Anilite

Chemical formula: Cu<sub>7</sub>S<sub>4</sub>

Anilite is a rare copper sulfide that has a metallic, coppery-red color on fresh surfaces, but quickly tarnishes and blackens upon exposure to air.

## Characteristics Anilite is a mineral belonging to the copper sulfide group. It most often forms massive, granular, or earthy aggregates. It is rarely observed in the form of small, platy crystals with a pseudohexagonal outline, not exceeding a few millimeters. A characteristic feature of anilite is its instability in contact with air – a freshly exposed surface has a metallic luster and a coppery-red to brownish color, but it very quickly becomes covered with a dark gray or black, dull tarnish, which is a result of oxidation. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 3. It is opaque and exhibits a metallic luster on fresh fracture surfaces. The density of anilite is quite high, approximately 5.45 g/cm³. ## Colors and Varieties The dominant color of fresh anilite is coppery-red, brownish-red, or brown. Due to oxidation, its surface quickly turns dark gray, bluish-black, or even completely black. No color or commercial varieties are distinguished. ## History and Name Anilite was first described in 1969 by Japanese mineralogists. Its name comes from the Ani mine in Akita Prefecture, Japan, which is its type locality. It was recognized by the International Mineralogical Association (IMA) in the same year. ## Uses Anilite, due to its rarity and small accumulations, has no significance as a copper ore. However, it is an object of interest for collectors specializing in rare minerals or sulfide systematics.

Properties

Mohs hardness
3
Luster
Metallic
Streak
Black
Density
5.45
Cleavage
Good on {001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification A key diagnostic feature of anilite is the rapid change in color and luster on a fresh fracture – from metallic, coppery-red to dull, black. Its relatively low hardness (about 3) and co-occurrence with other copper sulfides are also helpful in identification. ## Distinguishing from Similar Minerals Anilite can be confused with other copper sulfides, such as chalcocite, djurleite, or bornite. It differs from chalcocite and djurleite in the color of its fresh fracture (they are gray). Bornite also tarnishes, but its tarnish is usually colorful and iridescent (purple, blue), whereas the tarnish on anilite is uniformly dark gray to black. Final distinction of these minerals often requires advanced methods, such as X-ray diffraction (XRD). ## Crystal Forms Anilite crystals are extremely rare. If they occur, they take the form of small, thin plates, often with outlines resembling hexagons. However, it usually forms massive, granular aggregates or coatings.

Geological environment

## Genesis Anilite is a mineral of low-temperature hydrothermal origin. It forms in zones of secondary enrichment (supergene enrichment) of copper ore deposits, where it replaces other, earlier-formed copper sulfides. It is a relatively unstable mineral that readily transforms into other sulfides (e.g., djurleite, chalcocite) or copper sulfates under oxidizing conditions. ## Mineral Associations It most commonly co-occurs with other copper sulfides, such as chalcocite, djurleite, digenite, and covellite. Bornite, chalcopyrite, and pyrite may also accompany it. ## Localities The most important and classic locality, from which the best-formed specimens originate, is the Ani mine in Japan. This mineral has also been reported in many other places around the world, including the Butte mines in Montana (USA), the Sudbury region in Canada, the Lubin and Polkowice mines in Poland, and deposits in Chile and Argentina.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with visible, even if small, anilite crystals, which is extremely rare. For massive specimens, a large, freshly exposed fracture surface showing the characteristic coppery color, before the mineral completely blackens, increases its value. Well-documented locality and association with other rare sulfides are also important. ## Popular Localities Undoubtedly, the most desirable and classic locality for this mineral is its type locality – the Ani mine in Japan. Specimens from there, especially those with crystals, fetch the highest prices and are considered the best in the world.

Care and storage

## Cleaning Anilite is a very sensitive and unstable mineral. Its cleaning should be limited to an absolute minimum. If necessary, a very soft brush can be used to remove loose contaminants. All contact with water and other liquids should be avoided. ## What to Avoid Contact with water, chemicals, and even humid air must be absolutely avoided, as these accelerate the oxidation and decomposition process of the mineral, leading to its blackening and loss of luster. It should not be heated or exposed to prolonged light. ## Storage Anilite specimens require special storage conditions. They are best kept in sealed, dust-free containers (e.g., "perky box" type) with a desiccant (silica gel). Storage in stable, low humidity and temperature is crucial for preserving its original appearance.

External references

Sources

Read more