Feroxyhyte

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

Feroxyhyte is an iron(III) oxyhydroxide, forming very fine-grained, brownish masses and coatings, often found in iron-manganese concretions.

## Characteristics Feroxyhyte is an iron(III) oxyhydroxide, one of the polymorphs of goethite and lepidocrocite. It occurs almost exclusively in cryptocrystalline or very fine-grained aggregates, forming earthy masses, coatings, and efflorescences. It rarely forms microscopic, hexagonal platelets. Due to its structure, it does not form visible crystals macroscopically. It is a main component of some deep-sea iron-manganese concretions and is found in soils and sediments formed by the rapid oxidation of iron compounds. ## Physical Properties This mineral is opaque and characterized by a dull or earthy luster. Its density is approximately 4.2 g/cm³. Hardness is difficult to determine due to its earthy or powdery form. A significant physical characteristic of feroxyhyte is its ferrimagnetism at room temperature, which distinguishes it from most other iron oxyhydroxides. ## Colors and Varieties Feroxyhyte ranges in color from yellowish-brown, through reddish-brown, to dark brown, almost black. Its color depends on the degree of hydration and particle size. No named varieties are distinguished. ## History and Name The name feroxyhyte, given in 1976 by F.V. Chukhrov and his colleagues, refers to its chemical composition: the presence of iron (Latin: *ferrum*), oxygen (*oxygen*), and hydroxyl groups (*hydroxyl*). It was recognized by the International Mineralogical Association (IMA) as a distinct mineral in the same year. ## Applications Feroxyhyte has no direct industrial applications. However, it is an object of scientific interest in geology, oceanography, and soil science as an indicator of oxidation and weathering processes. It is also studied in the context of its magnetic properties.

Properties

Luster
Dull
Streak
Yellow-brown
Density
4.2
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Identifying feroxyhyte is very difficult and virtually impossible based on visual characteristics alone. Its appearance resembles common mixtures of iron oxides, collectively referred to as limonite. A key diagnostic feature, verifiable in field conditions, is its magnetism – unlike goethite, it is attracted to a magnet. However, definitive confirmation of its identity requires specialized tests, primarily X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Feroxyhyte can be confused with goethite, lepidocrocite, and maghemite. - **Goethite** is visually very similar but is not magnetic at room temperature. - **Lepidocrocite** also does not exhibit magnetism and often has a more vibrant, orange-red color. - **Maghemite** is strongly magnetic, but has a different diffraction pattern and usually forms under different conditions, often at higher temperatures. ## Crystal Forms Feroxyhyte does not form macroscopic crystals. It occurs exclusively as cryptocrystalline, earthy, powdery aggregates, or as a component of spheroidal concretions and nodules.

Geological environment

## Genesis Feroxyhyte forms under low-temperature and strongly oxidizing conditions, as a result of rapid precipitation from iron-rich solutions. It is a metastable mineral that eventually transforms into more stable phases, such as goethite or hematite. Its formation is often associated with the activity of iron-oxidizing microorganisms. ## Mineral Associations It most often co-occurs with other iron and manganese oxides and hydroxides. Typical associated minerals include goethite, lepidocrocite, vernadite, and birnessite. It is a major component of deep-sea iron-manganese nodules. ## Localities As a mineral, it is widely distributed in specific environments worldwide. It occurs abundantly on ocean floors (Pacific, Atlantic, Indian Ocean) in the form of polymetallic nodules. It is also found in soils and lake sediments in many places around the world, as well as a product of weathering of iron-rich rocks. The type locality is located in the former USSR.

Rarity

Rare

For collectors

## Quality Criteria Feroxyhyte is not a mineral valued by typical collectors due to the lack of well-formed crystals and its unattractive appearance. Its value is primarily scientific. For specialized collections, well-documented samples with confirmed purity (by XRD analysis) or manganese nodules rich in feroxyhyte with precisely determined collection localities are most desirable. ## Popular Localities There are no popular localities in the collector's sense. Specimens mainly come from scientific research, e.g., from oceanographic cruises conducting seafloor dredging in the Pacific or Atlantic.

Care and storage

## Cleaning Feroxyhyte specimens are very delicate and sensitive. They should only be cleaned dry, using a soft brush to remove dust. Contact with water is not recommended, as it can initiate phase transformations. ## What to Avoid Feroxyhyte is a metastable mineral. Heating should be absolutely avoided, as it transforms into hematite at temperatures above 100°C. It is also sensitive to moisture and chemicals. Prolonged exposure to sunlight and temperature changes can accelerate its slow, spontaneous transformation into more stable goethite. ## Storage Collectible feroxyhyte specimens, most often in the form of concretions or rock fragments with coatings, should be stored under stable conditions. A dry, cool, and dark place is recommended, preferably in a closed display box away from heat and moisture sources.

External references

Sources

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