Omsite

Chemical formula: Ni²⁺₂Fe³⁺(OH)₆[Sb⁵⁺(OH)₆]

Omsite is a very rare nickel, iron, and antimony hydroxide, forming microscopic, hexagonal crystals of a greenish-yellow color.

## Characteristics Omsite is a hydrated nickel, iron, and antimony hydroxide, belonging to the cualstibite structural group. It occurs as extremely small, hexagonal, tabular crystals, rarely exceeding 50 micrometers in diameter and a few micrometers in thickness. These crystals form rosette-like or randomly oriented aggregates on the surface of other minerals. Due to its microscopic size, its visual features are difficult to observe without specialized equipment. ## Physical Properties Omsite crystals exhibit a greenish-yellow color and a vitreous luster. They are transparent to translucent. Hardness is too difficult to measure due to the size of the crystals, but it is estimated to be around 4 on the Mohs scale. The density calculated from the chemical formula and unit cell parameters is 4.35 g/cm³. ## Colors and Varieties This mineral has been observed exclusively in one color: greenish-yellow. No varieties have been distinguished. ## History and Name Omsite was discovered in the Oms mine, located in the Pyrénées-Orientales region of France. Its name comes directly from this locality. The mineral was approved by the International Mineralogical Association (IMA) in 2012 under number 2012-021. It was described by a team of mineralogists led by S. J. Mills in 2012. ## Applications Omsite has no industrial or commercial applications. Its significance is purely scientific and as a collector's item, being a unique and rare representative of its mineral group.

Properties

Mohs hardness: 3
Color: bright yellow to amber yellow
Luster: Vitreous
Streak: yellow
Density: 3.378
Cleavage: <mi>001</mi>
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of omsite is only possible using advanced laboratory methods, such as Raman spectroscopy, X-ray diffraction (XRD), and chemical analysis using an electron microprobe (EDS/WDS). Visually, in the field or in a collection, it is impossible to distinguish from many other secondary minerals of similar appearance and microscopic size. Its characteristic greenish-yellow color and occurrence in association with nickel and antimony minerals in serpentinites can be an indication, but not a basis for identification. ## Distinguishing from Similar Minerals Omsite can be confused with other minerals from the cualstibite group or similar secondary hydroxides. It is distinguished from jamborite by the presence of antimony. From other antimonate minerals, it is distinguished by its specific chemical composition (presence of nickel and iron) and unique crystal structure, confirmable only by XRD. ## Crystal Forms Omsite forms very small, hexagonal, tabular crystals, often with a six-sided outline. These crystals group into rosette-like or irregular aggregates on the surface of the host rock.

Geological environment

## Genesis Omsite is a secondary mineral, forming in the oxidation zones of ore deposits. In its typical locality (Oms mine), it formed as a result of hydrothermal processes occurring within serpentinized ultramafic rocks (serpentinites), which were cut by later quartz-carbonate veins. It forms from the weathering of primary nickel and antimony minerals in an iron-rich environment. ## Mineral Associations This mineral co-occurs with other rare secondary minerals. In its type locality, it was found in association with trechmannite, orpiment, stibiconite, nickelhexahydrite, gersdorffite, ullmannite, as well as with quartz, calcite, and magnesite. ## Localities The only confirmed and described occurrence of omsite in the world is its type locality: the old nickel and copper mine in Oms, in the Pyrénées-Orientales department, Occitanie region, France.

Rarity

Very rare

For collectors

## Quality Criteria The quality of an omsite specimen is assessed almost exclusively based on the abundance and visibility of microscopic crystals on the rock matrix. The most desirable specimens are those where crystal aggregates are numerous, form distinct rosettes, and have an intense, greenish-yellow color. The aesthetics of the matrix and the presence of well-identified associated minerals, which confirm the paragenesis, are also important. Due to its microscopic nature, the purity and size of individual crystals are of secondary importance – the overall appearance of the aggregate under magnification is what matters. ## Popular Localities The only source of omsite specimens is its type locality – the Oms mine in France. Material from this location is extremely rare and sought after by specialized collectors of rare minerals and "micromounters."

Care and storage

## Cleaning Specimens containing omsite should be handled with extreme caution due to the microscopic size and fragility of the crystals. Cleaning is not recommended, as any mechanical intervention (even a stream of water or compressed air) can irrevocably damage or dislodge the crystals from the matrix. If absolutely necessary, one can attempt to remove dust by very carefully immersing the specimen in distilled water, without any movement. ## What to Avoid Avoid all forms of mechanical cleaning, ultrasonics, and contact with chemicals, including acids and bases. As a hydroxide, the mineral is potentially sensitive to pH changes. High temperatures, which can lead to dehydration and structural damage, should also be avoided. ## Storage Omsite specimens should be stored exclusively in sealed, stable "micromount" boxes to protect them from dust, shocks, and accidental touching. Display should only occur under a microscope, in controlled humidity conditions, and away from direct sunlight.

External references

https://www.mindat.org/min-43284.html