Ashoverite

Chemical formula: Zn<sup>2+</sup>(OH)<sub>2</sub>

Ashoverite is a rare polymorph of zinc hydroxide, forming colorless or white, tabular crystals with a pearly luster.

## Characteristics Ashoverite is one of five polymorphic varieties of zinc hydroxide (Zn(OH)₂). It occurs as very small, tabular or platy crystals, reaching sizes up to 2 mm, which often arrange themselves into rosette-like or fan-shaped aggregates. It is a secondary mineral, formed by the oxidation of zinc sulfides, mainly sphalerite. Crystals are typically colorless to white, less commonly with a pale green or bluish tint. Due to its rarity and small crystal size, it is primarily of interest to specialized collectors. ## Physical Properties Ashoverite crystals exhibit perfect cleavage in one direction, which, combined with their low hardness (approximately 3 on the Mohs scale), makes them very brittle and delicate. The luster on cleavage surfaces is distinctly pearly, while on other faces it can be vitreous. The mineral is transparent to translucent. ## Colors and Varieties Ashoverite is most often colorless or white. Specimens with a pale blue or pale green coloration are occasionally found, likely caused by copper impurities. No named varieties of this mineral are distinguished. ## History and Name The mineral was discovered in the Milldam mine, in Ashover, Derbyshire, England. Its name, approved in 1988, comes directly from the type locality. It was described by A. M. Clark, E. E. Fejer, G. C. Jones, and D. R. C. Kempe. Ashoverite is a tetragonal variety of zinc hydroxide, which distinguishes it from its other polymorphs: sweetite, wülfingite, spertiniite, and zincite (although the latter is an oxide, it is sometimes included in this group). ## Uses Ashoverite has no industrial application. Its significance is purely scientific and collectible.

Properties

Mohs hardness
3
Luster
Pearly
Streak
White
Density
3.41
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Tetragonal

Diagnostic features

## Identification Ashoverite is difficult to identify without specialized equipment. It is distinguished by tetragonal, tabular crystals with a characteristic pearly luster on cleavage planes. Its occurrence is limited to the oxidation zones of zinc-bearing deposits. It reacts with acids. ## Distinguishing from Similar Minerals It can be confused with other Zn(OH)₂ polymorphs, such as sweetite (also tetragonal, but with a different crystal habit - dipyramidal) or wülfingite (orthorhombic). It is distinguished from hydrozincite by its lack of reaction with cold acids and different crystal habit. Differentiation from similar secondary minerals often requires X-ray diffraction (XRD) analysis. ## Crystal Forms It forms thin, tabular crystals with a square or octagonal outline. These crystals often combine into fan-shaped, rosette-like, or almost spherical aggregates. Less common are forms resembling prismatic columns.

Geological environment

## Genesis Ashoverite is a secondary mineral, formed under low-temperature conditions in the oxidation zones of ore deposits, mainly in caverns and voids within carbonate rocks (limestones and dolomites). It forms as a result of the weathering of sphalerite (ZnS) in an alkaline environment. ## Mineral Associations It most commonly co-occurs with other secondary zinc and lead minerals. Its typical associations include: sweetite, hydrozincite, calcite, galena, sphalerite, anglesite, and cerussite. ## Localities The most important and classic locality, from which the best specimens originate, is the Milldam mine in Ashover, Derbyshire, England. This mineral has also been confirmed in several other places worldwide, including the Sterling Hill and Franklin mines in New Jersey (USA), the Preguiça mine in Portugal, and mines in the Laurion region in Greece.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic, rosette-like aggregates are most highly valued. Transparency and clarity of the crystals, as well as their size, are important – specimens with crystals exceeding 1 mm are already considered significant. A contrasting matrix, such as dark limestone, enhances visual appeal. Specimens from the type locality (Ashover, England) are the most sought after. ## Popular Localities By far the most valued and well-known specimens come from the Milldam mine in Ashover, England. Localities in the USA (Sterling Hill) and Greece (Laurion) have also provided collectible material, but it does not match the quality and fame of specimens from the type locality.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Due to extreme delicacy and low hardness, contact with water and any tools, such as brushes, which could damage the fragile crystals, should be avoided. ## What to Avoid Contact with acids and other chemicals, which would rapidly dissolve the mineral, must be absolutely avoided. The crystals are very soft and brittle, susceptible to mechanical damage from the slightest touch. They should be protected from vibrations and temperature changes. ## Storage It is recommended to store specimens exclusively in sealed, padded "micromount" boxes to protect them from dust, mechanical damage, and air exposure. Display should be in stable conditions, away from sources of vibration and direct sunlight.

External references

Sources

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