Aragonite
Aragonite is CaCO3, hardness 3.5 to 4, density 2.95, crystals to 30 cm. Calcite is the same chemistry at hardness 3 and 2.71, with a cleavage aragonite lacks.
Aragonite is calcium carbonate, CaCO3, orthorhombic and pseudohexagonal, hardness 3.5 to 4, measured density 2.95, with crystals to 30 cm. It is trimorphous with calcite and vaterite and converts to calcite over geological time. The Handbook of Mineralogy records it as fluorescent red or yellow under both longwave and shortwave ultraviolet.
In short
- Same chemistry as calcite, different mineral. Both are CaCO3. Aragonite is orthorhombic at hardness 3.5 to 4 and density 2.95; calcite is hexagonal at hardness 3 and density 2.7102. Vaterite is the third polymorph, and its IMA country of first description is the United Kingdom.
- The cleavage is the giveaway. Calcite has a perfect rhombohedral cleavage and breaks into little rhombs. Aragonite's {010} cleavage is merely distinct and it does not produce rhombs at all.
- Pseudohexagonal trillings are diagnostic. Repeated contact twinning on {110} gives columnar groups with a six-sided outline — a crystal that looks hexagonal but is three orthorhombic individuals sharing faces.
- It is unstable and it knows it. The Handbook states plainly that aragonite converts to calcite over geological time, which is why ancient aragonite is scarce and why shell material in old rock is usually calcite now.
- It fluoresces in both bands. Red or yellow under longwave and shortwave ultraviolet, phosphorescent as well, which puts it in a small group of species where the Handbook names both bands explicitly.
| Property | Aragonite | Calcite | What to do with it |
|---|---|---|---|
| Crystal system | Orthorhombic, pseudohexagonal | Hexagonal | Look at the outline: a six-sided prism may be either |
| Hardness | 3.5 to 4 | 3 | A copper coin at about 3.5 marks calcite and struggles on aragonite |
| Density | 2.95 | 2.7102 | A 9 per cent difference — measurable, but only on clean material |
| Cleavage | {010} distinct; {110} and {011} imperfect | {1011} perfect — breaks into rhombs | The single best test. Rhombs mean calcite |
| Habit | Prismatic, acicular with chisel-like terminations, coralloidal, pisolitic | Over 1000 forms recorded, to 7 m | Coralloidal and pisolitic forms are aragonite's |
| Twinning | Repeated on {110}, pseudohexagonal trillings | On {0112}, {1011}, {0001}, {0221} | A six-sided prism with re-entrant lines is a trilling |
| Fluorescence | Red or yellow, LW and SW; phosphorescent | Red, blue, yellow and others, SW and LW | Neither is diagnostic — both fluoresce widely |
| Third polymorph | Vaterite, CaCO3, IMA country United Kingdom, 1962 | — | Rare, and not something you will meet in a cabinet |
Rhombs mean calcite: the one test that actually settles it
If you take one thing from this page, take this. Calcite has a perfect rhombohedral cleavage. Aragonite does not. A calcite fragment struck with anything breaks into small rhombohedra, over and over, down to grit. An aragonite fragment breaks on a distinct {010} cleavage and two imperfect ones, and the pieces are not rhombs.
That test works on broken material, on a chip in a box, and on the underside of a specimen where the damage will not show. It works when the crystal form is absent and it works when the colour is uninformative, which between them covers most of the cases where people are unsure. Our note on cleavage or fracture covers how to read the surfaces.
Hardness is the second test and it is weaker than it looks. Calcite at 3 defines its own point on the Mohs scale; aragonite at 3.5 to 4 sits just above. A copper coin, conventionally about 3.5, will mark calcite and will be marginal on aragonite — which is a real distinction, but it is one test running across a gap of half a point, and gaps of half a point are where hardness testing goes wrong. Density is a 9 per cent difference at 2.95 against 2.7102 and is worth doing if you can weigh the specimen properly.
The pseudohexagonal trilling, and why the crystal is lying to you
Aragonite is orthorhombic. Its crystals frequently look hexagonal. Both statements are true and the mechanism is twinning.
Repeated contact twinning on {110} joins three orthorhombic individuals into a columnar group with a six-sided outline — a trilling. The angle works out close enough to 120 degrees that the composite looks like a single hexagonal prism, and a great many aragonite specimens in old collections are labelled as something hexagonal for exactly that reason.
How to read it: look for the seams. A true hexagonal prism has six equivalent faces meeting at clean edges. A trilling has three composition planes running down it, often visible as fine re-entrant lines or as a difference in lustre between adjacent faces, and the striations parallel to [001] that the Handbook describes as polysynthetic lamellae. A six-sided carbonate prism with lines running down the faces is aragonite; a six-sided carbonate prism with nothing running down the faces needs the cleavage test.
Why aragonite is rarer in old rock than it should be
The Handbook's occurrence paragraph opens with four words that explain a great deal: Converts to calcite over geologic time.
Aragonite is the metastable polymorph at ordinary surface conditions. It forms readily — as a primary precipitate in warm marine water, as ooids and carbonate mud, as the hard parts of an enormous range of marine organisms, in evaporites, in hot-spring sinter and as cave dripstone — and then, given long enough, it turns into calcite. That is why fossil shell in ancient limestone is calcite even though the animal built it out of aragonite, and it is why most collectable aragonite is geologically young.
There is one important exception and it is worth knowing because it changes what a specimen means. Aragonite is characteristic of high-pressure, low-temperature metamorphism — the blueschist facies — where it is associated with pumpellyite, lawsonite, glaucophane and quartz. In that setting it is not young at all; it survived because the pressure kept it stable. An aragonite from a blueschist terrane is a different kind of object from an aragonite out of a cave, and the locality is what tells you which you have. That is one more argument for the position our page on specimens without a locality takes.
Where the good crystals come from, including Cumbria
The Handbook notes that aragonite has many localities but that fine crystals are uncommon. The classics: Molina in Guadalajara Province, Spain, which with the Aragon region gave the mineral its name in 1791; Racalmuto, Cianciana and Agrigento in Sicily; Špania Dolina in Slovakia; the Erzberg near Eisenerz in Styria and Leogang in Salzburg; the Touissit mine near Oujda and Tazouta near Sefrou in Morocco; and large crystals from Tsumeb.
The British entry is Frizington and Cleator Moor in Cumbria — the west Cumberland iron field, which is also the source of much of the country's best hematite. Aragonite from there is part of the same ore-field story.
In the United States, caves at Bisbee in Arizona; large crystals near Lake Arthur and Santa Rosa in New Mexico; and the Passaic mine at Sterling Hill, Ogdensburg, New Jersey — which puts aragonite in the Franklin district, alongside the willemite that made the area's reputation. Both species fluoresce, in different colours and both bands, and a Sterling Hill piece carrying both is a genuinely instructive specimen to own.