Hloušekite

Chemical formula: (Ni₂+,Co₂+)Cu₂+₄(As₅+O₄)₂(As₅+O₃OH)₂·₉H₂O

Hloušekite is a very rare, hydrated nickel, cobalt, and copper arsenate, forming acicular crystals of a turquoise-blue color.

## Characteristics Hloušekite is a complex, hydrated arsenate belonging to the geminite group. It occurs as small, radial or spherical aggregates, which are composed of fine, acicular or bladed crystals. Individual crystals rarely exceed 1 mm in length. The mineral is characterized by a vivid, turquoise-blue or greenish-blue color, which makes it visually recognizable despite its small size. ## Physical Properties Hloušekite crystals are brittle, with a Mohs hardness of approximately 3. The luster is vitreous, and on aggregate surfaces, it can be silky. The mineral is translucent. Its density, calculated from the formula and unit cell parameters, is 3.55 g/cm³, but due to its microcrystalline form, direct measurement is impossible. ## Colors and Varieties This mineral occurs in a uniform range of colors from turquoise-blue to bluish-green. No color or commercial varieties have been distinguished. ## History and Name Hloušekite was recognized as a new mineral species by the International Mineralogical Association (IMA) in 2012. Its formal description was published in 2014. The mineral is named in honor of Jan Hloušek (1950–2014), a Czech mineralogist and collector, a specialist in minerals from Jáchymov, who first found this mineral. ## Applications Due to its extreme rarity and small crystal size, hloušekite has no industrial applications. It is of purely scientific and collectible interest, being a desirable acquisition for specialized collections of rare minerals.

Properties

Mohs hardness: 3
Luster: Vitreous to silky
Streak: Light blue
Density: 3.55
Cleavage: Perfect on {010}
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Hloušekite can be preliminarily identified based on its characteristic acicular habit, radial aggregates, and vivid, turquoise-blue color. Its location and co-occurrence with other rare arsenates from Jáchymov are also key indicators. However, definitive and certain identification requires advanced analytical methods, such as chemical analysis (EDS) and X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Visually, it can be confused with other blue or green secondary arsenates, such as geminite, lavendulan, or members of the mixite group. Geminite has a similar chemical composition but a different crystal structure. Lavendulan usually has a more purplish hue. Distinguishing it from these minerals without specialized equipment is practically impossible. ## Crystal Forms It forms acicular or thin, bladed crystals elongated along the [100] axis. These crystals group into radial, stellate, or spherical aggregates, and also form thin crusts on the host rock.

Geological environment

## Genesis Hloušekite is a secondary mineral, forming in the oxidation zones of polymetallic hydrothermal ores. It crystallizes as a result of the weathering of primary ores rich in arsenic, nickel, cobalt, and copper under low-temperature and low-pressure conditions. ## Mineral Associations This mineral occurs in association with other rare arsenates. At the type locality (Jáchymov), it was found in association with geminite, lavendulan, arsenolite, erythrite, annabergite (coettigite-parasymplesite series), and native arsenic. ## Localities It is a mineral with a very limited distribution. The main and best-known locality is the Geister vein in the Rovnost mine complex in Jáchymov (Karlovy Vary region, Czech Republic), which is its type locality. In addition, single finds have been reported in several other locations worldwide, but specimens from Jáchymov remain exemplary.

Rarity

Very rare

For collectors

## Quality Criteria As a mineral occurring exclusively in microscopic specimens, its collectible value depends on several factors. Highly valued are rich crystalline aggregates of intense, turquoise color, with well-developed, radial aggregates. The aesthetic arrangement on a small rock matrix and the presence of rare associated minerals, which increase the scientific and visual value of the specimen, are also of great importance. ## Popular Localities For collectors, the only significant source of hloušekite is its type locality – the historic Rovnost mine in Jáchymov, Czech Republic. Specimens from this location are most sought after due to their historical and scientific significance.

Care and storage

## Cleaning Specimens should only be dry-cleaned, using a soft brush or compressed air from a safe distance to remove dust. Avoid all contact with water and other liquids, which could damage or dissolve the mineral. ## What to Avoid As a hydrated arsenate, hloušekite is sensitive to high temperatures, which can cause dehydration and destruction of its structure. Avoid ultrasonic cleaners, all chemical agents, acids, and detergents. The mineral is toxic due to its arsenic content – wash hands after each contact with the specimen. It is recommended to avoid prolonged exposure to direct sunlight. ## Storage The best way to store specimens is to place them in a closed, stable container, such as a "micromount" box, which protects delicate crystals from mechanical damage, dust, and sudden changes in humidity.

Sources

IMA Mineral List (CNMNC)