Is aragonite the same as calcite
No. Both are CaCO3, but aragonite is orthorhombic at density 2.95 and calcite hexagonal at 2.7102. On a 50 g piece that is 1.50 g of difference in water.
No. Is aragonite calcite? Chemically yes, as a mineral no: both are CaCO3, but aragonite is orthorhombic, hardness 3.5 to 4, density 2.95, while calcite is hexagonal, hardness 3, density 2.7102, with a perfect rhombohedral cleavage aragonite lacks. They are polymorphs, listed as two species. Cleavage separates them first; density, an 8.85% gap, confirms it.
In short
- Same formula, different species. Aragonite and calcite are both CaCO3 and both are on the September 2026 IMA-CNMNC list as separate species, aragonite with a Spanish type locality dated 1791 and calcite dated 1836. The Handbook calls them trimorphous with vaterite. 43 of the 176 Handbook sheets in this site's reference set record a polymorph relationship of some kind; this is the one a collector meets most.
- Test cleavage first, density second, and nothing else. Rhombs on a broken edge mean a rhombohedral carbonate. No rhombs, with a density near 2.95, means aragonite. Hardness runs across a half-point gap, the recorded fluorescence colours overlap, and acid attacks both, so none of those three chooses between them.
- Worked through: a 50.00 g piece weighs 33.05 g in water as aragonite and 31.55 g as calcite. The 1.50 g gap is 150 steps on a 0.01 g scale. The cut-off falls at a measured specific gravity of 2.83. Dolomite, at 2.86, lands on the aragonite side of that line, which is why cleavage has to come first.
- Is aragonite fluorescent? The sheet says so without a may. Aragonite reads fluorescent red or yellow under LW and SW UV, phosphorescent; calcite reads may fluoresce red, blue, yellow, and other colors under both. They are 2 of 11 sheets in the 176 that name both bands, so a lamp cannot choose between them.
- Is aragonite rare? As a mineral, no; as fine crystals, yes, and old ones are rarer still. The Handbook's distribution line opens Many localities, but fine crystals are uncommon, and its occurrence line opens Converts to calcite over geologic time. That phrase, converts to, appears on 1 of the 176 sheets.
| Criterion | Aragonite | Calcite | Dolomite | Does it choose? |
|---|---|---|---|---|
| Formula | CaCO3 | CaCO3 | CaMg(CO3)2 | No. Aragonite and calcite are identical |
| Crystal system | Orthorhombic, pseudohexagonal | Hexagonal | Hexagonal | Only on a well-formed crystal |
| Cleavage | {010} distinct; {110}, {011} imperfect | {1011} perfect: rhombs | {1011} perfect: rhombs | Yes, first. Rhombs rule out aragonite |
| Hardness | 3.5 to 4 | 3 | 3.5 to 4 | Weakly. A half-point gap, and dolomite matches aragonite |
| Density, measured | 2.95 | 2.7102 | 2.86 | Yes, second. 8.85% between the two polymorphs |
| 50.00 g piece, weight in water | 33.05 g | 31.55 g | 32.52 g | Yes on a 0.01 g scale, on solid material |
| Fluorescence as recorded | Red or yellow, LW and SW; phosphorescent | May fluoresce red, blue, yellow and others, SW and LW | May fluoresce white to pink under UV | No |
| Habit | Prismatic, acicular with chisel-like terminations; coralloidal, pisolitic; trillings on {110} | Over 1000 forms noted, to 7 m | Crystals of {1011}, may have curved faces; saddle-shaped aggregates | Suggestive only |
Is aragonite calcite? The criteria, in the order that settles it
Aragonite is not calcite, and the reason is structure rather than chemistry. Both are calcium carbonate, CaCO3. The Handbook of Mineralogy records aragonite as orthorhombic and pseudohexagonal and calcite as hexagonal, and records both as trimorphous with each other and with vaterite. The IMA-CNMNC Master List lists aragonite and calcite as two species. So a label reading aragonite (calcite) is wrong in the way pyrite (marcasite) would be wrong.
How common is the situation? We counted. Across the 176 Handbook sheets in this site's reference set, converted with pdftotext -layout and the Polymorphism & Series field read by regular expression, 43 record a polymorph relationship: 23 say dimorphous, 11 trimorphous, and 9 use polymorphous or name polymorphs outright. Several pairs sit wholly inside the set: aragonite and calcite, microcline and orthoclase, pyrite and marcasite, zoisite and clinozoisite, realgar and pararealgar. The same question has its own page for the iron sulphides, whether marcasite is the same as pyrite.
The order is the useful part. Cleavage first, because it is free and it eliminates every rhombohedral carbonate at once: calcite's {1011} cleavage is perfect and a broken edge shows small rhombs; aragonite's best cleavage is merely distinct and does not make them. Reading a broken surface is covered on cleavage or fracture. Density second, because it is the only number with a gap big enough to measure at home. Everything else is supporting colour, and the aragonite species page covers the trilling habit that makes aragonite look hexagonal.
The worked density test: 50 grams, two answers, a cut-off at 2.83
A density test between aragonite and calcite comes down to 1.50 g on a 50 g specimen. Method, so you can substitute your own weight. Specific gravity is weight in air divided by the loss of weight in water. A piece weighing 50.00 g in air has a volume of 50.00 ÷ 2.95 = 16.95 cm³ if it is aragonite, and 50.00 ÷ 2.7102 = 18.45 cm³ if it is calcite. Suspended in water it therefore weighs 50.00 − 16.95 = 33.05 g as aragonite and 50.00 − 18.45 = 31.55 g as calcite.
The gap between the densities is (2.95 − 2.7102) ÷ 2.7102 = 8.85%. The midpoint is 2.83, which on this specimen is a water weight of 32.33 g. Near aragonite's value, each 0.01 g of error in the water weight moves the result by about 0.0017, so a 0.01 g scale has a very wide margin on solid material.
Now the trap. Dolomite, CaMg(CO3)2, is recorded at density 2.86 and hardness 3.5 to 4. It sits above the 2.83 cut-off and matches aragonite's hardness exactly, so a density-only rule calls dolomite aragonite. It also has a perfect rhombohedral cleavage, which is why the cleavage test goes first and the density test second. The decision rule, stated once: rhombs and about 2.71 is calcite; rhombs and about 2.86 is dolomite; no rhombs and about 2.95 is aragonite.
Where it fails: coralloidal, pisolitic and fibrous aragonite traps air and reads light, and a crust on matrix measures the matrix. The equipment, the minimum specimen weights and the porous-material caveat are on how to measure specific gravity at home, and the same arithmetic separates two dyed carbonates on whether magnesite is the same as howlite.
Is aragonite fluorescent, or UV reactive? Yes, and so is calcite
Aragonite's sheet records fluorescence in both ultraviolet bands and does not hedge it. The wording is Cathodoluminescent, fluorescent red or yellow under LW and SW UV, phosphorescent. Calcite's sheet hedges: May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent, cathodoluminescent, thermoluminescent, rarely triboluminescent.
Across the 176 sheets, 31 mention fluorescence and 11 name both shortwave and longwave: adamite, aragonite, calcite, grossular, marialite, meionite, mimetite, pyromorphite, saleeite, scolecite and willemite. The count is ligature-aware, because pdftotext renders the fl ligature on older sheets as a stray glyph and a plain search for fluoresc misses ten of them. Two of the eleven are the pair on this page, with overlapping colours. So a UV lamp answers is aragonite UV reactive and cannot answer is it aragonite or calcite. The general mechanism is on fluorescent minerals explained.
Is aragonite rare, and who each identification is wrong for
Aragonite is common as a substance and uncommon as good crystals. The Handbook's distribution field opens Many localities, but fine crystals are uncommon, and its occurrence field lists warm marine precipitate, shells and skeletons, evaporites, hot-spring sinter, cave dripstone and blueschist-facies rocks. It opens with Converts to calcite over geologic time. Searching all 176 sheets for the phrase converts to returns one sheet: aragonite's. Where the conversion keeps the crystal shape the result is calcite after aragonite, a paramorph, covered on what a pseudomorph is.
Who the answer calcite is wrong for: anyone holding six-sided columns with lines running down the faces and no rhombs on a broken edge. That is the pseudohexagonal aragonite trilling, and old labels call it calcite constantly. Who the answer aragonite is wrong for: anyone whose piece shows rhombs, whatever its shape or density. That is calcite or dolomite. Who neither answer suits: a specimen from old rock showing an aragonite shape that breaks into rhombs, because that may be the paramorph, and the honest label names both.
Acid is left out on purpose. Both species are calcium carbonate, so an acid test that attacks one attacks the other, and it damages the specimen either way; the carbonate problem is on how to remove calcite from a specimen. The rest of this section is indexed at the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route, and well-formed aragonite from a named Handbook locality is on it.