Pokrovskite

Chemical formula: Mg<sub>2</sub>CO<sub>3</sub>(OH)<sub>2</sub>

Pokrovskite is a rare magnesium hydroxycarbonate, forming fine, chalky-white aggregates and coatings.

## Characteristics Pokrovskite is a rare mineral from the carbonate group, chemically a basic magnesium carbonate. It most often occurs as very fine-grained, earthy or chalk-like masses, as well as thin coatings and crusts. Due to the nature of its aggregates, individual crystals are extremely rare and appear as microscopic, thin plates or lamellae. It typically forms soft, porous aggregates of white or yellowish color. ## Physical Properties This mineral is very soft, with a Mohs hardness of about 1. It has a low specific gravity, ranging from 2.35 to 2.41 g/cm³. The luster is dull or earthy, and the mineral itself is opaque. It is so soft that it can be scratched with a fingernail. ## Colors and Varieties Pokrovskite is usually white, chalky-white, sometimes with a yellowish or grayish tint, which may be caused by impurities. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Pokrovsk-Ural Massif in the Urals, Russia. It was described and recognized as a new mineral in 1984 by L.P. Ivanova, G.A. Maltseva, and N.V. Zvyagina. ## Uses Pokrovskite has no industrial application. Its significance is purely scientific and as a collector's item, as a rare representative of magnesium carbonates.

Properties

Mohs hardness
1
Luster
Dull
Streak
White
Density
2.35-2.41
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Pokrovskite can be recognized by its characteristic white, earthy, or chalk-like aggregates. It is extremely soft (hardness approx. 1) and light. A key diagnostic feature is its violent reaction with cold hydrochloric acid, which distinguishes it from many other white minerals. ## Distinguishing from Similar Minerals It can be confused with other white, earthy minerals such as chalk (a variety of calcite), kaolinite, hydromagnesite, or artinite. It is distinguished from chalk and hydromagnesite by the absence of fluorescence under UV light. It is distinguished from kaolinite by its reaction with acid. Artinite forms acicular or fibrous aggregates, while pokrovskite is usually earthy. However, accurate identification requires advanced methods, such as X-ray diffraction (XRD). ## Crystal Forms Well-formed crystals are rare and appear as microscopic, hexagonal plates or lamellae. It usually forms massive, earthy, chalk-like, or porous aggregates and thin coatings.

Geological environment

## Genesis Pokrovskite is a secondary mineral, formed in the weathering (oxidation) zones of ultramafic rocks, such as serpentinites and dunites. It forms as a result of the action of carbon dioxide-rich waters on magnesium-rich minerals, e.g., serpentine or olivine, under conditions of low temperatures and pressures. ## Mineral Associations It often co-occurs with other magnesium minerals and weathering products of ultramafic rocks. The most common associated minerals include: magnesite, hydromagnesite, artinite, brucite, serpentine, and relics of olivine and pyroxene. ## Localities The most important occurrences of this rare mineral are in Russia, at its discovery locality in the Pokrovsk-Ural Massif in the Urals. It is also known from Kazakhstan (Altai Massif), as well as from several places in Europe, including the ophiolite complex in Aetoraches, Greece, and the Eugui magnesite mine in Spain.

Rarity

Rare

For collectors

## Quality Criteria The quality of a pokrovskite specimen primarily depends on its purity and abundance on the rock matrix. Since this mineral rarely forms visible crystals, the most valued specimens are rich, pure white coatings and crusts covering a significant area of the host rock. Contrast with a dark matrix (e.g., serpentinite) enhances visual appeal. Specimens from a well-documented, classic locality (Urals) are more desirable. ## Popular Localities The most classic and highly prized specimens by collectors come from the type locality – the Pokrovsk-Ural Massif in Russia. Material from other confirmed localities, such as Spain or Greece, is also valuable due to the rarity of this mineral.

Care and storage

## Cleaning Pokrovskite specimens are extremely delicate and fragile. Mechanical cleaning should be avoided. If absolutely necessary, a very soft brush can be used to remove loose dust, with extremely careful movements. Wet cleaning is not recommended, as porous aggregates can absorb water and disintegrate. ## What to Avoid Avoid contact with water and all chemicals, especially acids, with which it reacts violently, releasing carbon dioxide. The mineral is very soft, so it should be protected from any contact with harder objects, impacts, and vibrations. Store away from moisture. ## Storage It is recommended to store specimens in closed, padded collector boxes to protect them from mechanical damage, dust, and moisture. Do not place adhesive labels on the specimen or touch it directly with fingers.

Sources

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