Stergiouite

Chemical formula: CaZn₂+₂(As₅+O₄)₂·₄H₂O

Stergiouite is a hydrated calcium zinc arsenate, forming colorless, platy crystals with a pearly luster, found in the Lavrion mines in Greece.

## Characteristics Stergiouite is a hydrated calcium zinc arsenate. This mineral forms very small, thin, platy crystals, reaching up to 0.2 mm in size, which often arrange into rosette-like or spherical aggregates. The crystals are colorless and transparent, exhibiting a characteristic pearly luster on the platy surfaces and a vitreous luster on other faces. ## Physical Properties Stergiouite is characterized by low hardness, approximately 2 on the Mohs scale, which means it is very soft and susceptible to scratching. Its crystals are flexible. The density calculated based on the formula and unit cell parameters is 3.43 g/cm³. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 2006 under the number IMA2006-033. Its name honors Vassilis Stergiou (1958-2017), a Greek mineral collector and photographer specializing in Lavrion minerals, who found the first samples of this mineral. It was described in detail in 2008 by a team of researchers, including Nikita V. Chukanov. ## Uses Due to its extreme rarity and microscopic crystal sizes, stergiouite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized micromount mineral collectors.

Properties

Mohs hardness: 2
Luster: Pearly, Vitreous
Streak: White
Density: 3.43
Cleavage: Perfect on {010}
Fracture: Micaceous
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Stergiouite is recognized by its unique appearance: it forms colorless, very thin, platy crystals with a strong pearly luster, often gathered into small, rosette-like aggregates. Key factors are its occurrence locality (Hilarion mine in Lavrion) and its association with other rare arsenates. ## Differentiation from similar minerals It can be confused with other microscopic, colorless secondary minerals, such as gypsum or talc, due to similar luster and low hardness. However, the unique crystal habit (thin plates in rosettes), flexibility, and specific geochemical environment (oxidized zone of a polymetallic deposit rich in arsenic) allow for its differentiation. Final identification requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Crystal forms The crystals are thin-platy, with a bladed or lamellar habit, dominated by the {010} face. They typically form rosette-like, fan-shaped, or spherulitic aggregates up to 0.4 mm in diameter.

Geological environment

## Genesis Stergiouite is a secondary mineral that forms in the oxidation zone (gossan) of polymetallic (Zn-Pb-Ag) deposits. It crystallizes in voids within arsenic-rich rocks as a result of low-temperature hydrothermal processes. ## Mineral associations This mineral occurs in association with other, often rare, secondary minerals. In its type locality (Hilarion mine), it was found in association with annabergite, gypsum, quartz, sericite, sphalerite, galena, pyrite, and goethite. ## Localities The only confirmed occurrence of stergiouite in the world is its type locality: Hilarion Mine (shaft no. 3), Kamariza, in the Lavrion district, Attica region, Greece.

Rarity

Extremely rare

For collectors

## Quality criteria The quality of stergiouite specimens is primarily assessed based on the richness and aesthetics of the crystal aggregates. Samples with well-formed, distinct rosettes or spherulites that contrast with the rock matrix are most highly valued. The size of the aggregates (though always microscopic) and the absence of damage also influence value. The clarity and transparency of individual crystals within the aggregate are an additional asset. ## Popular localities The only source of stergiouite specimens is the Hilarion mine in Lavrion, Greece. Specimens from this locality are highly sought after by collectors specializing in systematic minerals and rare micromounted minerals.

Care and storage

## Cleaning Stergiouite specimens should be handled with utmost care. Due to the microscopic size and fragility of the crystals, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. Avoid contact with water and any chemicals. ## What to avoid The mineral is very soft (hardness 2), so any physical contact that could scratch or damage it should be avoided. It must be protected from moisture, acids, and other chemical agents. As an arsenate, mineral dust is toxic – avoid inhaling it and wash hands after any contact. ## Storage Stergiouite should be stored exclusively in specialized micromount boxes, which protect it from dust, moisture, and mechanical damage. Display should only be done under a microscope.

Sources

IMA Mineral List (CNMNC)