Mcalpineite

Chemical formula: Cu<sup>2+</sup><sub>3</sub>Te<sup>6+</sup>O<sub>6</sub>

Mcalpineite is a very rare copper tellurate, forming microscopic, hexagonal crystals with an intense black color and metallic luster.

## Characteristics Mcalpineite is a copper tellurate with a very complex structure. It occurs as extremely small, hexagonal, tabular crystals, rarely exceeding 0.2 mm in size. It most often forms rosette-like or spherulitic aggregates, as well as thin coatings and crusts on host rocks. Its crystals have an intense black color and a strong, metallic luster, which makes it distinctive despite its small size. ## Physical Properties This mineral is opaque. Due to the microscopic size of the crystals, precise determination of hardness and density is difficult. The hardness is estimated at approximately 3 on the Mohs scale. The density calculated based on the chemical formula and unit cell parameters is 6.16 g/cm³. ## History and Name The mineral's name honors Frank McAlpine (born 1932), an American mineral collector from Eureka, Utah, who first found specimens of this mineral. Mcalpineite was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 1991. The discovery was made on material from the Centennial Eureka Mine in the Tintic District, Utah, USA.

Properties

Mohs hardness
3
Luster
Metallic
Streak
Black
Density
6.16
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Identification of mcalpineite is based on its characteristic appearance – black, metallic-lustered, hexagonal crystals forming small rosettes or spherulites. Due to microscopic sizes, final confirmation requires advanced analytical methods, such as energy-dispersive X-ray spectroscopy (EDS/EDX) to verify chemical composition (presence of copper and tellurium) and X-ray diffraction (XRD) to confirm crystal structure. ## Distinguishing from Similar Minerals In localities where it occurs, it may be confused with other black, metallic secondary minerals, such as tenorite (CuO) or other tellurates. However, mcalpineite is distinguished by its characteristic, hexagonal crystal morphology. Certain differentiation from similar, rare minerals requires laboratory analysis. ## Crystal Forms It forms very small, tabular, hexagonal crystals, often grouped into rosette-like or spherulitic (spherical) aggregates. It also occurs as thin crusts and coatings.

Geological environment

## Genesis Mcalpineite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic ore deposits that are rich in tellurium. It forms as a result of the oxidation of primary tellurides in the presence of copper-rich solutions. ## Mineral Associations This mineral co-occurs with other secondary tellurium minerals and sulfates. In the type locality (Centennial Eureka Mine), it was found in association with quartz, barite, pyrite, enargite, as well as rarer minerals such as: native tellurium, emmonsite, mackayite, jensenite, leisingite, frankhawthorneite, and utahite. ## Localities It is an extremely rare mineral, known from only a few locations worldwide. The most important and best-documented occurrences are: - Centennial Eureka Mine, Tintic District, Juab County, Utah, USA (type locality). - Empire Mine, Tombstone District, Cochise County, Arizona, USA. - Aga Mine, Goodsprings District, Clark County, Nevada, USA.

Rarity

Very rare

For collectors

## Quality Criteria As a micromineral, the collector's value of mcalpineite primarily depends on the quality and definition of the crystals, rather than their size. Specimens with sharp, well-formed, hexagonal crystals forming aesthetic, rosette-like aggregates are most prized. The contrast with the matrix rock and the richness of associated minerals, especially other rare tellurates, are also important. The purity of the specimen (lack of damage) is crucial. ## Popular Localities By far the most sought-after specimens by collectors come from the type locality – the Centennial Eureka Mine in Utah, USA. Material from this location is historically the most important and serves as the standard for this mineral.

Care and storage

## Cleaning Due to the extreme rarity and delicacy of specimens, mechanical cleaning is not recommended. Any contaminants, such as dust, should be removed only with compressed air (from a safe distance) or a very soft brush. Do not use water or any chemical agents. ## What to Avoid Avoid contact with chemicals, especially acids, which can rapidly destroy the mineral. Specimens are very brittle and sensitive to any shocks, ultrasounds, and temperature changes. Do not expose them to direct sunlight. ## Storage Mcalpineite specimens, as microminerals, should be stored exclusively in specialized, sealed "micromount" boxes that protect them from dust, mechanical damage, and moisture. Store under stable room conditions.

Sources

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