Odinite
Chemical formula: (Fe<sup>3+</sup>,Mg,Al,Fe<sup>2+</sup>)<sub>2.5</sub>(Si,Al)<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>
Oudinite is a clay mineral from the kaolinite-serpentine group, forming fine-grained, green aggregates, found mainly in marine sediments.
Description
## Characteristics Oudinite is a mineral from the kaolinite-serpentine group, being an iron analogue of amesite. It occurs as extremely fine-grained, compact or earthy aggregates, and also as a component of small, spherical grains (ooids). Due to its microcrystalline nature, it never forms macroscopic, well-formed crystals. Its color ranges from dark green, through greenish-gray, to almost black. ## Physical Properties As a clay mineral, oudinite is very soft. It is characterized by a dull or earthy luster and is translucent to opaque. The calculated density for this mineral is approximately 3.05 g/cm³. ## Colors and Varieties This mineral occurs in shades of green, most often dark and muted. No color varieties or trade names are distinguished for it. ## History and Name The name, approved in 1988, comes from Gilles Serge Odin (born 1944), a French sedimentologist from Pierre and Marie Curie University in Paris. He was honored for his extensive research on green marine clays, including glauconite and the verdine facies, in which oudinite is a key component. The type locality (locus typicus) is the Îles de Los in Guinea. ## Applications Oudinite has no industrial applications. Its significance is mainly scientific, as an indicator of conditions prevailing in sedimentary environments. It is also an object of interest for collectors specializing in mineral systematics.
Diagnostic features
## Identification Identifying oudinite based on visual characteristics is practically impossible. Reliable identification requires advanced analytical techniques, primarily X-ray diffraction (XRD) and chemical analysis. In the field, its green color, earthy form, and occurrence in a specific geological environment – in shallow-marine sediments rich in green grains – can be an indication. ## Distinguishing from Similar Minerals Oudinite can be easily confused with other green clay minerals, such as glauconite, celadonite, or minerals from the chlorite group (e.g., chamosite). Glauconite usually forms brighter, more lustrous grains. The final distinction of these minerals is possible only by laboratory methods. ## Crystal Forms Oudinite is a cryptocrystalline mineral. It occurs in the form of very fine flakes and lamellae, which form compact, earthy, or dusty aggregates. It often also forms small, spherical or oval aggregates (pellets, ooids) with a diameter of less than a millimeter.
Geological environment
## Genesis Oudinite is an authigenic mineral, meaning it crystallizes directly at the site of sedimentation. It forms during early diagenesis in shallow, tropical marine environments (lagoons, estuaries) with a low rate of sediment accumulation. It is characteristic of the so-called verdine facies. ## Mineral Associations This mineral co-occurs with quartz, calcite, goethite, and other clay minerals. ## Localities The most important and typical occurrence is in the sediments of the Îles de Los off the coast of Guinea. Its presence has also been found in shelf sediments off the coasts of French Guiana, Gabon, and in the Gulf of Guinea.
Rarity
Rare
Collector aspects
## Quality Criteria Since oudinite does not form aesthetic crystals, its collector's value is mainly related to its scientific and documentary value. Rich, as pure as possible samples from well-described localities, especially from the type locality, are prized. A specimen is more valuable if it has been subjected to analysis confirming its identity (e.g., XRD). ## Popular Localities Oudinite specimens are practically not found on the commercial collector's market. The most sought-after samples come from the type locality in Guinea and are mainly available in the collections of scientific institutions.
Care and storage
## Cleaning Oudinite specimens are extremely delicate and fragile. Wet cleaning should be avoided, as water can cause the aggregate to disintegrate. To remove dust, at most a soft brush or a gentle stream of air from a photographic blower can be used. ## What to Avoid This mineral is sensitive to all mechanical impacts, so touching, scratching, and vibrations should be avoided. It should be protected from contact with water and chemicals. Heating can lead to the loss of structural water and a change in its properties. ## Storage It is recommended to store specimens in closed, stable containers, e.g., "micromount" boxes, which protect against dust, moisture, and mechanical damage.