Coiraite

Chemical formula: (Pb₂+,Sn₂+)₁₂.₅As₃+₃Fe₂+Sn₄+₅S₂-₂₈

A very rare mineral from the sulfosalt group, being a complex sulfide of lead, tin, iron, and arsenic.

## Characteristics Coiraite is an extremely rare mineral belonging to the sulfosalt group. It forms small, anhedral (irregular) grains or film-like aggregates, rarely developing tiny, tabular crystals not exceeding a few hundred micrometers in size. It has a metallic luster and is completely opaque. Its color ranges from steel-gray to black. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5-3. It is characterized by high density (approximately 5.67 g/cm³), which is typical for lead-bearing minerals. The streak, or color of the powdered mineral, is black. ## Colors and Varieties Coiraite occurs in uniform, dark colors from gray to black. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1991. Its name honors Dr. Zulema F. Coira, an Argentine geologist, for her contributions to the study of the Pirquitas deposit in Argentina, which is also the type locality for coiraite. ## Uses Due to its extreme rarity, coiraite has no industrial application. It is solely an object of scientific interest and is valued in advanced collections of systematic minerals and micromounts.

Properties

Mohs hardness: 2.5-3
Luster: Metallic
Streak: Black
Density: 5.67
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of coiraite is extremely difficult and practically impossible without advanced analytical methods, such as electron microprobe (EDS/WDS). A preliminary indication may be its occurrence in a specific paragenesis of tin-bearing minerals in the Pirquitas deposit, metallic luster, and black streak. ## Distinguishing from similar minerals Coiraite is visually indistinguishable from many other gray, metallic sulfosalts with which it co-occurs, such as franckeite, cylindrite, pirquitasite, or hocartite. Certain differentiation requires chemical composition analysis. ## Crystal forms It most often occurs as anhedral grains and lamellar aggregates, intergrown with other minerals. It rarely forms imperfect, lath-like or tabular crystals with a monoclinic habit.

Geological environment

## Genesis Coiraite is a hydrothermal mineral. It forms in polymetallic, tin-silver ore veins, developing under low to moderate temperature conditions. ## Mineral associations This mineral occurs in close association with other, often rare, sulfosalts and sulfides. Its typical associated minerals include: pirquitasite, hocartite, franckeite, cylindrite, stannite, cassiterite, arsenopyrite, pyrite, sphalerite, and quartz. ## Localities The only confirmed and well-documented locality of coiraite in the world is its type locality - the Pirquitas mine in Jujuy Province, Argentina.

Rarity

Extremely rare

For collectors

## Quality criteria For collectors, the most important criterion is the certainty of mineral identification, confirmed by chemical analysis. The value of a specimen depends on the richness of coiraite aggregates on the rock matrix and its association with other rare minerals from this locality. Due to the microscopic nature of its occurrences, it is primarily a mineral for micromount collectors. ## Popular localities The only source of collectible specimens is the Pirquitas mine in Argentina. Specimens from this locality are highly sought after by specialists and scientific institutions.

Care and storage

## Cleaning Coiraite specimens should only be dry-cleaned, using a soft brush to remove dust. All contact with water and chemical agents, which can lead to its oxidation and decomposition, should be avoided. ## What to avoid The mineral is sensitive to moisture and acidic environments (even sweat from hands), which can cause its slow degradation. It is soft and brittle, so it should be protected from impacts and scratches. It should not be heated or exposed to prolonged sunlight. ## Storage It is recommended to store specimens in a dry, stable environment, preferably in tightly sealed containers (e.g., perky boxes) to limit air and moisture access. Due to the lead and arsenic content, hands should be washed after contact with the mineral.

Sources

IMA Mineral List (CNMNC)