Marécottite

Chemical formula: Mg(H<sub>2</sub>O)<sub>6</sub>]<sub>3</sub>[(U<sup>6+</sup>O<sub>2</sub>)<sub>4</sub>O<sub>3</sub>(OH)(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>]<sub>2</sub>·8H<sub>2</sub>O

Marécottite is a very rare, secondary uranyl mineral, forming microscopic, yellow, platy crystals.

## Characteristics Marécottite is a hydrated magnesium uranyl sulfate. It occurs as very small, platy or bladed crystals, rarely exceeding 0.2 mm in length. These crystals form rosette-like or radial aggregates, as well as thin coatings and crusts on rock surfaces. Due to the microscopic size of the crystals, its visual features are difficult to observe without magnification. ## Physical Properties Marécottite crystals exhibit a vitreous luster. The mineral is transparent to translucent. Hardness and density have not been precisely determined due to the small size and rarity of samples. ## Colors and Varieties Marécottite has a characteristic, bright yellow color. No varieties have been described. ## History and Name The mineral was described in 2004 by Jakub Plášil and his co-workers. The name comes from the Marécotte Valley in the canton of Valais, Switzerland, where its type locality – the La Creusaz uranium mine – is located. It was approved by the International Mineralogical Association (IMA) under number 2003-014. ## Uses Marécottite, like other secondary uranium minerals, has no industrial application. It is of purely scientific interest and is an object of interest for specialized collectors of rare minerals and micromounts.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light yellow
Density
3.35
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Identifying marécottite in the field is practically impossible. Identification requires specialized laboratory equipment, such as a polarizing microscope, and analytical methods (e.g., X-ray diffraction XRD, Raman spectroscopy) to determine its crystal structure and chemical composition. A visual feature, observed under a microscope, is its bright yellow color and characteristic habit of thin, platy crystals. ## Distinguishing from Similar Minerals Marécottite can be confused with many other yellow secondary uranyl minerals, such as uranophane, meta-autunite, or zippeite. Distinguishing it from these minerals based on visual characteristics is unfeasible. Key is the examination of chemical composition (presence of magnesium and sulfates) and crystallographic data. ## Crystal Forms Crystals are very small, thin, and platy, often elongated, with outlines resembling rectangular or bladed shapes. They usually form rosette-like, fan-shaped, or radial aggregates, as well as thin coatings on the host rock.

Geological environment

## Genesis Marécottite is a secondary mineral, forming in the oxidation (weathering) zones of uranium deposits. It forms as a result of sulfate-rich waters acting on primary uranium minerals, such as uraninite. This process occurs under low temperature and pressure conditions, usually within rock voids and fractures. ## Mineral Associations At the type locality (La Creusaz, Switzerland), marécottite co-occurs with gypsum, uranophane-alpha, johannite, zippeite, and also with uraninite as the primary mineral. ## Localities This is an extremely rare mineral. Apart from the type locality in the La Creusaz mine in the Marécotte Valley, Switzerland, its occurrence has been confirmed in only a few other places worldwide, including the Svornost mine in Jáchymov (Czech Republic) and the Blue Lizard mine in San Juan County, Utah (USA).

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of marécottite specimens is primarily assessed based on the richness and aesthetics of crystal aggregates visible under a microscope. Samples with well-formed, sharp crystals forming visually attractive groups, such as rosettes, are most highly valued. It is also important that the crystals are undamaged and set on a contrasting rock matrix. Due to the microscopic nature of the mineral, the size of individual crystals is not a key criterion, in contrast to the abundance of their occurrence on a given rock fragment. ## Popular Localities The most known and valued specimens, which also constitute the type material, come from the La Creusaz mine in Switzerland. Samples from Jáchymov and Utah are also sought after by specialized collectors due to the extreme rarity of this mineral.

Care and storage

## Cleaning Marécottite specimens should not be wet-cleaned. Due to its water solubility, contact with water can lead to the destruction of the crystals. Only gentle dust removal with a soft brush or a photographic air blower is permissible, preferably under a microscope. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. It should not be heated or exposed to direct sunlight, which can cause dehydration and decomposition. ## Storage Specimens should be stored in stable conditions, in sealed, closed containers (so-called micromount boxes), to protect them from dust, moisture, and mechanical damage. Due to its uranium content, the mineral is radioactive and should be stored with appropriate precautions, away from areas of permanent human habitation.

Sources

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