Petedunnite

Chemical formula: CaZn<sup>2+</sup>Si<sub>2</sub>O<sub>6</sub>

Petedunnite is a rare mineral of the pyroxene group, a zinc analog of diopside, characteristic of metamorphosed zinc deposits in Franklin, New Jersey.

## Characteristics Petedunnite is a mineral belonging to the clinopyroxene group, being the zinc analogue of diopside and hedenbergite. It usually forms grains or aggregates embedded in rock, ranging in color from dark green to almost black. Well-formed crystals are rarely observed. It is a hard and relatively dense mineral with a vitreous luster. ## Physical Properties Petedunnite is characterized by a Mohs hardness of 6 and a density of approximately 3.77 g/cm³. It has a vitreous luster, and its transparency ranges from translucent on thin edges to completely opaque in thicker sections. It exhibits good cleavage in two directions, typical of pyroxenes. ## Colors and Varieties This mineral occurs in uniform, dark colors. Its color is usually dark green, greenish-black, or black. No named color varieties or commercial varieties are distinguished. ## History and Name The name petedunnite, approved in 1987, honors Pete J. Dunn (1942–2017), a distinguished American mineralogist from the Smithsonian Institution, who dedicated a large part of his career to studying the unique mineralogy of the Franklin and Sterling Hill deposits in New Jersey. It was first identified in material originating from the Franklin mine. ## Applications Due to its extreme rarity, petedunnite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for advanced collectors of rare and systematic minerals.

Properties

Mohs hardness
6
Luster
Vitreous
Streak
Light gray
Density
3.77
Cleavage
Good on {110}
Fracture
Uneven to conchoidal
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of petedunnite based on visual characteristics is practically impossible. Its appearance does not distinguish it from other dark pyroxenes, such as hedenbergite or augite. A key diagnostic clue is its origin from the type locality (Franklin or Sterling Hill) and its co-occurrence with other zinc minerals. Final confirmation requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), to confirm the presence of zinc in the pyroxene structure. ## Distinguishing from Similar Minerals Petedunnite is visually identical to hedenbergite and dark varieties of diopside. The difference lies in the dominance of zinc over iron and magnesium at a specific position in the crystal structure, which is impossible to verify without specialized equipment. It can also be confused with augite or aegirine. ## Crystal Forms It most often occurs as anhedral (lacking its own faces) to subhedral grains, forming granular aggregates in the host rock. Well-formed, prismatic crystals with a typical pyroxene habit are exceptionally rare.

Geological environment

## Genesis Petedunnite is a mineral of metamorphic origin. It forms under high-temperature conditions within skarns, which formed as a result of the metamorphism of zinc-rich carbonate sediments. Its occurrence is closely associated with the metamorphosed zinc ore deposit in the Franklin Marble formation. ## Mineral Associations This mineral occurs in association with other, often rare, minerals characteristic of the Franklin and Sterling Hill deposits. The most common associations include: willemite, franklinite, zincite, calcite, as well as other pyroxenes, such as diopside and hedenbergite. ## Localities The type locality and main occurrence of petedunnite is the Franklin mine in Sussex County, New Jersey, USA. It has also been identified in the nearby Sterling Hill mine. Beyond these two localities, its occurrence has been confirmed in only a few other places in the world, including Japan.

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the value of a petedunnite specimen primarily depends on its confirmed analytical identity. Specimens are usually small and inconspicuous. Those that show richer aggregates of the mineral, visible in contrast to the host rock, are prized. Samples with visible, even partially formed crystals, are exceptionally desirable. A significant added value is the association with other rare Franklin minerals, such as esperite or larsenite. ## Popular Localities The only source of collectible specimens are the historic Franklin and Sterling Hill deposits in New Jersey, USA. Material from these mines, closed for years, is only available on the secondary market.

Care and storage

## Cleaning Petedunnite specimens should be cleaned carefully, using a soft brush and distilled water. Due to the possible presence in a fragile matrix, the use of ultrasonic cleaners, which could damage the specimen, is discouraged. ## What to Avoid The mineral is sensitive to strong acids. Contact with chemicals, sudden temperature changes, and prolonged exposure to direct sunlight, which may affect the color of associated minerals, should be avoided. ## Storage It is recommended to store specimens in closed boxes or display cases, which protect them from dust and accidental damage. Stable humidity and temperature conditions should be ensured.

Sources

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