Arsenic

Chemical formula: As

Native arsenic is a semimetal crystallizing in the trigonal system, known for forming reniform or spherulitic aggregates with a metallic luster.

## Characteristics Native arsenic is a chemical element and a semimetal that occurs in crystalline form in nature. It most often forms massive, granular, reniform, or botryoidal aggregates. Characteristic forms include those with a concentric structure, resembling onion skins when fractured. A freshly fractured surface has a tin-white or light gray color and a strong metallic luster, but it quickly tarnishes and becomes covered with a dark gray or black patina upon exposure to air. ## Physical Properties Arsenic is relatively soft, with a Mohs hardness of 3.5. It is brittle, and its density is approximately 5.73 g/cm³. It exhibits perfect cleavage in one direction. The luster on a fresh surface is metallic but quickly becomes dull. ## Colors and Varieties Typical native arsenic has a color ranging from tin-white to steel-gray. Some aggregates, especially reniform and botryoidal ones, may show iridescent tarnish on weathered surfaces. A variety of arsenic is scherbenkobalt, characterized by a lamellar, concentric structure. ## History and Name The name "arsenic" comes from the Greek word *arsenikon* (ἀρσενικόν), meaning orpiment (arsenic sulfide), and also from the word *arsenikos* (ἀρσενικός), meaning "masculine" or "strong," in reference to the potent properties of its compounds. As an element, it was known in antiquity, and its properties were described by Albertus Magnus around 1250. ## Uses Native arsenic primarily has scientific and collector's significance. Its compounds are used in industry for the production of pesticides, herbicides, wood preservatives, and in metallurgy as a component of lead and copper alloys. Due to its toxicity, its use is increasingly restricted.

Properties

Mohs hardness
3.5
Luster
Metallic
Streak
Black
Density
5.73
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Native arsenic can be identified by its metallic luster on a fresh surface, relatively high density, and characteristic rapid tarnishing to a dark gray or black color. When struck or heated, it emits an intense garlic odor. It often occurs in reniform or spherical aggregates with a concentric structure. ## Distinguishing from Similar Minerals Arsenic can be confused with native antimony, which is visually very similar but has a slightly lower density and is more stable in air. Dyscrasite (antimonial arsenic) is practically impossible to distinguish without chemical analysis. From other metallic-lustered minerals like galena or sphalerite, it is distinguished by the lack of regular crystal forms and the garlic odor. ## Crystal Forms Well-formed crystals are extremely rare and usually appear as rhombohedra or pseudocubic crystals. Most commonly found are massive, granular, crustal, botryoidal, reniform, and stalactitic aggregates, often with a radial or concentric internal structure.

Geological environment

## Genesis Native arsenic forms primarily under hydrothermal conditions, in ore veins at medium and low temperatures. It forms in a reducing environment, often in association with silver, cobalt, and nickel minerals. It can also form as a result of volcanic processes or contact metamorphism. ## Mineral Associations It often co-occurs with minerals such as: native antimony, native bismuth, native silver, skutterudite, nickeline, safflorite, löllingite, realgar, orpiment, galena, sphalerite, pyrite, as well as calcite, barite, and quartz. ## Localities Historically important and well-known localities for native arsenic include mines in the Ore Mountains (Schneeberg, Freiberg, Annaberg) in Germany and Jáchymov in the Czech Republic. Other significant occurrences are Kongsberg in Norway, Brosso mine in Italy, Sainte-Marie-aux-Mines in France, Akita in Japan, and some localities in Romania (Săcărâmb) and the USA (Sterling Hill, New Jersey).

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, though rare, crystals. Aesthetic reniform and botryoidal aggregates are also highly valued, especially those of substantial mass and exhibiting a concentric structure on fracture. Specimens from historical, classic localities, such as the German Ore Mountains, are particularly sought after. A fresh, metallic luster is desirable, although patina is a natural feature of the mineral. ## Popular Localities Among the most esteemed localities from which classic arsenic specimens originate are Schneeberg and Freiberg in Saxony (Germany). Specimens from these locations are considered exemplary. Material from Jáchymov (Czech Republic) and Kongsberg (Norway) is also highly valued in collections worldwide.

Care and storage

## Cleaning Native arsenic specimens should generally not be cleaned with water. Only gentle dusting with a soft brush is permissible to remove dust. Contact with water accelerates the oxidation and tarnishing process of the surface. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. Arsenic is sensitive to moisture and atmospheric oxygen, which cause it to darken. It should not be heated, as it then releases toxic fumes with a characteristic garlic odor. Due to its toxicity, inhalation of dust should be avoided, and hands should always be washed after handling the mineral. ## Storage Native arsenic specimens should be stored in a dry place, preferably in tightly sealed containers or display boxes, to limit exposure to air and moisture. Display should be in closed showcases. It should be stored away from minerals sensitive to chemical reactions.

External references

Sources

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