Oreillyite

Chemical formula: Cr<sub>2</sub>N

O'Reillyite is an extremely rare chromium nitride, forming microscopic, hexagonal crystals with a metallic luster, found in kimberlites and peridotite xenoliths.

## Characteristics O'Reillyite is a mineral from the nitride group, chemically corresponding to chromium(II) nitride. It occurs as extremely small, hexagonal crystals, not exceeding several tens of micrometers in size. It typically forms inclusions in other minerals, mainly in diamonds, olivine, and pyrope. Due to its size, its visual features are observable only under high magnification. ## Physical Properties O'Reillyite crystals exhibit a strong, metallic luster. The mineral is opaque. Its hardness and density have not been precisely measured due to the microscopic size of the grains, but they are estimated based on the properties of synthetic chromium nitride. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 2018. Its name honors Prof. Suzanne Y. O'Reilly, an Australian geologist and geochemist from Macquarie University in Sydney, for her contributions to research on the Earth's mantle and diamond-forming processes. It was discovered in peridotite xenoliths from the Orroroo kimberlite pipe in South Australia.

Properties

Luster
Metallic
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Identification of o'reillyite is possible only through advanced laboratory techniques, such as scanning electron microscopy (SEM) combined with chemical composition analysis (EDS) or Raman spectroscopy. It is distinguished by its metallic luster and hexagonal crystal shape visible under very high magnification. ## Differentiation from Similar Minerals Visually (on a micro scale), it may resemble other opaque, metallic inclusions in diamonds or mantle minerals, such as sulfides (pyrite, pyrrhotite) or oxides (magnetite, ilmenite). Definitive differentiation requires chemical analysis, which will show the presence of only chromium and nitrogen. ## Crystal Forms It forms small, hexagonal, tabular or platy crystals. It occurs as single, isolated grains or small clusters of inclusions within other minerals.

Geological environment

## Genesis O'Reillyite is a mineral of deep origin, forming under high pressure and temperature conditions prevalent in the Earth's upper mantle. It forms in a strongly reducing environment, where oxygen availability is very low, allowing for the crystallization of nitrides instead of oxides. Its presence is an indicator of specific geochemical conditions in the diamond formation zone. ## Mineral Associations This mineral occurs as an inclusion in diamonds. It is also found in peridotite xenoliths (mantle rocks) brought to the surface by kimberlitic magmas. It coexists with minerals such as diamond, olivine, pyrope, enstatite, chromite, and other rare mantle minerals. ## Localities Its occurrence has been confirmed in the Orroroo kimberlite pipe in South Australia (type locality) and as inclusions in diamonds from the Koffiefontein mine in the Republic of South Africa.

Rarity

Extremely rare

For collectors

## Quality Criteria O'Reillyite is not a mineral collected for its own sake as standalone specimens. Its value is purely scientific. Host specimens in which it is found (e.g., diamonds, xenoliths) are evaluated according to their own criteria. The presence of confirmed o'reillyite as an inclusion in a diamond can significantly increase its value for research institutions and specialized collections due to its extreme rarity and scientific importance.

Care and storage

## Cleaning Due to its occurrence as microscopic inclusions in other hard minerals (such as diamond or olivine), o'reillyite does not require and is not subject to separate cleaning. The host specimen should be cleaned according to the recommendations for the dominant mineral. ## What to Avoid Aggressive chemicals and acids that could damage the host mineral should be avoided. There are no specific recommendations regarding temperature or light for o'reillyite itself, but the care guidelines for the host mineral should be followed. ## Storage Specimens containing o'reillyite should be stored in stable conditions, protected from dust and mechanical damage. Valuable samples, especially those with diamonds, require specialized containers and security measures.

Sources

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