Wavellite

Description

Wavellite is a hydrated aluminum phosphate mineral, known for its distinctive radiating, spherical, or hemispherical aggregates of acicular (needle-like) crystals. These aggregates, often referred to as 'starbursts' or 'botryoidal' forms, can be quite beautiful and are highly prized by mineral collectors. Its color varies, commonly appearing in shades of green, yellow-green, white, colorless, brown, or black. It typically forms in low-temperature environments, often as a secondary mineral in metamorphic rocks or phosphate deposits.

How to Identify

Color

Typically green, yellow-green, white, colorless, brown, or black. Often translucent to opaque.

Luster

Vitreous (glassy) to pearly on cleavage surfaces.

Texture

Often forms radiating, spherical, or hemispherical aggregates of fine acicular crystals, giving a botryoidal or mammillary texture.

Crystal Form

Orthorhombic system. Crystals are typically acicular (needle-like) to prismatic, often forming characteristic radiating aggregates, spheres, or hemispheres. Individual crystals are rare and usually very small.

Cleavage

Good on {110} and {010}, but rarely observed due to the fine-grained, radiating habit.

Geological Environment

Found in low-temperature hydrothermal veins, in fissures and cavities of aluminous metamorphic rocks (e.g., phyllites, slates), and as a secondary mineral in phosphate-bearing igneous or sedimentary rocks. Often associated with limonite, variscite, crandallite, and other secondary phosphates.

Key Facts

• Hardness: 3.5-4 on the Mohs scale
• Specific Gravity: 2.30-2.37 g/cm³
• Crystal System: Orthorhombic
• Color: Green, yellow-green, white, colorless, brown, black
• Luster: Vitreous to pearly
• Transparency: Transparent to translucent to opaque
• Fracture: Uneven to subconchoidal
• Cleavage: Good on {110} and {010}, but rarely observed due to habit
• Composition: Hydrated aluminum phosphate with hydroxyl and fluoride. Al3(PO4)2(OH,F)3·5H2O

Quick Check

• Color: Green, yellow-green, white, colorless, brown, black
• Luster: Vitreous to pearly
• Streak: White

Physical Characteristics

• Crystal Habit: Typically forms radiating, spherical, or hemispherical aggregates of acicular to prismatic crystals; also botryoidal, mammillary, stalactitic, or massive.
• Cleavage Type: Good on {110} and {010}, but often obscured by the radiating habit.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly.

Formation

Wavellite is a secondary mineral, typically forming in low-temperature hydrothermal veins, in fissures and cavities of aluminous metamorphic rocks, or as a weathering product of phosphate-bearing igneous or sedimentary rocks. It often occurs in association with limonite, variscite, crandallite, and other secondary phosphate minerals. The presence of aluminum, phosphate, and water, along with a slightly acidic to neutral pH, are crucial for its formation.

Usage

Wavellite has no significant industrial or commercial uses. It is primarily a mineral sought after by collectors for its aesthetic radiating crystal habits and attractive colors. Some specimens are cut and polished as cabochons, but its relative softness limits its use in jewelry.

Age Distribution

Wavellite forms in secondary environments, often associated with the weathering of phosphate-rich rocks or hydrothermal alteration. Its formation is not tied to a specific geological age but rather to the conditions present for its precipitation.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Phosphates

Group

Wavellite Group

Crystal System

Orthorhombic

Chemical Formula

Al3(PO4)2(OH,F)3·5H2O

Composition

Aluminum phosphate hydroxide fluoride hydrate. Contains Aluminum (Al), Phosphorus (P), Oxygen (O), Hydrogen (H), and Fluorine (F).

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