Wiluite

Chemical formula: Ca₁₉(Al,Mg)₁₃(B,&#₉₇₂₃;,Al)₅(SiO₄)₁₀(Si₂O₇)₄(O,OH)₁₀

Wiluite is a rare, dark green silicate from the vesuvianite group, distinguished by its complex chemical composition with the presence of boron and intricate crystal structure.

## Characteristics Wiluite is a complex boron and calcium silicate belonging to the vesuvianite group. Its name is sometimes mistakenly used as a synonym for green vesuvianite, but wiluite is a distinct mineral, recognized due to the significant boron content in its structure. Typical specimens form well-developed, prismatic or pyramidal crystals with a square cross-section. They often occur as granular aggregates or radial clusters. Characteristic of wiluite is its dark green, almost black color, although brownish specimens also occur. ## Physical Properties Wiluite is characterized by a hardness of 6 on the Mohs scale, placing it in the same category as feldspars. It has a vitreous luster and is a relatively dense mineral, with a specific gravity of approximately 3.36 g/cm³. Crystals can be transparent to translucent, but due to their intense, dark color, most specimens appear opaque. ## Colors and Varieties The dominant and most typical color of wiluite is a dark, deep green, often with an olive or brownish tint. Some specimens can be almost black. There are no formal color varieties or trade names for this mineral. ## History and Name The mineral's name comes from its discovery locality along the Vilyui River in Yakutia (Sakha Republic), Russia. It was first described and recognized as a distinct mineral species in 1998 by a team of mineralogists: L.P. Vergasova, S.K. Filatov, V.V. Ananiev, T.F. Semenova, and P.C. Burns. Previously, these specimens were often mistakenly identified as vesuvianite. ## Uses Wiluite has no industrial applications. Its significance is limited solely to the scientific and collecting spheres, where it is valued as a rare member of the vesuvianite group, originating from a classic locality.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Colourless
Density: 3.36
Cleavage: Poor cleavage on {100}.
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Wiluite is most easily recognized by its characteristic dark green color, tetragonal crystal habit (short prisms terminated by pyramids), and vitreous luster. However, the association with its locality is key – the vast majority of specimens come from the Vilyui River region in Russia. A hardness of 6 on the Mohs scale and the absence of distinct cleavage are also helpful features. ## Distinguishing from Similar Minerals Wiluite is most often confused with vesuvianite, to which group it belongs. Differentiation is extremely difficult without advanced chemical analysis (to confirm the presence of boron). Typically, specimens from the type locality in Russia, with a dark green color, are classified as wiluite. It can also be confused with dark varieties of epidote or diopside, but these differ in crystal habit and hardness. ## Crystal Forms Wiluite crystals are typically tetragonal, developing forms of short, columnar prisms with a square cross-section, often terminated by pyramid faces. It also occurs as granular aggregates and in the form of radial or fibrous aggregates embedded in the host rock.

Geological environment

## Genesis Wiluite is a mineral formed in contact metamorphism and metasomatism processes. It forms in skarns – rocks created at the contact of magmatic intrusions (most often granitoid) with calcium carbonate-rich sedimentary rocks, such as limestones or dolomites. High temperature and pressure, as well as the presence of fluids rich in silica, aluminum, boron, and other elements, lead to its crystallization. ## Mineral Associations This mineral often co-occurs with other minerals typical of a skarn environment. Its most common associates include vesuvianite, grossular (a variety of garnet), diopside, wollastonite, as well as calcite and quartz. ## Localities The most important and classic occurrence of wiluite, from which the best specimens originate, is the Vilyui River basin in Yakutia (Sakha Republic), Russia. This is the so-called type locality. Specimens from this region serve as a standard for this mineral and are most sought after by collectors.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized wiluite specimens are those that form sharp, well-developed and terminated crystals with an intense, dark green color and strong luster. Crystals grown on a contrasting rock matrix are particularly sought after. Crystal size is very important – larger, undamaged specimens fetch significantly higher prices. Transparency, though rare for this mineral, also increases its value. ## Popular Localities The only commercially significant and historically important source of wiluite is its type locality in the Vilyui River basin in Russia. Specimens from this location are considered classic and most desirable in the collector's market.

Care and storage

## Cleaning Wiluite specimens should be cleaned carefully, using a soft brush and distilled water. Mild soap can be used, but it must be thoroughly rinsed off. Ultrasonic cleaners are not recommended, as they can damage crystals, especially if they contain internal fractures. ## What to Avoid The mineral is sensitive to strong acids, which can damage it. Sudden temperature changes should be avoided, as they can cause cracking. Prolonged exposure to intense sunlight is not recommended, although there is no evidence of it fading. ## Storage Wiluite is a relatively hard and stable mineral, so it can be stored under standard conditions. It is best kept in separate boxes or on stands to prevent scratching by harder minerals (e.g., quartz, topaz). Protect from dust and chemicals.