Auriacusite

Chemical formula: Fe₃+Cu₂+(As₅+O₄)O

Auriacusite is a rare iron and copper arsenate, forming aggregates of fine, acicular crystals with a characteristic golden-yellow color.

## Characteristics Auriacusite is an iron and copper arsenate, occurring as fine, acicular or bladed crystals. It forms radial aggregates, rosettes, or spherulitic clusters; less commonly, it appears as individual, prismatic crystals. Its most distinctive feature is its intense, golden-yellow to orange-yellow color, to which it owes its name. ## Physical Properties Auriacusite crystals are very small, usually not exceeding 1 mm in length. This mineral is characterized by a vitreous luster and is transparent to translucent. Its Mohs hardness is approximately 3, and its density is approximately 4.85 g/cm³. It is a brittle mineral. ## Colors and Varieties This mineral occurs in shades from golden-yellow to orange-yellow. No varieties have been distinguished. ## History and Name The name auriacusite comes from the Latin words *aurum* (gold) and *acus* (needle), which directly refers to its typical color and crystal habit. It was first described in 2011 by Stuart J. Mills and co-workers, and approved as a new mineral by the International Mineralogical Association (IMA2011-044). The type locality is the Maria-Josefa mine in Spain. ## Uses Auriacusite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Pale yellow
Density: 4.85
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Characteristic features of auriacusite include its golden-yellow color, vitreous luster, and occurrence as fine, acicular crystals forming radial aggregates. Due to the small size of the crystals, a microscope or loupe is often necessary for its identification. ## Distinguishing from Similar Minerals It can be confused with other secondary arsenates of similar color, such as mimetite or pharmacosiderite. Auriacusite differs from them in crystal habit (thin needles in radial clusters) and specific environment of occurrence. However, final confirmation requires advanced analytical methods, such as Raman spectroscopy or EDS analysis. ## Crystal Forms Auriacusite crystallizes as very fine, acicular or bladed crystals, which almost always occur in radial or spherulitic aggregates. Single, well-formed crystals are extremely rare.

Geological environment

## Genesis Auriacusite is a secondary mineral, formed in the oxidation zones (gossans) of polymetallic deposits rich in arsenic. It forms as a result of the weathering of arsenopyrite and other sulfides in the presence of copper minerals. ## Mineral Associations It most commonly co-occurs with goethite, pharmacosiderite, scorodite, olivenite, conichalcite, arseniosiderite, and quartz. ## Localities The most important and well-documented auriacusite localities in the world are: - Maria-Josefa mine, Rodalquilar, Andalusia, Spain - type locality. - Clara mine, Wolfach, Black Forest, Germany. - Cap Garonne mine, Le Pradet, Var, France.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized auriacusite specimens are those with rich, well-formed radial aggregates of intense, golden-yellow color. Contrast with the rock matrix, most often dark goethite or quartz, is important. Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. Large, well-defined spherulites or rosettes on an aesthetic matrix achieve the highest value. ## Popular Localities The most sought-after specimens by collectors come from the type locality - the Maria-Josefa mine in Spain, as well as from the Clara mine in Germany, which is renowned for supplying high-quality microminerals.

Care and storage

## Cleaning Auriacusite specimens should be cleaned with the utmost care, using compressed air to remove dust. Due to its brittleness and potential reactivity, the use of water, and especially chemical solutions, is discouraged. Any contact with liquids can lead to damage of the delicate crystals. ## What to Avoid Avoid ultrasonic cleaners, all chemicals (acids, bases, solvents), high temperatures, and sudden temperature changes. The mineral is brittle, so it must be protected from impacts and vibrations. As an arsenate, it should be stored away from moisture to prevent potential chemical reactions. ## Storage It is recommended to store specimens in closed, stable "micromount" containers, which protect them from dust, mechanical damage, and moisture. Avoid exposure to direct sunlight, although there is no evidence of fading.

Sources

IMA Mineral List (CNMNC)