Feitknechtite

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

A rare manganese oxyhydroxide, forming black, fine-grained aggregates, difficult to distinguish from other manganese oxides.

## Characteristics Feitknechtite is a manganese (III) oxyhydroxide and a polymorph of groutite. It most commonly occurs as fine-grained, earthy, or sooty masses, as well as coatings and crusts. Well-formed crystals are extremely rare and appear as microscopic, hexagonal plates. The mineral is opaque and has a black color with a metallic to submetallic luster. ## Physical Properties It is typically soft, with a Mohs hardness of 1-3 for aggregates. It is a brittle mineral. The density calculated from the crystal structure is approximately 3.68 g/cm³. Due to its occurrence as fine-grained masses, properties such as cleavage and fracture are not observed. ## Colors and Varieties Feitknechtite is uniformly black. No colored varieties or trade names are distinguished. ## History and Name The mineral's name, approved in 1964, honors Walter Feitknecht (1899-1975), a Swiss chemist from the University of Bern, for his fundamental research on the chemistry and crystal chemistry of metal oxides and hydroxides, including manganese compounds. ## Applications Feitknechtite has no industrial applications. Its significance is limited to scientific research (mineralogy, chemistry) and as an object of interest for specialized collectors of rare minerals and microminerals.

Properties

Mohs hardness: 1-3
Luster: Metallic to Sub-Metallic
Streak: Brownish-black
Density: 3.68
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Hexagonal

Diagnostic features

## Identification Identifying feitknechtite based on visual characteristics is practically impossible. Its black color, low hardness, and black streak are common to many manganese oxides. Definitive identification requires advanced analytical methods, primarily X-ray diffraction (XRD), which allows distinguishing it from its polymorph, groutite, and other oxides. ## Distinguishing from Similar Minerals Feitknechtite is most commonly confused with other black, opaque manganese oxides and hydroxides, such as groutite, pyrolusite, hausmannite, birnessite, or cryptomelane. Differentiation from groutite (orthorhombic system) is only possible with XRD, as feitknechtite crystallizes in the hexagonal system. Pyrolusite is usually harder (6-6.5) and often forms more distinct crystal habits. ## Crystal Forms Crystals are extremely rare, microscopic, and appear as hexagonal plates. Typically, this mineral forms massive, earthy, sooty, or powdery aggregates and thin coatings on other minerals.

Geological environment

## Genesis Feitknechtite is a secondary mineral, forming in the oxidation zones of manganese deposits. It forms as a result of weathering and alteration of primary manganese minerals, such as rhodonite or rhodochrosite. It can also form in sedimentary environments, for example, in lacustrine or marine manganese concretions. ## Mineral Associations It often co-occurs with other manganese minerals. Its typical associations include groutite, hausmannite, jacobsite, pyrolusite, birnessite, cryptomelane, as well as calcite and franklinite (at the type locality). ## Localities The type locality, where this mineral was first identified, is the Franklin Mine in New Jersey, USA. Other known occurrences include Långban in Sweden and some manganese deposits in Switzerland and Japan. It is likely more widespread but often remains unrecognized in mixtures of manganese oxides.

Rarity

Rare

For collectors

## Quality Criteria For feitknechtite, aesthetics are of secondary importance. Specimens whose identity has been confirmed by analytical methods (e.g., XRD) are most highly valued. Key criteria include the abundance of the mineral aggregate on the rock matrix, the presence of rare associated minerals, and origin from classic, well-known localities. ## Popular Localities Specimens from the type locality – Franklin, New Jersey, USA – are most sought after by collectors. Material from this mine is considered historically and scientifically most important. Samples from Långban, Sweden, known for its unique mineralogy, are also valuable.

Care and storage

## Cleaning Feitknechtite specimens are very delicate and should be handled with care. For dust removal, it is best to use compressed air or a very soft, dry brush. Wet cleaning, especially with detergents, should be avoided. ## What to Avoid The mineral is soft and susceptible to scratches and abrasion. Contact with acids and other chemicals that could damage it should be avoided. Cleaning in ultrasonic cleaners is prohibited. ## Storage It is recommended to store specimens in closed collector boxes to protect them from dust, moisture, and mechanical damage. Exposure to vibrations and shocks should be avoided.

Sources

IMA Mineral List (CNMNC)