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, in contrast to calcite's trigonal system. This difference in crystal structure leads to distinct physical properties. Aragonite is typically found in acicular (needle-like), columnar, fibrous, or radiating crystal habits, often forming pseudohexagonal twins. It is less stable than calcite at standard temperature and pressure and will slowly convert to calcite over geological time, a process known as aragonite-calcite inversion. This inversion is accelerated by heat and pressure.

How to Identify

Color

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

Luster

Vitreous (glassy) to resinous.

Texture

Often fibrous, columnar, or radiating aggregates. Individual crystals can be prismatic or acicular.

Crystal Form

Orthorhombic system. Commonly forms pseudohexagonal twins, radiating aggregates, stalactitic, columnar, or fibrous masses. Acicular (needle-like) crystals are characteristic.

Cleavage

Distinct cleavage in one direction ({010}), and imperfect in two others ({110} and {011}). This is often difficult to observe in fine-grained or fibrous aggregates.

Geological Environment

Found in evaporitic deposits, hot spring deposits (tufa), speleothems (cave formations), mollusk shells, coral skeletons, serpentinites, and as a secondary mineral in altered igneous and metamorphic rocks. Also forms in high-pressure, low-temperature metamorphic facies (e.g., blueschist facies).

Key Facts

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

Quick Check

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

Physical Characteristics

• Crystal Habit: Acicular, prismatic, columnar, fibrous, radiating, stalactitic, massive. Often forms pseudohexagonal twins.
• Cleavage Type: Distinct in one direction ({010}), imperfect 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 forms under specific conditions of lower temperature and higher pressure than calcite, or in environments with high magnesium concentrations that inhibit calcite precipitation. Common formation environments include: biogenic precipitation (e.g., in mollusk shells, coral skeletons), speleothems (cave formations like stalactites and stalagmites), hot springs (forming tufa), serpentinites, and as a diagenetic mineral in sedimentary rocks, particularly in evaporitic settings or deep-sea sediments. It can also form in hydrothermal veins and as a secondary mineral in altered igneous and metamorphic rocks.

Usage

Aragonite has several uses. It is a common component in marine organisms' shells and skeletons, playing a crucial role in marine ecosystems. Industrially, it is used as a raw material for cement, lime, and as a filler in plastics and paints. High-quality specimens are prized by mineral collectors. It is also used in water treatment to remove heavy metals and in some agricultural applications as a soil conditioner. Its unique crystal structure makes it a subject of research in materials science.

Age Distribution

Found in geological formations ranging from the Precambrian to the present day, often associated with relatively recent low-temperature, low-pressure 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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