Millerite
Nickel sulfide (NiS)
A nickel sulfide famous for delicate brass-yellow hairlike crystals that form radiating sprays inside cavities and geodes.
- Mohs hardness
- 3-3.5
- Color
- Pale brass-yellow to bronze, often iridescent tarnish
- Type
- mineral
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Overview
Millerite is a nickel sulfide (NiS) instantly recognizable for its extremely slender, brass-yellow to bronze, hairlike or needle-like crystals. These delicate fibers often form radiating tufts and matted sprays lining cavities, earning the nicknames "hair pyrites" and "capillary pyrites."
The crystals are thin and brittle, with a bright metallic luster that frequently develops an iridescent gray or greenish tarnish. Despite its fragility, millerite is a striking and popular collector mineral.
Named after the British mineralogist William Hallowes Miller, it is also a minor ore of nickel where it accumulates in quantity.
Formation & geology
Millerite typically forms at low temperatures, often as a secondary mineral lining cavities, vugs, and geodes in carbonate rocks, coal seams, and hydrothermal veins. The classic occurrence is as radiating sprays of fine needles inside geodes, such as those in the limestones and dolomites of the U.S. Midwest.
It crystallizes in the trigonal system and is found alongside other nickel minerals, pyrite, marcasite, calcite, and quartz. Notable localities include the Halls Gap area of Kentucky, the Keokuk geode region of Iowa/Illinois/Missouri, Glamorgan in Wales, and various nickel deposits worldwide.
How to identify it
The defining feature is the habit: extremely fine, brass-yellow to bronze hairlike crystals in radiating or matted clusters, usually inside a cavity. They are flexible-looking but brittle and easily damaged.
The streak is greenish-black, hardness is low (Mohs 3-3.5), and luster is bright metallic. Look-alikes include brassy pyrite and marcasite (which form blocky or bladed crystals, not hairs) and golden rutile or goethite needles (which are harder and not nickel sulfides). The slender capillary habit combined with brass color is essentially diagnostic.
Uses & significance
Millerite is a minor ore of nickel where it occurs in workable concentrations, but its primary value is as a collector's specimen. The delicate radiating sprays inside geodes are highly prized and command good prices when undamaged.
It has no gemstone use because the crystals are far too fine and fragile, and it has little metaphysical tradition. Specimens require careful handling and protective storage, as even gentle contact can break the fragile needles.
Frequently asked questions
Why is millerite called hair pyrites?
Because it forms extremely fine, hairlike brass-yellow crystals that resemble strands of hair, often in radiating tufts.
Where is millerite commonly found?
Frequently inside geodes and cavities in limestone, dolomite, and coal-related rocks, with famous occurrences in the U.S. Midwest and Wales.
Is millerite magnetic?
No, millerite itself is not magnetic; the brassy color can suggest pyrite, but its hairlike habit and nickel content distinguish it.
How do you tell millerite from pyrite?
Millerite forms slender hairlike needles and is softer, while pyrite forms cubic or blocky crystals and is much harder (Mohs 6-6.5).
Millerite guides
In-depth guides for identifying, valuing, and understanding Millerite.
