AWARUITE
Awaruite – Ni3Fe – is a rare alloy. It generally does not constitute an ore; however, a low-grade awaruite deposit is currently being developed in British Columbia (Canada), with the awaruite disseminated within an ultramafic/ophiolitic complex.
It may contain Cu, Co, S, P, and Si. There is an Ir-bearing variety.
An important characteristic of awaruite is that it is strongly magnetic.
1. Characteristics
Crystal system: Cubic hexaoctahedral
Color: Silvery-white to grayish-white. Does not tarnishes.
Habit: Massive grains, flakes, chips in rock, pebbles in placers. Rare crystals, up to 4 mm.
Cleavage: No.
Tenacity: Malleable and flexible.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 5
Parting: No.
Streak: No information available.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 7.8 – 8.22
2. Geology and Deposits
Awaruite is a product of the serpentinization of ultramafic igneous rocks (e.g., chromitites, peridotites, ophiolites) resulting from low-temperature (~<200°C) regional metamorphism. During the serpentinization process, Ni and Fe are released, creating conditions favorable for the formation of awaruite. The Ni may originate from olivine and enstatite – which contain approximately 0.2% Ni – or from pentlandite.
Awaruite can also be found in alluvial deposits (placers) resulting from the weathering and erosion of these serpentinized rock masses.
It occurs in meteorites, though it is rare.
3. Mineral Associations
In serpentinized ultramafic rocks, it is associated with minerals characteristic of this paragenesis, such as chromite, serpentine (lizardite), nephrite, magnetite, hematite, pyrrhotite, chalcocite, wairauite, native copper, and nickel-bearing copper. There, it also associates with other Ni minerals such as millerite, pentlandite, orcelite, heazlewoodite, oregonite, violarite, and mackinawite.
Associated secondary minerals include goethite (limonite), cuprite, and malachite.
In meteorites, it forms rims or regular intergrowths with kamacite. There, it also associates with graphite, schreibersite, and allabogdanite.
In placers, it is associated with native gold, magnetite, epidote, and garnet (andradite) (Josephine County, Oregon, USA).
4. Transmitted Light Microscopy
Not applicable, as awaruite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Awaruite easily takes a very good, exceptionally high-quality polish, unlike magnetite, which is difficult to polish and yields a poorer finish. Awaruite’s polishing hardness is slightly higher than that of heazlewoodite, similar to that of pentlandite, and lower than that of magnetite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Pure white; the outer, younger portions may exhibit a slight bluish tint. Depending on the composition, it may display a pale green tint.
Compared to the color of magnetite, the color of awaruite is intense white.
Compared to the color of chromite, the color of awaruite is intense white.
Compared to the color of heazlewoodite, the color of awaruite is white, lacking the yellow tint found in heazlewoodite.
Compared to the color of nickeline, the color of awaruite is lighter.
Compared to the color of native bismuth, the color of awaruite is darker.
Pleochroism: No.
Reflectivity: 65%. (Very high!)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: few other minerals, considering the paragenesis.
Magnetite exhibits a lower-quality polish and a gray-brown color; it is not white.
Magnetic platinum, wairauite, and native iron, if occurring as very small grains, are similar.
General Characteristics:
Grain shape: awaruite exhibits significant variations in habit, including those listed below:
– skeletal or elongated tabular crystals, rarely acicular crystals or
nearly perfect cubes a few microns in diameter,
– granular masses with grains forming a friable fabric intergrown with
abundant silicates (likely a pseudomorph),
– narrow veins within magnetite and silicates,
– compact masses with inclusions of serpentine and chlorite, occasionally intergrown with native copper and up to eight different sulfides,
– rhythmic botryoidal or reniform masses, sometimes alternating with native copper,
– pseudomorphs after magnetite and possibly other minerals,
– fine dust disseminated in silicates,
– irregular rims around pentlandite,
– with heazlewoodite, the two (awaruite and heazlewoodite) surrounded by magnetite.
No tarnishing occurs on polished sections.
Cleavage is absent.
Substitutions: awaruite replaces various other minerals, including sulfides and silicates, but especially magnetite and possibly trevorite.
Recrystallization may be observed in some cases in the outer portions of the grains (“nuggets”), which have been strongly impacted by fluvial transport.
Pseudomorphs after pentlandite are known.
Intergrowths with heazlewoodite, resembling a network, may occur.
Replacement relicts – composed of graphite, native iridium, and pentlandite – may be present within the awaruite.
Inclusions of graphite and native copper may occur.
