Aphthitalite
Chemical formula: K<sub>3</sub>Na(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>
Aphthitalite is a rare potassium sodium sulfate, forming characteristic, thin crusts and efflorescences in volcanic and evaporitic environments.
Properties
- Mohs hardness
- 3
- Luster
- Vitreous
- Streak
- White
- Density
- 2.66
- Cleavage
- Good on {1010}, Poor on {0001}
- Fracture
- Sub-Conchoidal
- Transparency
- Transparent to Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Aphthitalite can be identified by its occurrence as crusts and efflorescences in specific environments, such as volcanic fumaroles or evaporite deposits. Its vitreous luster, low hardness, and salty, bitter taste are characteristic. It reacts with water. ## Distinguishing from Similar Minerals It can be confused with other colorless or white sulfates and halides, such as halite, sylvite, thenardite, or gypsum. It differs from halite and sylvite by the lack of perfect cubic cleavage and a different crystallographic system. It is significantly harder than gypsum (gypsum has a hardness of 2). Thenardite has similar properties but crystallizes in the orthorhombic system. ## Crystal Forms Most often, it forms thin crusts, efflorescences, and granular aggregates. Single, small, thick-tabular crystals with a hexagonal outline, often grouped into aggregates, are rarely found.
Geological environment
## Genesis Aphthitalite is a mineral of evaporitic or sublimatic origin. It forms as a product of crystallization from brines in drying lakes and seas (marine and terrestrial evaporites). It is also a typical product of sublimation from hot volcanic gases (exhalations) in fumarole areas. ## Mineral Associations In volcanic environments, it co-occurs with thenardite, sylvite, halite, and hematite. In evaporite deposits, it is accompanied by halite, sylvite, thenardite, mirabilite, gypsum, anhydrite, as well as various borates and nitrates. ## Localities Significant occurrences of aphthitalite associated with volcanic activity are known from Vesuvius and Etna in Italy. It also occurs in evaporite deposits in Searles Lake Valley in California (USA), in salt deposits in Germany (Stassfurt), and in Chile.
Rarity
Not very common
For collectors
## Quality Criteria For collectors, the most desirable specimens are those with well-formed, even if small, transparent crystals with a distinct luster. Rich efflorescences and crusts on a contrasting matrix, for example, on volcanic lava, are also valued. The aesthetics of the specimen and minimal damage are important, which can be difficult to achieve due to the mineral's brittleness. ## Popular Localities The most prized specimens by collectors come from classic volcanic localities, such as Vesuvius in Italy, from which historical and well-formed crystals originate. Specimens from the Kīlauea volcano fumaroles in Hawaii and from deposits in Searles Lake, California, are also sought after.
Care and storage
## Cleaning Aphthitalite specimens should only be dry-cleaned, using a soft brush to remove dust. Due to its water solubility, contact with any liquid is absolutely not recommended. ## What to Avoid Avoid contact with water, water vapor, and high humidity, which can cause the mineral to dissolve or damage its surface. Aphthitalite is a brittle and soft mineral, therefore it requires careful handling to avoid scratches and mechanical damage. ## Storage Aphthitalite specimens should be stored in a dry environment, preferably in airtight containers (e.g., "membrane boxes") or in display cases with a desiccant (silica gel). Protect from dust and sudden temperature changes.