Gratonite
Chemical formula: Pb<sup>2+</sup><sub>9</sub>As<sup>3+</sup><sub>4</sub>S<sup>2-</sup><sub>15</sub>
Gratonite is a rare lead-arsenic mineral, forming characteristic, spindle-shaped crystals with a metallic luster.
Properties
- Mohs hardness
- 2.5
- Luster
- Metallic
- Streak
- Black with reddish tint
- Density
- 6.19-6.22
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Gratonite can be identified by its characteristic, spindle-shaped or slender prismatic crystals with vertical striations. Metallic luster, high density, and a black streak with a reddish tint are key diagnostic features. It occurs in paragenesis with other sulfosalts in hydrothermal deposits. ## Distinguishing from Similar Minerals Gratonite is sometimes confused with other acicular sulfosalts, such as jamesonite, boulangerite, or zinkenite. Distinguishing it based on visual characteristics is very difficult and often requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). The characteristic spindle-shaped crystal habit, if well-visible, can be a helpful clue. ## Crystal Forms Gratonite crystals are typically slender, prismatic, often tapering towards the ends, giving them a spindle-shaped appearance. Crystal surfaces are usually deeply striated along the vertical axis. More rarely, it forms radial aggregates or fibrous masses.
Geological environment
## Genesis Gratonite is a mineral of hydrothermal origin. It forms in medium- to low-temperature ore veins rich in lead, arsenic, and sulfur. It crystallizes in the late stages of ore mineral formation. ## Mineral Associations This mineral often co-occurs with other sulfosalts and sulfides. The most common associated minerals include enargite, tennantite, galena, pyrite, sphalerite, chalcopyrite, as well as barite and quartz. ## Localities The most important and type locality for gratonite is the Excelsior mine in Cerro de Pasco, Peru, which yields the world's best specimens. It is also known from the Quiruvilca mine in Peru. Outside of Peru, its occurrence has been reported in small quantities in Hotazel, South Africa, and in mines in Japan (Akenobe mine) and the United States (Nevada).
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharply terminated crystals with a strong metallic luster. Isolated crystals or groups of crystals embedded in a contrasting matrix, such as quartz or barite, are particularly sought after. Crystal size is also an important factor – specimens with crystals exceeding one centimeter in length are considered exceptional. ## Popular Localities Undoubtedly, the most desirable gratonite specimens come from the type locality – Cerro de Pasco, Peru. Crystals from this mine are considered exemplary for this mineral species and command the highest prices in the collector's market.
Care and storage
## Cleaning Gratonite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its arsenic and lead content, inhalation of dust should be avoided, and hands should be washed after each contact with the mineral. Wet cleaning is not recommended, and if necessary, a minimal amount of distilled water should be used, and the specimen dried immediately. ## What to Avoid Avoid contact with chemicals, especially acids, which can damage the mineral's surface. Gratonite is soft and brittle, so it must be protected from impacts and scratches. Prolonged exposure to moisture can lead to slow oxidation. ## Storage Collector's specimens of gratonite are best stored in closed, padded boxes or display cases to protect them from mechanical damage and dust. It is recommended to store them in a dry place, away from sources of moisture.