Is aragonite a polymorph of calcite?
Yes. Aragonite and calcite are both Ca(CO3) but separate species with different crystal structures, hardness 3.5 to 4 against 3. Vaterite is the third.
Yes. Aragonite is a polymorph of calcite: both are Ca(CO3) on the September 2026 IMA list, aragonite orthorhombic at hardness 3.5 to 4 and density 2.95, calcite hexagonal at 3 and 2.7102. Strictly they are polymorphs of each other, with vaterite as the third: the IMA list holds exactly three Ca(CO3) rows.
In short
- The definition, sourced. The IMA nomenclature guidelines (Nickel and Grice, Canadian Mineralogist 36, 913–926, 1998) define polymorphic minerals as those with essentially the same chemical composition but different crystal structures, and treat them as different species if the structures are topologically different. Aragonite and calcite fit: one formula, two structures, two species.
- The same-formula test, by script. Over the September 2026 IMA-CNMNC Master List, exactly three parsed rows have the formula Ca(CO3): aragonite, calcite and vaterite. A raw line scan finds the same three. Each of the three Handbook of Mineralogy sheets calls the other two trimorphous with it.
- What fails the test. Monohydrocalcite, Ca(CO3)·H2O, and ikaite, Ca(CO3)(H2O)6, carry the same calcium and carbonate plus 15.25% and 51.92% water by mass: different formulas, so not polymorphs. witherite, strontianite and cerussite share aragonite’s group name on their sheets but are Ba, Sr and Pb carbonates: not polymorphs either.
- Separating the three, from the sheets. Aragonite is biaxial (−), birefringence 0.155; calcite uniaxial (−), 0.172; vaterite uniaxial (+), 0.094 to 0.100. A 20.00 g piece reads 13.22 g in water as aragonite, 12.62 g as calcite and 12.13 g as vaterite (measured density 2.54).
- Documented failure: vaterite. Its sheet gives fibres to 0.1 mm in spherulitic aggregates, no hardness, and says identification depends on correspondence of the X-ray powder pattern with that of synthetic material. No hand test identifies a polymorph that small. How aragonite is told from calcite in hand specimens is on is aragonite the same as calcite.
| Mineral | Formula (IMA) | IMA status, country | Same formula as calcite? | Polymorph? | System (sheet) | Hardness | Density | 20.00 g piece in water | Optical class and birefringence (sheet) | Sheet line |
|---|---|---|---|---|---|---|---|---|---|---|
| Aragonite | Ca(CO3) | G, 1791, Spain | Yes | Yes | Orthorhombic, pseudohexagonal | 3.5 to 4 | 2.95 | 13.22 g | Biaxial (−); 1.530 to 1.685, 0.155 | Trimorphous with calcite and vaterite; aragonite group |
| Calcite | Ca(CO3) | G, 1836, unknown | - | Yes | Hexagonal | 3 | 2.7102 | 12.62 g | Uniaxial (−); ω 1.658, ε 1.486, 0.172 | Trimorphous with aragonite and vaterite; series with rhodochrosite; calcite group |
| Vaterite | Ca(CO3) | A, 1962 s.p., United Kingdom | Yes | Yes | Hexagonal | n.d. | 2.54 (calc. 2.65) | 12.13 g (12.45 g calc.) | Uniaxial (+); ω 1.550, ε 1.644 to 1.650, 0.094 to 0.100 | Trimorphous with aragonite and calcite |
| Monohydrocalcite | Ca(CO3)·H2O | G, 1964, Kyrgyzstan | No: 15.25% water | No | Not read | - | - | - | - | - |
| Ikaite | Ca(CO3)(H2O)6 | A, 1962-005 | No: 51.92% water | No | Not read | - | - | - | - | - |
| Witherite | Ba(CO3) | G, 1789, United Kingdom | No: barium | No | Orthorhombic | 3 to 3.5 | 4.22 to 4.31 | Not repeated (see witherite) | - | Aragonite group |
| Strontianite | Sr(CO3) | G, 1791, United Kingdom | No: strontium | No | Orthorhombic | 3.5 | 3.76 | Not repeated (see witherite) | - | Aragonite group |
| Cerussite | Pb(CO3) | G, 1845, Italy | No: lead | No | Orthorhombic | 3 to 3.5 | 6.55 | Not repeated (see witherite) | - | Aragonite group |
What does polymorph mean? The IMA guideline, applied to aragonite
A polymorph is the same chemical composition in a different crystal structure, and the IMA counts topologically different structures as separate species. The 1998 guidelines of the IMA Commission on New Minerals and Mineral Names (Nickel and Grice, Canadian Mineralogist 36, 913–926) say polymorphic minerals are those that have essentially the same chemical composition but different crystal structures, and that the polymorphic forms are regarded as different species if their structures are topologically different. Where the structures share essentially the same topology, differing only by a distortion or by order and disorder, the guidelines do not treat them as separate species.
Applied. Aragonite’s Handbook sheet gives orthorhombic, pseudohexagonal, point group 2/m 2/m 2/m; calcite’s gives hexagonal, point group −3 2/m (overbar read from the rendered sheet); vaterite’s gives hexagonal, point group 6/m 2/m 2/m. Three formulas the same, three structures different, and the IMA list keeps all three as species. So aragonite is a polymorph of calcite, and calcite equally of aragonite; neither is a form of the other.
Shells count. The same guidelines name aragonite in the shells of molluscs as a biogenic substance that also forms by geochemical processes, and is therefore regarded as a valid mineral. The polymorph question does not change with the source of the material.
How many calcium carbonates are polymorphs? The same-formula test
Three. A script over the September 2026 IMA list finds exactly three rows whose formula is Ca(CO3): aragonite, calcite and vaterite.
Method. The list was converted with pdftotext -layout; a row parser read 6150 rows (the list states 6239 species), and a raw scan for any line containing Ca(CO3) followed by a space was run as a cross-check. Both give aragonite, G, 1791, Spain; calcite, G, 1836, country unknown; and vaterite, A, 1962 s.p., United Kingdom. A second pass for formulas built only of calcium, carbon, oxygen and hydrogen adds monohydrocalcite, Ca(CO3)·H2O, and ikaite, Ca(CO3)(H2O)6, together with seven rows whose carbon sits in organic anions (oxalates such as whewellite and weddellite, a formate, a citrate and others) and which have no CO3 group.
The rule, stated once. Same formula, different structure: polymorph. Same formula plus water: a hydrate, a different composition, not a polymorph; worked on standard atomic weights, monohydrocalcite is 15.25% water and ikaite 51.92%. Same group name, different metal: not a polymorph. The sheets file aragonite, witherite, strontianite and cerussite in the aragonite group, but their IMA formulas are Ca, Ba, Sr and Pb carbonates; the calcite sheet’s series with rhodochrosite is a substitution between two formulas, again not polymorphism. The pyrite–marcasite pair on is marcasite the same as pyrite and the carbon pair on is diamond a mineral pass the same test.
How do the three polymorphs differ? Numbers from the sheets
Optics separate all three in principle: aragonite is biaxial, calcite and vaterite uniaxial, and vaterite is the one that is optically positive. From the sheets: aragonite biaxial (−), α 1.530, β 1.681, γ 1.685, birefringence 0.155; calcite uniaxial (−), ω 1.658, ε 1.486, birefringence 0.172 (the ε was read from the rendered sheet, where the text layer drops the letter); vaterite uniaxial (+), ω 1.550, ε 1.644 to 1.650, birefringence 0.094 to 0.100.
Weight. Measured densities are 2.95, 2.7102 and 2.54 (vaterite calculated 2.65). Worked as 20 − 20/D, a 20.00 g piece reads 13.22 g in water as aragonite, 12.62 g as calcite and 12.13 g as vaterite: 0.60 g and then 0.49 g apart. Hardness is 3.5 to 4 for aragonite and 3 for calcite; the vaterite sheet gives none. The cleavage-first rule and the 50 g density test for the aragonite–calcite pair are on is aragonite the same as calcite and are not repeated here; the specific-gravity method is on how to measure specific gravity at home.
Where the test fails: vaterite and altered crystals
Vaterite passes the same-formula test, and its sheet offers no hand test for it. Its Handbook sheet gives very finely fibrous crystals to 0.1 mm, typically in spherulitic aggregates; hardness not determined; colourless. Under Chemistry it says identification depends on correspondence of the X-ray powder pattern with that of synthetic material. Its occurrences include a carbonated calcium silicate hydrogel at Ballycraigy, Ireland, and weathering crusts in the Hatrurim Formation, Israel, where it is listed with calcite and aragonite. A label reading vaterite without an X-ray result records a hope, not a determination.
Shape is not structure either. The aragonite sheet’s occurrence line opens: converts to calcite over geologic time. A crystal can keep aragonite’s outline after its substance is calcite; that case, calcite after aragonite, is covered on what is a pseudomorph. The formula test cannot see it, because the formula never changed. Smithsonite, Zn(CO3), is a different case again: a different formula, covered on is smithsonite the same as aragonite.
What should the label say?
Name the species, not the relationship. A piece is aragonite, calcite or vaterite; polymorph describes how the three relate, not what the specimen is. A label reading aragonite (calcite) mixes two species, and an old label reading calcite on a pseudohexagonal column may be either; recording an old name beside a corrected one is covered on how to label and catalogue a mineral collection. The species pages are aragonite and calcite, and the rest of this section is at the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.