Volkonskoite

Chemical formula: Ca<sub>0.3</sub>(Cr<sup>3+</sup>,Mg)<sub>2</sub>(Si,Al)<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>·4H<sub>2</sub>O

Volkonskoite is a rare, green clay mineral from the smectite group, valued for its intense, emerald color derived from chromium.

## Characteristics Volkonskoite is a clay mineral from the smectite group, being a hydrated chromium, magnesium, and calcium silicate. It occurs as earthy, compact, or gel-like masses, often filling veins and fissures in sedimentary rocks. Its most distinctive feature is its intense, emerald-green or dark-green color, which it owes to its high chromium content. After drying, specimens become hard, brittle, and cracked, resembling dried mud. ## Physical Properties This mineral is very soft, with a Mohs hardness of 2-2.5. It becomes brittle upon drying. It has a dull or waxy luster. It is opaque, and its density ranges from 2.27 to 2.4 g/cm³. Like other clay minerals, it exhibits swelling properties when in contact with water. ## Colors and Varieties Volkonskoite occurs in shades from light green and emerald green to dark green, almost black. Color is its main attribute. No named varieties are distinguished, and color variability depends mainly on the degree of hydration and impurities. ## History and Name The mineral's name, given in 1830 by August Alexander Kämmerer, honors the Russian prince and minister of the court, Pyotr Mikhailovich Volkonsky (1776-1852). It was discovered in the Perm Krai in Russia. ## Uses Due to its intense and stable color, volkonskoite was historically used as a high-quality, stable green mineral pigment for artistic paints. However, its rarity and small deposits limit broader commercial application. Currently, it is primarily a mineral valued by collectors.

Properties

Mohs hardness
2-2.5
Luster
Waxy
Streak
Light green
Density
2.27-2.4
Cleavage
Perfect on {001}
Fracture
Conchoidal
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of volkonskoite are its intense emerald-green color, earthy or compact form, and low hardness. Its behavior after drying—becoming hard and cracked—is also characteristic. It swells upon contact with water. ## Distinguishing from Similar Minerals It can be confused with other green secondary minerals, such as malachite, chrysocolla, or various clay minerals from the chlorite group. Malachite reacts with hydrochloric acid, effervescing, which volkonskoite does not. Chrysocolla often has a more bluish tint. From other green clay minerals, it is distinguished by its very high chromium content, which can be confirmed by chemical analysis. ## Crystal Forms Volkonskoite does not form macroscopic crystals. It occurs as earthy, compact, reniform masses, or as gel-like masses filling fissures and voids in the host rock.

Geological environment

## Genesis Volkonskoite is a secondary mineral, forming in the weathering zone of sedimentary rocks, such as marls, sandstones, and conglomerates. Its formation is associated with hydrothermal or weathering decomposition of chromium-rich minerals (e.g., chromite) in the presence of organic matter, which provides reducing conditions. ## Mineral Associations It often co-occurs with calcite, quartz, gypsum, celadonite, pyrite, and various iron and manganese oxides. ## Localities The most important and historical localities are in Russia, in the Perm Krai along the Kama River (e.g., Yefimyatskaya Mountain). Occurrences have also been reported in Utah, USA (Emery and Grand counties), Israel, and Bavaria, Germany.

Rarity

Rare

For collectors

## Quality Criteria Specimens with an intense, uniform, emerald-green color are most sought after by collectors. The thickness and continuity of the volkonskoite vein in the host rock, as well as an interesting contrast with the surrounding rock, also enhance its appeal. Specimens preserved in a stable, uncracked form are more highly valued. ## Popular Localities Classic and most prized specimens come from historical localities in the Perm Krai, Russia. Specimens from Utah, USA, are also available on the collector's market and valued for their quality.

Care and storage

## Cleaning Volkonskoite specimens are sensitive to water. Washing should be avoided, and dust should be removed very carefully with a soft brush or compressed air from a safe distance. ## What to Avoid Contact with water and other liquids must be absolutely avoided, as they can cause swelling, and subsequently cracking and disintegration of the specimen during drying. Also avoid sudden changes in humidity and temperature. The mineral is very soft and brittle, susceptible to mechanical damage. ## Storage Volkonskoite requires storage in stable conditions, preferably in a dry environment. It is recommended to keep it in closed collector boxes or display cases to protect it from dust and humidity changes. Avoid exposing it to direct sunlight.

Sources

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