Analcime
Analcime is NaAlSi2O6·H2O, hardness 5 to 5.5 and density 2.24 to 2.29. Cubic trapezohedra and almost no cleavage separate it from every other zeolite here.
Analcime is a sodium aluminosilicate, NaAlSi2O6·H2O, hardness 5 to 5.5 and density 2.24 to 2.29. A specimen crystallises as trapezohedra to 25 cm on matrix, with very poor {100} cleavage. Of the 109 species sheets in this site's reference set, 15 record no usable cleavage, and analcime is the only zeolite among them.
In short
- Every other zeolite this site covers cleaves perfectly; analcime does not. Natrolite, scolecite and mesolite are perfect on {110}; heulandite, stilbite and stellerite are perfect on {010}. Analcime is very poor on {100}, which in practice means you will not see a cleavage at all.
- The trapezohedron is the identification. Crystals are commonly {211} trapezohedra to 25 cm — an equant, many-faced form that none of the fibrous or platy zeolites approaches. Shape carries this species in a way it carries almost no other.
- Hardness 5 to 5.5 puts it at the top of the zeolite range. Heulandite and stilbite are 3.5 to 4; natrolite, scolecite and mesolite are 5 to 5.5. A steel point will not mark analcime cleanly and will mark heulandite easily.
- It forms a series with pollucite, the caesium aluminosilicate, which is the same framework with caesium in place of sodium. That is a chemical relationship a collector will never see in a hand specimen, and it is why analcime analyses sometimes carry caesium.
- It is weakly piezoelectric and weakly electrostatic when rubbed or heated — the property the name records. Analcime is from the Greek for weak, for exactly that feeble charge.
| Species | Formula | Hardness | Density | Cleavage | Habit |
|---|---|---|---|---|---|
| Analcime | NaAlSi2O6·H2O | 5 to 5.5 | 2.24 to 2.29 | Very poor on {100} | Trapezohedra, equant, to 25 cm |
| Natrolite | Na2Al2Si3O10·2H2O | 5 to 5.5 | 2.20 to 2.26 | Perfect on {110} | Acicular to fibrous sprays |
| Scolecite | CaAl2Si3O10·3H2O | 5 to 5.5 | 2.25 to 2.29 | Perfect on {110} | Prismatic, often twinned |
| Mesolite | Na2Ca2Al6Si9O30·8H2O | 5 | 2.26 | Perfect on {110} | Very fine acicular, hair-like |
| Heulandite | (Ca,Na2)Al2Si7O18·6H2O | 3.5 to 4 | 2.10 to 2.20 | Perfect on {010} | Coffin-shaped tabular plates |
| Stilbite | NaCa2Al5Si13O36·14H2O | 3.5 to 4 | 2.19 | Perfect on {010} | Sheaf-like bundles |
| Stellerite | CaAl2Si7O18·7H2O | Not determined | 2.13 | Perfect on {010} | Tabular, often in fans |
A survey of the reference set: 15 species with no usable cleavage, and one zeolite among them
Method. On 20 September 2026 the Handbook of Mineralogy sheet for every species this site covers was converted to text and the Cleavage field extracted by regular expression. That gives 109 sheets after removing duplicates, mirror copies and non-species files. Each was classed on the word the Handbook itself uses.
The result: 57 of 109 record perfect cleavage, 9 record good, 15 record none, poor, indistinct or not observed, and 28 record something in between — distinct, in traces, interrupted, or no cleavage field at all.
The fifteen with nothing usable are analcime, benitoite, chalcocite, chalcopyrite, crocoite, olivenite, phosgenite, pyrite, quartz, scheelite, schorl, seligmannite, vesuvianite, willemite and wulfenite. That list contains exactly one zeolite.
This matters because zeolites are hard to tell apart and cleavage is one of the few properties that genuinely separates them. Natrolite, scolecite and mesolite cleave perfectly on {110}; heulandite, stilbite and stellerite perfectly on {010}. Analcime alone gives you nothing to split — and that absence, combined with the trapezohedral habit, is as close to a single-property determination as this group allows. The same survey underwrites how to split rocks in the field.
Why the shape does the work here and almost nowhere else
Identification by crystal form is usually bad practice. Habit varies with the conditions of growth, most species show several forms, and a collector who identifies on shape alone will be wrong regularly.
Analcime is the exception in this group, for a structural reason. It is cubic — or tetragonal, orthorhombic or monoclinic but pseudocubic, depending on the degree of aluminium-silicon ordering — and its dominant form is the trapezohedron {211}, an equant polyhedron with 24 faces. Nothing else in the zeolite cabinet is equant. Natrolite and mesolite are needles, scolecite is prismatic, heulandite is a tabular coffin shape, stilbite grows in sheaves.
Leucite and garnet also form trapezohedra, and both are far harder to exclude than the zeolites are: garnet is 6.5 to 7.5 and much denser, leucite is a feldspathoid of volcanic rocks. Density is the check that settles it — analcime at 2.24 to 2.29 is among the lightest silicates this site covers, and grossular garnet, for comparison, sits at 3.594.
Cornwall, Scotland and the basalt cavities
Analcime forms in the groundmass or vesicles of silica-poor intermediate and mafic igneous rocks — typically basalts and phonolites — from late-stage hydrothermal solutions, or disseminated through deuteric alteration. It also forms in lake beds, altered from pyroclastics or clays, and authigenically in sandstones and siltstones. Its listed associates are zeolites, prehnite, calcite, quartz and glauconite.
Two British occurrences are on the Handbook's own list of localities for outstanding specimens: the Dean quarry at St Keverne on the Lizard peninsula, Cornwall, and occurrences around Glasgow in Dumbartonshire, Scotland. That is an unusual pairing for this site — Cornwall appears here for a basalt zeolite rather than for the tin and copper suite that normally puts it on these pages.
Elsewhere: Aci Castello in the Cyclopean Islands and Val di Fassa and Alpe di Siusi in Trentino-Alto Adige, Italy; large crystals from Krasnoyarsk, Russia, and from Mont Saint-Hilaire, Quebec; Breidhdalsheidhi in Iceland; the Lake Superior district in Michigan; Bergen Hill and West Paterson in New Jersey; Table Mountain in Jefferson County, Colorado; the Bay of Fundy district in Nova Scotia; and Flinders in Victoria, Australia.
Type material is in the Natural History Museum in Paris. There is no catalogue on this site and nothing here is offered for sale; a Dean quarry or Mont Saint-Hilaire trapezohedron is a wanted list matter, and the rest of the section is indexed under mineral species.
Keeping analcime, and the zeolite water problem
Zeolites hold water in channels through the framework, and that water can leave. The heulandite page records dehydration as a real risk for the platy zeolites; analcime's single water molecule per formula unit is more tightly held than the six to fourteen carried by heulandite, stilbite and stellerite, and analcime is correspondingly the most robust zeolite on these shelves.
It is still worth keeping out of a spotlit cabinet and away from a radiator. Heat is the agent that drives zeolite water off, and a closed display case under a halogen lamp reaches temperatures people consistently underestimate — the argument set out on lighting a mineral display cabinet.
The mechanical risk is different from the chemical one. At hardness 5 to 5.5 analcime is harder than a knife blade, but trapezohedra sit on a basalt matrix by a small contact area and snap off cleanly under their own weight if a specimen is picked up by the crystal. Lift by the matrix. The general rules are on handling and storage.