Millerite Mineral Overview

• Mineral name: Millerite
• Mineral class: Sulfide
• Chemical formula: NiS (nickel sulfide)
• Crystal system: Trigonal
• Mohs hardness: 3–3.5
• Color range: Brass-yellow, pale gold
• Luster: Metallic
• Streak: Greenish-black
• Cleavage: Poor
• Fracture: Uneven
• Specific gravity: Approximately 5.3–5.5
• Transparency: Opaque
• Common locations: United States, Germany, Czech Republic, Canada
• Uses: Nickel ore, collector mineral
• Similar minerals: Pyrite, chalcopyrite, nickeline

Naming and Classification

Millerite is a nickel sulfide mineral, named after William Hallowes Miller, a British crystallographer.

It is one of the important minerals containing:

• Nickel (Ni)

Although not the primary nickel ore (that role is usually held by pentlandite), millerite is still a significant secondary source.

Physical and Optical Characteristics

Millerite typically occurs as:

• Fine, needle-like (acicular) crystals
• Radiating sprays
• Hair-like tufts or bundles

Its color is usually brass-yellow to pale gold, often brighter and more delicate-looking than pyrite.

The luster is metallic and can appear silky when crystals are fibrous.

With a Mohs hardness of 3–3.5, it is relatively soft.

Crystal Habit (Key Feature)

Millerite is best known for:

• Extremely fine, hair-like metallic needles
• Radiating or tufted clusters

These can resemble:

• Golden fibers
• Metallic “fur” or sprays

This is one of the most distinctive habits among sulfide minerals.

Chemical Composition and Structure

Millerite is composed of:

• Nickel (Ni)
• Sulfur (S)

It has a relatively simple composition compared to many sulfide minerals.

Its trigonal crystal structure supports the formation of:

• Slender, elongated crystals

Formation and Geological Occurrence

Millerite forms in several geological environments:

Hydrothermal Veins

• Deposited from nickel-rich fluids

Secondary Environments

• Alteration of other nickel minerals

Sedimentary Settings

• Occasionally found in limestone cavities

Typical formation conditions include:

• Low to moderate temperatures
• Nickel-rich environments

It is commonly associated with:

• Calcite
• Dolomite
• Other nickel sulfides

Common Locations

Millerite is found in several notable regions:

• United States: Missouri (Joplin area – famous for crystal sprays)
• Germany: Classic European localities
• Czech Republic: Well-known specimens
• Canada: Nickel mining regions

Missouri specimens are especially prized for their delicate crystal sprays.

Uses and Practical Significance

Millerite is used for:

Nickel Ore

• Contributes to nickel extraction

Collector Mineral

• Highly valued for:
  • Unique needle-like crystals
  • Bright metallic appearance

It has limited industrial importance compared to other nickel ores.

Similar and Related Minerals

Millerite can resemble:

• Pyrite: More cubic and less fibrous
• Chalcopyrite: More golden-yellow and less needle-like
• Nickeline: Reddish metallic rather than yellow

The fine, hair-like crystal habit is the key distinguishing feature.

Identification Notes for Collectors

• Look for golden, hair-like metallic crystals
• Note soft hardness (3–3.5)
• Observe metallic luster with silky appearance in clusters
• Check association with carbonate rocks
• Distinguish from pyrite by crystal shape

Millerite is a nickel sulfide mineral, recognized for its delicate, needle-like metallic crystal formations, making it one of the most visually distinctive and collectible nickel minerals.