Fluorapophyllite-(NH_4_)

Chemical formula: (N<sup>3-</sup>H<sub>4</sub>)Ca<sub>4</sub>(Si<sub>8</sub>O<sub>20</sub>)F·8H<sub>2</sub>O

Fluorapophyllite-(NH₄) is a rare mineral from the apophyllite group, distinguished by the presence of the ammonium ion in its structure.

## Characteristics Fluorapophyllite-(NH₄) is the ammonium analog of fluorapophyllite-(K) and belongs to the apophyllite group. It forms tabular or prismatic crystals, often terminated by pseudopyramidal faces. It typically occurs as small, colorless or white crystals, growing on other minerals in rock cavities. It is a transparent to translucent mineral, with a vitreous or pearly luster, especially visible on cleavage planes. ## Physical Properties This mineral has a hardness ranging from 4.5-5 on the Mohs scale. Its density is approximately 2.35 g/cm³. It exhibits perfect cleavage in one direction, which is typical for minerals of this group. The luster is vitreous, and on cleavage surfaces, it is pearly. ## Colors and Varieties Fluorapophyllite-(NH₄) is usually colorless or white. No colored varieties or trade names are distinguished, which is due to its extreme rarity and lack of commercial significance. ## History and Name The mineral's name refers to its chemical composition – it indicates the dominance of fluorine over the hydroxyl group and the presence of the ammonium ion (NH₄) as the dominant monovalent cation. It was formally described as a new mineral species in 2019 by Atali A. Agakhanov and co-workers. Type specimens come from the Dike D intrusion, located in the Murun complex on the Olyokma Plateau in Russia. ## Uses Due to its extreme rarity, small crystal size, and recent discovery, fluorapophyllite-(NH₄) has no industrial applications. It is of purely scientific interest and is an object of interest for specialized collectors of rare minerals.

Properties

Mohs hardness
4.5-5
Luster
Vitreous, Pearly
Streak
White
Density
2.35
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Tetragonal

Diagnostic features

## Identification Amateur identification of fluorapophyllite-(NH₄) is practically impossible and requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS/WDS). Preliminary identification is based on the typical apophyllite crystal habit (tabular, pseudocubic), perfect cleavage, and occurrence in a specific geological environment. ## Distinguishing from Similar Minerals It is visually indistinguishable from other minerals of the apophyllite group, such as fluorapophyllite-(K), fluorapophyllite-(Na), or hydroxyapophyllite-(K). The differences lie in the dominant cation (ammonium, potassium, sodium) or anion (fluorine, hydroxyl group), which can only be determined by laboratory analysis. ## Crystal Forms It forms well-developed, small crystals with a tabular or short-prismatic habit. Characteristic forms resemble cubes or octahedra, which are actually a combination of prism and pinacoid or bipyramid faces.

Geological environment

## Genesis Fluorapophyllite-(NH₄) forms under hydrothermal conditions in the late stages of crystallization from fluids rich in silica, calcium, fluorine, and nitrogen. Its formation is associated with the presence of organic matter in sedimentary rocks, which releases ammonia during metamorphic processes. ## Mineral Associations This mineral co-occurs with other minerals formed under similar conditions. In the type locality (Murun complex), it was found in association with quartz, calcite, tinaksite, tokkoite, and yuksporite. ## Localities The only confirmed and scientifically described locality of fluorapophyllite-(NH₄) is the Dike D intrusion in the Murun massif (Murunskii) on the Olyokma Plateau, in Yakutia, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a specimen primarily depends on its analytical confirmation, as it is a mineral impossible to identify visually. The most valuable specimens are those with well-formed, sharp crystals, even if they are small. The aesthetic composition with associated minerals (matrix) is also important. ## Popular Localities The only source of specimens is the type locality in Russia. Material from this location is extremely difficult to obtain and appears on the collector's market very sporadically, mainly in specialized circles.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Distilled water may be used, but prolonged soaking should be avoided. After wet cleaning, the specimen should be immediately and thoroughly dried. ## What to Avoid Contact with acids and other chemicals should be absolutely avoided. The mineral is sensitive to high temperatures, which can lead to its dehydration and structural damage. It should not be heated or exposed to direct, intense sunlight. ## Storage It is recommended to store in stable conditions, away from heat and moisture sources. It is best kept in a closed display box to protect it from dust and mechanical damage. Due to its perfect cleavage, it is prone to delamination upon impact.

Sources

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