Petersite-(La)

Chemical formula: Cu₂+₆La(PO₄)₃(OH)₆·₃H₂O

Petersite-(La) is a very rare mineral from the mixite group, forming blue-green, acicular crystals and radial aggregates.

## Characteristics Petersite-(La) is a hydrated copper and lanthanum phosphate, belonging to the mixite group. It occurs as fine, acicular or fibrous crystals, which often form radial, spherical aggregates or crusts. Its structure is delicate and brittle. Due to the small size of the crystals, individual features are most often observable under magnification. ## Physical Properties Petersite-(La) crystals exhibit a vitreous to silky luster, depending on the form of occurrence. The mineral is translucent. Its Mohs hardness is approximately 3, and its density is estimated at 3.45 g/cm³. ## Colors and Varieties It is characterized by a vivid, blue-green color. No varieties have been distinguished. ## History and Name The mineral's name honors Joseph J. Peters (1942-2019), curator of the mineralogical collection at the American Museum of Natural History in New York, for his contributions to mineralogy. The "(La)" suffix in the name indicates lanthanum as the dominant rare earth element in its chemical composition. The mineral was first described in 1982. ## Uses Petersite-(La) has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from the mixite group.

Properties

Mohs hardness: 3
Luster: Vitreous to silky
Streak: Light green
Density: 3.45
Cleavage: None
Fracture: Uneven
Transparency: Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Characteristic features of petersite-(La) include its blue-green color, acicular crystal habit, and formation of radial aggregates. Occurrence in the oxidation zones of copper deposits, in association with other secondary phosphate minerals, is a key indicator. However, definitive identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other minerals from the mixite group, such as mixite, agardite-(Y), or petersite-(Y). Distinguishing them based on visual features is practically impossible and requires confirmation of the dominant rare earth element. It may also resemble other secondary copper minerals with similar color and form, e.g., chalcanthite or some forms of malachite. ## Crystal Forms It forms very fine, elongated, acicular crystals, which almost always occur as radial or spherical aggregates. It is less commonly found as silky crusts on the host rock.

Geological environment

## Genesis Petersite-(La) is a secondary mineral, forming in the oxidation (weathering) zones of copper ore deposits. Its formation is associated with the interaction of phosphate- and lanthanum-rich waters with primary copper minerals. ## Mineral Associations It co-occurs with other secondary minerals, such as malachite, pseudomalachite, libethenite, and also with quartz. ## Localities The most important and type locality for this mineral is the "Grube Prinz Leopold" copper mine in Lautertal, Odenwald mountains (Hesse, Germany). This is the only confirmed and well-documented locality for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens of petersite-(La) are characterized by an intense, blue-green color and well-formed, radial aggregates on a contrasting rock matrix. Due to the microscopic size of the crystals, the aesthetics and richness of the crust are crucial. Specimens without mechanical damage, with clearly visible aggregate structure, are the most sought after. ## Popular Localities The only source of collector specimens is the historical type locality – the "Prinz Leopold" mine in Germany. Material from this location is extremely rare and highly sought after by specialized collectors.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Contact with water is inadvisable due to the delicate and soluble nature of the mineral. Ultrasonic cleaners and steam cleaning should be avoided. ## What to Avoid Avoid contact with all chemicals, including acids and detergents. The mineral is sensitive to high temperatures and can be damaged by prolonged exposure to strong sunlight. Due to its brittleness, it is very susceptible to mechanical damage. ## Storage It is recommended to store specimens in closed, stable containers (micromount type) to protect them from dust, moisture, and physical damage. Avoid storing in places with fluctuating temperature and humidity.

Sources

IMA Mineral List (CNMNC)