Posnjakite
Chemical formula: Cu<sup>2+</sup><sub>4</sub>S<sup>6+</sup>O<sub>4</sub>(OH)<sub>6</sub>·H<sub>2</sub>O
Blue-green, secondary copper sulfate forming small, platy crystals and coatings.
Properties
- Mohs hardness
- 2-3
- Luster
- Vitreous
- Streak
- Light blue
- Density
- 3.3-3.4
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Posnjakite is primarily identified by its characteristic, intense blue or blue-green color, its occurrence as small, platy crystals forming rosettes and coatings, and its association with other secondary copper minerals. ## Distinguishing from Similar Minerals It can be confused with other secondary copper sulfates, such as brochantite, langite, or wroewolfeite. It differs from brochantite by its lighter color and crystal habit (brochantite forms needles). From langite, which is its polymorph, it is difficult to distinguish without X-ray diffraction (XRD) analysis, although langite has a slightly more greenish hue. Wroewolfeite has a similar habit but usually a more orange-red color. ## Crystal Forms Crystals are very small, thin-platy, with a hexagonal outline, often grouped into rosette-like or radial aggregates. It also occurs as coatings, crusts, and earthy masses.
Geological environment
## Genesis Posnjakite is a secondary mineral, forming in the oxidation (weathering) zones of copper ore deposits, especially copper sulfides. It forms as a result of sulfate-rich waters acting on primary minerals under low temperature and near-neutral pH conditions. It often crystallizes in voids and fissures in rocks. ## Mineral Associations It most commonly co-occurs with other secondary copper minerals, such as brochantite, langite, malachite, azurite, as well as gypsum, jarosite, and goethite. The primary mineral from which it weathers is usually chalcopyrite or bornite. ## Localities Significant localities with well-formed specimens include the Tsumeb mine in Namibia, mines in the Bisbee and Morenci areas in Arizona (USA), deposits in Kazakhstan (type locality), as well as numerous occurrences in Cornwall (Great Britain) and the Urals (Russia). In Poland, its presence has been noted in the copper ore mines of the LGOM (e.g., Lubin, Polkowice).
Rarity
Not very common
For collectors
## Quality Criteria Specimens with well-formed, sharp crystals of intense blue color, forming aesthetic rosettes or rich coatings on a small rock matrix, are most highly valued. Contrast with the color of the host rock (e.g., white quartz) significantly enhances the attractiveness of the specimen. Crystal size, although usually microscopic, is also an important factor – larger crystals are rarer and more desirable. ## Popular Localities Collectors particularly value specimens from classic localities such as Tsumeb (Namibia) and Bisbee (Arizona, USA), which have yielded some of the world's best posnjakite crystals. Specimens from Kazakhstan and Russia are also sought after.
Care and storage
## Cleaning Due to its softness and delicacy, specimens should be cleaned very carefully. It is best to use compressed air to remove dust. If washing is necessary, only distilled water and a very soft brush are permissible, without applying pressure. Ultrasonic cleaners should be avoided. ## What to Avoid Contact with all acids and detergents, which can dissolve the mineral, should be avoided. Posnjakite is sensitive to high temperatures, which can lead to its dehydration and color change. It should be protected from prolonged exposure to direct sunlight. ## Storage Specimens, especially those with well-formed microcrystals, should be stored in closed "micromount" boxes to protect them from dust and mechanical damage. It does not require humidity control, but extreme conditions should be avoided.