Ahlfeldite
Chemical formula: Ni<sup>2+</sup>Se<sup>4+</sup>O<sub>3</sub>·2H<sub>2</sub>O
Ahlfeldite is a very rare hydrated nickel selenite, forming characteristic greenish-brown crusts and aggregates.
Properties
- Mohs hardness
- 2-2.5
- Luster
- Vitreous to Resinous
- Streak
- Pale green
- Density
- 3.35-3.84
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Translucent to Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Helpful features for identifying ahlfeldite include its characteristic greenish-brown color, botryoidal or spherulitic aggregate habit, and specific occurrence environment. Its association with other rare selenites is a strong diagnostic indicator. ## Distinguishing from Similar Minerals Ahlfeldite can be confused with other secondary nickel or selenium minerals. It differs from *annabergite* (nickel arsenate) usually by a less intense, more brownish color. From structurally similar selenites, such as pink *cobaltomenite* or blue *chalcomenite*, it differs distinctly in color. However, definitive identification requires advanced analytical methods, such as EDS spectroscopy or X-ray diffraction (XRD). ## Crystal Forms It most often forms crusts, botryoidal and spherulitic aggregates. Small, prismatic crystals are very rare and usually do not exceed a few millimeters in length.
Geological environment
## Genesis Ahlfeldite is a secondary mineral that forms in the oxidation (weathering) zones of hydrothermal ore deposits. Its formation is associated with the simultaneous presence of nickel and selenium minerals (e.g., clausthalite) in the host rock. ## Mineral Associations It often co-occurs with other rare selenites, such as cobaltomenite, chalcomenite, molybdomenite, as well as common minerals of the oxidation zone, like limonite and goethite. ## Localities The most important and type locality for ahlfeldite is the Pacajake mine in the Potosí Department, Bolivia. It is also known from the Virgen de Surumi mine (also in Bolivia), and from a few localities in the Harz Mountains in Germany and La Rioja Province in Argentina.
Rarity
Very rare
For collectors
## Quality Criteria The collector appeal of ahlfeldite depends on the intensity and purity of its color, the richness of crusts on the rock matrix, and the size of the aggregates. Specimens where ahlfeldite forms aesthetic compositions with other, color-contrasting selenites, e.g., with pink cobaltomenite, are most highly valued. ## Popular Localities Specimens from Bolivian localities, especially from the Pacajake mine, are considered the best in the world. They represent the global standard for this mineral and are most sought after by collectors specializing in rare minerals.
Care and storage
## Cleaning Ahlfeldite specimens should only be dry-cleaned, using a soft brush or compressed air to remove dust. The mineral is somewhat water-soluble, so contact with water is not recommended. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. Ahlfeldite is soft and brittle, susceptible to scratches and mechanical damage. It should also be protected from high humidity and sudden temperature changes. ## Storage It is recommended to store specimens in stable, dry conditions. The best solution is to place the mineral in a sealed "perky box" to protect it from dust, moisture, and mechanical damage.