Liskeardite

Chemical formula: (Al,Fe<sup>3+</sup>)<sub>32</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>18</sub>(OH)<sub>42</sub>(H<sub>2</sub>O)<sub>22</sub>·52H<sub>2</sub>O

A rare aluminum and iron arsenate, forming soft, earthy or fibrous aggregates of pale blue, greenish, or white color.

## Characteristics Liskeardite is a secondary mineral from the arsenate group, chemically a hydrated aluminum and iron arsenate. It occurs as very fine-grained, almost cryptocrystalline aggregates. Typical specimens are in the form of earthy or chalky coatings and masses, as well as delicate, fibrous aggregates. Due to its structure, it is a very brittle and soft mineral. ## Physical Properties Its hardness is low, estimated at about 3 on the Mohs scale, but due to its earthy or fibrous structure, specimens feel much softer to the touch. The luster is usually dull or earthy, and in the case of fibrous aggregates, it can be silky. The mineral is translucent to opaque. Its density is approximately 2.6 g/cm³. ## Colors and Varieties Liskeardite takes on characteristic, pale shades – from bluish-white and pale blue to greenish-white and white. Yellowish-white coatings are also sometimes found. No commercial or color varieties are distinguished. ## History and Name The mineral's name comes from its discovery location and first description – the area around the town of Liskeard in Cornwall (United Kingdom). It was described as a new mineral species in 1878 by the British mineralogist Nevil Story-Maskelyne. ## Uses Liskeardite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness
3
Luster
Dull
Streak
White
Density
2.6
Cleavage
None
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Liskeardite is identified by its characteristic appearance – pale blue or greenish, earthy or fibrous coatings. Key features include low hardness, dull luster, and occurrence in the oxidation zones of ore deposits rich in arsenic. ## Distinguishing from Similar Minerals It can be confused with other secondary minerals of similar color and habit. It is distinguished from variscite by its lower hardness and often more bluish hue. Chrysocolla is usually harder and has a vitreous or waxy luster. Mawellite, although sometimes bluish, forms characteristic radial aggregates. Final distinction from other rare arsenates or phosphates requires advanced chemical analysis. ## Crystal Forms It does not form macroscopic, well-developed crystals. It occurs exclusively in the form of cryptocrystalline, earthy, powdery, or very fine fibrous aggregates.

Geological environment

## Genesis It is a typical secondary mineral, forming in the oxidation zones (so-called gossans) of hydrothermal ore deposits that contain arsenic minerals, mainly arsenopyrite. It forms as a result of the action of surface waters on primary minerals in the presence of aluminum-rich rocks, such as shales. ## Mineral Associations It most often co-occurs with other secondary arsenic minerals, such as scorodite, pharmacosiderite, and chenevixite. It is also accompanied by iron oxides and hydroxides (goethite, limonite) and quartz. ## Localities The most important and classic occurrence is its type locality – Marke Valley mine in the Liskeard area, Cornwall (United Kingdom). It is also known from several localities in Saxony (Germany), e.g., in the Schneeberg area. It is a very rare mineral, reported from a few localities worldwide.

Rarity

Very rare

For collectors

## Quality Criteria As a mineral occurring mainly in microscopic form, liskeardite is primarily valued by micromineral collectors. The value of a specimen is determined by the richness of the coating on the rock matrix, the intensity and purity of the pale blue color, and aesthetic associations with other rare secondary minerals. ## Popular Localities The most sought-after by collectors are historical specimens from the type locality in Cornwall, UK, which serve as the standard for this mineral.

Care and storage

## Cleaning The mineral is extremely sensitive to water and mechanical factors. To remove dust, use only a soft, dry brush or carefully blow it away with a stream of air from a safe distance. Absolutely avoid contact with water and other liquids. ## What to Avoid Avoid water, all chemicals, ultrasonic cleaners, and abrasion. As a hydrated mineral, it is sensitive to changes in humidity and high temperatures, which can lead to its dehydration and destruction. Due to its arsenic content, avoid inhaling dust and wash hands after each contact with the specimen. ## Storage Liskeardite specimens are best stored in closed, airtight containers (e.g., micromount boxes) that protect them from dust, moisture, and mechanical damage. Stable environmental conditions should be ensured, away from direct sunlight and heat sources.

Sources

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