Kapustinite

Chemical formula: Na<sub>6</sub>Zr<sup>4+</sup>Si<sub>6</sub>O<sub>16</sub>(OH)<sub>2</sub>

Kapustinite is a rare mineral from the eudialyte group, forming small, hexagonal crystals with a vitreous luster and yellowish color, found in alkaline massifs.

## Characteristics Kapustinite is a complex zirconium and sodium silicate, belonging to the eudialyte group. It forms small, hexagonal, tabular or short-prismatic crystals, rarely exceeding 1 mm. It usually occurs as granular aggregates embedded in rock. It is a transparent to translucent mineral with a vitreous luster. ## Physical Properties Its Mohs hardness is approximately 5. The density is estimated at 2.84 g/cm³. It does not exhibit cleavage, and its fracture is uneven. Some specimens may show weak, yellow fluorescence under ultraviolet light. ## Colors and Varieties Kapustinite is most often yellow, yellowish-brown, or almost colorless. No named varieties are distinguished. ## History and Name The mineral was named in honor of Anatoly Yakovlevich Kapustin (1939–2020), a Russian mineralogist who specialized in the study of alkaline massifs and was the first to find this mineral in the 1970s. It was formally described and approved as a new mineral species by the IMA in 2003 (IMA2003-014). ## Uses Due to its rarity and small crystal size, kapustinite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness
5
Luster
Vitreous
Streak
White
Density
2.84
Cleavage
None
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Kapustinite is difficult to identify visually without specialized equipment. Key features include the hexagonal outline of crystals, yellowish color, vitreous luster, and occurrence in a specific geological environment (agpaitic pegmatites). Certain identification requires advanced methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other minerals from the eudialyte group, such as eudialyte itself, as well as other yellow silicates occurring in similar parageneses (e.g., catapleiite). Differentiation is mainly possible based on chemical composition analysis (kapustinite is poor in calcium and rare earth elements compared to eudialyte) and crystallographic data. ## Crystal Forms It forms tabular or short-prismatic crystals with a hexagonal outline. It occurs as single, scattered crystals or small, granular aggregates within the host rock.

Geological environment

## Genesis It is a mineral of magmatic origin, crystallizing in the late stages in ultrabasic, agpaitic pegmatites and hydrothermal rocks associated with nepheline-syenite massifs. ## Mineral Associations It most commonly co-occurs with microcline, nepheline, aegirine, eudialyte, lorenzenite, lamprophyllite, lomonosovite, sodalite, and catapleiite. ## Localities The most important and best-documented localities worldwide are the alkaline massifs on the Kola Peninsula in Russia, especially the Khibiny and Lovozero massifs (e.g., Koashva Mountain, Rasvumchorr Mountain). Outside Russia, its occurrence has also been reported in the Ilímaussaq complex in Greenland and the Mont Saint-Hilaire massif in Quebec, Canada.

Rarity

Very rare

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, transparent crystals of intense yellow color, set on a contrasting matrix (e.g., white microcline or green aegirine). Due to their small size, the quality of the crystal itself, rather than the size of the entire specimen, is crucial. Specimens with more than one well-formed crystal are particularly desirable. ## Popular Localities By far the most prized and classic specimens come from the Khibiny and Lovozero massifs on the Kola Peninsula in Russia. Specimens from Mont Saint-Hilaire and Ilímaussaq are also sought after but less commonly available on the collector's market.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air or very gently with a soft brush. Water and any chemical agents should be avoided, as the mineral is soluble in acids. ## What to Avoid Contact with acids (HCl, HNO₃), which easily decompose it, must be strictly avoided. It should be protected from moisture, ultrasound, and sudden temperature changes. It is brittle and sensitive to impact. ## Storage It is recommended to store specimens in stable conditions, in closed "micromount" boxes, protecting them from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight.

Sources

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