Zincaluminite

Chemical formula: (Zn<sup>2+</sup>,Al)<sub>9</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>18</sub>·nH<sub>2</sub>O

Zinc-alumina is a rare, hydrated zinc and aluminum sulfate, forming small, hexagonal crystals and rosette aggregates of pale blue or white color.

## Characteristics Zinc-alumina is a secondary mineral from the sulfate group, occurring in the oxidation zones of zinc ore deposits. It forms very small, tabular crystals with a hexagonal outline, which often combine into characteristic, rosette-like or spherulitic aggregates. Due to the small size of the crystals, its features are most often observed under magnification. It is also found in the form of coatings and crusts. ## Physical Properties Zinc-alumina crystals exhibit a pearly luster on basal surfaces (largest faces) and vitreous on others. It is a very soft mineral, with a Mohs hardness of approximately 2.5, meaning it can be scratched with a fingernail. It is light, and its density is only slightly greater than that of water. It is transparent to translucent. ## Colors and Varieties Zinc-alumina is most often pale blue, bluish-green, or white. No named varieties are distinguished. ## History and Name The mineral's name, given in 1881 by Bertrand and Damour, directly refers to its chemical composition – the content of zinc (Latin: *zincum*) and aluminum (Latin: *aluminium*). It was discovered in Laurion, Greece, which is its type locality. ## Uses Zinc-alumina has no industrial application. It is of purely scientific and collector's interest as a rare secondary mineral.

Properties

Mohs hardness
2.5
Luster
Pearly, Vitreous
Streak
White
Density
2.29
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Zinc-alumina is most easily recognized by its characteristic, rosette-like aggregates composed of very small, hexagonal tablets. The pale blue color, pearly luster on the largest crystal faces, and occurrence in paragenesis with other secondary zinc minerals are key indicators. It reacts with acids. ## Distinguishing from Similar Minerals It can be confused with other secondary zinc and aluminum sulfates, such as niedermayrite or ktenasite, which often have similar colors and forms. Precise differentiation of these minerals is usually impossible without advanced studies, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms Crystals are tabular, with a hexagonal outline. They most often occur as rosette-like, spherulitic (spherical), or fan-shaped aggregates. It also forms thin crusts and coatings on other minerals.

Geological environment

## Genesis It is a secondary mineral, formed as a result of weathering (oxidation) of sulfidic zinc deposits, especially in aluminum-rich rocks. It forms under low temperature and pressure conditions, often in voids and fractures of host rocks. ## Mineral Associations It most commonly co-occurs with other secondary minerals of the oxidation zone, such as: hydrozincite, aurichalcite, smithsonite, hemimorphite, gypsum, as well as with sulfides (sphalerite, galena) and minerals of the surrounding rocks. ## Localities The most important occurrences of this rare mineral are the mines in the Laurion area in Attica (Greece), where it was discovered. It is also known from several other places in the world, including the Ojuela Mine in Mapimí (Mexico), the Kabwe (Broken Hill) Mine in Zambia, and some localities in Germany and Austria.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp rosettes of intense, pale blue color, contrasting with the rock matrix, are most valued by collectors. Due to the microscopic size of the crystals, the size and aesthetics of entire aggregates are crucial. Confirmation of the locality is also important, especially if the specimen comes from the type locality (Laurion). ## Popular Localities Specimens from Greek Laurion are a historical standard and classic for this mineral. Well-formed aggregates from the Ojuela Mine in Mexico are also highly prized.

Care and storage

## Cleaning Specimens should only be cleaned very carefully, using a soft brush to remove dust. Contact with water is absolutely not recommended, as the mineral is partially soluble in it. ## What to Avoid Avoid all contact with water and other liquids, including cleaning agents. The mineral is very soft and brittle, so it must be protected from impacts, scratching, and vibrations. High temperatures, which can cause dehydration and structural damage, should be avoided. ## Storage It is recommended to store specimens in closed, stable containers (e.g., "membrane box" type or boxes with soft lining), in a dry place, away from heat sources and direct sunlight. Consider using desiccants near the specimen.

Sources

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