Uranophane

Description

Uranophane is a hydrated calcium uranyl silicate mineral. It is one of the most common secondary uranium minerals and is characterized by its distinctive yellow to yellowish-green color and often fibrous or acicular crystal habit. It is strongly radioactive due to its high uranium content. Uranophane is typically found as coatings, crusts, or radiating aggregates in the oxidized zones of uranium deposits. It is a member of the uranophane group of minerals, which includes uranophane-beta (a dimorph) and other related uranyl silicates.

How to Identify

Color

Typically lemon-yellow to yellowish-green, sometimes straw-yellow or brownish-yellow. The color is often vibrant and characteristic.

Luster

Silky to vitreous, sometimes dull or earthy in massive forms.

Texture

Often fibrous, acicular (needle-like), or radiating aggregates. Can also be massive, earthy, or as powdery coatings.

Crystal Form

Monoclinic. Crystals are typically acicular to fibrous, elongated along [010], often forming radiating aggregates, tufts, or felted masses. Individual crystals are usually very small.

Cleavage

Perfect on {100}, good on {001}. This can be difficult to observe due to the small crystal size and fibrous habit.

Geological Environment

Oxidized zones of uranium ore deposits, particularly those associated with granitic or pegmatitic rocks, sandstones, or other host rocks containing primary uranium minerals. It forms as a secondary alteration product.

Key Facts

• Hardness: 2.5-3 on the Mohs scale
• Specific Gravity: 3.81-3.98 g/cm³
• Crystal System: Monoclinic
• Color: Lemon-yellow, yellowish-green, straw-yellow, brownish-yellow
• Luster: Silky, vitreous, dull, earthy
• Transparency: Transparent to translucent
• Fracture: Uneven to splintery
• Cleavage: Perfect on {100}, good on {001}
• Composition: Hydrated Calcium Uranyl Silicate

Quick Check

• Color: Yellow to yellowish-green
• Luster: Silky to vitreous
• Streak: Pale yellow to yellowish-white

Physical Characteristics

• Crystal Habit: Acicular, fibrous, radiating aggregates, tufts, felted masses, coatings, crusts, massive, earthy, powdery.
• Cleavage Type: Perfect basal cleavage on {100}, good cleavage on {001}. Often difficult to observe due to fine-grained nature.
• Fracture Type: Uneven to splintery.
• Tenacity: Brittle.
• Luster Type: Silky to vitreous in crystalline forms; dull to earthy in massive or powdery forms.

Formation

Uranophane is a secondary uranium mineral, typically forming in the oxidized zones of uranium ore deposits. It results from the alteration of primary uranium minerals (like uraninite) in the presence of calcium-bearing solutions and silica. The alteration process involves the dissolution of primary uranium minerals, followed by the precipitation of uranophane under oxidizing and slightly acidic to neutral conditions. It often occurs as a coating or crust on other minerals, or as radiating aggregates in fractures and cavities.

Usage

Uranophane is primarily of scientific interest as an indicator mineral for uranium mineralization and as a subject of study in actinide geochemistry and environmental remediation. It is not used commercially due to its radioactivity and relatively low uranium content compared to primary ores. However, its presence can indicate the potential for economically viable uranium deposits nearby.

Age Distribution

Uranophane is a secondary mineral, forming from the alteration of primary uranium minerals. Its age is therefore dependent on the age of the primary uranium mineralization and the subsequent alteration events, which can span from Precambrian to recent.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates (Nesosilicates, specifically Uranyl Silicates)

Group

Uranophane Group

Crystal System

Monoclinic

Chemical Formula

Ca(UO2)2(SiO3OH)2·5H2O

Composition

Calcium (Ca), Uranium (U), Oxygen (O), Silicon (Si), Hydrogen (H). Contains approximately 56.8% UO3, 6.3% CaO, 14.2% SiO2, and 12.7% H2O.

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