Aragonite

Description

Aragonite is a carbonate mineral, one of the two common naturally occurring crystal forms of calcium carbonate (CaCO3), the other being calcite. It crystallizes in the orthorhombic system, distinguishing it from calcite's trigonal system. Aragonite often forms in acicular (needle-like), columnar, fibrous, or radiating crystal habits. It can also occur in botryoidal, stalactitic, or coralloidal (e.g., 'Flos Ferri') forms. Its color is typically white, colorless, or gray, but can also be yellow, green, blue, or violet due to impurities. Aragonite is metastable at standard temperature and pressure and tends to alter to calcite over geological time, a process that can be accelerated by heat and water. This transformation is a key factor in the preservation of fossils, as aragonitic shells often recrystallize to calcite.

How to Identify

Color

Typically white, colorless, or gray. Can also be yellow, green, blue, violet, or brown due to impurities.

Luster

Vitreous (glassy) to resinous.

Texture

Can be fibrous, columnar, granular, or massive. Often forms radiating aggregates or acicular crystals.

Crystal Form

Orthorhombic system. Commonly forms prismatic, acicular, or tabular crystals. Also found in botryoidal, stalactitic, and coralloidal (e.g., 'Flos Ferri') habits. Twinning is common, often forming pseudo-hexagonal prisms.

Cleavage

Distinct cleavage in one direction ({010}), poor in two others ({110} and {011}). Often difficult to observe due to fibrous or fine-grained habits.

Geological Environment

Found in sedimentary environments (e.g., marine and freshwater precipitates, ooids, shells), hydrothermal veins, hot springs, caves (speleothems), serpentinites, and as a component of some igneous rocks (e.g., carbonatites).

Key Facts

• Hardness: 3.5-4 (Mohs scale)
• Specific Gravity: 2.93-2.95 g/cm³
• Crystal System: Orthorhombic
• Color: White, colorless, gray, yellow, green, blue, violet, brown
• Luster: Vitreous to resinous
• Transparency: Transparent to translucent
• Fracture: Subconchoidal to uneven
• Cleavage: Distinct on {010}, poor on {110} and {011}
• Composition: Calcium Carbonate (CaCO3)

Quick Check

• Color: White, colorless, gray, yellow, green, blue, violet, brown
• Luster: Vitreous to resinous
• Streak: White

Physical Characteristics

• Crystal Habit: Prismatic, acicular, tabular, fibrous, columnar, radiating, botryoidal, stalactitic, coralloidal (Flos Ferri). Often twinned, forming pseudo-hexagonal prisms.
• Cleavage Type: Distinct in one direction ({010}), poor in two others ({110} and {011}).
• Fracture Type: Subconchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy) to resinous.

Formation

Aragonite is a polymorph of calcium carbonate (CaCO3), meaning it has the same chemical composition as calcite but a different crystal structure. It typically forms at lower temperatures and pressures than calcite, often in sedimentary and low-temperature metamorphic environments. Key formation conditions include: precipitation from marine and freshwater environments (e.g., in shells of marine organisms, ooids, speleothems), hydrothermal veins, hot springs, and as a component of some serpentinites. It is metastable at surface conditions and will slowly convert to calcite over geological time, especially in the presence of water.

Usage

Aragonite has limited industrial uses compared to calcite due to its relative scarcity and metastability. It is primarily used as a decorative stone, for carving, and as a collector's mineral. Biogenic aragonite is a significant component of coral reefs and mollusk shells. In some specialized applications, it is used in water treatment to remove heavy metals and in some pharmaceutical preparations. Its unique crystal habits make it a popular specimen for mineral collectors.

Age Distribution

Aragonite forms in various geological settings and is not restricted to a specific geological age. It can be found in modern and ancient environments.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Carbonates

Group

Aragonite Group

Crystal System

Orthorhombic

Chemical Formula

CaCO3

Composition

Calcium Carbonate

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