Groutite

Chemical formula: Mn<sup>3+</sup>O(OH)

Groutite is a rare manganese hydroxide, forming lustrous, black prismatic or acicular crystals, prized by collectors.

## Characteristics Groutite is a manganese(III) hydroxide and belongs to the diaspore group. It typically forms small, prismatic or acicular crystals, often aggregated into radial or fan-like clusters. Specimens range in color from black to dark brown and are characterized by a strong, almost metallic luster on crystal faces, and a resinous luster on fracture surfaces. It is polymorphic with feitknechtite and manganite. ## Physical Properties This mineral is relatively hard, with a Mohs hardness of 5.5. It is brittle, and its density is approximately 4.14 g/cm³. It exhibits perfect cleavage in one direction. The luster is described as submetallic to adamantine. ## Colors and Varieties The dominant color of groutite is black, sometimes with a dark brown or reddish tint, especially in thin fragments where it becomes translucent and shows a reddish-brown color. No color or commercial varieties are distinguished. ## History and Name The mineral's name honors Frank Fitch Grout (1880–1954), a professor of geology and mineralogy at the University of Minnesota, who made significant contributions to the study of manganese ore deposits in the Cuyuna Range. The mineral was first described in 1945 based on samples from the Mahnomen mine in the Cuyuna Range, Minnesota, USA. ## Uses Due to its rarity and small accumulations, groutite has no industrial significance. However, it is a sought-after mineral prized by collectors specializing in rare and systematic minerals.

Properties

Mohs hardness
5.5
Luster
Submetallic, Adamantine
Streak
Reddish-brown
Density
4.14
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Opaque, translucent in thin fragments
Crystal system
Orthorhombic

Diagnostic features

## Identification Groutite can be identified by its black color, submetallic or adamantine luster, and characteristic prismatic or acicular crystals, often forming radial aggregates. A reddish-brown streak is a key diagnostic feature. ## Distinguishing from Similar Minerals Groutite is sometimes confused with manganite, with which it is polymorphic. However, manganite has a lower hardness (4) and a darker, brownish-black streak. It is distinguished from goethite by its higher hardness and black color (goethite is usually brown). Other black manganese oxides, such as pyrolusite, are usually much softer and smudge fingers. ## Crystal Forms Crystals are most often elongated, prismatic along the [001] axis, often with vertical striations. They occur as single, well-formed crystals or as radial, fan-like, and fibrous aggregates. Granular masses are also found.

Geological environment

## Genesis Groutite is a secondary mineral, formed by the oxidation of other manganese minerals at low temperatures. It forms in hydrothermal veins and in the oxidation zones of manganese ore deposits. It can also form from the alteration of manganese-rich sediments. ## Mineral Associations It most commonly co-occurs with other manganese minerals, such as pyrolusite, manganite, hausmannite, braunite, as well as with quartz, barite, calcite, and hematite. ## Localities The most important localities worldwide include the type locality in the Cuyuna Range, Minnesota (USA), as well as mines in the Wessels and N'Chwaning areas in Kuruman (Kalahari Desert, South Africa), which yield some of the best specimens. It also occurs in Långban, Sweden, in Ilfeld in the Harz Mountains (Germany), and in the Bachkoun mine in Morocco.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed, lustrous crystals of intense black color. Radial aggregates forming aesthetic "suns" or fans on a contrasting matrix are highly valued. The absence of damage and crystal size are also important – specimens with crystals exceeding a centimeter in length are rare and sought after. ## Popular Localities The most famous and desirable groutite specimens come from the manganese mines in the Kalahari Desert, South Africa, especially from the Wessels mine. Specimens from the type locality in Minnesota (USA) are mainly of historical significance.

Care and storage

## Cleaning Groutite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always thoroughly dry the specimen after cleaning. ## What to Avoid Avoid contact with acids and other strong chemicals that can damage the mineral. Groutite is brittle, so it must be protected from impacts and falls. It should not be heated or subjected to ultrasound. ## Storage Groutite specimens are best stored in separate, padded boxes to prevent scratches and mechanical damage. They should be protected from dust and moisture. Exposure to direct sunlight is not recommended, although the mineral is stable.

External references

Sources

Read more