Wakabayashilite

Chemical formula: (As,Sb)₆As₄S₁₄

A rare arsenic and antimony sulfide, forming characteristic, flexible crystals of intense lemon-yellow color and silky luster.

## Characteristics Wakabayashilite is an antimony and arsenic sulfosalt, known for forming unusual, acicular or fibrous crystals. They typically occur as tangled, woolly, or felt-like aggregates, with individual crystals resembling thin hairs. The most distinctive feature of wakabayashilite is its flexibility and pliability – individual fibers can be bent, which is very unusual for minerals. Specimens have an intense, lemon-yellow to orange-yellow color. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1.5. It has a silky luster, and a resinous luster on fresh surfaces. It is translucent to opaque. Its density is significantly higher than average, at approximately 4.02 g/cm³. The crystals are distinctly flexible and elastic. ## Colors and Varieties Wakabayashilite occurs in uniform, bright shades from lemon-yellow, through golden-yellow, to orange-yellow. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1970 by A. Kato, K. Sakurai, and K. Ohsumi. Its name honors the Japanese mineralogist and crystallographer, Professor Yaichiro Wakabayashi (1874–1943), who first studied specimens of this mineral from the Nishinomaki mine in Japan as early as 1928, mistakenly identifying them as orpiment. ## Uses Due to its extreme rarity, small crystal size, and low hardness, wakabayashilite has no industrial applications. It is solely an object of interest for collectors of rare minerals and scientific institutions.

Properties

Mohs hardness: 1.5
Color: Lemon-yellow to orange-yellow
Luster: Silky
Streak: Orange-yellow
Density: 3.98
Cleavage: Good on {11-20}
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features of wakabayashilite are its unique form – flexible, elastic, acicular or fibrous crystals forming tangled aggregates. The intense lemon-yellow color, silky luster, and very low hardness (1.5 on the Mohs scale) are also characteristic. Reaction to light (darkening) can be an additional clue. ## Distinguishing from Similar Minerals Wakabayashilite is sometimes confused with orpiment, with which it often co-occurs. Orpiment has a similar color but forms tabular or prismatic crystals and has perfect cleavage, and its crystals are not flexible. Other yellow sulfides, like orpiment, also do not exhibit elasticity. ## Crystal Forms Crystals are strongly elongated in the form of needles or hairs, often with a hexagonal cross-section. They usually form dense, tangled, felt-like or woolly aggregates and crusts. More rarely, they occur as radial aggregates.

Geological environment

## Genesis Wakabayashilite is a low-temperature hydrothermal mineral. It forms in ore veins rich in arsenic and antimony. It crystallizes from hydrothermal solutions or as an alteration product of other arsenic minerals. ## Mineral Associations It most commonly co-occurs with realgar and orpiment. It is also accompanied by stibnite, calcite, quartz, getchellite, and other rare arsenic and antimony sulfosalts. ## Localities The most important and classic localities for this mineral are the Nishinomaki mine in Gunma Prefecture, Japan (type locality), and deposits in Nevada, USA, especially in the Getchell, Twin Creeks, and Turquoise Ridge mines in Humboldt County. It is also known from the Jas Roux mine in France and the Balcooma deposit in Queensland, Australia.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, long, and flexible crystals, forming rich, dense aggregates of intense, lemon-yellow color. A contrasting association with other minerals, such as red realgar or white calcite, significantly enhances the visual appeal and value of the specimen. Size and lack of damage to the delicate fibers are also important. ## Popular Localities The most famous localities, providing the best quality collector specimens, are in Nevada (USA), particularly from the Getchell and Twin Creeks mines. Specimens from these locations are characterized by exceptionally beautiful, bright yellow, felt-like aggregates, often on a contrasting matrix.

Care and storage

## Cleaning Wakabayashilite specimens are extremely delicate and sensitive. Mechanical cleaning is not recommended. Only very careful dust removal with a soft brush or a stream of compressed air from a distance is permissible. Avoid contact with water and all chemicals. ## What to Avoid The mineral is sensitive to sunlight, which can cause fading and gradual decomposition. High temperatures, humidity, and all chemicals, especially acids and bases, should be avoided. Due to its low hardness and flexibility, it is very susceptible to mechanical damage – even touch can destroy delicate aggregates. ## Storage It should be stored in a closed, darkened container (e.g., a "micromount" box), away from sources of light, heat, and humidity. It is best displayed under controlled conditions, protected from dust and direct lighting. Due to its arsenic content, hands should be washed after contact with the mineral.