Millerite is a nickel sulfide mineral with the chemical formula NiS. It crystallizes in the trigonal system and typically forms as acicular (needle-like) crystals, often radiating in sprays or velvety crusts.
Properties:
- Composition: Nickel sulfide (NiS)
- Color: Brass-yellow to bronze-yellow, often with iridescent tarnish
- Streak: Greenish-black
- Luster: Metallic
- Hardness: 3-3.5 on the Mohs scale
- Density: 5.3 - 5.6 g/cm³
- Crystal System: Trigonal
- Cleavage: Perfect, parallel to [0001]
- Fracture: Uneven
- Habit: Typically acicular (needle-like) crystals, often radiating in sprays or tufts. Can also be found as massive or disseminated grains.
- Occurrence: Millerite is a relatively rare mineral, typically found in low-temperature hydrothermal veins, as a secondary mineral in sulfide deposits, and in serpentinites.
Occurrence and Formation:
Millerite is typically found in association with other nickel minerals and sulfides, such as pyrite, pyrrhotite, and pentlandite. It forms under relatively low-temperature conditions in hydrothermal veins, often as a result of the alteration of other nickel-bearing minerals. It can also be found in serpentinites, formed from the alteration of ultramafic rocks. Notable occurrences include Germany, Wales, Czech Republic, Canada, and the United States.
Uses:
Millerite is not a major ore of nickel due to its relatively scarce occurrence. However, it serves as a collector's mineral and has scientific importance in understanding the geological processes involved in the formation of sulfide deposits.
Distinguishing Features:
Millerite is distinguished from other brass-yellow sulfides by its characteristic acicular crystal habit and perfect cleavage. Its association with other nickel minerals and its occurrence in specific geological environments can also aid in its identification.