Petersite-(Y)

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

Petersite-(Y) is a rare secondary mineral from the mixite group, forming characteristic blue-green, acicular crystals in radial aggregates.

## Characteristics Petersite-(Y) is a hydrated copper and yttrium phosphate, belonging to the mixite group. It occurs as fine, acicular or hair-like crystals, which most often form radial, spherical, or hemispherical aggregates. Individual crystals are usually too small to be observed with the naked eye, and their beauty is revealed only under magnification. The structure of these aggregates gives specimens a silky or slightly fibrous appearance. ## Physical Properties Petersite-(Y) crystals are brittle. The Mohs hardness is approximately 3, and the density ranges between 3.41 and 3.47 g/cm³. The mineral exhibits a vitreous luster, which in the case of compact aggregates can appear silky. ## Colors and Varieties This mineral is characterized by a vibrant, blue-green to emerald-green color. No color or commercial varieties are distinguished. ## History and Name The mineral's name honors Joseph J. Peters (1942-2019), curator of mineralogical collections at the American Museum of Natural History in New York, for his contributions to mineralogy. The "(Y)" suffix in the name indicates the dominance of yttrium in its chemical composition. The mineral was first described in 1982 from samples collected at the Cedar Mountain mine in Nevada, USA.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Light green
Density: 3.41-3.47
Cleavage: None
Fracture: Uneven
Transparency: Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Key diagnostic features include its characteristic blue-green color, acicular crystal habit, and the formation of radial, spherical aggregates. Petersite-(Y) often occurs in vugs and fissures within quartz or rhyolite rocks. ## Distinguishing from Similar Minerals It can be confused with other minerals from the mixite group, such as agardite-(Y) or mixite. Distinguishing them without advanced chemical analysis (e.g., EDS) is practically impossible. Visually, turquoise can also be similar, but it rarely forms radial acicular aggregates and is significantly harder (5-6 on the Mohs scale). ## Crystal Forms Crystals are hexagonal, very elongated in the form of needles or hairs. They almost always occur as radial, spherical, or hemispherical aggregates, as well as coatings and crusts on the host rock.

Geological environment

## Genesis Petersite-(Y) is a secondary mineral, forming in the oxidation (weathering) zones of copper ore deposits, in the presence of rare earth elements, especially yttrium. It forms as a result of hydrothermal solutions acting on rocks rich in these elements. ## Mineral Associations It often co-occurs with other secondary minerals such as chrysocolla, malachite, agardite, mixite, as well as quartz, hematite, and jarosite. ## Localities The most important and classic locality (type locality) is the Cedar Mountain mine in Mineral County, Nevada, USA. Other known localities include the Eisenzecher Zug mine in Siegen, North Rhine-Westphalia, Germany, and the Lavrion area in Greece.

Rarity

Rare

For collectors

## Quality Criteria The most prized petersite-(Y) specimens are characterized by an intense, vibrant blue-green color. Well-formed, spherical aggregates with a silky luster, contrasting against a light host rock (most often quartz), are highly valued. The size of the aggregates and the absence of damage to the delicate needles also significantly increase the collector's value of the specimen. ## Popular Localities Specimens from the type locality in Nevada, USA, are by far the most sought after by collectors, considered exemplary for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only a soft brush to remove dust. Due to its brittleness and solubility in acids, contact with water and chemicals is inadvisable. If liquid is necessary, use only isopropyl alcohol in minimal amounts and allow for rapid evaporation. ## What to Avoid Absolutely avoid contact with water, acids (even weak ones, like vinegar), and other cleaning agents. The mineral is sensitive to high temperatures and mechanical shocks. Prolonged exposure to strong sunlight can potentially cause fading. ## Storage It is recommended to store specimens in closed, stable containers (e.g., "membrane box" type boxes) or display cases, away from dust, humidity, and direct sunlight. They should be protected from vibrations and impacts that could damage the delicate needles.

Sources

IMA Mineral List (CNMNC)