Scolecite
Scolecite is a zeolite, hardness 5 to 5.5, density 2.25 to 2.29, in radiating sprays to 30 cm. Mesolite is one natrolite plus two scolecite, atom for atom.
Scolecite is a calcium zeolite, hardness 5 to 5.5 and measured density 2.25 to 2.29, in slender prismatic crystals to 30 cm that are square in cross section and gather into radiating sprays. It is twinned on {100} as contact or penetration twins, commonly. Its name is from the Greek for worm, for the way it curls when heated.
In short
- Mesolite is arithmetically one natrolite plus two scolecite. Add Na2Al2Si3O10·2H2O to twice CaAl2Si3O10·3H2O and the result is Na2Ca2Al6Si9O30·8H2O — the mesolite formula, atom for atom. Its name means “middle” for exactly this reason.
- Hardness and density separate none of the three. Scolecite is 5 to 5.5 and 2.25 to 2.29, natrolite 5 to 5.5 and 2.20 to 2.26, mesolite 5 and 2.26. The ranges overlap on both properties.
- Twinning is what is left, and the laws differ. Scolecite twins on {100} with [001] as axis, commonly; mesolite characteristically on {010} or {100}; natrolite on {110}, {011} and {031}.
- Crystals reach 30 cm, slender, striated along the length and frequently square in cross section, forming the radiating sprays that make Indian zeolite specimens what they are.
- The name is a behaviour, not a place. From the Greek for worm, because it curls as it dehydrates in a flame — which is also a warning about what dry heat does to a zeolite in a cabinet.
| Property | Scolecite | Mesolite | Natrolite | Does it separate them? |
|---|---|---|---|---|
| Formula | CaAl2Si3O10·3H2O | Na2Ca2Al6Si9O30·8H2O | Na2Al2Si3O10·2H2O | Yes, but only by analysis |
| Hardness | 5 to 5.5 | 5 | 5 to 5.5 | No |
| Density | 2.25 to 2.29 | 2.26 | 2.20 to 2.26 | No — all three overlap |
| Crystal system | Monoclinic, pseudotetragonal | Orthorhombic | Orthorhombic, pseudotetragonal | Not by eye |
| Cleavage | {110}, perfect | {110}, perfect | {110}, perfect | No |
| Twinning | {100}, axis [001], common | {010} or {100}, characteristic | {110}, {011}, {031} | Yes — the only property that does |
| Largest crystals | 30 cm | 20 cm | 1 m | Broadly |
The arithmetic behind the name mesolite
The Handbook of Mineralogy says mesolite is named from the Greek for middle, alluding to its composition intermediate between natrolite and scolecite. That is usually read as a loose statement about a solid solution. It is not. It is exact, and you can check it in a line.
Natrolite is Na2Al2Si3O10·2H2O. Scolecite is CaAl2Si3O10·3H2O. Add one natrolite to two scolecite:
Sodium: 2 + 0 = 2. Calcium: 0 + 2 = 2. Aluminium: 2 + 4 = 6. Silicon: 3 + 6 = 9. Framework oxygen: 10 + 20 = 30. Water: 2 + 6 = 8.
Na2Ca2Al6Si9O30·8H2O — which is the mesolite formula on the IMA-CNMNC master list, atom for atom, with nothing left over. Mesolite is not merely intermediate in composition; it is an ordered 1:2 intergrowth of the two end members at the structural level, and its name has said so from the beginning.
The practical consequence is one a collector can use. Because mesolite is ordered rather than a random solid solution, the three are separate species rather than points on a continuum, and a specimen is one of them rather than something in between. That is why the determination is worth making, and why habit and twinning are the properties to make it with.
Why the physical properties are useless here, and what to use instead
This is the second natural group on this site where the standard tests fail outright. Our stilbite note found five of six rows in its comparison with heulandite saying the test does not work. The natrolite group is the same story with different numbers.
Hardness runs 5, 5 to 5.5, and 5 to 5.5. Density runs 2.26, 2.25 to 2.29, and 2.20 to 2.26 — all three ranges overlap, and mesolite's single value sits inside both of the others. All three have perfect {110} cleavage. All three are colourless to white with a vitreous lustre, silky when fibrous. A hydrostatic weighing, which resolves most of the questions on this site, resolves nothing at all here.
What is left is twinning and habit, and they do work. Scolecite is twinned on {100} about [001], commonly and visibly, as contact or penetration twins, and its prisms are characteristically square in cross section. Mesolite is characteristically twinned on {010} or {100} and tends strongly to hairlike tufts and fine fibres. Natrolite twins on {110}, {011} and {031}, and forms the stellate and interlacing groups the species is known for, reaching a metre in length where the other two do not.
Our note on mineral twinning covers reading twin laws off a specimen. For this group it is not a refinement, it is the identification.
What the name warns you about
Scolecite is named from the Greek for worm, alluding to its curling action when heated. Hold a fibre in a flame and it writhes as the water leaves the structure.
That is a nineteenth-century blowpipe test and nobody should run it on a specimen worth keeping. But the behaviour behind it does not need a flame, and this is the part that matters in a cabinet: a zeolite's water is structural, it is held loosely, and it leaves under dry conditions as well as hot ones.
The practical consequences are modest but real. Keep zeolites out of direct sunlight and away from spotlights and heat sources; a sealed case with aggressive desiccant is the wrong environment for them, even though it is right for halite. Delicate radiating sprays lose cohesion as they dry, and a spray that has been in a hot window for a decade does not look like one that has not. Our note on lighting a display cabinet covers the heat side of this, which is the one collectors usually miss while worrying about light.
Localities, and why nearly every fine scolecite is Indian
Scolecite forms primarily in cavities in basalt, and secondarily in gneisses, amphibolites and in dykes and laccoliths from syenitic and gabbroic magmas. Its associates are the other zeolites, prehnite and calcite.
The Handbook names exceptional crystals from the Teigarhorn, Beruförður, Iceland — which is also the country of first description on the IMA list — and then the localities collectors actually meet: the Poona, Nasik and Bombay districts of Maharashtra, India, the Deccan basalt quarries that supply the great radiating sprays. Also Ben More on the Isle of Mull and Talisker Bay on Skye in Scotland; the Malta Valley in Carinthia; the Fallital-Giuv area in Graubünden; Washington State, on the south fork of the Toutle River and at the Pioneer quarry near Bremerton; Charcas in San Luis Potosí; and large crystals near Bento Gonçalves in Rio Grande do Sul.
The Scottish occurrences are genuine and historic and they do not compete with the Indian material on quality, which is worth saying plainly on a site with a British emphasis. Mull and Skye scolecite is a real part of the Hebridean zeolite suite and is collected for what it is rather than for how it displays.
We keep no stock list. If you hold Scottish zeolites with old labels, or Indian material with a named quarry rather than just a district, the wanted list sets out what we look for. Our species notes cover the rest of the zeolite group.