Humberstonite
Chemical formula: K<sub>3</sub>Na<sub>7</sub>Mg<sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>6</sub>(N<sup>5+</sup>O<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O
Humberstonite is a rare, water-soluble, hydrated sulfate and nitrate of potassium, sodium, and magnesium, forming colorless, tabular crystals.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.35
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Transparent
- Crystal system
- Trigonal
Diagnostic features
## Identification A key diagnostic feature of humberstonite is its water solubility. In field conditions, this can be checked by placing a small fragment on the tongue – it will have a salty, bitter taste (though this is not a recommended method). The characteristic tabular habit of crystals and its occurrence in a specific geological environment – in desert nitrate deposits – are also distinctive. ## Distinguishing from Similar Minerals It can be confused with other soluble sulfates and nitrates, such as nitratine, niter, or halite. Differentiation requires precise chemical analysis (EDS/XRD), as physical characteristics are very similar. However, humberstonite stands out due to its complex chemical formula containing potassium, sodium, and magnesium simultaneously. ## Crystal Forms Humberstonite forms thin, tabular crystals with a hexagonal outline. They often occur as radial or rosette-like aggregates, as well as coatings and crusts on other minerals.
Geological environment
## Genesis Humberstonite is an evaporite mineral. It forms in extremely arid, desert climatic conditions as a result of the evaporation of waters rich in nitrates and sulfates. It forms as a secondary product in the upper layers of saltpeter deposits (known as caliche). ## Mineral Associations It co-occurs with other evaporite minerals typical of Chilean nitrate deposits. It is most commonly associated with nitratine, niter, blödite, halite, darapskite, and glauberite. ## Localities The most important and classic localities for this mineral are in the Atacama Desert in Chile, particularly in the Antofagasta region. It is known from historical saltpeter mines such as Oficina Lautaro (type locality), Oficina Pedro de Valdivia, Oficina Maria Elena, and Oficina Pampa Blanca.
Rarity
Very rare
For collectors
## Quality Criteria The most prized humberstonite specimens are those with well-formed, sharp, transparent crystals forming aesthetic, rosette-like or radial aggregates. The absence of mechanical damage and a minimal amount of intergrowths with other minerals that could obscure the clarity of the composition are important. Contrast with a darker matrix rock also enhances the specimen's attractiveness. ## Popular Localities Specimens from classic, historical localities in the Antofagasta region of Chile, such as Oficina Lautaro and Oficina Pedro de Valdivia, are by far the most sought after by collectors. Material from these sites is considered exemplary for this species.
Care and storage
## Cleaning The mineral is water-soluble, so it must absolutely not be wet cleaned. To remove dust, a very soft brush or compressed air from a safe distance can be used very carefully. ## What to Avoid Avoid all contact with water, water vapor, and high air humidity, which can cause dissolution or damage to the crystals. Keep away from chemicals. It is a very delicate and brittle mineral. ## Storage Humberstonite specimens must be stored in a dry environment. The best solution is a sealed, transparent container (a "perky box") with a desiccant (e.g., silica gel). Avoid sudden temperature changes.