Aragonite: meaning, properties and lore

The mineral

Aragonite is a widespread mineral and the stable form of calcium carbonate at high pressures. Its chemical formula is CaCO3, and it has the same formula as calcite and vaterite but a different crystal structure.¹²

Aragonite is harder and has a higher specific gravity than calcite, the commoner form of calcium carbonate. Its observed density is 2.947 g/cm3, close to a calculated density of 2.944 g/cm3. Aragonite is less stable than calcite at room temperature and probably inverts to calcite over geologic time, even under normal conditions.¹²

Aragonite crystallizes in an orthorhombic unit cell with space group Pmcn and four formula units per cell. Its cell measures a = 4.9598 Å, b = 7.9641 Å and c = 5.7379 Å at 25 °C. The structure is isostructural with the carbonates of large divalent cations such as barium, strontium and lead.²

In aragonite the calcium ion is coordinated to nine oxygens, with Ca–O distances from 2.414 Å to 2.653 Å. Compared with six in calcite, this higher cation coordination number is a defining feature. The carbonate group is nonplanar, with the carbon atom 0.026 Å out of the plane of the oxygen atoms.⁵²

Infrared absorption studies describe aragonite-type structures as expecting six infrared bands, because the structure removes degeneracies in the vibrations of the carbonate ion. Aragonite-type CaCO3 shows good evidence for splitting of the antisymmetric stretching band. Changes in vibrational frequencies between calcite and aragonite are small, particularly in the stretching force constants.⁵

In aragonite-type carbonates the in-plane vibrational frequency shifts to higher energies as the cation becomes smaller. The authors of that infrared study concluded that these changes are primarily due to anion-anion forces rather than anion-cation interactions, and ascribe the shifts to repulsion between closed electron shells of oxygen atoms.⁵

Aragonite is found in serpentine and other basic rocks, in sediments, and in iron-ore deposits. It occurs in the oxidized zone of ore minerals, where lead substitutes for calcium. It is always found in deposits formed at low temperatures near the surface of the Earth, such as caves as stalactites.¹

Aragonite sometimes forms as crystals lining the hollow interior of geodes, alongside quartz, calcite and other minerals. It is also the mineral normally found in pearls. Mexican onyx is an aragonite stalagmite found in veins at Tecalco, Mexico.¹⁴

Aragonite is calcium carbonate, with the formula CaCO3, and it rates 3.5 to 4 on the Mohs hardness scale. Aragonite-Calcite stones sold commercially as Onyx-marble contain about 80% aragonite.³⁶

History and lore

Sahagun said an inferior kind of chalchihuitl came from quarries near Tecalco, and this appears to have been identical with the Mexican onyx, which is aragonite. Ancient Mexicans made figures, ornaments and beads from this aragonite material, which is greatly valued today as an ornamental stone.⁴

The mineral's presence in pearls and shells links it to marine life, and it is an important element in the shells and tests of many marine invertebrates.⁷¹

Bragg's 1924 paper on the structure of aragonite gave the atomic positions that Dickens and Bowen later refined.²

Birthstone and zodiac links

Aragonite is an important biomineral found in coral, clam shells, gallstones and otoliths. This biological role is a fact about the mineral, not a birthstone or modern modern modern zodiac link. Practitioners have not been recorded in the claims as tying the stone to a birthstone list, an anniversary, or a modern modern modern zodiac sign.²

Aragonite saturation is thought to be one of several physical factors influencing the distribution of mesophotic coral ecosystems. This scientific observation concerns the sea and reefs, and it is separate from any crystal-practice link to the stone.⁸

What crystal practitioners say

Practitioners associate aragonite with growth and renewal. According to the crystal therapy writer of The Healing Crystals First Aid Book, aragonite is said to help with a re-harmonisation of growth. Crystal healers in the same tradition describe it as enhancing the renewed absorption of water in worn ligaments.⁶

Users are advised to place or affix a section, slice or tumbled stone of aragonite on the affected spot. The First Aid Book names the knee joint as a place to put a tumbled stone, or to fix it with a bandage. Practitioners also advise taking gem essence or gem water made from the stone.⁶

Crystal healers consider aragonite with signature bands, a calcium carbonate rich in water crystal, to be probably the number one choice for slipped discs. Aragonite with bands is also listed as applied for knee problems and growth problems.⁶

Crystal healers recommend aragonite, with calcium, for osteoporosis because it encourages absorption of calcium in the intestine. Practitioners advise wearing it as a necklace or pendant for a longer period with direct body contact, together with other calcium crystals.⁶

Aragonite from Armenia was used for votive purposes in one pendant, according to the illustrated caption in The Curious Lore of Precious Stones.⁴

What testing has found

Scientific reviews have found no rigorous trials confirming the therapeutic value of crystal healing. Beyond placebo, no proof of a positive effect has been established, and crystal healing is generally regarded as harmless and without toxicity.⁹

In 2001, British psychologist Christopher French tested 80 volunteers who held crystals and meditated, asking them to distinguish genuine crystals from fakes.⁹

The French team concluded that the effect came from the power of suggestion rather than the crystals. Some researchers say crystals likely do not directly act on physical and psychological problems, but may have indirect benefits explained by the placebo effect.¹⁰

Pleasant feelings from crystal healing are attributed by believers to the strong placebo effect and cognitive bias. Permutt, a crystal therapy practitioner, says there is no proven link between crystals and the body. Crystal healers and other alternative medicine practitioners often show empathy and emphasise discussion with the patient.⁹¹⁰