Metahohmannite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>O(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·4H<sub>2</sub>O

Metahohmannite is a rare, secondary iron sulfate, forming aggregates of fine, acicular crystals of orange color.

## Characteristics Metahohmannite is a hydrated iron sulfate, occurring as aggregates of fine, fibrous or acicular crystals. It also forms efflorescences and crusts. Individual crystals are usually too small to be seen with the naked eye, and the material has the appearance of felted or matted aggregates. ## Physical Properties Metahohmannite crystals are characterized by a silky luster. It is a very soft mineral, with a Mohs hardness of approximately 2.5. Due to its fibrous nature, it is brittle. ## Colors and Varieties Metahohmannite most often takes on an orange, orange-yellow, or yellow-brown color. No varieties are distinguished. ## History and Name The mineral's name refers to its relationship with hohmannite. Metahohmannite is its less hydrated (dehydrated) form, indicated by the Greek prefix "meta-". Hohmannite, in turn, was named after the Chilean mining engineer, Richard Hohmann. Metahohmannite was first described by Arthur F. H. O. Hillebrand in 1889. ## Uses Metahohmannite has no industrial significance. It is a mineral primarily of interest to collectors specializing in rare secondary minerals or mineral systematics.

Properties

Mohs hardness
2.5
Luster
Silky
Streak
Orange-yellow
Density
2.2
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Triclinic

Diagnostic features

## Identification Metahohmannite is identified by its characteristic orange color, fibrous or acicular aggregate form, and silky luster. A key feature is its genesis – occurrence in the oxidation zones of sulfide deposits in desert areas. Reaction to moisture (dissolution, alteration) is also a diagnostic feature. ## Distinguishing from similar minerals It can be confused with other secondary iron sulfates of similar color, such as hohmannite, butlerite, parabutlerite, or fibroferrite. Hohmannite is darker, more brownish, and contains more water. Butlerite and parabutlerite have different crystal structures. Certain distinction of these minerals often requires chemical or X-ray diffraction (XRD) analysis. ## Crystal forms It forms aggregates of fine, elongated, acicular or capillary crystals. It often occurs as matted, felted aggregates, as well as efflorescences and coatings on other minerals.

Geological environment

## Genesis Metahohmannite is a secondary mineral, formed by the oxidation of iron sulfides (mainly pyrite) in very dry, desert conditions. It forms as a product of the evaporation of acidic, sulfate-rich mine waters or as an efflorescence on the walls of old mine workings. ## Mineral associations It often co-occurs with other iron sulfates, such as hohmannite, amarantite, butlerite, parabutlerite, fibroferrite, copiapite, melanterite, and pyrite. ## Localities The most important localities for this mineral are in Chile, in the Atacama Desert, from where its holotype originates (Chuquicamata mine). It is also known from the Island Mountain mine in California (USA) and from other dry, desert regions of the world where suitable conditions for its formation exist.

Rarity

Rare

For collectors

## Quality criteria Specimens with rich, well-formed aggregates of fibrous crystals with intense, orange color are most valued by collectors. Association with other rare sulfates and aesthetic placement on the matrix rock are also important. Due to the fragility and sensitivity of the mineral, specimens undamaged during transport are much more highly valued. ## Popular localities The best and most classic metahohmannite specimens come from its type locality – the Chuquicamata mine in Chile. Specimens from this location are considered exemplary for the species.

Care and storage

## Cleaning The mineral is extremely sensitive to water and moisture. It must absolutely not be cleaned wet. To remove dust, a very soft brush or a stream of compressed air from a distance can be used very carefully. ## What to avoid Avoid all contact with water, water vapor, and high humidity, which can cause it to dissolve or transform into another mineral. Protect from high temperatures and direct sunlight. ## Storage Metahohmannite specimens must be stored in sealed, dry containers, preferably with a desiccant (e.g., silica gel). They should be kept in stable conditions, away from sources of heat and light.

Sources

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