Sidwillite

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

Sidwillite is a very rare, hydrated molybdenum oxide, forming microscopic, yellow crystals in the oxidation zones of molybdenum deposits.

## Characteristics Sidwillite is a hydrated molybdenum oxide, occurring extremely rarely. It forms very small, platy or acicular crystals, which typically do not exceed 0.2 mm in length. It is observed as individual crystals, rosette-like aggregates, or powdery coatings on the surface of other minerals. Due to the microscopic size of the crystals, its visual characteristics are difficult to assess without specialized equipment. ## Physical Properties Sidwillite crystals are flexible and can be bent. The hardness is very low, estimated at approximately 2 on the Mohs scale. The calculated density is 3.12 g/cm³. The mineral exhibits a strong, vitreous luster. ## Colors and Varieties Sidwillite has a characteristic, pale yellow to canary yellow color. No varieties are distinguished. ## History and Name The mineral was named in honor of Sid Williams (1933–2003), an American mineralogist and geologist with Phelps Dodge Corporation, in recognition of his contributions to the study of oxidation zones of ore deposits. It was approved by the IMA in 1979, and its description was published in 1981. The type locality is the Lake Como and Red Mountain mines in Colorado, USA. ## Uses Sidwillite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals and scientists studying the weathering processes of molybdenum deposits.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light yellow
Density
3.12
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of sidwillite is based on its characteristic pale yellow color, microscopic, platy crystals, and occurrence in the oxidation zones of molybdenum deposits. Due to the small crystal size, definitive identification requires advanced analytical methods such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals Sidwillite can be confused with other secondary molybdenum minerals, such as molybdite (MoO₃) or koechlinite (Bi₂MoO₆). Molybdite is anhydrous and crystallizes in the orthorhombic system. Koechlinite is a bismuth molybdate. Precise distinction of these minerals under amateur conditions is practically impossible and requires specialized laboratory analyses. ## Crystal Forms Sidwillite forms elongated, platy or acicular crystals, often flattened. They occur as individual, dispersed crystals or form small, radial aggregates and rosettes.

Geological environment

## Genesis Sidwillite is a secondary mineral, formed by the oxidation (weathering) of primary molybdenum sulfides, mainly molybdenite (MoS₂). It forms under low temperature and pressure conditions, in dry or semi-dry climates, where it crystallizes from aqueous solutions rich in molybdenum. ## Mineral Associations This mineral co-occurs with other minerals of the oxidation zone. It is most commonly associated with molybdenite (as a primary mineral), jordisite, quartz, kaolinite, and other rare molybdates like koechlinite. ## Localities Sidwillite is an extremely rare mineral, known from only a few localities worldwide. The most important occurrences are the Lake Como (Hinsdale County) and Red Mountain (Clear Creek County) mines in Colorado, USA, which are its type localities. It has also been found in the Freedom No. 2 mine in Marysvale, Utah, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of sidwillite specimens is primarily assessed based on the richness and visibility of crystals on the rock matrix. Specimens with clearly formed, abundant clusters of pale yellow crystals, contrasting with the substrate, are most valued. Due to the microscopic nature of the mineral, the ability to observe crystals under magnification is crucial. The size of the crystals, even if measured in fractions of a millimeter, significantly increases the value of the specimen. ## Popular Localities The most sought-after specimens by collectors come from the type localities in Colorado, USA, especially from the mines in the Red Mountain area. These are historically the most important and serve as the standard for this mineral.

Care and storage

## Cleaning Sidwillite specimens are extremely delicate and sensitive. Any mechanical cleaning should be avoided. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. Contact with water is inadvisable due to the risk of dissolution. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is very soft and brittle, susceptible to mechanical damage. It should be protected from high temperatures, which can cause dehydration and structural change. ## Storage Specimens should be stored in sealed, stable "micromount" containers to protect them from dust, moisture, and physical damage. It is best to keep them in a dry place, away from heat sources and direct sunlight.

Sources

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