Anthonyite

Chemical formula: Cu<sup>2+</sup>(OH)<sub>2</sub>·3H<sub>2</sub>O

Anthonyite is a very rare, secondary copper mineral, forming characteristic lavender-blue, tabular crystals.

## Characteristics Anthonyite is a hydrated copper hydroxide, occurring as small, tabular or bladed crystals. It typically forms rosette-like or radial aggregates, as well as coatings and crusts. Its most characteristic feature is its lavender-blue to violet-blue color. It is a very soft and delicate mineral. ## Physical Properties Anthonyite crystals exhibit perfect cleavage in one direction, causing them to easily separate into thin, flexible lamellae. It has a pearly luster on cleavage surfaces and a vitreous luster on other faces. It is a light mineral, with a density of approximately 2.97 g/cm³ and a hardness of 2 on the Mohs scale, making it susceptible to scratching with a fingernail. ## Colors and Varieties Anthonyite occurs in a characteristic shade from lavender-blue to violet-blue. No color or commercial varieties are distinguished. ## History and Name The mineral was discovered in the Centennial Mine in Houghton County, Michigan (USA). Its name, approved in 1963, honors John Williams Anthony (1920–1992), a professor of mineralogy at the University of Arizona, for his contributions to the mineralogy of that state. ## Uses Due to its extreme rarity, small crystal size, and delicacy, anthonyite has no industrial applications. It is solely an object of interest for collectors of rare minerals.

Properties

Mohs hardness
2
Luster
Pearly, Vitreous
Streak
Pale blue
Density
2.97
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of anthonyite include its unique lavender-blue color, tabular crystal habit, perfect cleavage in one direction, and pearly luster on cleavage planes. Its extreme softness (hardness 2) is also an important clue. ## Distinguishing from Similar Minerals Anthonyite can be confused with other secondary blue copper minerals. It differs from chalcanthite by its insolubility in cold water and different crystal habit. Unlike azurite, it is much softer and has a different hue (lavender, not azure or Berlin blue). ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline, often grouped into rosette-like or radial aggregates. It also occurs as small scales, coatings, and crusts on the surface of the host rock.

Geological environment

## Genesis Anthonyite is a secondary copper mineral that forms in the oxidation zone of copper deposits. Its formation is likely the result of reactions between copper-rich solutions and alkaline waters, often under specific microclimatic conditions, such as in old mine workings. It can crystallize on rock surfaces or on metal artifacts left in mines. ## Mineral Associations It most commonly co-occurs with native copper, cuprite, tenorite, malachite, paratacamite, and tremolite. Its presence is strongly associated with minerals of the oxidation zone of copper deposits. ## Localities The most important and classic anthonyite localities are in the USA, in mines on the Keweenaw Peninsula in Michigan (including the Centennial, Calumet, and Hecla mines). It is also known from several other locations worldwide, such as the Ajo Mine in Arizona (USA), the Atacama Desert in Chile, and Villahermosa in Mexico.

Rarity

Very rare

For collectors

## Quality Criteria The most prized anthonyite specimens are those with well-formed, sharp crystals of intense, lavender color, forming aesthetic, rosette-like aggregates. Contrast with a light matrix significantly enhances visual appeal. Crystal size is a crucial factor, as most are very small – specimens with crystals exceeding a few millimeters are exceptionally desirable. ## Popular Localities Specimens from the historical localities on the Keweenaw Peninsula in Michigan, USA, are considered the best in the world. They are regarded as typical and exemplary material for this mineral species.

Care and storage

## Cleaning Anthonyite is extremely delicate and sensitive to water. It should not be wet-cleaned. Only very careful removal of dust with a soft brush or a stream of compressed air from a distance is permissible. ## What to Avoid Contact with water, which can damage or dissolve it, must be strictly avoided. It is unstable in low humidity conditions and can dehydrate, losing its structure and color. It should be protected from acids, chemicals, high temperatures, and direct sunlight. ## Storage Anthonyite specimens must be stored in sealed, airtight containers (e.g., perky boxes) to maintain a stable humidity level. It should be kept away from heat and light sources, in a place with a constant temperature. Due to its softness, it should not come into contact with harder minerals.

External references

Sources

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