Chloromagnesite
Chemical formula: MgCl₂
Chloromagnesite is a rare, hygroscopic magnesium halide, occurring mainly as efflorescences and coatings in volcanic and sedimentary environments.
Properties
- Mohs hardness
- 1-2
- Color
- Colourless, white
- Luster
- Vitreous
- Streak
- White
- Density
- 2.325
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Chloromagnesite can be identified by its characteristic, delicate, white or colorless, acicular aggregates in the form of coatings and efflorescences. A key diagnostic feature is its extreme hygroscopicity – the mineral quickly absorbs water from the air and becomes damp or completely dissolves. It also has a bitter, salty taste. ## Distinguishing from Similar Minerals It can be confused with other soluble halides or sulfates that form similar efflorescences, such as halite or sylvite. Halite has a salty taste (without bitterness) and crystallizes in cubic forms. Sylvite has a salty-bitter taste but is less hygroscopic. Final differentiation often requires chemical analysis. ## Crystal Forms It crystallizes in the trigonal system, forming very fine, acicular or capillary crystals. Most often, it occurs as fibrous aggregates, radial aggregates, as well as delicate coatings and crusts.
Geological environment
## Genesis Chloromagnesite is a mineral of volcanic or evaporitic origin. It forms as a sublimation product in volcanic exhalations (fumaroles), where hot chlorine-rich gases react with magnesium-containing rocks. It is also found as a component of some salt evaporites, formed by the evaporation of saline lakes or seas in arid climates. ## Mineral Associations In volcanic environments, it co-occurs with minerals such as halite, sylvite, carnallite, and hematite. In salt deposits, it is accompanied by bischofite, carnallite, kieserite, and halite. ## Localities The most important chloromagnesite localities are volcanic fumaroles, including Vesuvius and Etna in Italy. It also occurs in potash salt deposits in Germany (e.g., near Stassfurt), in salt domes in Texas and Louisiana (USA), and in some saline lakes in China.
Rarity
Very rare
For collectors
## Quality Criteria Due to its extreme rarity and instability, any well-documented chloromagnesite specimen is valuable. The most desirable specimens are those with visible, even microscopic, crystalline forms, abundantly covering a fragment of the host rock. Crucially, the specimen must be preserved in a dry state, indicating proper protection since its acquisition. ## Popular Localities Classic and most prized specimens come from historical localities in Italy, such as Vesuvius. Material from salt deposits in Germany is also found in specialized collections.
Care and storage
## Cleaning Chloromagnesite specimens must absolutely not be cleaned wet. Any contact with water, even in the form of water vapor, will cause it to dissolve. Mechanical cleaning is inadvisable due to extreme fragility. ## What to Avoid Above all, moisture must be avoided. The mineral must be protected from exposure to air with normal humidity, water, alcohols, and other liquids. It is very unstable under typical atmospheric conditions. ## Storage The only safe storage methods are hermetically sealed containers, preferably with a desiccant (e.g., silica gel), or immersion in an anhydrous liquid such as paraffin. Exposure to open air leads to rapid deliquescence (dissolution) of the mineral.