Heazlewoodite

Heazlewoodite, Ni₃S₂, is a rare sulfur-poor nickel sulfide mineral found in serpentinitised dunite. It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallise in the trigonal crystal system. It has a hardness of 4 and a specific gravity of 5.82.

Paragenesis

Heazlewoodite is formed within terrestrial rocks by metamorphism of peridotite and dunite via a process of nucleation. Heazlewoodite is the least sulfur saturated of nickel sulfide minerals and is only formed via metamorphic exsolution of sulfur from the lattice of metamorphic olivine.

Heazlewoodite 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 ~4000ppm Ni and up to 2500ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe²⁺ and Mg²⁺).

During metamorphism, sulfur and nickel within the olivine lattice are reconsitituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration.When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomittant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.

In this environment, nickel sulfide mineralogy converts to the lowest-sulfur state available, which is heazlewoodite.

Heazlewoodite is also known to occur within meteorites in association with troilite and taenite.

Occurrence

Heazlewoodite is known from few ultramafic intrusions within terrestrial rocks. The Honeymoon Well ultramafic intrusive, Western Australia is known to contain heazlewoodite-millerite sulfide assemblages within serpentinised olivine adcumulate dunite, formed from the metamorphic process.

The mineral is also reported, again in association with millerite, from the ultramafic rocks of New Caledonia.

Heazlewoodite was first described in 1896 from Heazlewood, Tasmania, Australia.

See also

* Serpentinite
* Metamorphism
* Olivine
* Dunite or peridotite
* Millerite
* Polydymite
* Vaesite

References

* Guillon J.H. and Lawrence L.J., The opaque minerals of the ultramafic rocks of New Caledonia, "Mineralium Deposita", volume 8, 1973, pp. 115-126.
* [http://webmineral.com/data/Heazlewoodite.shtml Webmineral data]
* [http://www.mindat.org/min-1839.html Mindat with location data]
* [http://rruff.geo.arizona.edu/doclib/hom/heazlewoodite.pdf Mineral Data Publishing - PDF]