Lukrahnite
Chemical formula: CaCu<sup>2+</sup>Fe<sup>3+</sup>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH,H<sub>2</sub>O)<sub>2</sub>
Lukrahnite is a very rare calcium, copper, and iron arsenate, forming aggregates of tiny, tabular crystals of an intense green color.
Properties
- Mohs hardness
- 4.5
- Luster
- Vitreous
- Streak
- Light green
- Density
- 4.29
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Identification of lukrahnite is based on its characteristic emerald-green color, specific crystal habit (tiny, tabular or bladed) forming rosette-like aggregates, and its occurrence in association with other rare arsenates. Due to the small size of the crystals, visual identification is difficult and requires magnification. Final confirmation of identity requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Lukrahnite can be confused with other green arsenates from the tsumcorite group, such as gartrellite, helmutwinklerite, or tsumcorite. Distinguishing it from these minerals based on visual characteristics is practically impossible and requires specialized laboratory tests. Chemical analysis to determine the proportions of calcium, copper, and iron is crucial. ## Crystal Forms Lukrahnite crystallizes as very small, thin, tabular or bladed crystals, often with a rhombic outline. These crystals combine to form characteristic radial or rosette-like aggregates, and also create thin coatings on rock surfaces.
Geological environment
## Genesis Lukrahnite is a secondary mineral that forms in the oxidation zones (gossans) of polymetallic ore deposits rich in arsenic. It originates from the weathering of primary sulfides, mainly tennantite, in the presence of calcium, copper, and iron-bearing minerals. ## Mineral Associations This mineral co-occurs with other rare arsenates and secondary minerals. In its type locality (Tsumeb), it has been found in association with tennantite, anglesite, beudantite, gartrellite, ludlockite, olovorsite, schneiderhöhnite, tsumcorite, and willemite. ## Localities The only confirmed and described locality for lukrahnite in the world is its type locality – the Tsumeb mine in the Oshikoto region of Namibia. This is a historic site, known for its exceptional richness and diversity of rare secondary minerals.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of lukrahnite specimens is primarily assessed based on the richness and aesthetics of the crystal aggregates. The most prized specimens are those with well-formed, distinct rosettes or dense clusters of intensely green crystals, contrasting with the rock matrix. Due to the microscopic size of the crystals, their individual clarity or transparency are not key criteria. Association with other rare minerals from Tsumeb is also important. ## Popular Localities The only source of lukrahnite specimens is the historic, now inactive Tsumeb mine in Namibia. All specimens available on the collector's market originate from this single location and are part of old collections.
Care and storage
## Cleaning Lukrahnite specimens should be cleaned with the utmost care, exclusively using compressed air to remove dust. Due to the extreme delicacy and small size of the crystals, contact with water, brushes, or ultrasonics is highly risky and can lead to the destruction of the specimen. ## What to Avoid Contact with all chemicals, including acids and bases, which can dissolve the mineral, must be strictly avoided. Lukrahnite is sensitive to high temperatures and mechanical shocks. It should be protected from direct sunlight, which can cause fading. ## Storage Lukrahnite specimens, typically in the form of micromounts, should be stored under stable conditions, in closed, padded "micromount" boxes that protect them from dust, moisture, and mechanical damage. Display should be away from sources of vibration and direct light.