Chlorothionite
Chemical formula: K<sub>2</sub>Cu<sup>2+</sup>S<sup>6+</sup>O<sub>4</sub>Cl<sub>2</sub>
A rare potassium and copper sulfate and chloride, forming small, blue crystals in volcanic exhalation products.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.86
- Cleavage
- Perfect on {010}
- Transparency
- Transparent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Key diagnostic features of chlorothionite are its blue color, vitreous luster, low hardness, and occurrence in a specific environment – as a product of volcanic exhalations. Its complete solubility in water is a characteristic feature, although destructive to the sample. ## Distinguishing from Similar Minerals Chlorothionite is sometimes confused with other blue, secondary copper minerals. It is distinguished from chalcanthite by its crystallographic system (orthorhombic, not triclinic) and typical parageneses. Linarite, although similar in color, is much harder and does not dissolve in water. The key indicator, however, is its origin – it is almost always associated with volcanic fumaroles. ## Crystal Forms It forms thin to thick tabular crystals, often with a rectangular or hexagonal outline. It also occurs as short prisms. Crystals often form druses and small aggregates on the surface of the host rock.
Geological environment
## Genesis Chlorothionite is a typical fumarolic mineral. It forms by sublimation, i.e., direct crystallization from hot volcanic gases rich in chlorine, sulfur, potassium, and copper. It crystallizes on the walls of fumarole vents at temperatures ranging from 100 to 250°C. ## Mineral Associations This mineral co-occurs with other products of volcanic exhalations. Its typical associated minerals are eriochalcite, cotunnite, dolerophanite, as well as tenorite and chalcanthite. ## Localities The classic and most important locality, from which the best specimens originate, is Vesuvius in Campania, Italy. This is its type locality. The occurrence of chlorothionite has also been reported at the Tolbachik volcano on the Kamchatka Peninsula in Russia.
Rarity
Rare
For collectors
## Quality Criteria The most highly prized chlorothionite specimens are those with well-formed, sharp, and transparent crystals of an intense blue color. The contrast with the rock matrix and the size of the crystals are important – specimens with crystals exceeding 2-3 mm are already considered exceptional. Clear associations with other rare fumarolic minerals enhance their attractiveness. ## Popular Localities Specimens from historical finds at Vesuvius in Italy are by far the most sought after and valued by collectors. Material from the Tolbachik volcano is also valuable, but Vesuvius is considered the classic and best locality for this mineral.
Care and storage
## Cleaning Chlorothionite is a mineral extremely sensitive to water and moisture, as it is completely soluble. It must absolutely not be cleaned with water or any liquids. Dust can be removed very carefully with a soft, dry brush or by blowing it off with a stream of air from a photographic blower bulb. ## What to Avoid Any contact with water, water vapor, and humid air should be avoided. Storage in a humid place leads to rapid disintegration of the mineral. It should also be protected from sudden temperature changes, which could cause condensation of water vapor on the specimen's surface. ## Storage Chlorothionite specimens must be stored in sealed, airtight containers (e.g., "perky box" type). A desiccant, such as silica gel, is recommended inside the container to ensure a consistently dry environment. Exposure to open air should be avoided.