Achávalite
Chemical formula: FeSe
Achávalite is an extremely rare iron selenide, distinguished by its metallic luster and a tendency to develop an iridescent, purplish tarnish.
Properties
- Mohs hardness
- 2.5
- Luster
- Metallic
- Streak
- Black
- Density
- 7.2
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Hexagonal
Diagnostic features
## Identification Key diagnostic features of achávalite include its high density, reddish-brown color on a fresh fracture, characteristic purple tarnish, metallic luster, and low hardness. A black streak is also an important indicator. ## Distinguishing from Similar Minerals Achávalite is sometimes confused with bornite due to its similar, iridescent tarnish. However, bornite is a copper and iron sulfide, has a different structure and properties. It can also be confused with other rare selenides, such as klockmannite, from which differentiation under amateur conditions is practically impossible and requires chemical analysis (e.g., EDS). ## Crystal Forms It most often occurs as granular masses, compact aggregates, or as inclusions in other minerals, especially in calcite. Well-formed crystals in the form of small, hexagonal tablets or plates are extremely rare and small in size.
Geological environment
## Genesis Achávalite is a hydrothermal mineral. It forms at low temperatures in selenium-rich veins cutting through sedimentary rocks, such as sandstones. Its presence indicates specific geochemical conditions favorable for selenium concentration. ## Mineral Associations This mineral most often occurs in association with other equally rare selenides. Typical co-occurring minerals include klockmannite, umangite, berzelianite, clausthalite, as well as calcite, which often forms the rock matrix. ## Localities The most important and historical locality for achávalite is its type locality – the Cacheuta mine in Sierra de Cacheuta, Mendoza Province, Argentina. This is the main source of the highest quality collector specimens. Other reports of its occurrence are few and poorly documented.
Rarity
Extremely rare
For collectors
## Quality Criteria Achávalite's collector appeal is primarily defined by its rarity. Specimens rich in this mineral, exhibiting an intense, reddish-brown color on a fresh surface and a well-developed, iridescent, purplish tarnish, are most highly valued. Association with other rare selenides, forming unique parageneses, is also important. Since it most often occurs in compact form, specimens with visible, even small, crystalline forms are considered exceptional. ## Popular Localities The only significant source of collector specimens is the historic Cacheuta mine in Argentina. Specimens from this locality are most sought after by collectors specializing in rare minerals and systematics.
Care and storage
## Cleaning Achávalite is a very delicate and reactive mineral. Cleaning should be kept to an absolute minimum. Only very careful dust removal with a soft brush is permissible. Avoid all contact with water and chemical agents. ## What to Avoid This mineral is sensitive to moisture and oxidation, which cause tarnishing. Avoid touching with fingers, as moisture and oils from hands accelerate this process. As a selenide, its dust can be toxic, so avoid any activities that could lead to its formation (e.g., scratching, grinding). Protect from acids and other chemicals. ## Storage It is recommended to store achávalite specimens in airtight, sealed containers (e.g., "perky boxes") to limit air and moisture access, which will slow down the tarnishing process. Store in stable conditions, away from heat sources and direct sunlight.