Nickeltalmessite

Chemical formula: Ca<sub>2</sub>Ni<sup>2+</sup>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O

Nickeltalmessite is a rare calcium nickel arsenate, forming small, greenish crystals, and is the nickel analog of talmessite.

## Characteristics Nickeltalmessite is a hydrated calcium nickel arsenate belonging to the talmessite group. It occurs as very small, acicular or bladed crystals, rarely exceeding 1 mm in length. These crystals form radial aggregates, rosettes, or fibrous masses with a light green, yellowish-green, or almost white color. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Nickeltalmessite crystals exhibit a vitreous luster. It is a relatively soft mineral, with a Mohs hardness of 3. It is translucent to transparent. Its calculated density is approximately 4.03 g/cm³. ## Colors and Varieties This mineral occurs in shades of light green and yellowish-green, which is characteristic of the presence of nickel ions. Some specimens may be almost colorless or whitish. No commercial or colored varieties are distinguished. ## History and Name Nickeltalmessite was first described in 1960 by German mineralogists Hugo Strunz and Heinz H. Meixner. The name refers to its chemical composition – it is the nickel (Nickel-) analogue of the mineral talmessite (-talmessite), which in turn was named after its discovery locality in the Talmessi mine in Iran. ## Uses Nickeltalmessite has no industrial applications. Its significance is purely scientific and collectible, as a rare member of the arsenate group.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
White
Density
4.03
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Triclinic

Diagnostic features

## Identification Field identification of nickeltalmessite is practically impossible due to its rarity and microscopic size. In a collection, it is recognized by its light green color, acicular crystal habit forming radial aggregates, and its association with other nickel and cobalt arsenates. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or EDS spectroscopy. ## Distinguishing from Similar Minerals Nickeltalmessite can be confused with other secondary nickel arsenates, such as annabergite (more intensely green, forms coatings) or gersdorffite (metallic luster). It differs from them by subtle color shades, crystal form, and chemical composition. From talmessite, which is often colorless or white, it is distinguished by the presence of nickel and the associated greenish color. ## Crystal Forms Crystals are very small, acicular or bladed, elongated along one axis. They usually occur as radial or fibrous aggregates, as well as small rosettes on the surface of the host rock.

Geological environment

## Genesis Nickeltalmessite is a secondary mineral, formed in the oxidation zones (weathering) of ore deposits containing nickel and arsenic, mainly in carbonate rocks (dolomites, limestones). It forms as a result of reactions between arsenic-rich solutions and primary nickel minerals. ## Mineral Associations It most commonly co-occurs with other arsenates, such as annabergite, erythrite, talmessite, gersdorffite, nickeline, as well as with calcite, dolomite, and quartz. The presence of these minerals is an important clue when searching for and identifying nickeltalmessite. ## Localities The most important and classic localities for this mineral are Bou Azzer in Morocco and Schneeberg in Saxony, Germany. It is also known from the Aghbar mine in the Bou Azzer district. Occurrences have also been reported in several other places worldwide where suitable geochemical conditions exist, but these are less significant localities.

Rarity

Very rare

For collectors

## Quality Criteria As a "micromount" type mineral, the quality of a specimen is primarily determined by the sharpness, size, and development of the crystals, as well as their aesthetic arrangement on the rock matrix. The most prized specimens are those with distinct, radial aggregates of an intense, light green color, contrasting with the substrate. Association with other rare minerals is also important. ## Popular Localities The best and most sought-after nickeltalmessite specimens by collectors come from the Bou Azzer region in Morocco. Specimens from historical localities in Germany (Schneeberg) are also valued but much rarer on the market.

Care and storage

## Cleaning Due to its fragility and small crystal size, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used from a safe distance to remove dust. Avoid contact with water and chemicals. ## What to Avoid The mineral is sensitive to acids, which can damage it. Ultrasonic cleaners, high temperatures, and sudden pressure changes should be avoided. As an arsenate, the dust of this mineral is toxic, and inhalation should be avoided. ## Storage Nickeltalmessite specimens should be stored in a dry place, in enclosed "micromount" boxes, to protect them from dust and mechanical damage. Keep it away from minerals that might react with it.

Sources

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