Synadelphite

Chemical formula: Mn₂+₉(As₃+O₃)(As₅+O₄)₂(OH)₉·₂H₂O

Synadelphite is a rare manganese arsenite-arsenate, forming prismatic crystals with a characteristic, zoned color ranging from colorless to reddish-brown.

## Characteristics Synadelphite is a complex, hydrated manganese arsenite-arsenate, belonging to the hematolite group. This mineral forms elongated, prismatic crystals, often terminated pyramidally. A characteristic feature of many specimens is color zoning – their interiors can be colorless, while the outer parts take on shades from red, through brown, to almost black. In transmitted light, the crystals exhibit a color from colorless to pale brown or reddish. ## Physical Properties Synadelphite crystals have a hardness of 4.5 on the Mohs scale. Their luster is vitreous, and the density is approximately 3.57 g/cm³. The mineral can be transparent, translucent, or opaque, depending on the intensity of coloration and the presence of inclusions. It exhibits imperfect cleavage in one direction. ## Colors and Varieties The dominant colors are shades of red and brown, often with a darker, almost black outer surface of the crystals. Distinct zoning occurs, where crystal cores can be completely colorless. No named varieties of this mineral are distinguished. ## History and Name The name synadelphite comes from the Greek words *σύν* (syn - together) and *ἀδελφός* (adelphos - brother), which refers to its close association and co-occurrence with other similar manganese minerals at the discovery site. The mineral was first described in 1884 based on specimens found in the Moss mine in Nordmark (Sweden). ## Uses Synadelphite has no industrial application. It is solely a mineral of scientific and collector's interest.

Properties

Mohs hardness: 4.5
Luster: Vitreous
Streak: White to pale red
Density: 3.57
Cleavage: On {010}, imperfect.
Fracture: Irregular/Uneven,Conchoidal
Transparency: Transparent,Translucent,Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification It is distinguished by its prismatic crystal habit and characteristic, zoned coloration (colorless core, reddish-brown rim). A hardness of approximately 4.5 and co-occurrence with other rare manganese minerals in skarns are key diagnostic features. ## Distinguishing from Similar Minerals It can be confused with other minerals from the hematolite group, such as hematolite or allactite. Differentiation requires precise chemical (analysis of As³⁺ to As⁵⁺ ratio) or crystallographic studies. It differs significantly from eudialyte by its much lower hardness and different environment of occurrence. ## Crystal Forms Synadelphite forms columnar or prismatic crystals with a nearly square cross-section, often terminated by bipyramidal faces. It also occurs in the form of radial or granular aggregates.

Geological environment

## Genesis It is a secondary mineral, formed in manganese and iron ore deposits that have undergone contact metamorphism. It typically forms in skarns or metamorphosed carbonate rocks rich in manganese. ## Mineral Associations It most commonly co-occurs with other manganese minerals, such as hausmannite, allactite, hematolite, pyrochroite, magnussonite, phlogopite, barite, and calcite. ## Localities The most important and classic locality, from which the best specimens originate, is the deposits in the Långban and Nordmark area in Värmland, Sweden. This mineral has also been confirmed in the Sterling Hill mine in New Jersey (USA), which is the second most important locality worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued specimens are those with well-formed, sharp crystals with distinct luster and an attractive, reddish-brown color. Crystals exhibiting characteristic color zoning with a colorless interior are particularly sought after. Large, undamaged crystals on a contrasting matrix (e.g., on calcite) achieve the highest value. ## Popular Localities By far the most prized specimens come from the historic localities in Sweden – Långban and the Moss mine in Nordmark. Specimens from Sterling Hill in the USA, although also significant, are generally less common on the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided. ## What to Avoid As an arsenate, the mineral is sensitive to acids and other strong chemicals. Prolonged exposure to moisture, high temperatures, and direct sunlight, which can cause color changes, should be avoided. Due to its arsenic content, caution should be exercised, and hands should be washed after contact with the mineral. ## Storage It is recommended to store specimens in a dry place, in closed display boxes or cabinets, away from direct light. They should be protected from impacts and scratching by harder minerals.

External references

https://www.mindat.org/min-3852.html