Georgerobinsonite

Chemical formula: Pb<sup>2+</sup><sub>4</sub>(Cr<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>FCl

Georgerobinsonite is an extremely rare lead chromate, distinguished by its intense orange color and occurrence as microscopic crystals.

## Characteristics Georgerobinsonite is a secondary lead mineral that forms extremely small, but well-formed crystals. They typically occur as platy or prismatic crystals with an intense, orange to reddish-orange color. Due to their microscopic size, individual crystals are most often observed as radial or tangled aggregates, growing on the host rock. It is a transparent mineral with a luster described as adamantine to resinous. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2. It is also very dense, which is typical for lead-bearing minerals. Its luster is strong, adamantine, and the crystals are transparent. ## Colors and Varieties Georgerobinsonite occurs in a characteristic, uniform color from orange to reddish-orange. No color or commercial varieties are known. ## History and Name The mineral is named in honor of George W. Robinson (born 1946), an American mineralogist, curator, and professor, who made significant contributions to mineralogy, particularly in the study of minerals from the Great Lakes region. Georgerobinsonite was officially recognized by the International Mineralogical Association (IMA) in 2009. It was discovered in the Macquist Mine in Maricopa County, Arizona, USA. ## Uses Due to its extreme rarity and microscopic size, georgerobinsonite has no industrial applications. It is solely of scientific importance and is a coveted object for specialized collectors of rare minerals (so-called "micromounts").

Properties

Mohs hardness
2
Luster
Adamantine to resinous
Streak
Light orange
Density
6.46
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Georgerobinsonite is identified based on its unique chemical composition (lead chromate with fluorine and chlorine), intense orange color, form of microscopic, platy crystals, and co-occurrence with other rare secondary minerals in the oxidation zones of lead deposits. ## Distinguishing from similar minerals It can be confused with other secondary lead minerals of similar color, such as crocoite, wulfenite, or vanadinite. Differentiation requires advanced analytical methods, such as Raman spectroscopy or EDS analysis, due to the microscopic size of the crystals. Crocoite typically forms larger, prismatic crystals. Wulfenite most often takes the form of square, platy crystals. Vanadinite crystallizes in the hexagonal system, forming hexagonal prisms. ## Crystal forms Crystals are very small, reaching up to 0.2 mm in size. They are platy, flattened along the {010} plane, or less commonly prismatic. They often form radial aggregates or irregular clusters on the rock surface.

Geological environment

## Genesis Georgerobinsonite is a secondary mineral, formed in the oxidation zones of hydrothermal lead ore deposits. It forms as a result of reactions between solutions rich in chromium, fluorine, and chlorine with primary lead-bearing minerals, such as galena. It forms in desert conditions, in a dry and hot climate. ## Mineral associations This mineral co-occurs with other rare secondary minerals. In its type locality (Macquist Mine), it was found in association with cerussite, diboleite, fornacite, fluorite, galena, hemimorphite, mimetite, phosgenite, wulfenite, and vanadinite. ## Localities The only confirmed occurrence of georgerobinsonite in the world is its type locality – the Macquist Mine (Osborne Well), in the Vulture Mining District, Maricopa County, Arizona, USA.

Rarity

Extremely rare

For collectors

## Quality criteria The quality of georgerobinsonite specimens is primarily assessed based on the richness and aesthetics of the microcrystal aggregates on the rock matrix. Specimens with numerous, well-formed, intensely colored crystals forming visually attractive clusters are most valued. Contrast with the color of the host rock also increases the specimen's value. Due to its microscopic nature, the purity and size of individual crystals are less important than the overall appearance of the aggregate. ## Popular localities The only source of georgerobinsonite specimens is the Macquist Mine in Arizona, USA. All specimens available on the collector's market come from this single locality.

Care and storage

## Cleaning Georgerobinsonite specimens are extremely delicate and small. Mechanical cleaning is highly risky. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. Avoid contact with water and any chemicals. ## What to avoid The mineral is very soft (Mohs hardness 2), so any physical contact, including touching with fingers or tools, should be avoided. It should be protected from acids and other chemicals. Due to its lead content, avoid inhaling dust and wash hands after any contact. ## Storage Specimens should be stored exclusively in specialized, sealed micromount boxes, which protect them from dust, moisture, and mechanical damage. Store in a dry place, away from sources of vibration.

External references

Sources

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