Arsenoclasite
Chemical formula: Mn<sup>2+</sup><sub>5</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>4</sub>
Arsenoclasite is a rare, hydrated manganese arsenate, forming characteristic lamellar or fibrous aggregates of a reddish-brown color.
Properties
- Mohs hardness
- 5.5
- Luster
- Vitreous, Pearly
- Streak
- Pale red
- Density
- 4.16
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Arsenoclasite can be identified by its characteristic reddish-brown color, fibrous or lamellar aggregates, and pearly luster on cleavage surfaces. A key feature is its excellent cleavage in one direction, which causes the mineral to easily split into thin lamellae or fibers. The streak is pale red. ## Differentiation from Similar Minerals It can be confused with other manganese arsenates, such as sarkinite or allactite. Differentiation often requires advanced studies, e.g., X-ray diffraction (XRD). In field conditions, distinguishing it from sarkinite is difficult, although sarkinite more often forms better-developed, single crystals. ## Crystal Forms Arsenoclasite crystals are rare and usually appear as small, tabular or bladed forms, elongated in one direction. Most often, however, it forms fibrous, radial, rosette-like aggregates or massive clusters composed of thin lamellae.
Geological environment
## Genesis Arsenoclasite is a secondary mineral, forming in altered iron and manganese ore deposits rich in arsenic. It forms as a result of hydrothermal or metamorphic processes, filling fractures and cavities in host rocks. ## Mineral Associations It often co-occurs with other rare arsenate minerals and manganese oxides. Its typical associated minerals include sarkinite, allactite, magnussonite, hausmannite, barite, and calcite. ## Localities The most important and classic locality for arsenoclasite is its type locality - the Långban deposit in Värmland, Sweden. It is also known from the Sterling Hill mine in Ogdensburg, New Jersey (USA). These are the main localities from which specimens of collector's significance originate.
Rarity
Very rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-developed, radial aggregates or rosettes of intense, reddish-brown color. Rich veins of arsenoclasite contrasting with light host rock are also highly prized. Due to its rarity, even small but well-defined specimens are considered valuable. ## Popular Localities Undoubtedly, the most sought-after specimens come from the historic locality of Långban in Sweden, which has provided the highest quality samples of this mineral. Specimens from Sterling Hill in the USA are also valued, although they often appear as more massive aggregates.
Care and storage
## Cleaning Arsenoclasite specimens should be cleaned very carefully, only mechanically, using a soft brush to remove dust. Due to its arsenic content and excellent cleavage, avoid washing with water, which could penetrate crevices and weaken the aggregate structure. ## What to Avoid Contact with acids and other chemicals should be strictly avoided. The mineral is sensitive to impacts and pressure, which can cause it to break along cleavage planes. It should be protected from moisture and sudden temperature changes. ## Storage It is recommended to store specimens in stable conditions, in closed boxes or display cases, to protect them from dust and mechanical damage. Due to its fragility, it should not be stored in contact with harder minerals.