Mahnertite

Chemical formula: (Na,Ca,K)Cu<sup>2+</sup><sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>Cl·5H<sub>2</sub>O

Mahnertite is a very rare, secondary copper mineral with an intensely blue-green color, forming microscopic, tabular crystals.

## Characteristics Mahnertite is a hydrated arsenate of sodium, calcium, potassium, and copper with an additional chloride anion. It occurs as very small, thin, tabular crystals, rarely exceeding 0.2 mm. These crystals often form rosette-like or radial aggregates, as well as thin coatings and crusts on the surface of the host rock. Due to its microscopic size, it is a mineral primarily accessible to advanced micromount collectors. ## Physical Properties Mahnertite crystals exhibit a vitreous luster. They are transparent to translucent. The Mohs hardness is low, at 2, which means it is a very soft mineral. The density calculated based on the chemical formula and unit cell parameters is 3.31 g/cm³. ## Colors and Varieties This mineral is characterized by a vivid, blue-green color. The streak is light blue. No varieties of this mineral have been distinguished. ## History and Name The mineral is named in honor of Volker Mahnert (1943-2018), an Austrian zoologist and director of the Natural History Museum in Geneva, Switzerland, in recognition of his contributions to the museum's activities. The mineral was described by H. Sarp, P. Perroud, and J. Deferne in 1991. The type locality from which it originates is the Cap Garonne mine in France. ## Uses Mahnertite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light blue
Density
3.31
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of mahnertite is based on its characteristic blue-green color, the form of thin, tabular crystals forming rosette-like aggregates, and its occurrence in paragenesis with other secondary copper arsenates. Due to the microscopic size of the crystals, definitive confirmation requires advanced analytical methods such as X-ray diffraction (XRD) or EDS spectroscopy. ## Distinguishing from Similar Minerals It can be confused with other secondary, blue-green copper minerals such as geminite, lindackerite, or lavendulan. Differentiation with the naked eye is practically impossible. Mahnertite is distinguished by its specific chemical composition (presence of sodium, calcium, potassium, and chlorine) and unique crystallographic parameters. ## Crystal Forms Crystals are very small, thin, and tabular, with a hexagonal or octagonal outline. They typically occur as radial or rosette-like aggregates, as well as thin coatings on the rock.

Geological environment

## Genesis Mahnertite is a secondary mineral, formed in the oxidation zones (gossans) of polymetallic ore deposits rich in copper and arsenic. It forms as a result of the weathering of primary sulfides, mainly tennantite, in an environment rich in chloride ions, often originating from seawater or saline groundwater. ## Mineral Associations This mineral co-occurs with other rare, secondary arsenates. At the type locality (Cap Garonne), it was found in association with geminite, lindackerite, tennantite, chalcopyrite, covellite, olivenite, agardite-(Y), cyanotrichite, and gypsum. ## Localities The most important and best-documented occurrence of mahnertite is its type locality – the Cap Garonne mine in Le Pradet, Var, France. This is a historic copper mine known for its exceptional diversity of rare secondary minerals. Beyond France, its presence has also been reported in the Maria Josefa mine in Rodalquilar, Spain.

Rarity

Very rare

For collectors

## Quality Criteria For mahnertite collectors, the most important criteria are the richness and aesthetics of the crystal aggregates. Specimens with well-formed, distinct rosettes of intense, blue-green color, contrasting with the rock matrix, are most valued. Crystal size, although always microscopic, is also important – larger and better-formed tablets increase the value of the specimen. ## Popular Localities By far the most sought-after and classic mahnertite specimens come from the Cap Garonne mine in France. This locality has provided the highest quality samples of this mineral, serving as the type specimen for the species.

Care and storage

## Cleaning Mahnertite specimens should be cleaned with the utmost care, exclusively using compressed air to remove dust. Due to its softness and solubility, contact with water, and especially chemicals, should be avoided. Ultrasonic cleaners or brushes must not be used. ## What to Avoid The mineral is sensitive to acids and water, in which it can dissolve. It should be protected from high temperatures, which can lead to dehydration and destruction of the crystal structure. Avoid direct sunlight, which can cause fading over time. ## Storage It is recommended to store specimens in stable conditions, in closed, transparent "micromount" boxes, which protect delicate crystals from mechanical damage, dust, and sudden changes in humidity.

Sources

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