Picromerite
Chemical formula: K<sub>2</sub>Mg(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O
Picromerite is a hydrated potassium and magnesium sulfate, occurring as white or colorless crystals, often in the oxidation zones of salt deposits.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.19
- Cleavage
- Perfect on {110}
- Fracture
- Conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Picromerite can be identified by its occurrence in evaporitic environments, low hardness, vitreous luster, and, most importantly, its solubility in water. It has a characteristic bitter, salty taste, however, tasting minerals is not recommended and is dangerous. ## Distinguishing from Similar Minerals It can be confused with other soluble sulfates and halides, such as halite, sylvite, carnallite, or kainite, which occur under similar conditions. Differentiation often requires chemical analysis. It differs significantly from gypsum and anhydrite by its much lower hardness and water solubility. ## Crystal Forms Picromerite crystals are usually short, prismatic, often with a square or rectangular cross-section. It also occurs as granular, fibrous aggregates, and as efflorescences and coatings.
Geological environment
## Genesis Picromerite is a mineral of evaporitic origin. It forms as a result of the evaporation of saline lake waters or closed marine basins in a dry and hot climate. It crystallizes from brines rich in potassium and magnesium. It is also a product of sublimation in volcanic exhalations, such as at Vesuvius. ## Mineral Associations It co-occurs with other salt minerals (evaporites), such as halite, sylvite, kainite, carnallite, leonite, langbeinite, gypsum, and anhydrite. ## Localities Significant deposits and occurrences of picromerite are found in Germany (e.g., in potassium salt deposits in the Stassfurt region), in Austria (Hallstatt), in Italy (as a product of volcanic activity at Vesuvius), in the USA (in evaporites in New Mexico and Texas, e.g., in the Permian Basin), and in potassium mines in Russia.
Rarity
Not very common
For collectors
## Quality Criteria For collectors, the most valuable specimens are well-formed, transparent crystals with sharp edges and a vitreous luster. Specimens on a rock matrix are prized, especially when accompanied by other color-contrasting evaporite minerals. Large, single crystals are rare and highly valued. ## Popular Localities Classic specimens come from the historical German potassium salt deposits near Stassfurt. Specimens from volcanic exhalations from Vesuvius are also sought after by collectors due to their specific genesis.
Care and storage
## Cleaning Cleaning picromerite is very risky and generally not recommended due to its water solubility. If absolutely necessary, compressed air can be used to remove dust. Any contact with water or a humid environment will cause damage or complete dissolution of the specimen. ## What to Avoid Contact with water, water vapor, alcohol, and other liquids must be strictly avoided. The mineral is hygroscopic, meaning it absorbs moisture from the air, so it must be protected from high humidity. It is also very soft and brittle, susceptible to scratches and mechanical damage. ## Storage Picromerite specimens must be stored in a dry environment. It is best to place them in a tightly sealed container (e.g., a "perky box") with a desiccant (e.g., silica gel). Avoid exposure to open air, especially on humid days.