Poellmannite

Chemical formula: Ca₆Al₃(OH)₁₈[Na(H₂O)₆](S₆+O₄)₂·₆H₂O

Poellmannite is an extremely rare mineral of the ettringite group, forming colorless, acicular crystals in volcanic xenoliths.

## Characteristics Poellmannite is a complex, hydrated sulfate belonging to the ettringite supergroup. It exclusively forms microscopic, colorless, and transparent crystals with an acicular habit. They typically occur as radial or tangled aggregates, growing on the surface of the host rock. Due to the small size of the crystals, reaching a maximum length of 0.2 mm, a microscope is essential for observation. ## Physical Properties Poellmannite crystals are characterized by a vitreous luster and are fully transparent. Their Mohs hardness has not yet been determined. The density calculated based on the chemical formula and unit cell parameters is 1.93 g/cm³, but this has not been experimentally confirmed. ## Colors and Varieties This mineral is colorless. No colored varieties or trade names are known. ## History and Name The mineral's name, approved by the International Mineralogical Association (IMA) in 2017, honors Professor Johann Pöllmann (born 1957), a German mineralogist and chemist from Martin Luther University Halle-Wittenberg. He was recognized for his contributions to the study of the mineralogy and chemistry of cement phases, including minerals of the ettringite group. ## Applications Poellmannite, as an extremely rare mineral occurring in the form of microscopic crystals, has no industrial application. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals (so-called microminerals).

Properties

Luster: Vitreous
Streak: White
Density: 1.93
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Amateur identification of poellmannite is practically impossible due to the microscopic size of the crystals. Certain identification requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical microanalysis (EDS/WDS). For a specialist, the acicular habit of the crystals, vitreous luster, and co-occurrence with a characteristic assemblage of minerals in a specific geological environment can be a clue. ## Distinguishing from Similar Minerals Poellmannite can be confused with other acicular minerals of the ettringite supergroup, such as ettringite or thaumasite, which can form similar aggregates. Distinguishing them based on visual features is impossible. The key diagnostic feature is the unique chemical composition of poellmannite, particularly the presence of sodium and the absence of carbonates or silicates, which can only be determined by laboratory analysis. ## Crystal Forms Poellmannite crystallizes in the form of very fine, elongated acicular crystals. These crystals combine into characteristic aggregates with a radial structure or form tangled, felt-like clusters.

Geological environment

## Genesis Poellmannite is a high-temperature mineral, forming under specific conditions of contact metamorphism. It forms within calcareous xenoliths (fragments of foreign rocks) entrained by leucite-tephrite lava. These xenoliths underwent intense alteration under the influence of extreme magma temperatures. ## Mineral Associations This mineral occurs in association with other rare minerals typical of this environment, such as mayenite, wollastonite, gehlenite, larnite, and brownmillerite. ## Localities The only confirmed occurrence of poellmannite in the world is its type locality - the Caspar quarry near the Bellerberg volcano, near the town of Ettringen in the Eifel mountains (Rhineland-Palatinate, Germany).

Rarity

Extremely rare

For collectors

## Quality Criteria The collector's value of poellmannite specimens is assessed based on criteria typical for microminerals. The most important factor is the abundance and quality of crystals on the rock matrix – the denser and better-formed (sharp, undamaged) the acicular aggregates, the more valuable the specimen. Specimens where poellmannite co-occurs with other rare minerals from this locality, forming aesthetic and scientifically rich associations, are also highly prized. ## Popular Localities All known poellmannite specimens available on the collector's market come from a single location in the world – the Caspar quarry in the German Eifel region. This is the classic and only locality for this mineral.

Care and storage

## Cleaning Poellmannite specimens are extremely delicate and susceptible to damage. No mechanical (brushes, needles) or chemical cleaning is recommended. Attempting to remove dust or impurities will almost certainly destroy the microscopic crystals. It is safest to leave the specimen in its original state. ## What to Avoid As a hydrated mineral, poellmannite is sensitive to high temperatures, which can lead to its dehydration and structural damage. Contact with all chemicals, especially acids, as well as water and other liquids, should be avoided. It should be protected from direct sunlight and sudden temperature changes. ## Storage It is recommended to store specimens in sealed, closed "micromount" boxes, which protect them from dust, moisture, and mechanical damage. The box should be stored in a stable environment, away from sources of vibration and heat.

Sources

IMA Mineral List (CNMNC)