Molybdofornacite

Chemical formula: Cu²⁺Pb²⁺₂Mo⁶⁺O₄As⁵⁺O₄(OH)

Molybdenofornacite is a rare arsenate-molybdate lead-copper mineral, forming small, prismatic crystals of olive-green or yellowish-green color.

## Characteristics Molybdenofornacite is a complex mineral from the fornacite group, chemically classified as a lead and copper arsenate and molybdate. It occurs as small, elongated or tabular crystals, rarely exceeding a few millimeters in length. Crystals often form radial or fan-shaped aggregates, as well as crusts and coatings. Its characteristic, vivid color makes it recognizable despite its small size. ## Physical Properties This mineral is characterized by a hardness of 2-3 on the Mohs scale, making it relatively soft. It has a high luster, described as vitreous, resinous, or even adamantine. It is a heavy mineral due to its high lead content – its density is approximately 6.6 g/cm³. Crystals are typically transparent to translucent. ## Colors and Varieties Molybdenofornacite occurs in shades ranging from olive-green and yellowish-green to canary-yellow and brownish-green. No named varieties are distinguished, and color variability depends on subtle differences in chemical composition. ## History and Name The mineral's name refers to its chemical composition – it indicates the dominance of molybdenum over chromium and its structural similarity to fornacite. It was described as a new mineral in 1982 by P. J. Dunn, D. R. Peacor, W. B. Simmons Jr., and R. A. Ramik. The type locality, from which the first studied specimens originated, is the Tsumeb mine in Namibia. ## Uses Molybdenofornacite has no industrial application. Its significance is purely scientific and as a collector's item.

Properties

Mohs hardness: 3-4
Color: Light green, olive green
Luster: Adamantine
Streak: Pale Yellow
Density: 6.57
Cleavage: Good on {010}
Fracture: Conchoidal
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Molybdenofornacite is identified by its characteristic olive-green or yellowish-green color, high luster, and typical crystal forms – small, prismatic, often gathered in radial aggregates. High density is also an important diagnostic feature. It occurs in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals It can be confused with other secondary lead and copper minerals of similar color, such as fornacite, wulfenite, pyromorphite, or mimetite. - **Fornacite**: Practically impossible to distinguish visually. Requires chemical analysis (EDS/XRD) to determine the dominance of molybdenum over chromium. - **Wulfenite**: Typically forms tabular, square crystals and has a lower density. - **Pyromorphite/Mimetite**: Most often form hexagonal, barrel-shaped crystals and have lower luster and density. ## Crystal Forms Crystals are usually small, prismatic, and elongated, often with vertical striations on the faces. They occur as single, acicular crystals, but more often form fan-shaped, radial, or stellate aggregates, as well as crusts on the host rock.

Geological environment

## Genesis Molybdenofornacite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic hydrothermal deposits rich in lead, copper, arsenic, and molybdenum. It forms as a result of reactions between solutions rich in these elements and primary minerals such as galena, chalcopyrite, or tennantite. ## Mineral Associations It most commonly co-occurs with minerals typical of oxidation zones, such as dioptase, duftite, willemite, calcite, quartz, cerussite, anglesite, and its chromium analogue – fornacite. ## Localities The most important and well-known locality, which is also the type locality, is the Tsumeb mine in Namibia. The world's best specimens of this mineral originate from there. Other confirmed localities include the Mammoth-St. Anthony mine in Arizona (USA) and several places in Germany (e.g., in the Eifel region).

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized molybdenofornacite specimens are characterized by well-formed, sharp crystals with a vivid, intense color (pure green or yellow is preferred). The attractive arrangement of crystals on the host rock, forming aesthetic, radial aggregates, is of great importance. Contrast with other minerals, such as white calcite or green dioptase, significantly increases the specimen's value. Crystal size is crucial – specimens with crystals exceeding a few millimeters are exceptionally rare and sought after. ## Popular Localities By far the most valued and sought-after specimens by collectors come from the Tsumeb mine in Namibia. They are considered the global standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its softness, mechanical cleaning should be avoided. If liquid is necessary, it is recommended to use only distilled water and to immediately and gently dry the specimen. ## What to Avoid Contact with acids and other chemicals that can damage the mineral must be absolutely avoided. Molybdenofornacite is sensitive to high temperatures and mechanical shocks. As a lead and arsenic-bearing mineral, it is toxic – avoid inhaling dust and wash hands after any contact with the specimen. ## Storage It is recommended to store specimens in separate, sealed display boxes to prevent scratching by harder minerals and to protect them from dust. It should be kept away from heat and moisture sources. Due to its toxicity, it should be stored out of reach of children and pets.

External references

https://www.mindat.org/min-2734.html