Fluormayenite
Chemical formula: Ca<sub>12</sub>Al<sub>14</sub>O<sub>32</sub>[☐<sub>4</sub>F<sub>2</sub>]
Calcium aluminate from the mayenite supergroup, forming colorless, microscopic crystals in high-temperature metamorphic rocks.
Description
## Characteristics Fluormayenite is a rare mineral from the mayenite supergroup, being a hydrated calcium aluminate with fluorine. It typically occurs as very small, well-formed crystals, rarely exceeding fractions of a millimeter in size. It most often forms crystals with a rhombic dodecahedral habit {110}, sometimes in combination with tetrahedral faces. Specimens are usually colorless or white and transparent, with a vitreous luster. Due to its size, a microscope is essential for its observation. ## Physical Properties This mineral is characterized by a hardness of approximately 5.5 on the Mohs scale. It has a vitreous luster and is transparent to translucent. Its calculated density is 2.73 g/cm³. It does not exhibit cleavage, and its fracture is uneven. ## Colors and Varieties Fluormayenite is usually colorless or white. No colored varieties or trade names are known. ## History and Name The mineral's name, approved by IMA in 2013, refers to its chemical composition – the dominance of fluorine (Fluor) and its structural relationship with mayenite, another mineral from this group. It was first described by E. Galuskina and co-workers. The type locality is the Caspar quarry on the Bellerberg volcano in the Eifel region, Germany. ## Applications Fluormayenite has no industrial applications. It is solely an object of scientific interest and is valued by collectors specializing in rare minerals and micromounts.
Diagnostic features
## Identification Identification of fluormayenite based on visual characteristics is practically impossible due to its microscopic size and lack of distinctive colors. Definitive identification requires advanced analytical techniques, such as X-ray microanalysis (EDS/WDS) to confirm its chemical composition (presence of Ca, Al, O, F) and X-ray diffraction (XRD) to verify its crystal structure. ## Distinguishing from Similar Minerals Fluormayenite can be confused with many other colorless minerals found in the same environment, such as quartz, nosean, haüyne, and especially with other minerals from the mayenite group (e.g., chlormayenite). Differentiation from chlormayenite, which contains chlorine instead of fluorine, is only possible through chemical analysis. ## Crystal Forms It most often forms isometric, well-formed crystals, mainly in the form of rhombic dodecahedra {110}, less commonly tetrahedra. It occurs as single, dispersed grains in the host rock or in small aggregates.
Geological environment
## Genesis Fluormayenite is a mineral typical of high-temperature, low-pressure contact metamorphic environments. It forms in xenoliths of limestones and calc-silicate rocks that have been thermally altered by contact with hot magma, most often phonolitic or basaltic. ## Mineral Associations This mineral co-occurs with other rare minerals characteristic of rocks altered at high temperatures. Its most common associations include larnite, jennite, ettringite, afwillite, gehlenite, brownmillerite, as well as calcite and diopside. ## Localities The most important and well-known fluormayenite localities in the world are in the Eifel volcanic complex in Germany, particularly in the Caspar quarry on Bellerberg mountain near Ettringen and in Emmelberg. It has also been reported in the Hatrurim Formation in Israel and Jordan, where similar formation conditions prevailed.
Rarity
Rare
Collector aspects
## Quality Criteria For collectors, specimens with sharp, well-formed, and transparent microcrystals are of the greatest value. An ideal specimen should have clearly visible, isolated dodecahedral crystals, set on a contrasting matrix. Rich associations with other rare minerals from the locality are also highly prized. ## Popular Localities The vast majority of the best collector specimens come from the classic locality in the Eifel region of Germany. Material from the Caspar quarry is considered reference for this species.
Care and storage
## Cleaning Fluormayenite specimens, usually microcrystals on a matrix, should be cleaned very carefully. The safest method is to use a soft brush to remove dust. If necessary, distilled water can be used, but a strong stream should be avoided. Ultrasonic cleaners are strongly discouraged, as they can damage delicate crystals or the entire host rock. ## What to Avoid This mineral is susceptible to acids, which can damage or completely dissolve it. Contact with all household and laboratory chemicals should be avoided. It is stable under normal conditions, but should be protected from extreme temperatures. ## Storage The best way to store fluormayenite specimens is to place them in specialized micromineral boxes (micromounts). This protects fragile crystals from mechanical damage, dust, and potential loss. It does not require special climatic conditions.