What is Aragonite?
Aragonite (CaCO₃) is a calcium carbonate mineral and the less common polymorph of calcite — both share the same chemical formula but have different crystal structures. Aragonite crystallizes in the orthorhombic system rather than calcite’s trigonal system, producing distinctly different crystal habits. Named after the Aragon region of Spain where it was first identified, aragonite is beloved by collectors for its dramatic twinned crystal formations, particularly the spectacular “sputnik” clusters of radiating prismatic crystals.
Aragonite forms under specific conditions: lower temperatures, higher pressures, or in the presence of certain ions (particularly magnesium and strontium) that inhibit calcite crystallization. It is the primary mineral in mother-of-pearl (nacre), coral skeletons, and many mollusk shells. Over geological time, aragonite slowly converts to the more stable calcite polymorph, making well-preserved ancient aragonite specimens relatively rare.
Physical Properties and Identification
- Chemical formula: CaCO₃ (calcium carbonate)
- Crystal system: Orthorhombic
- Hardness: 3.5–4 on the Mohs scale
- Luster: Vitreous to resinous
- Specific gravity: 2.93 (slightly denser than calcite at 2.71)
- Cleavage: Distinct in one direction, poor in two others
- Streak: White
- Effervescence: Reacts with dilute hydrochloric acid (like all carbonates)
Aragonite is distinguished from calcite by its orthorhombic crystal form (calcite is trigonal), slightly higher specific gravity, and different cleavage. The classic cyclic twin — where three crystals join at 120° to create a pseudo-hexagonal prism — is diagnostic for aragonite and does not occur in calcite.
Famous Aragonite Localities
Molina de Aragón, Spain is the type locality and produces the classic cyclic-twinned pseudohexagonal prismatic crystals that define aragonite in most collectors’ minds. Spanish aragonite twins are textbook specimens found in virtually every mineral reference collection.
Morocco is the major commercial source of spectacular aragonite “sputnik” clusters — radiating balls of acicular (needle-like) crystals that resemble sea urchins or satellite antennae. Moroccan aragonite ranges from white to reddish-brown and is available in impressive sizes at accessible prices, making it a popular choice for beginning collectors.
Sicilia, Italy produces fine aragonite associated with native sulfur deposits, including excellent cyclic twins and prismatic crystals. The Girgenti (Agrigento) locality is historically famous. Minglanilla, Cuenca, Spain yields outstanding blue aragonite that is highly collectible. Mexico produces spectacular cave formations and crystallized specimens. China has emerged as an important source of well-crystallized aragonite at competitive prices. Namibia, Pakistan, and various US localities also produce notable material.
Collecting Aragonite: What to Look For
Crystal habit: Aragonite occurs in several collectible forms. Cyclic twins (pseudo-hexagonal prisms) are the classic collector form. Sputnik clusters (radiating acicular balls) are dramatic and displayable. Flos ferri (“flowers of iron”) — delicate coral-like branching formations found in iron mines — are rare and highly sought after.
Twinning quality: For twinned crystals, look for sharp, well-defined contact planes between the twin individuals. The pseudo-hexagonal outline should be clearly visible. Complete, undamaged twins with clean terminations are premium specimens.
Color: While white and colorless aragonite is most common, blue (Minglanilla), reddish-brown (Morocco), and yellow specimens exist. Blue aragonite is particularly desirable and commands higher prices.
Condition: Aragonite’s moderate hardness and cleavage make it vulnerable to damage. Examine specimens carefully for chipped crystal points and broken needles, especially on sputnik clusters where protruding needles are easily snapped.
Care, Handling, and Display
Aragonite requires careful handling. Its moderate hardness (3.5–4) and cleavage make crystals vulnerable to chipping. Sputnik clusters with protruding needles are particularly fragile — always support them from below and store in padded containers. Never stack aragonite specimens.
Clean aragonite with a soft brush only. As a carbonate mineral, aragonite dissolves in acid — never use vinegar, citric acid, or hydrochloric acid near your specimens. Avoid prolonged water exposure. Aragonite is not light-sensitive, so display lighting poses no risk.
Aragonite is metastable at Earth surface conditions and will eventually convert to calcite over geological time. However, this process is extremely slow under normal room conditions and is not a practical concern for collectors. Avoid exposing aragonite to high temperatures, which can accelerate the conversion.
For display, aragonite’s white to tan colors look best on dark backgrounds that provide contrast. Sputnik clusters are particularly dramatic when displayed on stands that allow light to reach all sides. Pair with calcite (its polymorph partner) for an educational display showing how the same chemistry creates different crystal forms.
Aragonite vs. Calcite: Understanding Polymorphs
Aragonite and calcite share the formula CaCO₃ but differ in crystal structure — aragonite is orthorhombic while calcite is trigonal. This makes them polymorphs. Calcite is the stable form at normal surface conditions, while aragonite forms preferentially at higher pressures, lower temperatures, or in magnesium-rich environments. Distinguishing them visually relies on crystal habit: aragonite forms orthorhombic prisms and cyclic twins, while calcite forms rhombohedra and scalenohedra. Calcite displays strong double refraction (place it on text and you see doubled letters); aragonite shows only weak birefringence.
Shop Aragonite at My Mineral Box
Explore our collection of aragonite specimens featuring dramatic sputnik clusters, classic Spanish twins, and unique crystal formations from worldwide localities. Every piece is selected for crystal quality and display appeal. Browse our full mineral collection today.
