Cranswickite
Chemical formula: MgS<sup>6+</sup>O<sub>4</sub>·4H<sub>2</sub>O
Cranswickite is a hydrated magnesium sulfate, forming white, water-soluble efflorescences and crusts in cold, dry conditions.
Properties
- Mohs hardness
- 2.5
- Luster
- Silky
- Streak
- White
- Density
- 1.873
- Cleavage
- Perfect on {100}, good on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Cranswickite is identified by its mode of occurrence – white, fibrous efflorescences and crusts. A key diagnostic feature is its water solubility and instability under room conditions. It forms in specific, cold and dry conditions, which is an important clue to its genesis. Final identification requires advanced methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other hydrated sulfates, such as epsomite (MgSO₄·7H₂O) or starkeyite (MgSO₄·4H₂O, monoclinic). Epsomite usually forms larger, better-developed crystals and contains more water. Starkeyite is a polymorph of cranswickite, and distinguishing it without crystallographic analysis is impossible. Other white, soluble sulfate efflorescences (e.g., mirabilite) may also look similar. ## Crystal Forms It forms very fine, acicular or fibrous crystals, rarely exceeding 1 mm in length. These crystals combine into delicate, woolly or felt-like aggregates, as well as brittle crusts and powdery coatings.
Geological environment
## Genesis Cranswickite is a secondary mineral, formed by the evaporation of magnesium sulfate-rich solutions under specific conditions – low temperature and low humidity. It is a product of weathering of magnesium-bearing rocks in a dry and cold climate. ## Mineral Associations At its type locality (Antarctica), it co-occurs with epsomite, starkeyite, and meridianiite. It may also accompany other evaporite minerals. ## Localities The most important and type locality is the vicinity of Stornes Lake in Prydz Bay, Princess Elizabeth Land, Antarctica. This is the only confirmed and well-documented location of this mineral in the world.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of a cranswickite specimen primarily depends on the abundance and purity of the material. Most valued are specimens with clearly visible, fluffy aggregates of fibrous crystals, covering as large an area of the host rock as possible. It is important that the specimen is free of impurities and shows no signs of dissolution or dehydration. ## Popular Localities The only source of cranswickite specimens is its type locality in Antarctica. For this reason, it is practically unavailable on the collector's market and is found almost exclusively in institutional and research collections.
Care and storage
## Cleaning The mineral is water-soluble, so wet cleaning is absolutely forbidden. Any contact with water, even a damp cloth, will cause its destruction. Cleaning is only possible mechanically, dry, using a very soft brush or a stream of compressed air from a distance to avoid damaging the delicate crystals. ## What to Avoid Avoid water, alcohol, and all other liquids. Cranswickite is unstable at standard room temperature and humidity – it easily dehydrates (loses water) and can transform into other magnesium sulfates, such as starkeyite (MgSO₄·4H₂O, monoclinic) or pentahydrite (MgSO₄·5H₂O). It should be protected from moisture and temperature changes. ## Storage Cranswickite specimens require storage in sealed, airtight containers (e.g., "perky box" type) with the addition of a desiccant (e.g., silica gel). It is best to store it in a cool and dry place to prevent its dehydration or dissolution. Due to its fragility, the container should protect it from shocks and mechanical damage.