Wulfenite

Description

Wulfenite is a lead molybdate mineral with the chemical formula PbMoO4. It is renowned for its vibrant colors, typically ranging from bright orange-red to yellow, but can also be found in shades of brown, gray, green, or even colorless. It commonly forms in thin, tabular, square, or pyramidal crystals, often with bevelled edges. Its adamantine to resinous luster and high specific gravity are characteristic. Wulfenite is a secondary mineral, meaning it forms from the alteration of pre-existing minerals, specifically in the oxidized zones of lead-molybdenum ore deposits. It is a member of the scheelite group of minerals.

How to Identify

Color

Typically bright orange-red, yellow, or yellowish-orange. Less commonly brown, gray, green, or colorless.

Luster

Adamantine to resinous, sometimes greasy.

Texture

Crystals are typically well-formed, often tabular and square, or pyramidal. Can also be granular or earthy.

Crystal Form

Tetragonal system, commonly forming thin, square, tabular crystals, often with bevelled edges. Also found as bipyramidal or prismatic crystals. Crystals can be very thin and delicate.

Cleavage

Distinct on {011}, but often difficult to observe due to its brittle nature and thin crystal habit.

Geological Environment

Oxidized zones of lead-bearing hydrothermal ore deposits, often associated with limestone or other carbonate host rocks. Found in association with minerals like cerussite, anglesite, galena, limonite, and vanadinite.

Key Facts

• Hardness: 2.5-3 on the Mohs scale (relatively soft).
• Specific Gravity: 6.5-7.0 (very heavy for a non-metallic mineral, due to lead content).
• Crystal System: Tetragonal.
• Color: Varies widely, most commonly yellow, orange, or orange-red. Can also be brown, gray, green, or colorless.
• Luster: Adamantine to resinous, sometimes greasy.
• Transparency: Transparent to translucent.
• Fracture: Uneven to subconchoidal.
• Cleavage: Distinct on {011}, but often imperfect and difficult to observe.
• Composition: Lead Molybdate (PbMoO4).

Quick Check

• Color: Yellow to orange-red (most common), brown, gray, green, colorless.
• Luster: Adamantine to resinous.
• Streak: White to yellowish-white.

Physical Characteristics

• Crystal Habit: Typically thin, tabular, square crystals, often with bevelled edges. Also bipyramidal, prismatic, granular, or earthy.
• Cleavage Type: Distinct {011}, but often imperfect.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Adamantine to resinous.

Formation

Wulfenite (PbMoO4) is a secondary mineral that forms in the oxidized zone (supergene zone) of lead-bearing hydrothermal ore deposits. It typically forms through the alteration of primary lead minerals (like galena, PbS) and molybdenum-bearing minerals (like molybdenite, MoS2) by circulating oxygenated groundwater. The lead is derived from the dissolution of galena, and the molybdenum from molybdenite or other primary molybdenum minerals. The presence of oxygen and carbonate (often from limestone host rocks) facilitates the formation of lead carbonates and subsequently, in the presence of molybdenum, wulfenite. It often occurs in arid or semi-arid environments where oxidation processes are more pronounced.

Usage

Wulfenite is primarily a collector's mineral due to its striking colors and crystal habits. It is also a minor ore of molybdenum, though not economically significant compared to molybdenite. Historically, it has been used as a source of lead and molybdenum in small-scale operations. Its aesthetic appeal makes it highly valued in the mineral specimen market.

Age Distribution

Wulfenite forms in the oxidized zones of lead-bearing hydrothermal ore deposits, which can range in age from Precambrian to Cenozoic, depending on the specific geological setting of the primary lead mineralization.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Molybdates (part of the Tungstate group, specifically a Molybdate of Lead)

Group

Scheelite Group

Crystal System

Tetragonal

Chemical Formula

PbMoO4

Composition

Lead (Pb) 56.43%, Molybdenum (Mo) 26.13%, Oxygen (O) 17.44% (by weight)

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