Richardsollyite

Chemical formula: Tl₁+Pb₂+As₃+S₂-₃

Richardsollyite is a rare lead, thallium, and arsenic sulfosalt, forming small, dark crystals with a metallic luster.

## Characteristics Richardsollyite is a very rare mineral from the sulfosalt group, chemically classified as a lead, thallium, and arsenic sulfide. It occurs as very small, tabular or prismatic crystals, rarely exceeding fractions of a millimeter. It usually forms aggregates or appears as single crystals embedded in the host rock. Its color ranges from black to steel-gray, and fresh fracture surfaces may exhibit a reddish tint. ## Physical Properties This mineral is characterized by a metallic luster. It is opaque. Due to its extreme rarity and small crystal size, many of its physical properties, such as hardness or density, have not been precisely determined. ## History and Name Richardsollyite was discovered in the Lengenbach quarry in the Binn Valley (Binntal) in Switzerland, which is its type locality. The mineral's name, given in 1967, honors Richard Harrison Solly (1851-1925), a British mineralogist and crystallographer from the University of Cambridge, who made significant contributions to the study of minerals from Lengenbach.

Properties

Luster: Metallic
Streak: Reddish
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying richardsollyite is extremely difficult and practically impossible without advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). Preliminary field identification relies on its co-occurrence with other rare sulfosalts in the specific geological environment of the Lengenbach quarry. Visual characteristics, such as black color, metallic luster, and small, tabular crystals, are merely indicative. ## Distinguishing from Similar Minerals Richardsollyite is visually indistinguishable from many other rare, black sulfosalts found in the same locality, such as hutchinsonite, imhofite, wallisite, or hatchite. Definitive differentiation is only possible based on chemical composition and crystallographic data. ## Crystal Forms It forms very small, poorly developed crystals with a tabular or short-prismatic habit, often with visible striations on the faces.

Geological environment

## Genesis Richardsollyite forms through hydrothermal processes in ore deposits embedded in metamorphic rocks. Its only known occurrence environment is dolomitic marbles (the so-called Lengenbach dolomites), where it crystallizes at low temperatures from solutions rich in arsenic, thallium, lead, and sulfur. ## Mineral Associations It co-occurs with other rare thallium and lead sulfosalts, such as hutchinsonite, hatchite, wallisite, imhofite, as well as realgar, orpiment, and pyrite. These minerals form complex parageneses within the Lengenbach dolomites. ## Localities The only confirmed locality for richardsollyite in the world is the Lengenbach quarry in the Binn Valley (Binntal), in the canton of Valais, Switzerland. It is an endemic mineral to this locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a richardsollyite specimen is determined almost exclusively by the degree of crystal development and size, which is a rarity. The most prized specimens are those with clearly visible, though still microscopic, crystals of a defined shape, embedded in a contrasting dolomitic matrix. The presence of other rare associated minerals can increase the scientific and collector's value of the specimen.

Care and storage

## Cleaning Due to its extreme rarity, small crystal size, and potential toxicity (thallium and arsenic content), richardsollyite specimens should only be cleaned by specialists. They usually do not require cleaning. If necessary, compressed air can be used to remove dust. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to chemical changes. It should be protected from high temperatures and mechanical damage. Due to the content of toxic elements, inhalation of dust should be avoided, and hands should be washed after any contact. ## Storage Specimens should be stored under stable conditions, in closed, airtight containers (e.g., "perky box" type), to limit air and moisture access and prevent accidental contact. Store away from sunlight, in a dry place.

Sources

IMA Mineral List (CNMNC)