Fehrite

Chemical formula: MgCu²⁺₄(S⁶⁺O₄)₂(OH)₆·₆H₂O

Fehrite is a very rare, hydrated magnesium copper sulfate with a characteristic turquoise color, belonging to the ktenasite group.

## Characteristics Fehrite is a mineral from the ktenasite group, chemically classified as a hydrated magnesium-copper sulfate. It forms very small, bladed or tabular crystals, which rarely exceed 0.2 mm in length. They usually occur as radial aggregates, rosettes, or spherulites. Its most characteristic feature is its intense, turquoise color. ## Physical properties Fehrite crystals are transparent and exhibit a vitreous luster. The hardness of the mineral has not yet been precisely determined, but for other minerals in the ktenasite group, it is approximately 2 on the Mohs scale. The density of fehrite has been calculated to be 2.73 g/cm³. The mineral exhibits a pale blue-green streak. ## Colors and varieties Fehrite occurs in a uniform, turquoise color. No color or commercial varieties have been distinguished. ## History and name The mineral was approved by the International Mineralogical Association (IMA) in 2019 under number 2019-022. Its name honors the Swiss amateur mineralogist, Karl T. Fehr (born 1954), in recognition of his contributions to Alpine and systematic mineralogy. It was discovered in the dumps of the Casualidad mine in Spain. ## Applications Due to its extreme rarity and microscopic crystal size, fehrite has no industrial applications. It is solely an object of scientific and collecting interest for specialized micromineral collectors.

Properties

Luster: Vitreous
Streak: Pale blue-green
Density: 2.73
Cleavage: on {001}
Fracture: Irregular/Uneven
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Fehrite is identified by its characteristic turquoise color, its occurrence in the form of small, radial aggregates, and its specific geological environment. However, final identification requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD), due to the microscopic size of the crystals and similarity to other secondary minerals. ## Distinguishing from similar minerals Fehrite can be confused with other minerals from the ktenasite group, such as ktenasite or niedermayrite, which have similar color and form of occurrence. Distinguishing them from each other is impossible without specialized chemical analysis. It can also be confused with other secondary copper minerals, e.g., aurichalcite, which, however, usually has a lighter, bluish hue. ## Crystal forms The mineral forms very small, bladed crystals, elongated along the [010] axis and flattened parallel to the {001} plane. These crystals combine into radial aggregates, rosettes, and spherulites, forming crusts on the host rock.

Geological environment

## Genesis Fehrite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic deposits. It forms under low-temperature conditions as a result of sulfate-rich waters acting on rocks containing copper and magnesium minerals. In the type locality, it formed in the dumps of a former mine, due to the weathering of ores in the presence of dolomite. ## Mineral associations This mineral co-occurs with other secondary sulfates and carbonates. In the type locality (Casualidad mine in Spain), it was found in association with gypsum, chalcanthite, epsomite, hexahydrite, ktenasite, antlerite, brochantite, malachite, and linarite. ## Localities The only confirmed occurrence of fehrite in the world is its type locality - the dumps of the former Mina Casualidad mine, located in Baños de Alhamilla, Pechina, in the province of Almería (Andalusia, Spain).

Rarity

Very rare

For collectors

## Quality criteria The quality of fehrite specimens is assessed primarily based on the richness and aesthetics of the crystal aggregates on the rock matrix. The most valued specimens are those with well-formed, distinct rosettes or spherulites of intense turquoise color, contrasting with the substrate. Due to the microscopic nature of the mineral, its association with rare accompanying minerals is also crucial. ## Popular localities The only source of fehrite specimens is its type locality in Spain. All specimens available on the collector's market come from this single location.

Care and storage

## Cleaning Fehrite specimens should be handled with the utmost care. Due to its probable low hardness and water solubility, wet cleaning is not recommended. Any contaminants can only be removed with compressed air from a safe distance. ## What to avoid Contact with water, acids, and other chemicals that could damage or dissolve the mineral should be strictly avoided. Fehrite, as a hydrated mineral, is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight, which could lead to its dehydration and destruction. ## Storage Fehrite specimens, typically in the form of micromounts, should be stored under stable conditions, in closed, airtight boxes (so-called micromount boxes). This protects them from dust, mechanical damage, and fluctuations in ambient humidity.