Terrywallaceite

Chemical formula: Ag₁+Pb₂+(Sb₃+,Bi₃+)₃S₂-₆

A rare lead, silver, antimony, and bismuth sulfosalt, forming metallic, acicular crystals in hydrothermal deposits.

## Characteristics Terrywallaceite is a complex sulfosalt with a metallic luster and a steel-gray to black color. It most commonly occurs as small, acicular or bladed crystals that form tangled or radiating aggregates. Individual crystals rarely reach large sizes, and specimens typically consist of rich clusters embedded in the host rock or overgrown on other minerals. ## Physical Properties This mineral is opaque and exhibits a strong, metallic luster. Its Mohs hardness is 3.5-4, making it relatively soft. The density calculated from its chemical composition and unit cell parameters is approximately 5.86 g/cm³, thus it is a heavy mineral. ## Colors and Varieties Terrywallaceite is monochromatic – its color ranges from steel-gray to almost black. No color or commercial varieties are distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2010. Its name honors Terrance C. Wallace, Jr. (born 1956), an American mineralogist and professor of geological sciences, for his contributions to mineralogy. It was discovered in the Uchucchacua mine in Peru. ## Uses Terrywallaceite has no industrial applications. Its significance is purely scientific and collectible, as a rare and well-defined mineral species.

Properties

Mohs hardness: 3.5-4
Luster: Metallic
Streak: Black
Density: 5.86
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features include: metallic luster, steel-gray color, black streak, and high density. Occurrence as acicular or bladed aggregates is typical. A key diagnostic clue is its origin from the Uchucchacua mine in Peru and co-occurrence with other rare sulfosalts. ## Distinguishing from Similar Minerals Correct identification of terrywallaceite is extremely difficult without advanced analytical methods. Visually, it is practically impossible to distinguish from many other sulfosalts with a similar appearance and occurring in the same environment, such as benavidesite, fizélyite, andorite, or uchucchacuaite. Certain identification requires chemical analysis (e.g., EDS) to confirm the presence and proportions of Ag, Pb, Sb, and Bi. ## Crystal Forms Crystals are elongated, acicular or bladed, often showing striations on their faces. They typically form disordered, tangled clusters, as well as radiating or subparallel aggregates.

Geological environment

## Genesis Terrywallaceite forms as a result of hydrothermal processes at low and medium temperatures. It is a component of polymetallic ore veins, where it crystallizes along with other sulfides and sulfosalts. ## Mineral Associations This mineral co-occurs with a wide range of other hydrothermal minerals. In its type locality (Uchucchacua mine), it has been found in association with benavidesite, fizélyite, miargyrite, pyrargyrite, uchucchacuaite, andorite, galena, sphalerite, pyrite, alabandite, calcite, and rhodochrosite. ## Localities The most important and globally known locality for terrywallaceite is its type locality – the Uchucchacua mine, Oyon Province, Lima Region in Peru. This is the main source of collector specimens.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens are those with rich, dense clusters of acicular crystals on a contrasting matrix, such as quartz or rhodochrosite. Samples with well-defined, even if small, individual crystals are also highly valued. The value of a specimen significantly increases when accompanied by other rare minerals, such as uchucchacuaite or benavidesite. ## Popular Localities The only source of valuable collector specimens is the Uchucchacua mine in Peru. Specimens from this locality are the standard for this mineral.

Care and storage

## Cleaning Specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, especially tap water, is not recommended. In absolute necessity, a small amount of distilled water can be used, and the specimen should be thoroughly dried immediately. ## What to Avoid Avoid ultrasonic cleaners, all chemical agents, acids, and detergents. The mineral is relatively soft and brittle, so it must be protected from impacts and scratches. Prolonged exposure to high humidity can lead to slow oxidation. ## Storage It is recommended to store specimens in a dry place, in closed, padded boxes, to protect them from mechanical damage and dust. It is best to keep it away from minerals that can react with sulfur.

Sources

IMA Mineral List (CNMNC)