Kupletskite-(Cs)

Chemical formula: Cs₂NaMn₂+₇Ti₄+₂(Si₄O₁₂)₂O₂(OH)₄F

A rare silicate from the astrophyllite supergroup, the cesium analog of kupletskite, forming characteristic, star-shaped aggregates of lamellar crystals.

## Characteristics Kupletskite-(Cs) is a mineral belonging to the astrophyllite supergroup, being the cesium analog of kupletskite. It forms thin, tabular or lath-like crystals, which almost always occur as radial, star-shaped, or sheaf-like aggregates. Its appearance is very characteristic, resembling "suns" or "stars" with a metallic luster embedded in the rock. The mineral's color is usually brown, reddish-brown, or brownish-golden. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 3. It is brittle and has perfect, unidirectional cleavage, which causes its lamellae to easily separate. The luster on cleavage surfaces is pearly, and on other surfaces, it is vitreous. Its density is about 3.36 g/cm³. It is translucent, and in thicker fragments, opaque. ## Colors and Varieties Kupletskite-(Cs) occurs in shades of brown, from golden and brownish-red to almost black. The name "kupletskite-(Cs)" indicates that it is a variety of kupletskite in which cesium (Cs) dominates a specific position in the crystal structure, replacing potassium. ## History and Name The mineral was recognized by the IMA in 1970. Its name honors Russian petrologists, Boris Mikhailovich Kupletsky (1894–1964) and his wife Elsa Maximillianovna Bohnstedt-Kupletskaya (1886–1950). The suffix "-(Cs)" refers to the dominance of cesium in its chemical composition. The type locality is the Darai-Pioz Glacier in Tajikistan. ## Applications Kupletskite-(Cs) has no industrial applications. However, it is a highly valued and sought-after collector's mineral due to its rarity, attractive forms, and origin.

Properties

Mohs hardness: 3
Luster: Vitreous to Pearly
Streak: Light brown
Density: 3.36
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Triclinic

Diagnostic features

## Identification The most important diagnostic feature of kupletskite-(Cs) is its habit in the form of radial, star-shaped aggregates of brown color and pearly or vitreous luster. Perfect cleavage in one direction, causing thin lamellae to separate, is also crucial. Its occurrence in alkaline pegmatites in association with other rare minerals is a strong indication. ## Distinguishing from Similar Minerals Kupletskite-(Cs) can be confused with other minerals from the astrophyllite group, such as astrophyllite (usually more golden-yellow) or kupletskite (potassium analog). Certain differentiation between them requires advanced chemical analyses (e.g., EDS). It differs from mica (e.g., biotite) by its greater brittleness, specific brown hue, and characteristic star-shaped aggregates. ## Crystal Forms Crystals are tabular, lath-like, elongated, and flattened. They almost always form aggregates: radial, stellate (star-shaped), spherulitic, or sheaf-like, often embedded in the host rock.

Geological environment

## Genesis It is an igneous mineral, typical of highly differentiated rocks saturated with alkali elements and rare earth elements, such as nepheline syenites and associated pegmatites. ## Mineral Associations Kupletskite-(Cs) co-occurs with many rare minerals typical of its formation environment, such as microcline, nepheline, aegirine, eudialyte, lorenzenite, reedmergnerite, polylithionite, and many others from the Darai-Pioz massif. ## Localities The most important and practically sole source of well-formed, collector-grade specimens is its type locality – the alkaline Darai-Pioz massif in the Alai Mountains of Tajikistan. This is one of the world's most famous localities for rare minerals.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with large, well-formed, complete "stars" or radial aggregates are most highly valued. An intense brown color and good luster are important. Contrast with a light host rock (e.g., with albite or nepheline) and the presence of other rare associated minerals enhance its attractiveness. ## Popular Localities The only place in the world from which specimens of interest to collectors originate is the Darai-Pioz massif in Tajikistan. Specimens from this locality are considered classics and benchmarks for this mineral.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners should not be used, as they could damage the delicate lamellae. ## What to Avoid The mineral is very brittle and has perfect cleavage, so it should be protected from impacts, falls, and scratching by harder minerals. Contact with acids and other chemicals should be avoided. Due to its brittleness, any attempts at processing are highly risky. ## Storage It is recommended to store specimens in separate, padded boxes or display cases, away from minerals of greater hardness. It should be protected from dust, which can accumulate between the lamellae of the aggregates.

Sources

IMA Mineral List (CNMNC)