Yakhontovite

Chemical formula: (Ca,Na,K)<sub>0.2</sub>(Cu<sup>2+</sup>,Fe<sup>3+</sup>,Mg)<sub>2</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>·3H<sub>2</sub>O

Yakhontovite is a very rare, blue or greenish-blue mineral from the smectite group, an hydrated copper silicate, closely related to montmorillonite.

## Characteristics Yakhontovite is a mineral from the smectite group, belonging to the silicates. It occurs as earthy, cryptocrystalline masses and aggregates, which can form crusts or fillings. Its structure is very fine-grained, meaning individual crystals are not visible to the naked eye. Visually, it presents as a material with a dull, earthy appearance, often in intense blue or greenish-blue colors. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1. This is a characteristic feature of clay minerals from the smectite group. It has a low density, and its luster is described as earthy or dull. It is opaque. ## Colors and Varieties Yakhontovite occurs in characteristic shades from light blue and sky blue to greenish-blue. No named varieties have been distinguished. ## History and Name The mineral's name honors the Russian mineralogist and crystallographer Liya Aleksandrovna Yakhontova (1925–2017), a professor at Moscow University. It was first described in 1986 after being found in the oxidation zone of the Komsomolskoye polymetallic deposit in Siberia, Russia.

Properties

Mohs hardness
1
Luster
Earthy
Streak
Light blue
Density
2.23
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Yakhontovite can be identified by its intense blue or greenish-blue color, very low hardness (it can be scratched with a fingernail), and earthy, dull appearance. It occurs in the form of masses, crusts, and coatings. ## Distinguishing from Similar Minerals It can be confused with chrysocolla, which often has similar colors and forms of occurrence. However, chrysocolla is significantly harder (2.5-3.5 on the Mohs scale). From other blue secondary copper minerals, such as azurite, it is distinguished by the lack of crystalline form and much lower hardness. Verification requires advanced methods, such as X-ray diffraction (XRD). ## Crystal Forms Yakhontovite is a cryptocrystalline mineral. It forms massive, earthy aggregates, crusts, coatings, and fillings of small fractures in the rock.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation (weathering) zones of sulfide deposits, mainly copper. It forms as a result of surface water acting on primary ore minerals in volcanic rocks, such as andesites and their tuffs. ## Mineral Associations It most commonly co-occurs with other minerals of the oxidation zone, such as gypsum, quartz, chrysocolla, as well as with primary minerals like pyrite, chalcopyrite, and bornite. ## Localities The most important and type locality (discovery site) is the Komsomolskoye deposit in Khabarovsk Krai, Russian Far East. Its occurrence has also been confirmed at the Freedom #2 mine in Marysvale County, Utah, USA.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of yakhontovite primarily depends on the intensity and purity of its blue color. Specimens forming thick, uniformly colored crusts on small fragments of matrix rock are most valued. Due to its rarity, any well-formed and mineral-rich specimen is valuable. ## Popular Localities The vast majority of specimens on the collector's market come from the type locality in Russia (Komsomolskoye deposit). Specimens from the USA are extremely rare and practically unavailable commercially.

Care and storage

## Cleaning As a very soft mineral potentially sensitive to water, yakhontovite should not be cleaned wet. The safest method is to use a soft brush to remove dust or compressed air from a safe distance. ## What to Avoid Contact with water should be strictly avoided, as it can cause swelling and disintegration of the sample, which is typical for minerals of the smectite group. All chemicals, ultrasound, and high temperatures should also be avoided. Due to its low hardness, it is extremely susceptible to scratches and mechanical damage. ## Storage Yakhontovite specimens are best stored in closed, dry containers (e.g., "membrane box" type) or under stable humidity conditions to prevent degradation. It should be protected from dust, shocks, and direct contact with harder minerals.

Sources

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