Tangdanite

Chemical formula: Ca₂Cu₂+₉(As₅+O₄)₄(S₆+O₄)₀.₅(OH)₉·₉H₂O

Tangdanite is a very rare, secondary copper mineral, forming aggregates of blue-green, platy crystals.

## Characteristics Tangdanite is a hydrated copper and calcium arsenate, belonging to the mixite group. It occurs as very fine, hexagonal, platy crystals, which typically form rosette-like or spherulitic aggregates. Individual crystals rarely exceed 0.2 mm in size. Due to its appearance and chemical composition, it is a mineral of interest mainly to collectors specializing in rare specimens or secondary minerals. ## Physical Properties Tangdanite crystals exhibit a vitreous luster. They are transparent to translucent. The mineral is relatively soft, with a Mohs hardness of approximately 3. It is brittle and has an uneven fracture. The calculated density is 3.55 g/cm³. ## Colors and Varieties Tangdanite is characterized by a vivid, blue-green color. The streak is light blue. No varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the Tangdan copper mine in Yunnan Province, China. It was officially recognized by the International Mineralogical Association (IMA) in 2011 (IMA2011-084). The first description was made by Chinese and Italian mineralogists, including Gu-Xiang-Xie. ## Uses Tangdanite has no industrial applications. Its significance is purely scientific and collectible.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Light blue
Density: 3.55
Cleavage: None
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Identification of tangdanite is based on its characteristic blue-green color, its occurrence as fine, platy crystals forming rosette-like aggregates, and its association with other secondary copper and arsenic minerals. The light blue streak is also a helpful feature. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or Raman spectroscopy. ## Distinguishing from Similar Minerals Tangdanite is visually almost impossible to distinguish from other blue-green arsenates of the mixite group, such as mixite, agardite, or zalesiite, without the use of specialized equipment. All these minerals form similar acicular or fibrous aggregates. Differences in chemical composition (the presence of calcium and sulfates in tangdanite) are key to their differentiation. ## Crystal Forms The mineral forms very small, hexagonal, platy crystals, most often occurring in radial, rosette-like, or spherulitic clusters and aggregates. Single, well-formed crystals are observed less frequently.

Geological environment

## Genesis Tangdanite is a secondary mineral, formed in the oxidation (weathering) zones of copper ore deposits that also contain arsenic minerals. It crystallizes from aqueous solutions under low temperature and pressure conditions, often precipitating in rock fractures and voids. ## Mineral Associations At the type locality (Tangdan mine), tangdanite occurs in association with minerals such as olivenite, conichalcite, malachite, azurite, tyrolite, brochantite, tenorite, gypsum, and goethite. ## Localities The only confirmed and described locality for tangdanite in the world is the Tangdan copper mine, in Dongchuan County, Kunming Prefecture, Yunnan Province, China.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of tangdanite specimens is primarily assessed based on the richness and aesthetics of the crystal aggregates. The most desirable specimens are those with clearly formed, lustrous rosettes or spherulitic clusters of intense, blue-green color, contrasting with the rock matrix. The size of the aggregates and the absence of mechanical damage also significantly increase their collectible value. ## Popular Localities The only source of tangdanite specimens is its type locality – the Tangdan mine in China. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Due to their extreme rarity and delicacy, tangdanite specimens should not be cleaned mechanically or chemically. Any such procedures carry the risk of damage. Only careful dust removal with a soft brush or a photographic air blower is permissible. ## What to Avoid Avoid contact with water, acids, detergents, and other chemicals. The mineral is brittle, so it must be protected from impacts, vibrations, and sudden temperature changes. As an arsenate, mineral dust is toxic – avoid inhaling it. ## Storage Specimens should be stored under stable conditions, preferably in a closed, padded "micromount" box, away from dust, humidity, and direct sunlight. This will protect the delicate crystals from damage and degradation.

Sources

IMA Mineral List (CNMNC)