Fluorapophyllite-(Cs)

Chemical formula: CsCa₄(Si₈O₂₀)F(H₂O)₈

A rare mineral from the apophyllite group, being the cesium analog of Fluorapophyllite-(K), distinguished by its occurrence as small, colorless crystals.

## Characteristics Fluorapophyllite-(Cs) is a mineral belonging to the apophyllite group, in which cesium (Cs) is the dominant alkali metal. It is the cesium analog of Fluorapophyllite-(K) and the sodium analog of Fluorapophyllite-(Na). It forms small, tabular or prismatic crystals, reaching up to 1 mm in size. It usually occurs as single, well-formed crystals or small aggregates thereof. It is colorless and transparent, which, combined with its small size, makes it difficult to observe with the naked eye. ## Physical Properties The mineral is characterized by a vitreous luster. Its Mohs hardness is approximately 4.5-5, and its density is slightly higher than that of other apophyllites, around 2.62 g/cm³. It exhibits perfect cleavage in one direction, which is typical for minerals of this group. ## Colors and Varieties Fluorapophyllite-(Cs) is colorless. No colored varieties or commercial names are known, which is due to its extreme rarity and microscopic crystal sizes. ## History and Name The mineral's name refers to its chemical composition and its relationship with other minerals in the group. The term "Fluorapophyllite" indicates the dominance of fluorine over hydroxyl groups and its belonging to the apophyllites. The suffix "-(Cs)" signifies that cesium is the dominant alkali metal in this particular mineral. It was formally described and approved by the International Mineralogical Association (IMA) in 2018 under number IMA2018-049. The discovery was made on material from the Vepice quarry in the Czech Republic. ## Applications Due to its extreme rarity and microscopic size, Fluorapophyllite-(Cs) has no industrial application. Its significance is purely scientific and collectible, being an object of desire for specialized micromount mineral collectors.

Properties

Mohs hardness
4.5-5
Color
Colorless
Luster
Vitreous
Streak
White
Density
2.54
Cleavage
Perfect on {001}
Fracture
Step-Like
Transparency
Transparent
Crystal system
Tetragonal

Diagnostic features

## Identification Identification of Fluorapophyllite-(Cs) is impossible without advanced analytical methods. Preliminary identification based on crystal morphology (tetragonal, tabular) and co-occurrence with other minerals (e.g., quartz, calcite, other apophyllites) is only possible under high magnification. Final confirmation requires chemical analysis, e.g., using an electron microprobe (EDS/WDS), to ascertain cesium dominance. ## Distinguishing from Similar Minerals It can be confused with other minerals from the apophyllite group, such as Fluorapophyllite-(K) or Fluorapophyllite-(Na), as well as with other colorless micromount minerals. Differences are impossible to determine visually and rely solely on chemical composition. ## Crystal Forms Crystals are most often tabular, flattened parallel to the basal plane {001}, or short, prismatic. They reach a maximum size of 1 mm.

Geological environment

## Genesis Fluorapophyllite-(Cs) is a low-temperature hydrothermal mineral. It forms in the late stages of crystallization in gas vesicles and fractures in volcanic rocks, such as basalts or phonolites. In its type locality (Vepice, Czech Republic), it occurs in calcite-quartz veins cutting basic rocks. ## Mineral Associations This mineral co-occurs with calcite, quartz, other minerals from the apophyllite group (Fluorapophyllite-(K), Fluorapophyllite-(Na)), as well as analcime, natrolite, and pyrite. ## Localities The only confirmed and described locality for Fluorapophyllite-(Cs) is the Vepice quarry, near the town of Ktiš, in the South Bohemian Region of the Czech Republic. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of Fluorapophyllite-(Cs) specimens is assessed based on the size and perfection of crystal development, as well as their abundance on the rock matrix. Specimens with clearly visible, sharp, transparent crystals, well-contrasted with the substrate, are most highly valued. Due to the microscopic nature of the mineral, the aesthetics of the entire "micromount" specimen are crucial. ## Popular Localities The only source of specimens is the Vepice quarry in the Czech Republic. Material from this locality is extremely rare and sought after by collectors specializing in microscopic and systematic minerals.

Care and storage

## Cleaning Micromount specimens of Fluorapophyllite-(Cs) generally do not require cleaning. If necessary, only compressed air should be used to remove dust. Contact with water and any chemicals that could damage the delicate crystals or co-occurring minerals should be avoided. ## What to Avoid Ultrasonic cleaners, which can destroy fragile crystals, should be avoided. The mineral is sensitive to acids. It should not be heated or exposed to sudden temperature changes. Store away from moisture. ## Storage Specimens should be stored exclusively in specialized "micromount" boxes that protect them from dust, mechanical damage, and moisture. The box should be properly labeled to avoid confusion with other minerals.

External references

Sources

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