Melliniite

Chemical formula: (Ni,Fe)<sub>₄</sub>P

Melliniite is an extremely rare nickel-iron phosphide, discovered in the Choteau iron meteorite in Montana, USA.

## Characteristics Melliniite is a mineral with a metallic appearance, occurring as microscopic, rounded or irregular grains. It typically forms inclusions within other meteoritic minerals, such as kamacite. Due to the extremely small size of the grains (usually below 50 micrometers), its visual features are difficult to observe without specialized equipment, such as an electron microscope. ## Physical Properties As an opaque mineral, melliniite is characterized by a metallic luster. Its hardness and density have not been precisely measured due to its small size and rarity. The calculated density is approximately 7.85 g/cm³. ## Colors and Varieties In reflected light (in microscopic studies), its color is described as creamy white with a slight pinkish tint. No varieties of this mineral have been distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2016. Its name honors Gianni Mellini (born 1949), an Italian mineralogist and crystallographer from the University of Siena, for his contributions to the study of extraterrestrial and terrestrial minerals. It was discovered in the Choteau iron meteorite, found in 1941 in Montana, USA. ## Uses Melliniite has no commercial or industrial applications. Its significance is purely scientific, serving as an indicator of processes occurring in the parent bodies of asteroids from which meteorites originate.

Properties

Luster: Metallic
Density: 7.85
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of melliniite is possible only through advanced laboratory techniques. This requires chemical analysis (e.g., using an electron microprobe - EDS/WDS) to determine its composition (nickel, iron, phosphorus) and electron or X-ray diffraction to confirm its crystal structure. Visual identification is not possible. ## Distinguishing from Similar Minerals In microscopic studies, melliniite must be distinguished from other nickel-iron phosphides found in meteorites, such as schreibersite ((Fe,Ni)₃P) or nickel-phosphide (Ni₃P). Differentiation is based on precise measurement of the metal-to-phosphorus ratio and structural analysis. ## Crystal Forms This mineral occurs as very fine, anhedral (lacking well-formed faces) or rounded grains, typically ranging from 5 to 50 micrometers in size.

Geological environment

## Genesis Melliniite is a mineral of extraterrestrial origin. It forms under strongly reducing conditions, with very low oxygen content, within the parent bodies of asteroids. It crystallizes during the very slow cooling of a phosphorus-rich iron-nickel melt, which is characteristic of the interiors of some asteroids. ## Mineral Associations Melliniite coexists with other minerals typical of iron meteorites. Its most common associates are kamacite (Fe-Ni alloy), taenite (Fe-Ni alloy), schreibersite ((Fe,Ni)₃P), and cohenite ((Fe,Ni,Co)₃C). ## Localities The only confirmed locality for melliniite worldwide is its type locality – the Choteau iron meteorite, belonging to the IAB group, found in Teton County, Montana, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, melliniite is not an item of collector trade in the form of isolated specimens. Its value is inextricably linked to the scientific and collector value of the meteorite itself in which it was found. For meteorite collectors, the presence of such a rare mineral can enhance the attractiveness and price of a specimen (slice or fragment), especially if its presence is confirmed by research. ## Popular Localities The only source of melliniite is the Choteau meteorite. Fragments of this meteorite are found in institutional and private collections, but are extremely rare on the market.

Care and storage

## Cleaning Due to its microscopic nature and occurrence as inclusions in meteorites, individual melliniite grains are not subject to cleaning. Meteorite specimens containing this mineral should be treated according to general meteorite conservation principles. ## What to Avoid Moisture and aggressive chemicals should be avoided, as they could damage both melliniite and its surrounding metallic matrix (mainly iron-nickel alloys). Iron meteorites are susceptible to rusting in humid environments. ## Storage Meteorite specimens containing melliniite should be stored in a dry environment, preferably in specialized containers with a desiccant (e.g., silica gel), to prevent corrosion.

Sources

IMA Mineral List (CNMNC)