Crowningshieldite

Chemical formula: (Ni<sub>0.9</sub>Fe<sub>0.1</sub>)S

Crowningshieldite is a very rare nickel-iron sulfide, forming hexagonal plates with a metallic luster.

## Characteristics Crowningshieldite is an extremely rare sulfide mineral, chemically classified as a nickel-iron sulfide. It occurs as very small, hexagonal or trigonal plates, reaching sizes up to several tens of micrometers. Its crystals are too small to be seen with the naked eye. It is observed as inclusions in other minerals, mainly in pentlandite. ## Physical Properties Due to the microscopic size of the crystals, most physical properties, such as hardness, density, cleavage, or fracture, have not been precisely determined. The mineral's luster is metallic, and the mineral itself is opaque. ## History and Name The mineral was named in honor of Roger Crowningshield (1920–2005), a renowned gemologist from the Gemological Institute of America (GIA), in recognition of his contributions to research on diamonds and gemstones. It was discovered in nickel mines in the Sudbury Complex in Ontario, Canada, and officially approved as a new mineral by the International Mineralogical Association (IMA) in 1994.

Properties

Luster
Metallic
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Identification of crowningshieldite is possible only using advanced laboratory techniques, such as scanning electron microscopy (SEM) with chemical composition analysis (EDS) or electron microprobe analysis (EMPA). Its characteristic features are its chemical composition with dominant nickel and a small admixture of iron, and the hexagonal shape of its microscopic plates. ## Distinguishing from similar minerals It can be confused with other nickel sulfides, such as millerite or pentlandite, in which it often occurs. Differentiation requires analysis of chemical composition and crystal structure. Unlike pentlandite, which crystallizes in the isometric system, crowningshieldite is trigonal. ## Crystal forms It forms very small, thin, hexagonal or trigonal plates with a maximum size of up to about 50 µm and a thickness of up to 5 µm.

Geological environment

## Genesis Crowningshieldite forms in magmatic nickel-copper sulfide deposits associated with mafic and ultramafic intrusions. It crystallizes in the late stages of sulfide magma crystallization. ## Mineral associations It most commonly occurs as inclusions in pentlandite. It also co-occurs with other sulfides, such as chalcopyrite and pyrrhotite. ## Localities Its occurrence has been confirmed in only a few locations worldwide. The type locality is the Sudbury area in Ontario, Canada (Creighton and Frood mines). It has also been found in the Bushveld Complex in South Africa and the Jinchuan intrusion in China.

Rarity

Extremely rare

For collectors

## Quality criteria Crowningshieldite does not form macroscopic specimens, and therefore is not an object of traditional collector trade. Its value is purely scientific. For research institutions and museums, polished sections (thin sections) are valuable, where the presence of the mineral has been analytically confirmed, and its relationships with other minerals are clearly visible under the microscope.

Care and storage

## Cleaning Due to its microscopic nature and occurrence as inclusions, individual crowningshieldite crystals are not subject to cleaning. Host rock specimens should be cleaned using methods appropriate for the dominant minerals (e.g., pentlandite), usually dry, using a soft brush to remove dust. ## What to avoid Avoid contact with strong acids and chemical reagents, which could damage both crowningshieldite and the surrounding minerals. As a sulfide, it may be susceptible to oxidation in humid environments. ## Storage Specimens containing crowningshieldite should be stored under stable conditions, in a dry environment, to prevent weathering processes. It is best to keep them in closed collector boxes, protected from dust and moisture.

Sources

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