Trébeurdenite

Chemical formula: Fe₂+₂Fe₃+₄O₂(OH)₁₀(CO₃)·₃H₂O

Trébeurdenite is a very rare iron hydroxycarbonate, forming microscopic, hexagonal plates with a metallic luster.

## Characteristics Trébeurdenite is a hydrated iron hydroxycarbonate with a complex chemical structure. It occurs as extremely small, hexagonal or pseudohexagonal platy crystals, rarely exceeding 20 micrometers in diameter. These crystals form rosette-like or spherulitic aggregates, as well as thin coatings and crusts. Due to the microscopic size of the crystals, its macroscopic features are difficult to observe without specialized equipment. ## Physical Properties Trébeurdenite crystals exhibit a metallic luster. Hardness and density have not been precisely measured due to the small size and fragility of the material. The mineral is opaque. ## Colors and Varieties Trébeurdenite has a characteristic black color, and in transmitted light under a microscope, it reveals a reddish-brown hue. No varieties have been distinguished. ## History and Name The mineral was discovered on the beach of Trébeurden in Brittany (France), in pebbles of a rock called Ploumanac'h dolerite. Its name, approved by the International Mineralogical Association (IMA) in 2011, comes directly from its type locality. It was described by a team of mineralogists led by Chi Ma and Anthony R. Kampf. ## Applications Trébeurdenite has no industrial application and is solely an object of scientific interest and a collector's item for specialized micromineral collectors.

Properties

Luster: Metallic
Streak: Reddish brown
Cleavage: Perfect on {0001}
Transparency: Opaque
Crystal system: Hexagonal

Diagnostic features

## Identification Identification of trébeurdenite is almost exclusively possible using advanced analytical methods, such as scanning electron microscopy (SEM) with EDS analysis or X-ray diffraction (XRD). In collector conditions, identification relies on knowledge of the type locality and the characteristic morphology of microscopic, black, hexagonal plates forming rosette-like aggregates. ## Distinguishing from Similar Minerals It can be confused with other dark-colored secondary iron minerals, such as magnetite, hematite, or goethite, which may co-occur with it. Differentiation without specialized analysis is practically impossible. The key is the hexagonal crystal morphology visible under high magnification. ## Crystal Forms It forms very small, thin, hexagonal or pseudohexagonal plates. These crystals often combine into rosettes, spherulites, or form thin, brittle coatings on the surface of the host rock.

Geological environment

## Genesis Trébeurdenite is a secondary mineral, formed as a result of hydrothermal processes or weathering affecting iron-rich igneous rocks, such as dolerites. It forms at low temperatures in rock cavities and fissures. ## Mineral Associations At the type locality, it co-occurs with minerals such as goethite, lepidocrocite, quartz, hibbingite, as well as minerals from the serpentine and chlorite groups. ## Localities The only confirmed and described locality of trébeurdenite in the world is its discovery site – the beach of Trébeurden, Côtes-d'Armor department, Brittany, France. The mineral is found there in dolerite pebbles.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a trébeurdenite specimen is assessed based on the abundance and development of its microscopic crystals. Samples with numerous, well-formed rosettes or dense crystalline coatings, visible under a microscope, are most valued. Contrast with the light background of the host rock is also important. ## Popular Localities The only source of specimens is the type locality in Trébeurden, France. Material from this location is highly sought after by collectors specializing in microminerals and type locality minerals.

Care and storage

## Cleaning Specimens containing trébeurdenite should generally not be cleaned mechanically or chemically. Dust can only be removed very carefully, using a photographic air blower or compressed air from a distance. Any contact with water or chemicals can damage the delicate crystals. ## What to Avoid Contact with water, acids, bases, and any cleaning agents must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. It should not be heated or exposed to direct sunlight. ## Storage Specimens should be stored under stable conditions, preferably in closed, airtight containers (so-called micromount boxes), to protect them from dust, moisture, and mechanical damage. Due to the microscopic size of the crystals, display without magnification is pointless.

Sources

IMA Mineral List (CNMNC)