Douglasite
Chemical formula: K₂Fe²⁺Cl₄·2H₂O
Douglasite is a rare, water-soluble potassium iron chloride, forming in salt deposits and as a product of volcanic exhalations.
Properties
- Mohs hardness
- 2
- Color
- Light green; brownish red on exposure.
- Luster
- Vitreous
- Streak
- White
- Density
- 2.16
- Cleavage
- Good on {110}
- Fracture
- Conchoidal
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification The key diagnostic feature of douglasite is its complete and rapid solubility in water. This mineral is also very soft and light. Its occurrence in paragenesis with other salts (halite, sylvite, carnallite) in evaporite deposits is a strong indicator. It reacts to taste – it is salty and bitter, however, taste testing is highly discouraged due to potential toxicity. ## Distinguishing from Similar Minerals Douglasite can be confused with other soluble salts, such as halite (NaCl) or sylvite (KCl). It is distinguished from them by its greenish tint (if present) and its co-occurrence with iron minerals. Under laboratory conditions, it can be identified by chemical analysis, which will show the presence of potassium and iron. ## Crystal Forms Douglasite crystals, though rare, adopt cubic or octahedral forms. Most often, however, it forms granular, fibrous aggregates, crusts, or occurs as efflorescences.
Geological environment
## Genesis Douglasite is a mineral of evaporitic origin. It forms in the final stages of brine crystallization in closed sedimentary basins, in a dry and hot climate. It crystallizes from solutions rich in potassium, iron, and chlorine. It is also found as a product of volcanic sublimation (exhalation), where it forms from hot gases near fumaroles. ## Mineral Associations It most often co-occurs with other salt minerals, such as halite, sylvite, carnallite, kieserite, and polyhalite. In volcanic environments, it may be accompanied by sal ammoniac. ## Localities The most important douglasite localities are in Germany, in potassium salt deposits in Saxony-Anhalt (Douglashall and Wintershall mines). It is also known from Italy, where it occurs as a product of volcanic exhalations at Vesuvius.
Rarity
Very rare
For collectors
## Quality Criteria The quality of douglasite specimens is primarily assessed based on their preservation and form. The most valued are rare, well-formed crystals with a visible greenish coloration. Since the mineral is extremely unstable, it is crucial that the specimen is as well-preserved as possible, without signs of dissolution. Association with other evaporite minerals, forming an aesthetic composition, also enhances its attractiveness. ## Popular Localities The vast majority of collector specimens come from historical localities in Germany, especially from mines in the Stassfurt region. Specimens from Vesuvius are also sought after, but are mainly of scientific importance.
Care and storage
## Cleaning This mineral **must not be wet-cleaned** due to its complete solubility in water. Any contact with water, even a damp cloth, will destroy the specimen. Cleaning can only be done mechanically, dry, using a very soft brush to remove dust. ## What to Avoid Absolute avoidance of contact with water, water vapor, and high air humidity is essential. Douglasite is hygroscopic, meaning it absorbs moisture from the surroundings, leading to its dissolution. It should also be protected from all chemicals and sudden temperature changes. ## Storage Douglasite specimens must be stored in completely dry conditions. The best solution is a sealed, airtight container (e.g., a "perky box") with a desiccant (e.g., silica gel). Exposure to open air, especially in climates with variable humidity, is impossible and leads to rapid degradation of the mineral.