Bassanite
Chemical formula: CaS<sup>6+</sup>O<sub>4</sub>·0.5H<sub>2</sub>O
Bassanite is a hydrated calcium sulfate, a mineral rarely found in nature, known primarily as a component of plaster of Paris.
Properties
- Mohs hardness
- 2
- Luster
- Vitreous, Silky
- Streak
- White
- Density
- 2.7
- Cleavage
- Perfect on {010}, Good on {100}
- Fracture
- Fibrous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Bassanite is most easily recognized by its characteristic form – white, felt-like or silky aggregates of fine, acicular crystals. Its occurrence in volcanic fumarole environments or as a product of gypsum dehydration is a strong diagnostic indicator. It reacts with water, slowly transforming into gypsum. ## Distinguishing from Similar Minerals Bassanite can be confused with gypsum (especially its fibrous variety – selenite) and anhydrite. It differs from gypsum by its lower water content and often more delicate, acicular form. Anhydrite usually forms more massive, granular, or tabular aggregates. Final distinction between these three minerals often requires X-ray diffraction (XRD) analysis. ## Crystal Forms It forms microscopic, elongated, acicular crystals. It occurs almost exclusively as fibrous aggregates, tangled masses (felt-like), crusts, and coatings.
Geological environment
## Genesis Bassanite forms under specific conditions of low humidity and temperatures above 65°C. It is a product of gypsum dehydration. Typical environments for its formation include: - Volcanic fumaroles, where hot volcanic gases dehydrate previously formed gypsum (as in the type locality at Vesuvius). - Dry, desert environments (evaporites), where gypsum dehydrates under the influence of sun and low humidity. - Burning mine dumps, where high temperatures transform gypsum. - More rarely in caves as a product of dehydration of gypsum speleothems. ## Mineral Associations It co-occurs with minerals typical of its formation environment, such as gypsum, anhydrite, calcite, celestine, sulfur (in volcanic environments), and other evaporites. ## Localities The most important bassanite localities worldwide include the volcanic products of Vesuvius (Italy), where it was discovered. It is also found in desert regions, e.g., in gypsum mines in Chile (Atacama Desert) and in some dry caves in New Mexico (USA). It also occurs on burning coal dumps, including in Pennsylvania (USA) and Germany.
Rarity
Rare
For collectors
## Quality Criteria Bassanite's collector appeal stems mainly from its rarity and specific form. The most prized specimens are those with well-visible, silky fibrous aggregates, forming fluffy or felt-like masses on the matrix rock. Contrast with a darker matrix enhances visual value. Due to the microscopic size of its crystals, their individual form is not a criterion for evaluation. ## Popular Localities Classic and most prized specimens come from the historical locality at Vesuvius in Italy. Specimens from dry caves in the USA and from the Atacama Desert in Chile are also sought after by collectors specializing in rare minerals.
Care and storage
## Cleaning Bassanite specimens are extremely delicate and sensitive to water. Wet cleaning should be avoided. Only very careful dust removal with a soft brush or a stream of compressed air from a distance is permissible. ## What to Avoid Absolutely avoid contact with water, which can cause its rehydration to gypsum, destroying the specimen's structure and appearance. It should be protected from moisture, chemicals, as well as shocks and impacts due to its low hardness and fragility. ## Storage Specimens should be stored in a dry place, preferably in closed, airtight containers or display cases with a desiccant (e.g., silica gel). Exposure should be away from sources of moisture and areas prone to temperature changes.