Petterdite

Chemical formula: Pb<sup>2+</sup>Cr<sup>3+</sup><sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>·H<sub>2</sub>O

Petterdite is a rare secondary mineral of the hydrotalcite group, distinguished by its characteristic violet-red color and occurrence as small, hexagonal platelets.

## Characteristics Petterdite is a hydrated lead chromium carbonate belonging to the hydrotalcite group. It forms very small, tabular or platy crystals with a hexagonal outline, rarely exceeding 1 mm in size. Most often, they occur as disseminated aggregates, rosettes, or thin coatings on the surface of the host rock. Its most characteristic feature is an intense, violet-red or purplish color. ## Physical Properties Petterdite crystals exhibit perfect cleavage in one direction, parallel to the basal plane of the hexagonal platelets. They are soft, with a Mohs hardness of only 2. The luster is dull, earthy, or pearly on cleavage surfaces. Due to the small size of the crystals, precise density determination is difficult, and the calculated value is approximately 4.15 g/cm³. ## Colors and Varieties This mineral is monochromatic and has no known varieties. Its color ranges from violet-red and purplish to pinkish-red. Color is one of its main diagnostic features. ## History and Name Petterdite was discovered at the Red Lead mine in Dundas, Tasmania (Australia). It was first described in 1999. The mineral is named in honor of William Frederick Petterd (1849–1910), an Australian naturalist, mineralogist, and collector who made significant contributions to the study of Tasmanian mineralogy. ## Uses Petterdite has no industrial applications. Its significance is purely scientific and as a collector's item, being a rare and sought-after mineral among specialists and advanced micromineral collectors.

Properties

Mohs hardness
2
Luster
Pearly, Earthy
Streak
Light reddish purple
Density
4.15
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Petterdite is relatively easy to identify based on its unique violet-red color, form of small hexagonal platelets, and characteristic association with other lead and chromium minerals such as crocoite, cerussite, and dundasite. Its extreme softness is also an important clue. ## Distinguishing from Similar Minerals Due to its specific color and habit, petterdite is difficult to confuse with other minerals. Some pink or purple secondary minerals, like cobaltoan calcite, usually have a different habit (e.g., botryoidal, reniform) and occur in different parageneses. Chemical analysis confirming the presence of lead and chromium is the ultimate proof. ## Crystal Forms This mineral forms thin, tabular crystals with a hexagonal outline. These crystals often group into rosette-like or fan-shaped aggregates, and also form thin coatings on other minerals.

Geological environment

## Genesis Petterdite is a secondary mineral, formed in the oxidation (weathering) zones of ore deposits containing lead minerals (mainly galena) and chromium minerals (chromite). It forms as a result of reactions of carbonate-rich solutions with primary minerals in the presence of water. ## Mineral Associations At the type locality (Dundas, Tasmania), petterdite coexists with minerals such as crocoite, cerussite, dundasite, phosgenite, anglesite, galena, sphalerite, pyrite, chromite, and goethite. This paragenesis is crucial for its identification. ## Localities The most important and best-known occurrence of petterdite is its type locality – the Red Lead mine and nearby mines in the Dundas area of Tasmania, Australia. This is the only place from which well-formed, collectible specimens of this mineral originate.

Rarity

Very rare

For collectors

## Quality Criteria The quality of petterdite specimens is primarily assessed based on the intensity and saturation of the violet-red color. The most prized specimens are those that display well-formed, hexagonal crystals forming rich aggregates or rosettes. Contrast with the host mineral and the presence of rare associated minerals, such as crocoite or dundasite, are also important. Due to the microscopic size of the crystals, specimens are evaluated under magnification. ## Popular Localities The only source of valuable collector specimens is the Dundas area in Tasmania (Australia), particularly the Red Lead mine. Specimens from this locality are considered the global standard for this mineral.

Care and storage

## Cleaning Petterdite specimens should be cleaned with utmost care, exclusively using compressed air to remove dust. Due to its extreme softness and fragility, contact with water, brushes, or other mechanical tools should be avoided. ## What to Avoid The mineral is sensitive to acids, which can damage or dissolve it. All chemicals, ultrasonics, and sudden temperature changes should be avoided. Specimens should be protected from direct sunlight, which could theoretically affect its color. ## Storage Petterdite, as a mineral occurring in the form of microcrystals, should be stored in specialized micromineral boxes that protect it from mechanical damage and dust. Storage in areas with high humidity should be avoided.

Sources

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