Aragonite Identification Guide

What Aragonite Looks Like

Aragonite is calcium carbonate (CaCO3), the same chemistry as calcite but a different crystal structure (orthorhombic, not trigonal). It commonly forms slender needle-like (acicular) crystals, radiating spiky clusters, columnar or coral-like masses ("flos ferri"), and pseudo-hexagonal twins. Colors include colorless, white, gray, yellow, brown, and orange; the popular banded "sputnik" and stalactitic forms come from Spain and Morocco. Aragonite also builds up many seashells and pearls.

• Color: colorless, white, yellow, orange-brown, blue-gray
• Luster: vitreous to slightly resinous
• Transparency: transparent to translucent
• Habit: needle-like sprays, radiating clusters, twinned pseudo-hexagonal prisms, coralloid masses

Step-by-Step Field ID Checklist

1. Note the habit: spiky radiating needles or pseudo-hexagonal twinned prisms favor aragonite over the blockier rhombs of calcite.
2. Do the acid test: a drop of dilute HCl produces brisk effervescence (it is a carbonate).
3. Check hardness: a steel knife scratches it (3.5-4), slightly harder than calcite.
4. Look at cleavage: aragonite lacks the perfect three-direction rhombohedral cleavage of calcite; it shows poor cleavage and tends to fracture.
5. Examine for twinning: repeated twinning that mimics hexagonal symmetry is characteristic.

Key Diagnostic Tests

• Mohs hardness: 3.5 to 4 (slightly harder than calcite's 3)
• Streak: white
• Cleavage: poor/distinct in one direction only (not the three perfect cleavages of calcite)
• Fracture: subconchoidal
• Specific gravity: about 2.9 to 3.0, slightly denser than calcite (~2.7)
• Acid: vigorous fizz in cold dilute hydrochloric acid
• Meigen's stain (lab): boiling in cobalt nitrate solution stains aragonite lilac/pink but leaves calcite unchanged, a definitive separator

Common Look-Alikes and How to Tell Them Apart

• Calcite: the key look-alike (same composition). Calcite is softer (3), shows perfect rhombohedral cleavage (breaks into rhombs), is less dense, and stays uncolored in the Meigen cobalt-nitrate test. Aragonite is harder, lacks rhombic cleavage, often needle-like, and stains lilac.
• Natrolite/other zeolites: also needle-like but do not fizz in acid; aragonite effervesces.
• Gypsum (selenite): softer (2, scratched by fingernail), no acid fizz.
• Strontianite/witherite: also fizzing carbonates with similar habit; require flame tests (strontium red, barium green) or density to separate.
• Celestine: can resemble pale aragonite but does not fizz and is denser.

Where Aragonite Is Typically Found

Aragonite forms in low-temperature, near-surface settings: hot-spring and cave deposits (some stalactites), oxidized ore zones, around iron deposits (the coralloid flos ferri of Styria, Austria), and in evaporite and biological systems (shells, corals, pearls). Famous crystal localities include Molina de Aragón, Spain (the type locality and source of the name), Morocco (orange sputnik clusters), Sicily, and Mexico.