Hydrotungstite

Chemical formula: W⁶⁺O₂(OH)₂·H₂O

Hydrotungstite is a rare, hydrated tungsten oxide, forming soft, earthy coatings and crusts with a characteristic green color.

## Characteristics Hydrotungstite is a hydrated tungsten oxide, most commonly occurring as earthy or microcrystalline aggregates, coatings, and crusts. It rarely forms small, tabular crystals. Its appearance is usually inconspicuous, and specimens have a dull or slightly silky luster. Due to its nature, it is a secondary mineral, formed as a result of the weathering of other tungsten minerals. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It is relatively heavy for its appearance, with a density reaching 4.6 g/cm³. It exhibits imperfect cleavage in one direction. It is translucent, especially in thin fragments. ## Colors and Varieties Fresh hydrotungstite specimens range in color from dark green to yellowish-green. Upon exposure to air and loss of water, its color changes to yellowish. No named varieties of this mineral are distinguished. ## History and Name The name hydrotungstite directly refers to its chemical composition – "hydro" from the presence of water in its structure and "tungstite" from its tungsten content. It was first described in 1940 based on specimens found in the San Antonio de Calacalani mine in Bolivia. ## Uses Hydrotungstite has no industrial significance. Its only importance is scientific and as a collector's item, as a rare product of tungsten mineral weathering.

Properties

Mohs hardness: 2
Luster: Silky
Streak: Yellowish-green
Density: 4.60
Cleavage: {010}, imperfect
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Hydrotungstite is identified by its characteristic green or yellowish-green color, very low hardness (can be scratched with a fingernail), and occurrence as earthy coatings and crusts on other tungsten minerals, mainly wolframite and scheelite. ## Distinguishing from Similar Minerals It can be confused with other secondary tungsten minerals, such as tungstite (WO₃·H₂O), which is usually more yellow and harder. It is distinguished from malachite by its significantly lower hardness and lack of reaction with acids. From other green mineral coatings (e.g., uranium minerals), it is distinguished by its geological context – occurrence in the oxidation zones of tungsten deposits. ## Crystal Forms It most often occurs as earthy, powdery aggregates or forms thin coatings. Very rarely, small, tabular crystals with monoclinic symmetry, grouped into small aggregates, are observed.

Geological environment

## Genesis It is a typical secondary mineral, formed in the oxidation (weathering) zones of tungsten ore deposits. It forms as a result of the slow decomposition of primary tungsten minerals, such as wolframite or scheelite, under the influence of surface waters. ## Mineral Associations It most commonly co-occurs with the minerals from which it forms, namely wolframite and scheelite. It is also accompanied by other products of their weathering, such as tungstite, ferritungstite, as well as quartz, limonite, and clay minerals. ## Localities The most important and historical locality is the San Antonio de Calacalani mine in Oruro, Bolivia, from where the first described specimens originated. It is also known from several other localities worldwide, including the Wolfram Camp mine in Queensland (Australia), Portugal, and several places in the United States (e.g., Arizona and Nevada).

Rarity

Rare

For collectors

## Quality Criteria The collector's appeal of hydrotungstite primarily depends on the intensity and purity of its green color. The most prized specimens are those in the form of distinct, rich coatings on a contrasting matrix (e.g., on dark wolframite or white quartz). Exceptionally rare and sought after are specimens with visible, even microscopic, crystals. Due to its instability, specimens that have retained their original, vibrant green color are more highly valued. ## Popular Localities Classic and most valued specimens come from the type locality in Bolivia (San Antonio de Calacalani).

Care and storage

## Cleaning Hydrotungstite specimens are extremely delicate and sensitive. They should only be cleaned with a very soft brush, dry, to remove dust. Avoid water and any chemical agents. ## What to Avoid Contact with water, which can alter its structure, must be absolutely avoided. The mineral is very soft, so it must be protected from scratches and impacts. Changes in humidity and exposure to air can cause a color change from green to yellow. ## Storage It is recommended to store specimens in closed, stable conditions, preferably in "membrane boxes" or airtight containers away from dust, humidity, and direct sunlight, to preserve their original color and form.

External references

https://www.mindat.org/min-1988.html