Karchevskyite

Chemical formula: Mg<sub>18</sub>Al<sub>9</sub>(OH)<sub>54</sub>Sr<sub>2</sub>(CO<sub>3</sub>)<sub>9</sub>(H<sub>2</sub>O)<sub>6</sub>(H<sub>3</sub>O)<sub>5</sub>

Karchevskyite is a rare mineral from the hydrotalcite group, forming soft, white aggregates with a pearly luster, found in ultramafic alkaline rocks.

## Characteristics Karchevskyite is a very rare, hydrated magnesium, aluminum, and strontium hydroxycarbonate, belonging to the hydrotalcite group. It occurs as fine, flexible, lamellar or fibrous crystals, which typically form rosette-like or spherulitic aggregates. Individual lamellae rarely exceed 1 mm. The mineral is soft, feeling like talc or graphite to the touch. Its typical appearance is white, chalky, or silvery masses with a pearly luster. ## Physical Properties The mineral is characterized by very low hardness, only 2 on the Mohs scale. It is extremely light, with a density of approximately 2.13 g/cm³. Karchevskyite lamellae are flexible but not elastic. The luster on cleavage surfaces is distinctly pearly, while on other surfaces it is dull or earthy. ## Colors and Varieties Karchevskyite is usually white, silvery-white, or colorless. No colored or commercial varieties have been distinguished. ## History and Name The mineral was discovered in the Afrikanda massif on the Kola Peninsula in Russia and described in 2001 by Victor N. Yakovenchuk and co-authors. The name honors the Russian mineral collector and geologist, Pavel Karchevsky (born 1962), who found the first samples of this mineral.

Properties

Mohs hardness
2
Luster
Pearly
Streak
White
Density
2.13
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Key diagnostic features include extreme softness (hardness 2), pearly luster on cleavage surfaces, flexible lamellae, and characteristic rosette-like or spherulitic aggregates. It feels slippery to the touch, similar to talc. It reacts with hydrochloric acid, releasing carbon dioxide bubbles. ## Distinguishing from Similar Minerals It can be confused with talc, but talc does not react with acid. Brucite can also have a similar appearance, but it has higher hardness (2.5) and does not contain carbonates. From other minerals of the hydrotalcite group (e.g., quintinite), it is distinguished by the presence of strontium, which requires advanced chemical analysis (EDS/WDS). ## Crystal Forms It forms very fine, lamellar or platy crystals, usually grouped into spherical aggregates (spherulites) or rosette-like forms. Single, well-formed crystals are extremely rare.

Geological environment

## Genesis Karchevskyite is a secondary mineral, formed by low-temperature hydrothermal processes. It forms in calcite-dolomite veins cutting ultramafic alkaline rocks, such as pyroxenites. It originates from the metasomatic alteration of earlier minerals. ## Mineral Associations It most commonly co-occurs with calcite, dolomite, olivine, titanite, magnetite, perovskite, as well as other minerals from the hydrotalcite group, such as quintinite and charmarite. ## Localities The most important and type locality is the Afrikanda massif on the Kola Peninsula in Russia. It is also known from the Poudrette quarry in Mont Saint-Hilaire, Quebec (Canada). These are the only confirmed, well-documented localities of this mineral in the world.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens are those with well-formed, distinct rosette-like or spherulitic aggregates, set on a contrasting rock matrix. Important factors include intense, pearly luster, cleanliness (absence of dirt), and the size of the aggregates. Due to its rarity, even small but well-defined specimens are valuable. ## Popular Localities By far the most sought-after specimens come from the type locality – the Afrikanda massif in Russia. Specimens from Mont Saint-Hilaire in Canada are also valued, but often appear as microscopic clusters.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its extreme softness and delicacy, contact with water and any chemical agents should be avoided. It is best to clean it dry. ## What to Avoid Water, acids, solvents, and ultrasound must be absolutely avoided. The mineral is very soft and easily damaged mechanically – even a stronger touch can destroy the delicate lamellae. It should be protected from high temperatures and humidity. ## Storage Karchevskyite requires storage in stable conditions, away from dust and moisture. The safest solution is a sealed "micromount" box or a display case away from harder minerals that could scratch or crush it.

Sources

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