Oberwolfachite

Chemical formula: SrFe<sup>3+</sup><sub>3</sub>(As<sup>5+</sup>O<sub>4</sub>)(S<sup>6+</sup>O<sub>4</sub>)(OH)<sub>6</sub>

Oberwolfachite is a rare strontium and iron sulfate-arsenate, forming tiny, yellowish-brown hexagonal crystals.

## Characteristics Oberwolfachite is a complex sulfate-arsenate belonging to the alunogen group. It occurs as very small, hexagonal, tabular or prismatic crystals, rarely exceeding 0.2 mm in length. These crystals often form rosette-like or spherical aggregates. It is a secondary mineral, formed in the oxidation zones of ore deposits. ## Physical Properties Oberwolfachite crystals exhibit a vitreous luster. Due to the small size of the crystals, many physical properties, such as hardness or density, have not been precisely determined. ## Colors and Varieties This mineral has a characteristic yellowish-brown to orange-brown color. No color varieties or commercial varieties are known. ## History and Name The name oberwolfachite comes from its discovery locality – the Clara mine near the town of Oberwolfach in the Black Forest (Germany). It was approved as a new mineral by the International Mineralogical Association (IMA) in 2015. It was described by a team of mineralogists, including Werner Krause and Jakub Plášil.

Properties

Luster
Vitreous
Streak
Pale yellow
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of oberwolfachite requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction (XRD) combined with chemical analysis (EDS), due to its small size and similarity to other minerals of the alunogen group. ## Distinguishing from similar minerals It can be confused with other arsenates and sulfates of the alunogen group, such as beudantite, segnitite, or gallobeudantite. Differentiation is almost exclusively possible based on detailed chemical composition analysis, especially the determination of strontium as the dominant element. ## Crystal forms It forms hexagonal, tabular crystals, often grouped into rosette-like or spherical aggregates on the surface of the host rock.

Geological environment

## Genesis Oberwolfachite is a secondary mineral that crystallizes in the oxidation zones (so-called iron caps) of polymetallic deposits. It forms as a result of the weathering of primary minerals containing arsenic, sulfur, and iron in a strontium-rich environment. ## Mineral associations It most commonly co-occurs with quartz, barite, goethite, hematite, and other rare arsenates and sulfates, such as arsenogoyazite, weilerite, or beudantite. ## Localities The only confirmed occurrence of oberwolfachite in the world is its type locality – the Clara mine, Rankach, Oberwolfach, Black Forest, Baden-Württemberg in Germany.

Rarity

Extremely rare

For collectors

## Quality criteria As a "micromount" type mineral, specimens with well-formed, sharp crystals forming aesthetic aggregates on a contrasting substrate (matrix) are most highly valued. The intensity and purity of the color also influence the value, although the quality of the crystals themselves, rather than their size (which is always small), is key. ## Popular localities The only source of specimens is the Clara mine in Germany, which is world-renowned for the occurrence of many rare secondary minerals.

Care and storage

## Cleaning Due to the extreme rarity and small size of the crystals, mechanical cleaning is not recommended. If necessary, compressed air can be used to remove dust. Contact with water and chemicals should be avoided. ## What to avoid All acids and detergents that could damage the mineral should be avoided. The crystals are brittle and susceptible to mechanical damage. It should not be subjected to ultrasonic cleaning. ## Storage Oberwolfachite specimens should be stored in stable conditions, in enclosed "micromount" containers, to protect them from dust, moisture, and physical damage.

Sources

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