Natropharmacoalumite
Chemical formula: NaAl₄(As⁵⁺O₄)₃(OH)₄·4H₂O
A rare sodium and aluminum arsenate, forming small, cubic crystals with a vitreous luster, found in the oxidation zones of ore deposits.
Properties
- Mohs hardness
- 2.5
- Color
- colorless
- Luster
- Vitreous
- Streak
- white
- Density
- 2.564
- Cleavage
- imperfect cleavage on {100}.
- Fracture
- Irregular/Uneven
- Transparency
- Transparent
- Crystal system
- Isometric
Diagnostic features
## Identification Natropharmacoalumite is identified primarily by its characteristic cubic crystal form and its occurrence environment – oxidation zones of arsenic-rich deposits. Vitreous luster and lack of color are also helpful features. However, final identification requires advanced analytical methods, such as chemical composition analysis (EDS) or X-ray diffraction (XRD), due to its similarity to other minerals of the pharmacoalumite group. ## Distinguishing from Similar Minerals It can be confused with other minerals from the same group, especially pharmacoalumite (potassium dominant) and pharmacosiderite (iron dominant). Visual differentiation is practically impossible and requires specialized studies. It differs from them in chemical composition, which translates into slightly different physical parameters, impossible to verify without equipment. ## Crystal Forms The mineral crystallizes in the isometric system, forming almost perfect cubes. Crystals are usually very small, rarely exceeding 1 mm. They occur as single, scattered crystals or in small aggregates on the rock surface.
Geological environment
## Genesis Natropharmacoalumite is a secondary mineral, formed in the oxidation (weathering) zones of arsenic-rich ore deposits, especially in arid and semi-arid climates. It forms as a result of the reaction of solutions rich in arsenic, aluminum, and sodium with primary minerals, such as arsenopyrite or enargite. ## Mineral Associations It often co-occurs with other secondary arsenate minerals. The most common associations include: pharmacosiderite, scorodite, mansfieldite, jarosite, quartz, and iron oxides (goethite, hematite). ## Localities The most important and well-documented occurrences of this rare mineral are: - Gold Hill Mine in Tooele County, Utah, USA – type locality. - Maria Josefa Mine, Rodalquilar, Almería, Spain – known for well-formed microcrystals. - Clara Mine in the Black Forest, Germany.
Rarity
Very rare
For collectors
## Quality Criteria The collecting appeal of natropharmacoalumite primarily depends on the quality and size of the crystals. Most valued are specimens with sharp, well-formed, transparent cubes, clearly standing out from the host rock. Color contrast with the matrix (e.g., a dark limonitic matrix) is also important. Due to the small size of the crystals, it is a mineral mainly for collectors specializing in "micromount" specimens. ## Popular Localities The most sought-after specimens by collectors come from the Maria Josefa mine in Spain, which has yielded some of the world's best microcrystals of this mineral. Specimens from the type locality in Gold Hill (USA) also have historical and collector's value.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to the small size and fragility of the crystals, wet cleaning is not recommended. If necessary, distilled water and compressed air can be used for drying, avoiding direct mechanical contact. ## What to Avoid Avoid contact with acids and other chemicals that may damage the mineral. The crystals are brittle and sensitive to ultrasound and shocks. Specimens should not be heated. ## Storage It is recommended to store specimens in stable conditions, in closed "micromount" boxes, which protect them from dust, mechanical damage, and sudden changes in humidity.