Arsenobenauite

Chemical formula: SrFe₃+₃(As₅+O₄)(As₅+O₃OH)(OH)₆

A rare strontium iron arsenate of the plumbogummite group, occurring in oxidation zones as yellow-brown microcrystals and crusts.

## Characteristics Arsenobenauite is a rare arsenate mineral belonging to the plumbogummite group within the alunite supergroup. Chemically, it is the arsenic analogue of benauite. It usually forms microscopic rhombohedral crystals, as well as earthy or dense coatings and crusts on rock surfaces. Its color ranges from yellowish, through yellow-brown, to greenish-yellow. ## Physical properties Due to its rarity and microscopic crystal size, macroscopic determination of arsenobenauite's physical properties is difficult. This mineral usually forms translucent crystals; however, in dense crusts, it becomes opaque. ## Colors and varieties Arsenobenauite occurs in shades of yellow, brown, and green. It has no distinct color varieties or commercial names. ## History and name The name of the mineral refers to its chemical composition dominated by arsenic and its direct structural relationship to benauite. It was described based on specimens from the Fuchs quarry in Germany, which serves as its type locality. ## Uses Due to its extreme rarity and formation of only microscopic aggregates, arsenobenauite has no industrial application. It is exclusively a rare object of interest for scientists and advanced systematic mineral collectors.

Properties

Crystal system: Trigonal

Diagnostic features

## Identification Macroscopic recognition of arsenobenauite is practically impossible due to its similarity to many other secondary iron and arsenic minerals. Reliable identification requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis, to confirm the presence of strontium, iron, and arsenic in appropriate proportions. ## Distinguishing from similar minerals Visually, this mineral can be confused with other members of the alunite supergroup (such as benauite, beudantite, or segnitite) and common iron hydroxides. Distinguishing it from benauite requires confirming the dominance of arsenic over phosphorus in the crystal structure. ## Crystal habits It forms very fine rhombohedral crystals. Most commonly, it occurs as microscopic aggregates, thin crusts, and earthy masses filling rock fissures.

Geological environment

## Origin Arsenobenauite is a secondary mineral. It forms in oxidation zones and in rock fractures (including rhyolites) rich in iron, manganese, strontium, and arsenic, as a result of the action of hydrothermal solutions or weathering processes. ## Mineral associations It co-occurs with other secondary minerals. At the type locality, it is accompanied by manganese and iron oxides as well as other rare arsenates. ## Localities This mineral is known from very few places worldwide. Its most important and best-documented occurrence is the type locality at the Fuchs quarry (Hartkoppe) in Sailauf, Aschaffenburg District (Lower Franconia, Bavaria, Germany).

Rarity

Extremely rare

For collectors

## Quality criteria The collector value of arsenobenauite relies almost entirely on its rarity and documented provenance. The most appreciated specimens are those with well-developed microcrystals that can be observed under magnification, as well as those from the type locality. ## Popular localities The only widely known locality sought by collectors, from which reference specimens originate, is the Fuchs quarry in Sailauf (Germany).

Care and storage

## Cleaning Specimens should be cleaned using only a gentle stream of compressed air. Washing in water, brushing, and using ultrasonic cleaners should be avoided due to the high fragility of the microcrystals and the earthy nature of some aggregates. ## What to avoid Contact with any chemicals, acids, and alkaline solutions must be strictly avoided. Specimens must be protected from mechanical damage, abrasion, and shocks. ## Storage Specimens should be stored in closed micromount boxes to protect them from dust and moisture. These boxes permanently protect delicate crusts from accidental touching and damage during viewing.

Browse minerals in the shop

External references

https://www.mindat.org/min-499365.html