Pekoite
Chemical formula: Cu<sup>1+</sup>Pb<sup>2+</sup>Bi<sup>3+</sup><sub>11</sub>S<sup>2-</sup><sub>18</sub>
Pekoite is a rare, lead-gray bismuth, lead, and copper sulfosalt mineral, forming acicular crystals and fibrous aggregates.
Properties
- Mohs hardness
- 3.5
- Luster
- Metallic
- Streak
- Black
- Density
- 7.23
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Pekoite can be identified by its characteristic form – acicular, often tangled crystals forming fibrous aggregates. Key features also include its lead-gray color, metallic luster, and co-occurrence with other bismuth sulfides and sulfosalts. It is soft and has a black streak. ## Distinguishing from Similar Minerals Pekoite is visually almost impossible to distinguish from many other acicular bismuth sulfosalts, such as aikinite, bismuthinite, gladite, or jamesonite. All these minerals have similar color, luster, and habit. Definitive identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal Forms Pekoite crystals are strongly elongated along the b-axis, taking the form of needles, blades, or hairs. They usually occur as disordered, tangled masses, fibrous aggregates, or radial aggregates. Single, well-formed crystals are rarely observed.
Geological environment
## Genesis Pekoite is a hydrothermal mineral. It forms in ore veins where solutions rich in bismuth, lead, copper, and sulfur crystallize at moderate temperatures. At its type locality (Peko mine), it is associated with magnetite ore bodies. ## Mineral Associations At the Peko mine, pekoite co-occurs with minerals such as chalcopyrite, pyrite, magnetite, hematite, bismuthinite, gladite, junoite, and native gold. In other localities, it is accompanied by various sulfides and sulfosalts. ## Localities The most important and type locality is the Peko mine, Tennant Creek, Northern Territory, Australia. This mineral has also been identified in several other places worldwide, including the Băița Bihor mine in Romania, in quartz veins at the Ikuno mine in Japan, and in the Czech Republic (Harrachov).
Rarity
Very rare
For collectors
## Quality Criteria For collectors, the most valuable specimens are those with clearly visible, well-formed acicular crystals, creating aesthetic, three-dimensional aggregates on a contrasting rock matrix (e.g., on quartz). Specimens rich in pekoite, where the mineral is the dominant component rather than just minor inclusions, are highly prized. Due to identification difficulties, specimens with analytically confirmed identity have significantly higher value. ## Popular Localities By far the most classic and sought-after specimens by collectors come from the type locality – the Peko mine in Australia. Material from other localities, such as Romania or Japan, is much rarer on the collector's market.
Care and storage
## Cleaning Pekoite specimens should be cleaned very carefully, preferably using compressed air to remove dust. Any contact with water or chemicals is inadvisable. If it is necessary to remove loose contaminants, a very soft brush can be used, proceeding with the utmost delicacy. ## What to Avoid Avoid contact with water, acids, and other chemicals that can react with sulfides, leading to their oxidation and destruction. The mineral is soft and brittle, so it must be protected from impacts, scratching, and vibrations. Prolonged exposure to humid air can cause its slow oxidation and darkening. ## Storage Collector specimens of pekoite, especially those with delicate, acicular crystals, should be stored in sealed, stable containers (e.g., "perky boxes") to protect them from mechanical damage, dust, and humidity changes. Avoid storage in areas with high humidity.