Penfieldite

Chemical formula: Pb<sup>2+</sup><sub>2</sub>Cl<sub>3</sub>(OH)

Penfieldite is a rare lead halide, forming colorless or white, hexagonal crystals, found mainly in the oxidation zones of lead deposits.

## Characteristics Penfieldite is a rare mineral from the halide group, specifically a hydrated lead chloride. It forms characteristic, elongated crystals with a hexagonal outline, which often taper towards the ends, resembling sharpened pencils. The crystals are typically colorless, white, or pale yellow and exhibit a strong, adamantine luster. It occurs as single, prismatic crystals or their radial aggregates on the surface of other minerals, most often in rock cavities. ## Physical Properties This mineral is relatively soft, with a hardness of about 3 on the Mohs scale. It is characterized by high density (6.58-6.61 g/cm³), which is typical for lead-bearing minerals. The crystals show a strong, almost adamantine luster on their faces, while on fracture surfaces, the luster is more resinous. It is transparent to translucent. ## Colors and Varieties Penfieldite is most often colorless or white. Specimens with a delicate bluish, greenish, or yellowish tint are also found, which is likely due to the presence of impurities. No named varieties of this mineral are distinguished. ## History and Name The name penfieldite was given in honor of Samuel Lewis Penfield (1856-1906), a professor of mineralogy at Yale University, who was the first to study this mineral. It was discovered in ancient Greek metallurgical slags in Laurium, Greece, where it formed as a result of the reaction of lead with seawater. The mineral was described in 1892. ## Uses Penfieldite has no industrial application due to its extreme rarity. However, it is a highly valued and sought-after collector's mineral.

Properties

Mohs hardness
3
Luster
Adamantine, Resinous
Streak
White
Density
6.58 - 6.61
Cleavage
Good on {1010}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Penfieldite can be identified by its characteristic shape of hexagonal, prismatic, and often pointed crystals. Key features also include very high density, strong, adamantine luster, and occurrence in a specific environment – oxidation zones of lead deposits or in ancient slags. ## Distinguishing from Similar Minerals Penfieldite is sometimes confused with other secondary lead minerals, such as phosgenite or cerussite. It differs from phosgenite by its hexagonal crystal system (phosgenite is tetragonal) and often more elongated crystals. Cerussite, in turn, often forms tabular crystals or reticulated aggregates, in contrast to the acicular forms of penfieldite. ## Crystal Forms Penfieldite crystals are hexagonal, usually in the form of elongated columns (prisms) with a hexagonal cross-section. They are often terminated by sharp pyramids, giving them the appearance of pointed needles. They occur as single crystals or form radial, star-like aggregates.

Geological environment

## Genesis Penfieldite is a secondary mineral formed in the oxidation zones of lead ores, especially in a chlorine-rich environment. It forms as a result of the action of saline waters on primary lead-bearing minerals. It is also found as a product of seawater reacting with ancient lead smelting slags. ## Mineral Associations This mineral often co-occurs with other secondary lead minerals, such as phosgenite, laurionite, paralaurionite, cerussite, anglesite, and cotunnite. ## Localities The most important and classic locality for penfieldite is the ancient slag heaps in Laurium, Greece. Other known localities include the Margarita mine in the Sierra Gorda region of Chile and the Kintore mine in Broken Hill, Australia. It has also been found in slags in Baratti, Italy, and on Tinos Island, Greece.

Rarity

Very rare

For collectors

## Quality Criteria The most prized by collectors are specimens with well-formed, sharp, and transparent crystals with a strong luster. Rich groups of crystals forming radial aggregates on a contrasting matrix are also highly valued. Crystal size is a key factor – specimens with crystals exceeding a centimeter in length are exceptionally rare and valuable. ## Popular Localities Classic specimens from Laurium, Greece, which serve as the type locality for this mineral, are by far the most sought after. High-quality crystals also come from the Margarita mine in Chile, which often stand out for their excellent transparency and form.

Care and storage

## Cleaning Penfieldite specimens should be cleaned with the utmost care. The safest method is to use compressed air to remove dust. If necessary, a soft brush can be used. The mineral is sparingly soluble in water, so prolonged contact with liquids, especially distilled water, should be avoided. ## What to Avoid Contact with acids and other chemicals must be strictly avoided. Penfieldite is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight, which can cause its degradation. Due to its lead content, inhalation of dust should be avoided, and hands should be washed after each contact with the mineral. ## Storage It is recommended to store specimens in stable conditions, preferably in closed, padded boxes or display cases, to protect them from mechanical damage and dust. It should be kept away from sources of moisture and in a place with a constant temperature.

External references

Sources

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