Robinsonite

Chemical formula: Pb₂+₄Sb₃+₆S₂-₁₃

Robinsonite is a rare, lead-gray lead-antimony sulfosalt, forming acicular or fibrous crystals with a metallic luster.

## Characteristics Robinsonite is a rare mineral from the sulfosalt group, composed of lead, antimony, and sulfur. It occurs as elongated, acicular or fibrous crystals, often forming radial or tangled aggregates. Its color ranges from steel-gray to lead-gray, and on a fresh surface, it exhibits a strong, metallic luster. Over time, its surface may tarnish and become covered with a darker patina. ## Physical Properties Robinsonite crystals are opaque and relatively soft, with a Mohs hardness of 2.5-3. The mineral is quite heavy due to its high lead content, with a density of approximately 5.3 g/cm³. It exhibits perfect cleavage in one direction, parallel to the crystal elongation, which makes it brittle and easily breaks into fibrous fragments. ## Colors and Varieties This mineral does not exhibit significant color variability. Its color is consistently steel-gray or lead-gray. There are no named color varieties or commercial variants. ## History and Name Robinsonite was first identified in 1952 from material originating from the Red Bird mine in Nevada (USA) and the Ivigtut mine in Greenland. The mineral is named in honor of Stephen C. Robinson (1911-2000), a Canadian mineralogist who first synthesized this compound in the laboratory before its discovery in nature. ## Uses Robinsonite has no industrial significance. Its occurrence is too rare, and quantities too small, for it to be an ore of lead or antimony. It is solely an object of interest for collectors of rare minerals and scientists studying sulfosalts.

Properties

Mohs hardness: 2.5-3
Luster: Metallic
Streak: Black
Density: 5.29-5.33
Cleavage: Perfect on {112}
Fracture: Uneven
Transparency: Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features of robinsonite include its fibrous or acicular form, steel-gray color with a metallic luster, and high density. Its excellent cleavage, causing it to break into elongated fragments, is also characteristic. It occurs in paragenesis with other lead sulfosalts. ## Distinguishing from Similar Minerals Robinsonite can be confused with other fibrous sulfosalts, such as jamesonite, boulangerite, or zinkenite. Zinkenite is usually darker and forms more compact, felted masses. Boulangerite often has a duller, leaden luster. Jamesonit can be difficult to distinguish without chemical analysis, although its crystals may show transverse striations. Certain identification most often requires advanced methods, such as X-ray diffraction (XRD). ## Crystal Forms Robinsonite forms prismatic, strongly elongated, acicular or hair-like crystals, often exhibiting longitudinal striations. These crystals group into radial, star-shaped, or haphazardly tangled aggregates, as well as fibrous and massive occurrences.

Geological environment

## Genesis Robinsonite is a hydrothermal mineral, forming in ore veins at relatively low to medium temperatures. It originates in environments rich in lead, antimony, and sulfur, often as a result of crystallization from hydrothermal solutions or metasomatism. ## Mineral Associations This mineral co-occurs with other lead and antimony sulfosalts, such as boulangerite, zinkenite, plagionite, semseyite, and jamesonite. It is also often accompanied by sphalerite, galena, pyrite, quartz, and carbonates (calcite, dolomite). ## Localities The most important robinsonite localities worldwide include the Quiruvilca mine in Peru, where well-formed crystals have been found, and the Red Bird mine in Nevada (USA), which is one of the type localities. Other known occurrences include Baia Sprie and Herja in Romania, Příbram in the Czech Republic, the Madoc mine in Ontario (Canada), and the Ivigtut mine in Greenland.

Rarity

Rare

For collectors

## Quality Criteria Most sought after by collectors are specimens with well-formed, sharp, and lustrous crystals, forming aesthetic, radial or star-shaped aggregates. Specimens where robinsonite contrasts with a light rock matrix, e.g., with quartz, are also highly valued. The size of the crystals and the richness of the aggregate on the matrix significantly increase the specimen's value. ## Popular Localities Specimens from the Quiruvilca mine in Peru are considered among the best in the world, having yielded large and well-formed crystal aggregates. Classic, though rarer, specimens from Romanian mines in the Maramureș region (e.g., Baia Sprie, Herja) are also prized.

Care and storage

## Cleaning Robinsonite specimens are very delicate and brittle. They should be cleaned only with a soft brush or very carefully using compressed air to remove dust. Avoid contact with water and any chemical agents. ## What to Avoid The mineral is sensitive to shocks and impacts, which can cause it to break into fine fibers. Ultrasonic cleaners, acids, detergents, and high temperatures should be avoided. Prolonged exposure to humid air can cause slow oxidation and tarnishing of the surface. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from mechanical damage, dust, and moisture. Due to the lead and antimony content, hands should be washed after handling the mineral.

Sources

IMA Mineral List (CNMNC)