Nikmelnikovite

Chemical formula: Ca₁₂Fe²⁺Fe³⁺₃Al₃(SiO₄)₆(OH)₂₀

Nikmelnikovite is a very rare mineral from the garnet group, a hydrated calcium, iron, and aluminum silicate, found in skarns in Russia.

## Characteristics Nikmelnikovite is a complex, hydrated silicate belonging to the hydrogarnets within the garnet supergroup. It forms very small, single, isometric crystals up to 0.3 mm in size, typically with a rhombic dodecahedral habit. Crystals are most often overgrown on other minerals, mainly calcite. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Nikmelnikovite crystals are characterized by a vitreous luster. The mineral is brittle, and its Mohs hardness is approximately 5. The density calculated based on the chemical formula and unit cell parameters is 3.43 g/cm³. It is a non-fluorescent mineral. ## Colors and Varieties Nikmelnikovite occurs in dark brown to almost black colors. No varieties have been distinguished. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 2018 under number 2018-038. Its name honors Nikolai Nikolaevich Melnikov (born 1938), a Russian scientist specializing in geomechanics and mining, director of the Institute of Mining Problems of the North at the Kola Science Centre of the Russian Academy of Sciences. It was discovered in archival material collected in the 1960s. ## Uses Nikmelnikovite has no practical application and is solely an object of scientific and collecting interest for specialized collectors of rare minerals.

Properties

Mohs hardness
5.5
Color
Brown to red
Luster
Vitreous
Streak
Light brown
Density
3.00
Cleavage
None
Fracture
Conchoidal
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of nikmelnikovite is possible only through advanced analytical methods, such as Raman spectroscopy, chemical analysis (EDS/WDS), and X-ray diffraction (XRD). Visually, it is practically impossible to distinguish from other dark garnets without appropriate magnification and analysis. It occurs as very small, dark brown to black, isometric crystals. ## Distinguishing from Similar Minerals It can be confused with other dark garnets from the andradite and grossular groups, such as schorlomite or melanite, which may occur in similar geological environments. Definitive differentiation requires specialized laboratory tests to confirm the chemical composition, especially the presence of hydroxyl (OH) groups, which is a characteristic feature of hydrogarnets. ## Crystal Forms Nikmelnikovite forms single, well-formed, isometric crystals, mainly in the form of a rhombic dodecahedron {110}. Crystal sizes do not exceed 0.3 mm.

Geological environment

## Genesis Nikmelnikovite forms in metamorphic rocks of the skarn type. In its type locality (Zhelezny mine), it was found in magnesium skarn, in cavities within calcite. Its formation is associated with metasomatic processes at the contact of a magmatic intrusion with carbonate sedimentary rocks. ## Mineral Associations This mineral co-occurs with calcite, andradite, diopside, forsterite, phlogopite, clinochlore, magnetite, perovskite, apatite, and baddeleyite. ## Localities The only confirmed occurrence of nikmelnikovite in the world is its type locality: Zhelezny mine (Rudnik Zheleznii) in the Kovdor region, on the Kola Peninsula in the Murmansk Oblast, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" mineral, the value of a specimen is primarily determined by the quality and development of the crystals, rather than their size. The most desirable specimens are those with sharp, undamaged crystals with a distinct luster, set on a contrasting matrix, for example, on white calcite. A rich association with other minerals is also important. ## Popular Localities The only source of specimens is the Zhelezny mine on the Kola Peninsula in Russia. The material comes mainly from historical collections, making it extremely difficult to acquire.

Care and storage

## Cleaning Due to its extreme rarity and small crystal size, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove loose dust particles. Avoid contact with water and chemicals. ## What to Avoid Avoid all acids and strong bases, which can damage the mineral. Crystals are brittle and susceptible to mechanical damage, so ultrasonics should be avoided. Do not heat the specimen due to the presence of hydroxyl groups in its structure. ## Storage Nikmelnikovite specimens should be stored under stable conditions, in a specialized "micromount" box, protected from dust, moisture, and physical damage. Avoid exposure to direct sunlight and sudden temperature changes.

External references

Sources

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