Prewittite

Chemical formula: KPb<sup>2+</sup><sub>1.5</sub>Zn<sup>2+</sup>Cu<sup>2+</sup><sub>6</sub>O<sub>2</sub>(Se<sup>4+</sup>O<sub>3</sub>)<sub>2</sub>Cl<sub>10</sub>

Prewittite is an extremely rare copper, lead, zinc, and potassium selenite, discovered in the fumaroles of Vesuvius in Italy.

## Characteristics Prewittite is a mineral with an exceptionally complex chemical composition, belonging to the selenite class. It forms very tiny, tabular crystals measuring no more than 0.2 mm. Due to the microscopic size of the crystals, its visual characteristics are difficult to observe without specialized equipment. It is a secondary mineral, formed under specific volcanic conditions. ## Physical Properties Prewittite crystals exhibit a luster described as vitreous to greasy. They are transparent to translucent. Hardness and density have not been precisely measured due to the small size and rarity of samples. ## Colors and Varieties This mineral occurs in green. No varieties have been distinguished. ## History and Name Prewittite was officially recognized as a new mineral by the International Mineralogical Association (IMA) in 2017 under the designation IMA2017-023. Its name honors Charles T. Prewitt (1933–2022), a distinguished American crystallographer and mineralogist from the University of Arizona, for his contributions to mineralogy and crystal chemistry. ## Uses Prewittite has no practical or commercial applications. Its significance is purely scientific and as a collector's item, being a unique and newly discovered mineral species.

Properties

Luster
Vitreous to greasy
Streak
Green
Cleavage
Perfect on {0001}
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of prewittite is possible only through advanced laboratory methods, such as X-ray diffraction (XRD) and chemical spectroscopy (EDS/WDS). Visual identification in the field or in a collection is impossible. ## Differentiation from Similar Minerals Prewittite occurs in association with many other rare selenites and chlorides from the Vesuvius fumaroles, such as eriochalcite, melanothallite, or belloite. Differentiation from these requires specialized instrumental analysis. ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline, occurring singly or in small aggregates on the surface of the host rock.

Geological environment

## Genesis Prewittite is a fumarolic mineral. It forms as a result of sublimation and reactions of volcanic gases rich in selenium, chlorine, copper, lead, and other metals, at temperatures of approximately 450-600°C. It crystallizes directly from the gas phase on the walls of active fumaroles. ## Mineral Associations This mineral co-occurs with other products of volcanic exhalations, such as hematite, belloite, eriochalcite, ilinskite, paratacamite, as well as newly discovered or rare selenites and sulfates. ## Localities The only confirmed locality of prewittite in the world is its type locality - the Arsénica fumarole in the La Fossa crater on Vulcano Island, part of the Aeolian Islands archipelago in Italy.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" mineral, the collector's value of prewittite primarily depends on the confirmation of its identity. The most desirable specimens are those where the microscopic crystals are well-formed, have a distinct green color, and are situated on a stable, contrasting rock matrix. The presence of associated minerals also increases the scientific and collector's value of the specimen. ## Popular Localities The only source of specimens is the type locality on Vulcano Island, Italy. Obtaining new samples is extremely difficult and dangerous due to volcanic activity and the toxicity of fumarolic gases.

Care and storage

## Cleaning Due to its extreme rarity, small size, and fragility, prewittite specimens should not be cleaned by any mechanical or chemical means. Any attempt at cleaning risks irreversible damage to the mineral. ## What to Avoid Avoid any contact with water, chemicals, ultrasound, and all forms of vibration and shock. The mineral is likely sensitive to changes in humidity and temperature. ## Storage Specimens should be stored exclusively in specialized, sealed "micromount" containers, protecting them from dust, moisture, and mechanical damage. Display is not recommended.

Sources

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