Bendadaite

Chemical formula: Fe<sup>2+</sup>Fe<sup>3+</sup><sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>·4H<sub>2</sub>O

Bendadaite is a rare, hydrated iron arsenate, forming characteristic, radial aggregates of dark green to black color.

## Characteristics Bendadaite is a rare secondary mineral from the arsenate group, belonging to the arthurite group. It occurs as small, acicular or bladed crystals, which most often form spherical or radial aggregates. Individual crystals are rare and reach a maximum of a few millimeters in length. Its appearance is very characteristic, resembling dark green or almost black "hedgehogs" or "spheres" growing on the host rock. ## Physical Properties Bendadaite crystals exhibit a luster ranging from vitreous to silky, depending on the form of occurrence. They are translucent at the edges, and in larger masses, practically opaque. This mineral is relatively soft, with a Mohs hardness of approximately 3. It is also quite brittle. ## Colors and Varieties Bendadaite occurs in shades from dark green, through greenish-black, to almost black. Sometimes it exhibits an olive-green hue, especially in the case of finer needles. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was discovered in the Bendada mine in Portugal, from which its name originates. It was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 2007. Its discovery was the result of a re-examination of historical specimens from this mine. ## Uses Bendadaite has no industrial application. Its significance is purely scientific and collectible, being a valued acquisition for specialists collecting rare minerals, especially arsenates.

Properties

Mohs hardness
3
Luster
Vitreous to silky
Streak
Light green
Density
3.16
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Bendadaite is most easily recognized by its characteristic form of occurrence – spherical or radial aggregates composed of very fine, acicular crystals. Its dark green to black color and occurrence in the oxidation zones of arsenic deposits are key indicators. ## Distinguishing from Similar Minerals It can be confused with other secondary iron arsenates, such as scorodite; however, scorodite usually forms better-developed, thicker crystals of a different form and often a lighter color. Similar aggregates can be formed by cacoxenite, which, however, usually has a yellow or brown color and occurs in a different geological environment. Accurate distinction from other minerals of the arthurite group often requires advanced chemical analysis. ## Crystal Forms Crystals are elongated, acicular or bladed, with pointed terminations. They almost always occur as radial, spherical or hemispherical aggregates, which can reach several millimeters in diameter.

Geological environment

## Genesis Bendadaite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially arsenopyrite. It forms as a result of the action of surface waters on primary arsenic and iron minerals under conditions of low temperature and pressure. ## Mineral Associations It most often co-occurs with minerals from which it forms or which it accompanies in the oxidation zone. These include arsenopyrite, pyrite, scorodite, pharmacosiderite, beudantite, as well as quartz and various iron oxides (e.g., goethite). ## Localities The most important and typical occurrence is the Bendada mine in the Guarda district, Portugal. This mineral has also been identified in several other locations worldwide, including the Clara mine in the Black Forest (Germany) and mines in the Cadia region in New South Wales (Australia).

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, distinct, spherical aggregates of intense, dark green color. Contrast with a light host rock (most often quartz) is important, as it highlights the form and color of bendadaite. The size of the aggregates and the lack of damage to the delicate needles significantly increase the value of the specimen. ## Popular Localities By far the most desirable and classic specimens come from the type locality – the Bendada mine in Portugal. Specimens from the Clara mine in Germany are also known, but the Portuguese ones set the standard for this mineral.

Care and storage

## Cleaning Bendadaite specimens should be cleaned very carefully, preferably using compressed air to remove dust. If necessary, a soft brush can be used. Contact with water, especially with detergents, is not recommended, as it can damage delicate crystals and potentially react with the mineral. ## What to Avoid Bendadaite is a mineral containing arsenic, so inhalation of dust should be avoided, and hands should be washed after each contact with a specimen. It should be protected from acids and other chemicals. It is brittle, so it must be protected from impacts and vibrations. It should not be heated or exposed to prolonged strong light. ## Storage It is recommended to store specimens in closed, stable containers or display cases, away from dust and moisture. It is best to place it in a separate box with a warning label about arsenic content to avoid accidental contact.

Sources

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