1733–1785), who discovered the mineral. Its species page is prehnite.
The boundary. The report also says what is not a zeolite: framework structures such as feldspars, nepheline, sodalites and scapolites, whose channels are too restricted for typical zeolitic behaviour. And it allows one relaxation: the framework may be interrupted by (OH,F) groups at a tetrahedron apex not shared with neighbouring tetrahedra, as in parthéite. Prehnite is neither listed under that clause nor accepted under any other.
Why prehnite is not a zeolite: the oxygen-count test
The arithmetic of a framework. A tetrahedron has four corner oxygens. In a framework every corner is shared with a neighbouring tetrahedron, so each oxygen counts half: 4 × ½ = 2.0 oxygens per tetrahedral atom. That is the requirement of Hey (1930), quoted in the 1997 report as O : (Si + Al) = 2. If one corner in four is not shared, the count is 3 × ½ + 1 = 2.5.
Method. The IMA list writes many silicates with the tetrahedral group in brackets: natrolite Na2(Si3Al2)O10, chabazite-Ca [Al4Si8O24]. A script took the bracketed group for each of 18 named rows of the September 2026 list (pdftotext -layout), counted its oxygens and its silicon plus aluminium, and divided. All 13 zeolite rows give exactly 2.0: natrolite, mesolite, scolecite, gonnardite, paranatrolite, thomsonite-Ca, analcime, leucite, stilbite-Ca, heulandite-Ca, stellerite, barrerite and chabazite-Ca. Prehnite, Ca2Al(Si3Al)O10(OH)2, gives 10 / 4 = 2.5.
Its company at 2.5. Muscovite, KAl2(Si3Al)O10(OH)2 on the same list, has the identical bracketed group, and fluorapophyllite-(K) and gyrolite also give 2.5. Neither apophyllite nor gyrolite is a zeolite either; see is apophyllite a zeolite and gyrolite. On the IMA list’s own grouping, prehnite counts like a mica, not like natrolite. The apophyllite page’s separate evidence, which reference volume each Handbook sheet cites, is linked rather than repeated.
When the oxygen count misleads: two documented failures
Failure one: the bare formula. The Handbook writes prehnite Ca2Al2Si3O10(OH)2, with no brackets. Count every silicon and aluminium against the ten oxygens and the answer is 10 / 5 = 2.0, the zeolite figure. The IMA list puts one aluminium outside the bracket. That placement is structural information (the prehnite sheet cites a 1990 crystal-structure study); it cannot be read off the bare formula. So the test is only as good as the grouping it is fed: worked on a formula without brackets, it cannot tell prehnite from natrolite.
Failure two: 2.0 is not enough. Albite, Na(AlSi3O8) on the IMA list, gives 8 / 4 = 2.0, and it is a feldspar, which the 1997 report names among frameworks that are not zeolites because any channels are too restricted. Leucite, K(AlSi2O6), also gives 2.0, and the report does include it in its list of zeolites. A ratio of 2.0 says framework; it does not say open channels. A ratio of 2.5 rules out the framework the definition starts from, which is the narrower claim this page makes. Feldspars are separated in the hand on how to identify feldspar.
The rule, stated once. Read the IMA list’s bracketed group, divide oxygens by silicon plus aluminium: above 2 by a clear margin, as prehnite’s 2.5 is, the mineral is not a zeolite framework; at 2.0, check the 1997 report’s list, because feldspars sit there too. The hand tests are on how to test mineral hardness properly and how to measure specific gravity at home.
Why prehnite is filed with the zeolites: when the answer changes
It grows with them. The prehnite sheet places it in veins and cavities in mafic volcanic rocks, as a secondary or hydrothermal mineral, and names its associates as zeolites, datolite, pectolite, calcite, epidote and copper. Its localities include Paterson and Bergen Hill, New Jersey, the Khandivali quarry, Bombay, and the Jeffrey mine, Asbestos, Quebec. A basalt-cavity plate carrying prehnite with stilbite or heulandite genuinely carries zeolites, and a label reading zeolites describes the plate, not the prehnite.
In the hand. Hardness 6 to 6.5 and density 2.80 to 2.95, per the sheet: a 20.00 g piece reads 12.86 to 13.22 g in water, against 10.91 to 11.15 g for natrolite. Those numbers separate prehnite from the zeolites on the plate. Weight does not separate it from pectolite, 2.84 to 2.90 and 12.96 to 13.10 g, and leaves only 0.02 g to datolite, 2.96 to 3.00 and 13.24 to 13.33 g, both non-zeolites from the same cavities; hardness does, at 6 to 6.5 against pectolite’s 4.5 to 5 and datolite’s 5 to 5.5, per their Handbook sheets.
Industrial use of the word. The 1997 report says zeolites in its sense are 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. The classification questions are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.