Aragonite

Aragonite is a calcium carbonate mineral with the same chemistry as calcite (CaCO3) but a different crystal structure — it is the orthorhombic polymorph, while calcite is trigonal. This makes the two minerals chemical twins with different shapes and properties.

Aragonite commonly forms radiating clusters of needle-like or columnar crystals (the 'sputnik' clusters), as well as fibrous, stalactitic, and coral-like masses. Colors range from white and colorless to warm yellow, orange, and brown.

It is the carbonate that most marine organisms use to build shells, coral skeletons, and pearls, though over geologic time aragonite tends to convert to the more stable calcite.

Aragonite forms in environments favoring its structure over calcite, typically lower temperatures, higher pressures, or specific water chemistries. It precipitates in caves (as flos ferri and stalactites), around hot and cold mineral springs, and in metamorphic blueschists formed at high pressure.

Biologically, marine animals — corals, many mollusks, and the nacre of pearls — secrete aragonite to build their hard parts, and it makes up much of modern carbonate sediment and reefs.

Fine crystal specimens come from Spain (Molina de Aragón, the namesake locality), Morocco, Mexico, and the Czech Republic, among others.

Aragonite is recognized by its radiating needle or columnar crystal clusters, hardness of 3.5-4, vitreous luster, white streak, and vigorous fizzing in dilute acid (as a carbonate).

It is slightly harder than calcite and lacks calcite's perfect rhombohedral cleavage, instead showing poor cleavage and often forming hexagonal-looking twinned prisms.

Look-alikes: Calcite is the main confusion — calcite is softer (3), shows perfect rhomb cleavage, and breaks into rhombs, while aragonite is harder and forms needles/prisms. Strontianite and witherite resemble it but are denser carbonates of strontium and barium. The acid test confirms it is a carbonate.

Aragonite has limited industrial use compared with calcite but is important biologically and decoratively. Brown banded 'onyx' aragonite and stalactitic forms are carved into ornaments, bookends, and tiles, and the colorful clusters are popular display specimens.

In aquariums, crushed aragonite sand is used to buffer water chemistry and supply calcium for reef tanks. It is the building block of natural pearls and nacre (mother-of-pearl).

Metaphysically, aragonite is associated with grounding, stability, and emotional balance. Its main scientific significance is as a recorder of past ocean chemistry and climate.