Wairauite

Chemical formula: CoFe

Wairauite is a rare, naturally occurring cobalt-iron alloy found in ultramafic rocks.

## Characteristics Wairauite is a natural alloy (metallic mixture) of cobalt and iron. It occurs as very fine, irregular grains, usually less than 100 micrometers in size, dispersed in a matrix of other minerals, most commonly serpentinites. Due to the microscopic size of the grains, it does not form crystals or specimens visible to the naked eye. In polished sections under a microscope, it has a metallic luster and a steel-gray color. ## Physical Properties This mineral is opaque and characterized by a metallic luster. Its Mohs hardness is 5, and its calculated density is very high, reaching 8.32 g/cm³, which is typical for metal alloys. ## Colors and Varieties The color of wairauite is uniform, steel-gray. No color varieties or commercial forms are distinguished. ## History and Name The mineral's name comes from the Wairau Valley in the Marlborough region of New Zealand, where its type locality (Red Hills) is situated. It was officially described and recognized as a new mineral in 1964. ## Applications Due to its extreme rarity and microscopic occurrence, wairauite has no industrial applications. It is solely of scientific interest and is sought after by specialized collectors of rare minerals.

Properties

Mohs hardness: 5
Luster: Metallic
Streak: Black
Density: 8.32
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Isometric

Diagnostic features

## Identification Identification of wairauite is impossible without specialized equipment. It requires analysis under a reflected light microscope (ore microscope) on a polished section and confirmation of chemical composition using methods such as electron microprobe (EDS/WDS). Its high density and metallic nature are clues, but do not allow for unambiguous identification. ## Distinguishing from Similar Minerals Under a microscope, it can be confused with other rare metal alloys, such as awaruite (Ni₃Fe) or pure cobalt or iron. Definitive differentiation requires chemical analysis to determine the cobalt-to-iron ratio. ## Crystal Forms It crystallizes in the isometric system, but it is never observed in the form of well-developed crystals. It occurs exclusively as anhedral (irregular), rounded grains and inclusions within other minerals.

Geological environment

## Genesis Wairauite forms under specific conditions in ultramafic rocks, such as dunites and peridotites, that have undergone serpentinization. This process occurs at low temperatures and involves reducing conditions, which allow for the precipitation of pure metals and their alloys instead of oxides or sulfides. ## Mineral Associations It most commonly co-occurs with minerals from the serpentine group (antigorite, lizardite, chrysotile), awaruite, chromite, magnetite, pentlandite, and chalcopyrite. ## Localities The most important and type locality is the serpentinized peridotites in Red Hills, Wairau Valley, New Zealand. Beyond this locality, its presence has been reported in only a few other places worldwide, including the ophiolite complex in the Polar Urals in Russia and in Japan.

Rarity

Very rare

For collectors

## Quality Criteria Since wairauite occurs as microscopic inclusions, there is no market for specimens in the traditional sense. Collectible value lies in possessing a sample (usually a polished section or a small rock fragment) with analytically confirmed presence of this mineral. The most valuable samples are those from the type locality in New Zealand, with the highest possible concentration of wairauite grains, documented by appropriate analyses and microscopic images.

Care and storage

## Cleaning Specimens containing wairauite (usually polished sections or fragments of host rock) should only be cleaned with a soft, dry cloth or a brush to remove dust. Do not use water or any chemical agents. ## What to Avoid As a natural iron alloy, wairauite is susceptible to oxidation (rusting). Contact with water, moisture, and chemicals that can accelerate this process should be strictly avoided. It should also be protected from acids and other aggressive substances. ## Storage Wairauite samples should be stored in a dry place, preferably in an airtight container or display case with a desiccant (e.g., silica gel) to minimize the risk of oxidation. Polished sections are best kept in special boxes that protect against scratching.

External references

https://www.mindat.org/min-4231.html