Aragonite: Same Chemistry as Calcite, Different Crystal

Short answer: aragonite is calcium carbonate, CaCO3, the same compound as calcite, packed into an orthorhombic instead of a trigonal structure. That makes it a polymorph: slightly harder (3.5 to 4), denser (about 2.93) and without calcite's perfect rhombohedral cleavage. Its six-sided "star" and "sputnik" clusters are twinned crystals, not true hexagons. Over time and with heat it turns into calcite. Natural colors include brown, white, yellow and blue; very vivid blues and purples deserve questions about dye or a mislabeled stone.

What is a polymorph, and why does aragonite matter?

Two minerals with one formula and two structures are polymorphs. Minerals.net puts the distinction plainly: calcium carbonate forms as both aragonite and calcite, and the two only differ in their crystallization, calcite in trigonal crystals and aragonite in orthorhombic ones. A third polymorph, vaterite, is rare. Minerals.net's calcite page adds that when crystals are too small to see, only optical or x-ray tests can tell the two apart, so fine-grained "calcite" and "aragonite" labels are sometimes simply guesses.

The structural difference shows up in properties you can measure:

Values are from Minerals.net, Webmineral and Geology.com on calcite. The cleavage is the most useful field test: break a calcite crystal and it splits into neat rhombohedral blocks, while aragonite fractures irregularly. A full side-by-side is planned for the aragonite vs calcite comparison; our calcite guide covers calcite itself.

Why aragonite becomes calcite

At the Earth's surface, calcite is the stable form and aragonite is metastable, meaning it survives only because the change is slow. The International Gem Society notes that heating to 400 °C converts aragonite into calcite, a change that happens naturally over millions of years. That is why old aragonite fossils and specimens are often partly or wholly calcite. Minerals.net calls this out for collectors: some aragonite crystals on the market are actually calcite pseudomorphs after aragonite, keeping the outward shape while the inside has converted.

Where does aragonite form?

In more places than its relative rarity suggests, usually where conditions briefly favor it over calcite:

• Hot springs. Minerals.net says aragonite often forms as a deposit from hot, mineral-rich springs, building mounds and crusts; its calcite page adds that at hot springs aragonite is the likely form, while most other cave and spring growths are calcite.
• Shells and pearls. The iridescent nacre of pearl and mother-of-pearl is a layer of aragonite secreted by mollusks, and corals build their skeletons from it. The Smithsonian Ocean portal explains why this matters today: aragonite is a delicate form of calcium carbonate that is 50 percent more soluble in seawater, which is why sea snails with aragonite shells are especially exposed to ocean acidification.
• Ore deposits, caves and evaporites. The IGS lists limestone caverns, hot springs and the oxidized zone of ore deposits as common settings. Delicate white flos ferri, a coral-like branching form, grows in iron mines such as Eisenerz in Austria.

The name comes from Spain. Abraham Gottlob Werner named the mineral after Molina de Aragón, the type locality, where fine twinned crystals still occur along the Gallo river. Other classic sources Minerals.net lists include Agrigento in Sicily, Frizington in Cumbria, Liupanshui in Guizhou, China, and in the United States the Kelly Mine in New Mexico and Bisbee, Arizona. Morocco's Tazouta mine, near Sefrou, is the big modern producer of reddish-brown twins; see the Morocco origins guide for that district.

What are sputnik and star aragonite?

Both are names for twinning, not separate minerals. Minerals.net explains that most large aragonite crystals are twinned growths of three individual crystals forming pseudohexagonal trillings, so a mineral that crystallizes in the orthorhombic system mostly shows hexagonal-looking prisms. Each trilling can be recognized by basal striations running in several directions, one set from each member crystal.

The trade names describe how those twins are arranged:

• Star aragonite. A single cluster of twinned prisms radiating from a common base, seen end-on as a six-sided star. Moroccan material is typically reddish brown, colored by included sediment; Minerals.net notes that sand inclusions give aragonite a brown color.
• Sputnik aragonite. A ball-shaped aggregate where twinned prisms point outward in all directions, like the spikes of the 1957 satellite. Minerals.net lists radiating aggregates and ball-like protrusions of pseudohexagonal crystals among aragonite's habits.

Good sputnik and star specimens have intact terminations on as many points as possible. Because the prisms are soft and brittle, almost every cluster has some chipped tips; what you are paying for is the number of clean points, the symmetry of the burst and the size.

Is blue aragonite natural or dyed?

It can be either, and the label alone will not tell you. Blue is a genuine aragonite color: the IGS lists blue aragonite from Laurium, Greece, and blue material from Chile, and Minerals.net includes blue among aragonite's natural colors. But the bright, uniform sky-blue tumbles and chunks sold cheaply as "blue aragonite" are not always what the name says. Some are another blue carbonate traded under the aragonite name (our calcite guide covers Caribbean calcite, which is also marketed as blue aragonite), and some are pale material dyed.

Dyeing works on stones that are porous or cracked. GIA's gem treatment overview describes dyeing as introducing colored dyes into porous or fractured gems, and notes that fractures are sometimes purposely induced by heating so an otherwise non-porous material will accept the dye. Fibrous, granular carbonate is exactly that kind of material. The same caution applies to bright purple, pink and green aragonite: purple is not among the natural colors in most mineral references, so ask the seller directly.

How to check a blue or purple piece:

1. Look into cracks and pits with a 10x loupe. Dye concentrates there as darker lines and spots. Natural color sits evenly in the crystal fibers.
2. Compare broken surfaces. A chip or rough end that is paler inside than on the surface points to dye.
3. Wipe a hidden spot with acetone on a cotton swab. Color on the swab means surface dye. This can dull a polish, so test somewhere invisible.
4. Use price as a signal. Natural blue aragonite crystals from classic localities are collector items; a bag of uniform electric-blue tumbles at a few dollars is a manufactured product.

How do you test and care for aragonite?

A steel knife or nail (about 5.5) scratches aragonite, and a copper coin (about 3) barely does, which separates it from harder look-alikes. Webmineral notes fluorescence in shortwave and longwave light, and the IGS adds that Sicilian material may phosphoresce green. A drop of cold dilute acid fizzes on it, which also means vinegar and citrus cleaners will etch it.

Care follows from softness and acid sensitivity. The IGS advises against steam and ultrasonic cleaning and recommends a soft brush, mild detergent and warm water, and storing aragonite apart from harder stones. Sputnik clusters are best dusted with a soft brush or air blower; fragile flos ferri should not be touched at all. Keep pieces away from heat, given the conversion to calcite at around 400 °C.

What is aragonite associated with in crystal traditions?

In crystal traditions, brown star and sputnik aragonite are associated with grounding and patience, and blue aragonite with calm communication. These are traditions and beliefs, not properties of the mineral.

Shopping the aragonite we stock

Decide whether you want a specimen or a shaped piece. Specimens show what makes aragonite interesting, the twinning; polished pieces trade that for color and form. For clusters, count intact points and check the base for stability. For blue and purple pieces, ask whether the color is natural and look for dye in cracks in the listing photos.

• Aragonite crystal cluster, sputnik radiating specimen: a radiating burst that shows the sputnik habit well; check the photos for unbroken terminations around the ball.
• Star Aragonite (Morocco): small Moroccan twinned clusters in the classic reddish-brown color, an inexpensive way to see trilling striations with a loupe.
• Polished aragonite tower: a rarer shaped piece, since solid masses large enough to cut are uncommon; inspect the tip and base for chips, as aragonite is soft.
• Blue Aragonite: low-cost blue pieces; a good candidate for the loupe-and-crack check described above.
• Large raw purple aragonite: an unusual color; ask about its origin and treatment before buying it as a reference specimen.

Frequently asked questions

Is aragonite the same as calcite?

Same chemistry, different structure. Both are calcium carbonate, CaCO3, but calcite is trigonal and aragonite orthorhombic. Aragonite is a little harder (3.5 to 4 versus 3), denser (about 2.93 versus 2.7) and lacks calcite's perfect rhombohedral cleavage. Over time it converts to calcite.

Why does aragonite look hexagonal if it is orthorhombic?

Because most large aragonite crystals are trillings: three crystals twinned together around a common axis. Their combined outline is six-sided, so the prisms look hexagonal. Striations running in several directions across the end face give the twinning away under a loupe.

What is sputnik aragonite?

It is a trade name for ball-shaped clusters of twinned aragonite prisms that radiate outward in every direction, recalling the spikes of the Sputnik satellite. Moroccan specimens are typically reddish brown. Value depends on intact points, symmetry and size.

Is blue aragonite dyed?

Sometimes. Natural blue aragonite exists, for example from Laurium, Greece, and Chile, but some cheap bright-blue material is dyed or is a different carbonate sold under the aragonite name. Look for color concentrated in cracks and ask the seller whether the color is natural.

Can aragonite go in water?

Brief rinsing in plain water is fine for solid pieces, but avoid soaking, acidic liquids, steam and ultrasonic cleaners. Clean with a soft brush, mild soap and warm water, dry it well, and keep fragile clusters dry and dusted instead.

Sources

• Minerals.net, The mineral aragonite: https://www.minerals.net/mineral/aragonite.aspx
• Minerals.net, The mineral calcite: https://www.minerals.net/mineral/calcite.aspx
• Webmineral, Aragonite mineral data: https://webmineral.com/data/Aragonite.shtml
• Geology.com, Calcite: https://geology.com/minerals/calcite.shtml
• International Gem Society, Aragonite Value, Price, and Jewelry Information: https://www.gemsociety.org/article/aragonite-jewelry-and-gemstone-information/
• Smithsonian Ocean, Ocean Acidification: https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification
• GIA, Gem Treatment: https://www.gia.edu/gem-treatment