| Millerite | |
|---|---|
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| General | |
| Category | Sulfide mineral |
| Formula | NiS |
| IMA symbol | Mlr[1] |
| Strunz classification | 2.CC.20 |
| Crystal system | Trigonal |
| Crystal class | Ditrigonal pyramidal (3m) (sameH-M symbol) |
| Space group | R3m |
| Unit cell | a = 9.607 Å, c = 3.143 Å; Z = 9 |
| Identification | |
| Colour | Pale brass-yellow to bronze-yellow, tarnishes to iridescence |
| Crystal habit | Typically acicular (needle-like) often in radial sprays – also massive |
| Cleavage | Perfect on {1011} and {0112} – obscured by typical form |
| Fracture | Uneven |
| Tenacity | Brittle; capillary crystals elastic |
| Mohs scale hardness | 3–3.5 |
| Luster | Metallic |
| Streak | Greenish black |
| Diaphaneity | Opaque |
| Specific gravity | 5.3–5.5 |
| Other characteristics | brittle and becomes magnetic on heating |
| References | [2][3][4][5][6] |
Millerite ornickel blende is anickelsulfide mineral,NiS. It is brassy in colour and has anacicular habit, often forming radiating masses and furryaggregates. It can be distinguished frompentlandite by crystal habit, its duller colour, and general lack of association withpyrite orpyrrhotite.
Millerite is a common metamorphic mineral replacingpentlandite withinserpentiniteultramafics. It is formed in this way by removal of sulfur from pentlandite or other nickeliferous sulfide minerals duringmetamorphism ormetasomatism.
Millerite is also formed from sulfur poorolivinecumulates by nucleation. Millerite is thought to form from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes.Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within thecrystal lattice, as contaminants and substituting for othertransition metals with similar ionic radii (Fe2+ and Mn2+).[citation needed]
During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization andtalc carbonate alteration. When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfurfugacity.
This forms disseminated needle like millerite crystals dispersed throughout the rock mass.
Millerite may be associated withheazlewoodite and is considered a transitional stage in the metamorphic production of heazlewoodite via the above process.
Millerite, when found in enough concentration, is a very important ore ofnickel because, for its mass as a sulfide mineral, it contains a higher percentage of nickel thanpentlandite. This means that, for every percent of millerite, an ore contains more nickel than an equivalent percentage of pentlandite sulfide.
Millerite forms an important ore constituent of the Silver Swan, Wannaway, Cliffs, Honeymoon Well, Yakabindie and Mt Keith (MKD5) orebodies. It is an accessory mineral associated with nickellaterite deposits inNew Caledonia.
Millerite is found as a metamorphic replacement ofpentlandite within the Silver Swan nickel deposit, Western Australia, and throughout the many ultramafic serpentinite bodies of theYilgarn craton,Western Australia, generally as a replacement of metamorphosed pentlandite. There is one known occurrence of millerite in South Africa, near Pafuri in theTransvaal. The deposit has never been commercially mined.[7]
It is commonly found as radiating clusters of acicular needle-like crystals in cavities in sulfide richlimestone anddolomite or ingeodes. It is also found in nickel-ironmeteorites, such as CKcarbonaceous chondrites.[8]
Millerite was discovered byWilhelm Haidinger in 1845 in the coal mines ofWales. It was named for British mineralogistWilliam Hallowes Miller. The mineral is quite rare in specimen form, and the most common source of the mineral is in the Halls Gap area ofLincoln County, Kentucky in theUnited States.
