Ardennite-(As)

Chemical formula: Mn<sup>2+</sup><sub>4</sub>Al<sub>4</sub>(AlMg)(As<sup>5+</sup>O<sub>4</sub>)(SiO<sub>4</sub>)<sub>2</sub>(Si<sub>3</sub>O<sub>10</sub>)(OH)<sub>6</sub>

Ardennite-(As) is a rare manganese and aluminum silicate and arsenate, forming yellow to brown, radial crystal aggregates.

## Characteristics Ardennite-(As) is a complex silicate and arsenate belonging to the ardennite group. It forms elongated, prismatic or acicular crystals, which often occur as radial or fan-shaped aggregates. Specimens typically range in color from yellow, through yellowish-brown, to reddish-brown. Crystals exhibit longitudinal striations on their faces. ## Physical Properties This mineral is characterized by a hardness of 6-7 on the Mohs scale. It has a vitreous luster, and a greasy or resinous luster on fracture surfaces. It is translucent to opaque. Its density is approximately 3.55-3.74 g/cm³. ## Colors and Varieties The dominant colors are honey-yellow, yellowish-brown, reddish-brown, and sometimes almost black. Color variability depends on impurities and crystallization conditions. No named commercial or color varieties are distinguished. ## History and Name The mineral's name refers to its original locality in the Belgian Ardennes and to the dominance of arsenic (As) in its chemical composition. It was originally described in 1872, and its redefinition as ardennite-(As) occurred in 2007 as part of the work on a new classification of the sorosilicate group. ## Applications Ardennite-(As) has no industrial applications. It is solely a mineral of scientific and collector's interest.

Properties

Mohs hardness
6-7
Luster
Vitreous, Resinous
Streak
Yellowish brown
Density
3.55-3.74
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Characteristic features of ardennite-(As) include its radial or fan-shaped aggregates composed of elongated, striated, yellowish-brown crystals. Its occurrence in manganese- and aluminum-rich metamorphic rocks is also an important indicator. ## Distinguishing from Similar Minerals It can be confused with other manganese silicates, such as piemontite or spessartine. It is distinguished from them by its typical crystal habit (elongated, radial aggregates), higher hardness, and specific occurrence environment. However, definitive confirmation requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Crystals are prismatic, elongated along one axis, often flattened. They typically form fan-shaped, radial, or sheaf-like aggregates. Single, well-formed crystals are rarely found.

Geological environment

## Genesis Ardennite-(As) forms as a result of low-grade metamorphism in sedimentary rocks rich in manganese, aluminum, and arsenic, such as quartzites and shales. It crystallizes under greenschist or glaucophane facies conditions. ## Mineral Associations It often co-occurs with minerals such as quartz, albite, hematite, braunite, piemontite, spessartine, and ottrelite. ## Localities The most important localities worldwide include Salmchâteau in the Ardennes (Belgium), where it was discovered, and deposits in the Piedmont region of Italy (e.g., in the Aosta Valley). It also occurs in Greece (Andros island) and Japan (Shiozawa mine, Gunma Prefecture).

Rarity

Rare

For collectors

## Quality Criteria The most highly valued by collectors are specimens with well-formed, fan-shaped or radial aggregates of intense, honey-yellow or reddish-brown color. Contrast with the surrounding host rock (matrix), most often white quartz, is also important. Large, undamaged aggregates are rare and sought after. ## Popular Localities The historical localities in Belgium (Salmchâteau) and Italy (Aosta Valley) are considered classic and yield the best specimens. Specimens from these locations set the standard against which finds from other parts of the world are compared.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. Compressed air may be used with caution. Avoid water and any chemical agents. ## What to Avoid The mineral is sensitive to acids. Ultrasonic cleaners should be avoided, as they can damage fragile crystals and aggregates. It should not be heated or exposed to sudden temperature changes. ## Storage It is recommended to store specimens in closed boxes or display cases to protect them from dust and mechanical damage. Due to its fragility, contact with harder minerals should be avoided.

Sources

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