HEAZLEWOODITE
Heazlewoodite – Ni3S2 – is a rare sulfide found in serpentinized dunites. It does not constitute an ore.
1. Characteristics
Crystal system: Trigonal trapezohedral.
Color: Light bronze to brass-yellow.
Habit: Massive, fine-grained. Small, rare crystals; tabular or pseudo-cubic rhombohedral.
Cleavage: No.
Tenacity: Brittle?
Twinning: Possible.
Fracture: Unequal.
Mohs Hardness: 4
Parting: No.
Streak: Light bronze.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.82
2. Geology and Deposits
Heazlewoodite occurs in serpentinized peridotites (dunites) as sparse, disseminated grains in minor amounts. It forms through the metamorphic exsolution of sulfur contained within the olivine structure. Thus, it likely originates from trace amounts of sulfur and nickel present in magmatic olivine that are displaced from the olivine structure during metamorphic processes. Magmatic olivine can contain up to ~4,000 ppm Ni and up to 2,500 ppm S, either as impurities or substituting for elements such as Fe²⁺ and Mg²⁺.
During metamorphism, Ni and S may combine – primarily during serpentinization and the formation of talc and carbonates – to form minerals such as millerite. Heazlewoodite represents the lowest-sulfur phase within the Ni-sulfide mineral assemblage.
Heazlewoodite also occurs in meteorites, including both iron meteorites and carbonaceous chondrites.
3. Mineral Associations
Heazlewoodite occurs with magnetite, garnet (andradite, grossular), pyroxenes (diopside), serpentine, perovskite, tremolite, sulfides (pyrrhotite, pentlandite, pyrite, sphalerite, chalcopyrite, bornite, chalcocite, millerite), native copper, chromite, bravoite, vaesite, goethite, and cuprite.
It is also associated with certain rare minerals such as zaratite, reevesite, and hellyerite (three hydrated Ni carbonates), awaruite (Ni3Fe), and shandite (Ni3Pb2S2).
4. Transmitted Light Microscopy
Not applicable, as heazlewoodite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: It takes a good polish easily. Its polishing hardness is lower than that of pentlandite, pyrrhotite, awaruite, and millerite, and higher than that of chalcopyrite and bornite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Cream-yellow to creamy-white.
Compared to the color of pyrite, the color of heazlewoodite is slightly darker and more yellow.
Compared to the color of pyrrhotite and pentlandite, the color of heazlewoodite is lighter and more yellow.
Compared to the color of awaruite, the color of heazlewoodite is more yellow.
Pleochroism: No.
Reflectivity: 57.78%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Moderate to very weak, ranging from brown and bluish-gray to lilac and greenish-gray (conflicting data in the literature).
Internal reflections: No.
May be confused with: Identifying heazlewoodite is difficult; it is necessary to consider the paragenesis and pay attention to its characteristic anisotropy.
Pyrite exhibits a similar reflection color, but its polish is much poorer, and it is isotropic.
General Characteristics:
Grain shape: generally massive or granular, occurring as isolated rounded grains. It may form aggregates resembling mosaics. When crystals are present, they are tabular or form pseudo-cubic rhombohedra. Dendritic or curved aggregates of heazlewoodite within pentlandite sometimes occur.
Cleavage is often clearly visible.
Parting becomes visible when heazlewoodite alters to millerite.
Intergrowths with pentlandite, magnetite, and awaruite occur.
Inclusions IN heazlewoodite can consist of pentlandite (lenticular), awaruite (cubic), millerite (“flames”), sphalerite, native nickel, bornite, and chalcocite.
Inclusions OF heazlewoodite occur in chalcopyrite, bornite, pentlandite, and chalcocite.
Lamellar twinning is not uncommon.
Veins of magnetite in heazlewoodite occur.
Substitutions 1: heazlewoodite is replaced by millerite (which forms tabular crystals in this case), or by magnetite, pentlandite, chalcocite or bornite.
Substitutions 2: heazlewoodite replaces magnetite.
Crusts around heazlewoodite may be of bravoite, shandite, magnetite, or perovskite.
Alteration of heazlewoodite can form bravoite, vaesite, millerite, pentlandite and goethite.
