Jensenite
Chemical formula: Cu<sup>2+</sup><sub>3</sub>Te<sup>6+</sup>O<sub>6</sub>·2H<sub>2</sub>O
Jensenite is a very rare copper tellurate, forming microscopic, emerald-green crystals with a platy habit.
Properties
- Mohs hardness
- 3-4
- Luster
- Vitreous
- Streak
- Light green
- Density
- 4.75
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Jensenite can be identified by its characteristic emerald-green color, platy crystal habit with a hexagonal outline, and occurrence as small rosettes or flakes. Its origin from very few, specific localities and co-occurrence with other copper tellurates and tellurides are also key. ## Distinguishing from Similar Minerals Jensenite is sometimes confused with other green, secondary copper minerals, such as brochantite, malachite, or other rare tellurates (e.g., quetzalcoatlite). It is distinguished from brochantite and malachite by its crystal habit (platy, not acicular or botryoidal) and geological environment. Identification and differentiation from other rare tellurates require advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Jensenite crystals are thin-platy, flattened parallel to (001), and have a pseudohexagonal (six-sided) outline. They usually form radial or rosetted aggregates, and also occur as single, overgrown crystals.
Geological environment
## Genesis Jensenite is a secondary mineral formed in the oxidation (weathering) zones of ore deposits where tellurium is present. It forms as a result of the oxidation of primary tellurides, most often in the presence of copper minerals. It forms under low temperature and pressure conditions, in voids and fissures in rocks. ## Mineral Associations This mineral co-occurs with other rare tellurium minerals, such as mcalpineite, xocomecatlite, quetzalcoatlite, frankhawthorneite, as well as native tellurium, copper, tenorite, quartz, and calcite. ## Localities The most important and typical locality for jensenite is the Centennial Mine in Calumet, Houghton County (Michigan, USA). This is a historic copper mine, known for the occurrence of many rare secondary minerals. Besides this site, its presence has been noted in only a few other places worldwide, including the Bambolla Mine in Moctezuma (Sonora, Mexico).
Rarity
Very rare
For collectors
## Quality Criteria The collector appeal of jensenite primarily depends on the quality and size of the crystals, which for this mineral means specimens exceeding fractions of a millimeter in diameter. The most prized are well-formed, sharply terminated crystals forming aesthetic, rosetted aggregates of an intense emerald-green color. Contrast with the rock matrix and the presence of rare associated minerals are also important. ## Popular Localities By far the most valued and sought-after specimens by collectors come from the type locality – the Centennial Mine in Michigan, USA. This is the classic and historic source of the world's best specimens of this mineral.
Care and storage
## Cleaning Jensenite specimens should be cleaned with the utmost care, using only compressed air to remove dust. Due to the extreme rarity and fragility of the crystals, mechanical methods (brushes) or chemical methods (water, solvents) are discouraged. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and sudden temperature changes. The crystals are very small and brittle, susceptible to mechanical damage. Specimens should not be exposed to direct sunlight, which can cause them to fade. ## Storage Jensenite specimens, as microminerals, should be stored in specialized, sealed boxes (so-called micromount boxes) that protect them from dust, moisture, and physical damage. Display should only be done under a microscope.