Rhabdoborite-(Mo)

Chemical formula: Mg<sub>12</sub>Mo<sup>6+</sup><sub>1.33</sub>O<sub>6</sub>(BO<sub>3</sub>)<sub>6</sub>F<sub>2</sub>

An extremely rare magnesium and molybdenum borate, forming microscopic, acicular crystals in magnesian skarns.

## Characteristics Rhabdoborite-(Mo) is a very rare borate mineral, being the molybdenum analogue of rhabdoborite-(V) and rhabdoborite-(W). It forms colorless, transparent crystals with an acicular or hair-like habit, not exceeding 0.5 mm in length and only a few micrometers in thickness. It occurs as radial or tangled aggregates, and also as single needles grown on other minerals. Its name refers to the characteristic rod-like (Greek *rhabdos*) form of the crystals and the dominant boron and molybdenum in its composition. ## Physical Properties The crystals exhibit a vitreous luster. Due to their small size and fragility, hardness has not been precisely measured. The density calculated from the chemical formula and unit cell parameters is 2.97 g/cm³. The mineral is non-fluorescent under ultraviolet light. ## Colors and Varieties The mineral is colorless, with a white streak. No colored or commercial varieties are known. ## History and Name It was officially recognized by the International Mineralogical Association (IMA) in 2018 (IMA2017-091). It was discovered at the Kopeysk magnesian skarn deposit, near the city of Kopeysk in the Chelyabinsk Oblast in the Southern Urals, Russia. This is its only known occurrence worldwide (type locality). The name comes from the Greek word *rhabdos* (ῥάβδος) meaning "rod" or "stick", which alludes to the acicular form of the crystals, and from the elements in its composition - boron and molybdenum.

Properties

Luster
Vitreous
Streak
White
Density
2.97
Cleavage
None
Fracture
Uneven
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of rhabdoborite-(Mo) based on visual characteristics is impossible. Due to its microscopic size and similarity to many other acicular minerals (e.g., from the ludwigite, szaibelyite group), the only reliable method of identification is advanced chemical analysis (energy-dispersive spectroscopy, EDS) and structural analysis (X-ray diffraction, XRD). A key clue is its occurrence in a specific mineral assemblage in magnesian skarns in the Urals. ## Distinguishing from Similar Minerals It can be confused with other acicular borates, such as szaibelyite, ludwigite, and especially with its vanadium and tungsten analogues. Differentiation requires chemical analysis to confirm the dominance of molybdenum. ## Crystal Forms The mineral forms exclusively acicular and hair-like crystals, collected in radial, stellate, or haphazardly tangled aggregates. Individual crystals are elongated along the crystallographic c-axis.

Geological environment

## Genesis Rhabdoborite-(Mo) forms under conditions of high-temperature contact metamorphism. It is a product of metasomatic processes within magnesian skarns that formed at the contact of dolomites with gabbroid intrusions. ## Mineral Associations This mineral occurs in close association with other borates and skarn minerals. It is most often accompanied by: ludwigite, szaibelyite, forsterite, calcite, dolomite, magnetite (magnesium-rich variety - magnomagnetite), as well as other rare borates such as shabynite or karlite. ## Localities The only confirmed occurrence of rhabdoborite-(Mo) in the world is the Kopeysk deposit in the Chelyabinsk Oblast in the Southern Urals, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" mineral, specimens with rich, dense clusters of well-formed, radial aggregates of acicular crystals are most highly valued. Additional value is provided by a contrasting matrix (e.g., white calcite) and the presence of other rare associated minerals. Due to its colorless nature, the main criterion is the form and richness of the aggregates, not the color. ## Popular Localities All specimens available on the collector's market come from the only known locality - the Kopeysk deposit in the Urals, Russia.

Care and storage

## Cleaning Rhabdoborite-(Mo) specimens are exclusively micromounts. They should be cleaned with the utmost care, preferably using compressed air from a safe distance to remove dust. Any contact with water or chemicals is not recommended. ## What to Avoid Avoid ultrasonic cleaners, all chemical solvents, acids, and sudden temperature changes. The crystals are extremely fragile and sensitive to shocks and abrasions. ## Storage Specimens should be stored exclusively in specialized "micromount" boxes, protected from dust, moisture, and vibrations. Exposure to strong light is not recommended, although there is no evidence of its degrading effect.

Sources

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