Molybdite

Chemical formula: Mo⁶⁺O₃

Molybdite is a rare molybdenum oxide, forming characteristic, acicular crystals of a bright yellow color, found in the oxidation zones of molybdenite deposits.

## Characteristics Molybdite is a hydrated molybdenum oxide, most often occurring as small, acicular or fibrous crystals. It forms radial aggregates, crusts, coatings, and earthy or powdery masses. Due to its delicate structure, specimens are extremely brittle. Well-formed, bladed crystals, which can reach several millimeters in length, are rarely found. ## Physical Properties Molybdite crystals are characterized by a hardness of 3-4 on the Mohs scale, making them a relatively soft mineral. They have a silky, pearly luster, and in the case of well-formed crystals, a adamantine luster. It is a light mineral, with a density of about 4.7 g/cm³. Crystals are usually translucent, less often transparent. ## Colors and Varieties This mineral exhibits characteristic colors ranging from canary yellow and straw yellow to yellowish-green, and is sometimes almost colorless. No commercial or colored varieties are distinguished. ## History and Name The name molybdite comes from the Greek word *molybdos* (μόλυβδος) meaning "lead," which referred to the historical confusion of molybdenite (from which molybdite forms) with galena and graphite. For many years, the term "molybdite" was incorrectly used to describe the similar-looking ferrimolybdite. It was not until 1963 that the International Mineralogical Association (IMA) disqualified "molybdite" as a synonym for ferrimolybdite and redefined it as pure, natural MoO₃. ## Uses Molybdite, due to its rarity and small accumulations, has no industrial significance as a molybdenum ore. However, it is an object of interest for collectors and an important indicator mineral in the search for molybdenum deposits.

Properties

Mohs hardness: 3-4
Color: colorless, light greenish yellow, light brown, also pale blue
Luster: Silky, Pearly, Adamantine
Streak: Light yellow to colorless
Density: 4.5
Cleavage: Perfect cleavage (<mi>100</mi>), distinct cleavage (<mi>001</mi>) (Cech 1964)
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Molybdite is most easily recognized by its characteristic canary-yellow color and typical forms of occurrence – radial or tangled aggregates of small, acicular crystals. It occurs in paragenesis with molybdenite, often as a coating on its surface. ## Distinguishing from Similar Minerals Molybdite is visually almost impossible to distinguish from ferrimolybdite without advanced chemical analyses (EDS, XRD). However, ferrimolybdite is much more common. Other yellow secondary minerals, such as autunite or carnotite, are radioactive, unlike molybdite. It is distinguished from native sulfur by its crystal form (acicular vs. orthorhombic) and environment of occurrence. ## Crystal Forms Crystals are usually elongated, acicular or fibrous, often flattened and bladed. They form characteristic radial, stellate aggregates, tangled masses resembling felt, as well as coatings and crusts.

Geological environment

## Genesis Molybdite is a typical secondary mineral, forming exclusively in the oxidation (weathering) zones of ore deposits containing molybdenite (MoS₂). It forms under strongly acidic and oxidizing conditions, as a result of the slow decomposition of primary molybdenum sulfides. ## Mineral Associations It most often co-occurs with the mineral from which it forms – molybdenite. It is also accompanied by other minerals of the oxidation zone, such as ferrimolybdite, wulfenite, powellite, as well as quartz, pyrite, chalcopyrite, and clay minerals. ## Localities Significant finds come from the Questa mine in New Mexico (USA), which has yielded some of the best crystalline specimens. Other important localities include the Climax mine in Colorado (USA), Knaben in Norway, mines in the Cinovec region in the Czech Republic, and some deposits in Chile and Peru.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued molybdite specimens are those with well-formed, macroscopic crystals of intense, canary-yellow color. Rich crusts of acicular crystals on a contrasting rock matrix, especially in association with lustrous molybdenite, are particularly sought after. Due to its rarity, even microscopic but well-formed crystals are valuable for collectors specializing in "micromount" type mounts. ## Popular Localities Specimens from the Questa mine in Taos County, New Mexico, USA, are considered among the best in the world. Classic, rich aggregates of acicular crystals originate from there. The historical locality in Knaben, Norway, has also provided good quality specimens.

Care and storage

## Cleaning Molybdite specimens are extremely delicate and brittle. Cleaning should be kept to an absolute minimum. If necessary, a very gentle stream of compressed air can be used to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Ultrasonic cleaners, mechanical cleaning (even with a soft brush), contact with water, and chemical solutions should be strictly avoided. The mineral is sensitive to shocks and pressure – even touching it with a finger can damage delicate needles. Store away from heat and moisture sources. ## Storage Molybdite requires storage in a sealed, stable "micromount" type box that protects it from dust, moisture, and mechanical damage. It should not be displayed in open showcases where it is exposed to air circulation and dust accumulation.