Pickeringite

Chemical formula: MgAl<sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>4</sub>·22H<sub>2</sub>O

Pickeringite is a hydrated magnesium aluminum sulfate, forming characteristic fibrous and acicular efflorescences on rocks.

## Characteristics Pickeringite is a sulfate mineral belonging to the halotrichite group. It forms characteristic, silky coatings and crusts, as well as fibrous or acicular aggregates. It rarely occurs as well-formed, hair-like crystals. It is a brittle and light mineral. Due to its high water content in the structure, it can lose water in the air and partially decompose. ## Physical Properties Pickeringite is very soft, with a Mohs hardness of 1.5. It has a low specific gravity, approximately 1.73-1.79 g/cm³. The luster is typically silky, especially in fibrous aggregates, or vitreous. It is a transparent to translucent mineral. ## Colors and Varieties This mineral is most often colorless or white. Impurities can give it yellowish, grayish, and less commonly pinkish or reddish-brown hues, which is associated with the presence of admixtures, mainly iron. No named varieties are distinguished. ## History and Name The name pickeringite comes from the name of the American linguist and philologist John Pickering (1777-1846). The mineral was first described in 1844 based on samples from the Tarapacá Province in Chile. ## Uses Pickeringite has no significant industrial importance. However, it is of interest to collectors specializing in secondary and rare minerals. Due to its bitter and astringent taste, it was formerly referred to as "magnesium salt."

Properties

Mohs hardness
1.5
Luster
Silky
Streak
White
Density
1.73-1.79
Cleavage
Perfect on {010}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Pickeringite is most easily recognized by its characteristic mode of occurrence – it forms delicate, silky, fibrous or acicular efflorescences and coatings. Key diagnostic features include its very low hardness, lightness, and easy solubility in water. It also has a characteristic bitter-salty taste. ## Distinguishing from Similar Minerals It can be confused with other secondary sulfates such as halotrichite, epsomite, or melanterite. Halotrichite (iron-aluminum sulfate) is visually very similar but often has a greenish tint and is associated with pyrite oxidation. Epsomite (Epsom salt) also forms acicular crystals but has a different chemical composition (magnesium sulfate). Accurate differentiation of these minerals often requires chemical analysis. ## Crystal Forms Pickeringite crystals are very rare and appear as small, hair-like or acicular forms, crystallizing in the monoclinic system. It usually occurs as fibrous aggregates, radial aggregates, as well as coatings, crusts, and efflorescences on rock surfaces.

Geological environment

## Genesis Pickeringite is a typical secondary mineral. It forms in weathering (oxidation) zones of sulfide-rich rocks, especially pyrite and marcasite, in the presence of aluminum- and magnesium-containing minerals. It forms as a product of evaporation of mine waters and solutions percolating through rocks. It is often found in dry, desert climates as efflorescences on the ground surface and in old mine workings and on mine dumps. ## Mineral Associations It most often co-occurs with other secondary sulfates such as halotrichite, alunogen, epsomite, melanterite, and gypsum. It is also associated with pyrite (as a primary mineral) and clay minerals. ## Localities Significant pickeringite localities are found in desert regions, e.g., in Tarapacá Province, Chile (type locality), and in many places in the western USA (Arizona, California, Nevada, Utah). It also occurs in old mines in Germany (Harz Mountains), the Czech Republic, Hungary (Eger), and in Italy near Vesuvius and on Vulcano island.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, long and delicate "hairs" or needles, forming fluffy, silky aggregates. Purity and whiteness of the specimen are important, although samples with natural coloration (e.g., pink from manganese) can also be attractive. Large, undamaged fibrous aggregates on a piece of matrix rock are more highly valued than broken fragments. ## Popular Localities Classic and prized specimens come from historical localities in Chile and Italy. Many good quality samples, often in the form of spectacular "tangles," are provided by old mines in the Czech Republic and Germany. Specimens from numerous localities in the United States also appear on the collector's market.

Care and storage

## Cleaning Pickeringite specimens are extremely sensitive to water, in which they readily dissolve. Wet cleaning is absolutely forbidden. Dust can be removed very carefully with a soft brush or a stream of dry, compressed air from a safe distance. ## What to Avoid Avoid all contact with water, moisture, and chemicals. The mineral is hygroscopic, meaning it absorbs moisture from the air, which can lead to its dissolution and destruction. It should be protected from high temperatures, which can cause the loss of crystallization water and structural breakdown. It is very brittle and delicate, susceptible to mechanical damage. ## Storage Pickeringite requires storage in conditions of low, controlled humidity. It is best kept in tightly sealed containers (e.g., "perky boxes") or in display cases with a desiccant (silica gel). Avoid exposing it to open air.

External references

Sources

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