Meymacite

Chemical formula: W<sup>6+</sup>O<sub>3</sub>·2H<sub>2</sub>O

Meymacite is a hydrated tungsten oxide, occurring as a weathering product of wolframite and scheelite, often in the form of yellow, earthy coatings.

## Characteristics Meymacite is a hydrated tungsten oxide that rarely forms distinct, well-crystallized individuals. It typically occurs as earthy, powdery, or porous aggregates and coatings on other tungsten minerals, such as wolframite or scheelite. Its appearance is inconspicuous, and identification requires specialized studies, as it is visually difficult to distinguish from other secondary tungsten minerals. ## Physical Properties Due to its mode of occurrence, precise determination of physical properties such as hardness or density is problematic. This mineral is very soft and brittle. It has a dull or earthy luster and is opaque. ## Colors and Varieties It ranges in color from light yellow to yellowish-brown. No named varieties of this mineral are recognized. ## History and Name The name Meymacite comes from the locality of Meymac in the Corrèze department, France, where this mineral was first described in the Puy-les-Vignes tungsten mine. For many years, the status of Meymacite as a distinct mineral species was questioned. It was considered a mixture or a synonym of other tungsten oxides, such as tungstite (WO₃·H₂O) or hydrotungstite (WO₃·2H₂O). Ultimately, in 2011, the International Mineralogical Association (IMA) discredited Meymacite as a distinct mineral, recognizing it as a mixture, primarily of tungstite.

Properties

Luster
Earthy
Streak
Yellow
Transparency
Opaque

Diagnostic features

## Identification Identification of Meymacite in practice is impossible without advanced analytical techniques, such as X-ray diffraction (XRD). In field conditions or in a collection, it can only be preliminarily identified as a "secondary tungsten mineral" based on its yellow color, earthy form, and co-occurrence with wolframite or scheelite. ## Distinguishing from Similar Minerals It is visually indistinguishable from other secondary, hydrated tungsten oxides, such as tungstite, hydrotungstite, or ferritungstite. All form similar yellow, earthy coatings in the oxidation zones of tungsten deposits. Definitive differentiation is possible only by laboratory methods. ## Crystal Forms It does not form macroscopic crystals. It occurs exclusively in the form of earthy, powdery aggregates, and also as pseudomorphs after wolframite or scheelite crystals, meaning it replaces the primary mineral while retaining its external form.

Geological environment

## Genesis It is a secondary mineral, formed in the oxidation (weathering) zones of tungsten ore deposits. It forms as a result of the chemical decomposition of primary tungsten minerals, mainly wolframite ((Fe,Mn)WO₄) and scheelite (CaWO₄), under the influence of oxygen-rich surface waters. ## Mineral Associations It co-occurs with the minerals from which it forms, namely wolframite and scheelite. It is also accompanied by other minerals of the oxidation zone, such as tungstite, hydrotungstite, ferritungstite, quartz, and iron oxides (limonite). ## Localities Historically, it was described from the Puy-les-Vignes tungsten mine near Meymac, France. Similar secondary tungsten oxides, often collectively referred to without precise identification, are found in the weathering zones of many tungsten deposits worldwide, for example, in Cornwall (United Kingdom), Portugal, Bolivia, or Nevada (USA).

Rarity

Very rare

For collectors

## Quality Criteria As a discredited mineral species, Meymacite has no collector's value as such. Historical specimens from the type locality (Meymac), labeled with an old tag, may have value. In collecting practice, these specimens are treated as "tungstite pseudomorphs after wolframite" or simply "secondary tungsten minerals." The attractiveness of such specimens depends on the quality and size of the primary wolframite or scheelite crystals that have been replaced by the yellow oxide mass.

Care and storage

## Cleaning Specimens containing Meymacite are extremely delicate and brittle. Mechanical cleaning is not recommended. If necessary, compressed air can be used to remove dust. Contact with water should be avoided, as it can alter the structure of hydrated oxides. ## What to Avoid Chemicals, ultrasonics, and all cleaning solutions should be strictly avoided. The mineral is sensitive to changes in humidity and temperature. It should be protected from direct sunlight, which can cause further dehydration and disintegration. ## Storage It is recommended to store specimens in closed, stable conditions, preferably in collector's boxes away from vibrations and heat sources. Due to its brittleness, it should not be stored in contact with harder minerals.

Sources

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