Ferrimolybdite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>(Mo<sup>6+</sup>O<sub>4</sub>)<sub>3</sub>·7H<sub>2</sub>O

Yellow iron molybdate, forming characteristic, silky coatings and fibrous aggregates in the oxidation zones of molybdenum deposits.

## Characteristics Ferrimolybdite is a hydrated iron molybdate, known for occurring as delicate, fibrous or acicular crystals. It forms radial aggregates, fluffy crusts, and earthy coatings. Its most characteristic feature is its intense, canary-yellow color, which makes it easily noticeable. Individual crystals are usually very small and gathered in dense aggregates that feel like felt or mineral wool to the touch. ## Physical Properties This mineral is extremely soft and brittle, with a Mohs hardness of only 1-2.5. It is also very light. The luster of aggregates is most often silky, which is typical for minerals with a fibrous structure, or dull and earthy in the case of coatings. Individual, rarely found crystals may exhibit a vitreous luster. It is translucent, and in compact masses, it becomes opaque. ## Colors and Varieties The dominant and almost sole color of ferrimolybdite is bright, canary yellow. Sometimes it can take on greenish-yellow, straw-yellow, or brownish hues, which is usually the result of impurities or partial dehydration. No color or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1907, directly refers to its chemical composition – it contains iron (Latin: *ferrum*) and molybdenum. For many years, it was confused with molybdite (MoO₃), which is a molybdenum oxide. Only precise chemical analyses allowed for the distinction of ferrimolybdite as a separate mineral species. Many historical specimens described as "molybdite" actually turned out to be ferrimolybdite. ## Uses Ferrimolybdite has no industrial application. However, it is an important indicator mineral for geologists searching for molybdenum deposits. For collectors, it is interesting due to its intense color and attractive, fibrous forms.

Properties

Mohs hardness
1-2.5
Luster
Silky
Streak
Pale yellow
Cleavage
Perfect on {010}
Fracture
Fibrous
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Key features for identifying ferrimolybdite are its canary-yellow color, very low hardness, and characteristic fibrous or acicular appearance of aggregates with a silky luster. Its occurrence in paragenesis with molybdenite is also an important indicator. ## Distinguishing from Similar Minerals Ferrimolybdite is sometimes confused with other secondary yellow minerals. - **Molybdite (MoO₃)** is visually almost identical, and a definitive distinction often requires chemical analysis. However, ferrimolybdite is much more common. - **Jarosite** has a similar color but usually forms fine-grained, loose coatings or small, trigonal crystals, rarely exhibiting a fibrous habit. - **Autunite** and other secondary uranium minerals may have a similar color but exhibit strong fluorescence under UV light and radioactivity, which ferrimolybdite lacks. ## Crystal Forms Crystals are acicular and elongated, almost always occurring as aggregates. Most often, it forms radial aggregates, fluffy coatings, crusts, and earthy masses. Well-formed, isolated crystals are extremely rare.

Geological environment

## Genesis Ferrimolybdite is a typical secondary mineral, formed in the oxidation (weathering) zone of ore deposits. It forms as a result of the chemical decomposition of primary molybdenum sulfides (mainly molybdenite) and iron (such as pyrite) under the influence of oxygen-rich surface waters. ## 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 **limonite** (goethite/hematite), **jarosite**, as well as primary sulfides: **pyrite** and **chalcopyrite**. Sometimes it occurs together with **molybdite**. ## Localities It is a widely distributed mineral in the weathering zones of molybdenum deposits worldwide. Important localities, from which classic specimens originate, include: Climax mine in Colorado and Questa deposit in New Mexico (USA), Knaben mine in Norway, as well as numerous porphyry copper and molybdenum deposits in Chile and Peru.

Rarity

Not very common

For collectors

## Quality Criteria Most valued by collectors are specimens with rich, dense aggregates of fibrous crystals of intense, canary-yellow color. Attractiveness is enhanced by a contrasting, dark rock matrix or co-occurrence with well-formed molybdenite crystals. Due to the mineral's brittleness, undamaged specimens are much more highly valued. ## Popular Localities Historical localities in the United States, especially from the Questa deposit in New Mexico and the Climax mine in Colorado, are considered classic, providing the best ferrimolybdite specimens. Beautiful, fibrous aggregates also come from Norway (Knaben).

Care and storage

## Cleaning Ferrimolybdite is extremely delicate and brittle. To remove dust, use only a soft brush or a stream of compressed air from a safe distance. Absolutely avoid contact with water, which can damage its structure. ## What to Avoid Avoid ultrasonic cleaners, all chemical agents, and sudden changes in temperature and humidity. The mineral is very sensitive to shocks and impacts – even slight pressure can destroy delicate, fibrous aggregates. ## Storage Ferrimolybdite specimens are best stored in closed, padded boxes or display cases to protect them from dust, mechanical damage, and direct light. Avoid placing other minerals on them.

External references

Sources

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