Potassicmendeleevite-(Ce)

Chemical formula: Cs₆K₆(Ce₂₂Ca₆)(Si₇₀O₁₇₅)(OH,F)₂₀·₁₅H₂O

Potassicmendeleevite-(Ce) is an extremely rare mineral of the mendeleevite group, a complex cesium, potassium, and cerium silicate, known exclusively from one locality in the world.

## Characteristics Potassicmendeleevite-(Ce) is a very complex silicate belonging to the mendeleevite group. It forms small, tabular or short prismatic crystals, usually with a hexagonal outline, reaching up to 1 mm in size. It occurs as single crystals or small aggregates. It is colorless to pale yellow and transparent. ## Physical Properties The mineral's hardness on the Mohs scale is 5. It has a vitreous luster and a white streak. The density calculated from the formula and unit cell parameters is 3.42 g/cm³. It exhibits good cleavage in one direction. ## Colors and Varieties The mineral is usually colorless, occasionally taking on pale yellow hues. Due to its extreme rarity, no color or commercial varieties are distinguished. ## History and Name The mineral's name reflects its chemical composition and group affiliation. The "Potassic" prefix indicates potassium as the dominant alkali element, "mendeleevite" honors the Russian chemist Dmitri Mendeleev, and the "-(Ce)" suffix (according to Levinson's classification) signifies that cerium is the dominant rare earth element in the structure. The mineral was approved by the IMA in 2010 (IMA2010-020), and its description was published in 2011. The type locality is the Darai-Pioz massif in Tajikistan. ## Uses Potassicmendeleevite-(Ce) has no industrial applications. Its significance is purely scientific and collectible, being an object of interest for specialized research institutions and micromount mineral collectors.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 3.42
Cleavage: Good on {0001}
Fracture: Uneven
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Identifying Potassicmendeleevite-(Ce) based on visual characteristics is impossible. Definitive identification requires advanced analytical techniques, such as energy-dispersive spectroscopy (EDS) or wavelength-dispersive spectroscopy (WDS) to confirm its unique chemical composition (high cesium, potassium, and cerium content), and X-ray diffraction (XRD) to determine its crystal structure. The only clue is its occurrence in a specific geological environment at the type locality. ## Distinguishing from Similar Minerals This mineral can be confused with other colorless or pale yellow silicates found in the same paragenesis, such as quartz, microcline, or other minerals from the mendeleevite group (e.g., Mendeleevite-(Ce)). Differentiation from these is only possible through chemical analysis. ## Crystal Forms Crystals are tabular or short-prismatic, often with a hexagonal outline. They usually occur as single, well-formed crystals grown on other minerals.

Geological environment

## Genesis Potassicmendeleevite-(Ce) forms under very specific geochemical conditions. Its genesis is associated with quartz-aegirine-microcline pegmatites that cut nepheline syenites within alkaline massifs. ## Mineral Associations This mineral co-occurs with a number of other, often rare minerals. The most common associations include aegirine, microcline, polylithionite, and quartz. It has also been found in association with pekovite and kapitsaite-(Y). ## Localities The only confirmed locality for this mineral in the world is its type locality – the Darai-Pioz glacier in the Alai Mountains of Tajikistan. This is a famous mineralogical site, known for the occurrence of many unique and rare mineral species.

Rarity

Extremely rare

For collectors

## Quality Criteria As a micromount mineral, specimens with sharp, well-formed, and undamaged crystals are most highly valued. Additional value is added by their attractive placement on a contrasting rock matrix, especially in association with other rare minerals. The size and transparency of the crystals also influence the quality assessment. ## Popular Localities The only source of Potassicmendeleevite-(Ce) specimens is the Darai-Pioz massif in Tajikistan. Specimens from this locality are highly sought after by collectors specializing in rare minerals and systematics.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. If necessary, distilled water can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to Avoid The mineral should be protected from contact with acids and other aggressive chemicals. As a hydrated mineral, it may be sensitive to high temperatures, which can lead to water loss and structural damage. Sudden temperature changes should be avoided. ## Storage It is recommended to store specimens in closed micromount boxes, which protect them from dust, mechanical damage, and sudden changes in humidity. Prolonged exposure to direct sunlight should be avoided.

Sources

IMA Mineral List (CNMNC)