Enargite

Chemical formula: Cu₃AsS₄

Enargite is a copper and arsenic sulfide, characterized by a dark, metallic luster and elongated, almost black, striated crystals.

## Characteristics Enargite is a copper and arsenic sulfosalt, belonging to the stannite group. It most commonly forms prismatic or tabular crystals with an almost square cross-section, which are characteristically striated parallel to the longest axis. It also occurs as granular, radial, or massive aggregates. Its color ranges from steel-gray to iron-black, often with a violet or bluish tarnish. It is an opaque mineral with a metallic luster. ## Physical Properties Enargite is a relatively soft and brittle mineral. Its Mohs hardness is 3, meaning it can be scratched by a copper wire. It has a metallic luster and is opaque. The density is approximately 4.45 g/cm³, which is a noticeable weight in hand for larger specimens. ## Colors and Varieties This mineral is typically steel-gray to iron-black. A dull, violet, or bluish tarnish may appear on surfaces exposed to air. There are no distinct color varieties or commercial varieties, although it sometimes forms intergrowths with other sulfides, which can affect its appearance. ## History and Name The name enargite comes from the Greek word "enarges" (ἐναργής) meaning "distinct" or "obvious," referring to its perfect cleavage. The mineral was first described in 1850 by the German mineralogist Johann Friedrich August Breithaupt based on specimens from the San Francisco mine in San Juan Province, Argentina. ## Uses Enargite is an important ore of copper, as well as a significant source of arsenic. Its exploitation is of great economic importance in some regions of the world, especially in large porphyry systems and epithermal deposits. For collectors, well-formed, lustrous crystals are particularly valuable.

Properties

Mohs hardness: 3
Color: Greyish-black to black, grey/rose-brown in reflected light.
Luster: Metallic
Streak: Black.
Density: 4.4
Cleavage: Perfect on {110}, imperfect on {100} and {010}, indistinct {001}.
Fracture: Irregular/Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Enargite can be identified by its steel-gray to black color, metallic luster, and characteristic, elongated, striated crystals with an almost square cross-section. An important diagnostic feature is its perfect prismatic cleavage. The streak is black. The mineral is also relatively soft (Mohs hardness of 3). ## Distinguishing from Similar Minerals Enargite is sometimes confused with other dark, metallic sulfides. It is distinguished from sphalerite by its crystal habit and perfect cleavage (sphalerite has cubic cleavage). It differs from stibnite by its hardness (stibnite is much softer, 2) and crystal habit. From tennantite and tetrahedrite, with which it often co-occurs, it is distinguished by its orthorhombic crystal system and prismatic habit (tennantite and tetrahedrite crystallize in the isometric system, forming tetrahedra). ## Crystal Forms Enargite crystals are typically columnar or tabular, often with an almost square cross-section. Vertical striations on the prism faces are characteristic. It also occurs in granular, massive, radial, or columnar aggregates.

Geological environment

## Genesis Enargite is a hydrothermal mineral of medium to high temperatures. It forms in ore veins and in porphyry copper deposits. It is a typical mineral for the advanced stage of magma crystallization in volcano-plutonic systems, often in zones with high sulfur activity (so-called high-sulfidation epithermal deposits). ## Mineral Associations Enargite often co-occurs with other sulfides and sulfosalts of copper and arsenic. The most common associated minerals include: pyrite, chalcopyrite, bornite, tennantite, tetrahedrite, covellite, luzonite, sphalerite, galena, as well as barite and quartz. ## Localities The most important and historical enargite localities worldwide include Butte, Montana (USA), where large quantities of this mineral were found. Beautiful specimens also come from the Quiruvilca and Pasto Bueno mines in Peru. Other significant localities include Bingham Canyon, Utah (USA), Chuquicamata, Chile, Tsumeb, Namibia, Bor, Serbia, and Mankayan, Philippines.

Rarity

Not very common

For collectors

## Quality Criteria Specimens with sharp, well-formed crystals and a strong, metallic luster are most valued by collectors. Large, single crystals or attractive crystal groups on matrix (most often quartz or pyrite) command the highest prices. The absence of damage, especially on the edges and terminations of brittle crystals, is important. Specimens from classic, historical localities, such as Butte, are particularly sought after. ## Popular Localities Peruvian mines, especially in the Huaron and Quiruvilca regions, currently supply many excellent enargite specimens to the collector market, often in association with pyrite or quartz. Historically, specimens from Butte, Montana, USA, were considered the best in the world; today, they are classics and rarities on the market.

Care and storage

## Cleaning Enargite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If washing is necessary, use a small amount of distilled water and immediately dry the specimen thoroughly. Ultrasonic cleaners should be avoided, as they can damage brittle crystals. ## What to Avoid Enargite is sensitive to acids and strong chemicals. Prolonged exposure to moisture can lead to its slow oxidation and the formation of secondary arsenic and copper minerals, which damages the crystal surface. Avoid sudden temperature changes and direct sunlight, which can cause fading or surface alterations. ## Storage Enargite specimens are best stored in a dry place, in sealed containers or display cases, to protect them from dust and moisture. Due to its brittleness, avoid placing it in contact with harder minerals that could scratch it. Due to its arsenic content, it is recommended to wash hands after handling the mineral.