Allactite

Chemical formula: Mn<sup>2+</sup><sub>7</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>8</sub>

Allactite is a rare manganese arsenate, distinguished by its strong pleochroism and occurrence as small, dark red crystals.

## Characteristics Allactite is a rare, hydrated manganese arsenate. It forms small, tabular or short-prismatic crystals, often gathered in radial or divergent aggregates. It is usually transparent to translucent, with a vitreous luster, which can become slightly greasy on fracture surfaces. Its most characteristic optical feature is very strong pleochroism, i.e., a change in color depending on the direction of observation. ## Physical Properties The hardness of allactite on the Mohs scale is 4.5, making it a relatively soft mineral susceptible to scratching. Its density ranges from 3.83-3.97 g/cm³. It exhibits good cleavage in one direction. ## Colors and Varieties This mineral most often takes on colors from dark reddish-brown to reddish-brown, sometimes with a purple tint. Due to manganese oxidation, its surface may blacken. No color or commercial varieties are distinguished. ## History and Name The name allactite comes from the Greek word "allaktein" (to change), which is a direct reference to its strong pleochroism. The mineral was first described in 1884 by Hjalmar Sjögren based on specimens found in the Långban mine in Sweden. ## Applications Allactite has no industrial applications. It is solely an object of scientific interest and a valued collector's stone due to its rarity and unique optical properties.

Properties

Mohs hardness
4.5
Luster
Vitreous to Greasy
Streak
Gray to brownish-gray
Density
3.83-3.97
Cleavage
Good on {001}
Fracture
Uneven to Sub-Conchoidal
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of allactite is its strong pleochroism (color change from yellowish-brown to greenish-brown), visible under a polarizing microscope. In collector conditions, it is recognized by its dark, reddish-brown color, characteristic crystal forms, and co-occurrence with other manganese minerals from classic localities. ## Distinguishing from similar minerals Allactite is sometimes confused with other reddish or brownish manganese minerals. It is distinguished from hematite by its lower hardness and density and grayish streak (hematite has a cherry-red streak). Rhodonite is much harder (5.5-6.5). However, the ultimate confirmation is the study of optical properties. ## Crystal forms Allactite crystals are usually small, tabular or short-prismatic. They often form fan-shaped, radial, or rosette-like clusters and aggregates grown on other minerals.

Geological environment

## Genesis Allactite is a secondary mineral formed in metamorphosed zones of manganese and zinc ore deposits. It forms as a result of hydrothermal processes occurring within these deposits. ## Mineral associations This mineral often occurs in association with other manganese minerals, such as hausmannite, pyrochroite, tephroite, rhodonite, as well as with calcite and barite. ## Localities The most important and historical occurrences of allactite are the Långban mine in the Värmland region of Sweden, which is its type locality. Equally well-known and valued specimens come from the Franklin and Sterling Hill mines in New Jersey, USA.

Rarity

Rare

For collectors

## Quality criteria The most highly valued specimens are those with well-formed, sharp, and lustrous crystals of intense, reddish-brown color. The rock matrix and the presence of rare associated minerals are also of great importance. Radial aggregates on a contrasting background are particularly sought after. ## Popular localities Two localities are considered classic and provide the best specimens: Långban in Sweden and the Franklin and Sterling Hill deposits in the USA. Specimens from these locations fetch the highest prices and are the pride of advanced collections of rare minerals.

Care and storage

## Cleaning Allactite specimens should be cleaned very carefully, preferably with a soft brush dry. If necessary, distilled water can be used, but prolonged soaking should be avoided. Hands should be washed after contact with the mineral. ## What to avoid As a relatively soft mineral, allactite is susceptible to scratching, so contact with harder minerals should be avoided. It is sensitive to acids and strong detergents. Due to its arsenic content, inhalation of dust generated during any processing should be avoided. ## Storage It is recommended to store specimens in separate, closed boxes or display cases to protect them from dust, light, and mechanical damage. It should not be exposed in areas of high humidity.

External references

Sources

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