Julienite

Chemical formula: Na₂Co₂+(S₂-CN₃-)₄·₈H₂O

Julienite is a very rare, water-soluble cobalt thiocyanate, forming blue, acicular crystals.

## Characteristics Julienite is a hydrated sodium and cobalt thiocyanate. It forms very fine, acicular or hair-like crystals of an intensely blue color, often gathered into chaotic clusters, coatings, or woolly aggregates. Due to its extreme rarity and instability, it is a mineral known mainly from a single locality in the world and represents more of a mineralogical curiosity than a collector's item. ## Physical Properties Julienite crystals are extremely delicate and soft. This mineral is water-soluble, which is its characteristic and unusual feature. It has a luster described as silky. ## Colors and Varieties Julienite occurs exclusively in azure blue to dark blue. No varieties are known. ## History and Name The mineral was first described in 1928 by Alfred Schoep. The name commemorates Henri Julien, a Belgian mine administrator at Shinkolobwe, who first observed this mineral. ## Uses Julienite has no practical application and is of purely scientific interest and as a curiosity for specialized collectors.

Properties

Mohs hardness: 1-2
Luster: Silky
Streak: Blue
Density: 1.62
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Diagnostic features of julienite include its intensely blue color, acicular crystal habit, and, most importantly, its water solubility. The reaction to contact with a drop of water is immediate and allows for unambiguous identification. ## Distinguishing from Similar Minerals It can be confused with other secondary blue cobalt minerals, such as erythrite (although erythrite is typically pink, its cobalt-rich varieties can be blue) or some copper sulfates. The key distinguishing test is water solubility, which is not present in similar minerals. ## Crystal Forms Julienite forms very thin, acicular crystals, often elongated and flexible. They occur as tangled, woolly aggregates, coatings, or radiating clusters.

Geological environment

## Genesis Julienite is a secondary mineral, forming in the oxidation zone of uranium-cobalt-nickel deposits. Its formation is likely associated with the reaction of cobalt-containing solutions with organic matter (e.g., bitumens present in rocks), which could have been the source of thiocyanate ions. It is a product of very specific and rare chemical conditions. ## Mineral Associations At its type locality (Shinkolobwe), it co-occurs with uraninite, gypsum, calcite, and black cobalt oxides. ## Localities The only confirmed and well-described occurrence of julienite in the world is the Shinkolobwe (Kasolo) mine in Haut-Katanga Province, Democratic Republic of Congo. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a julienite specimen primarily depends on the abundance and intensity of the blue needles on the rock matrix. Specimens with rich, dense clusters of the mineral, covering a significant area of the host rock, are most valued. Due to its nature, the purity or size of individual crystals are not key criteria. ## Popular Localities All known collector specimens originate from a single location – the Shinkolobwe mine in the DRC. This mine has been closed and inaccessible for years, meaning new specimens do not appear on the market, and historical collections are extremely valuable.

Care and storage

## Cleaning This mineral **must not be cleaned** with water or any other liquids, as it is soluble in them. Specimens should only be cleaned mechanically, dry, using a soft brush or compressed air from a safe distance. ## What to Avoid Contact with water, water vapor, and high humidity must be strictly avoided, as these lead to its permanent destruction. It should also be protected from all chemicals and high temperatures. ## Storage Julienite specimens must be stored in conditions of controlled, low humidity. It is best to place them in a sealed, closable box (e.g., a "perky box") along with a moisture-absorbing agent, such as silica gel. Avoid exposure to direct sunlight.

Sources

IMA Mineral List (CNMNC)