Amstallite

Chemical formula: CaAl(Si,Al)<sub>4</sub>O<sub>8</sub>(OH)<sub>4</sub>·(H<sub>2</sub>O,Cl)

Amstallite is a very rare, hydrated calcium aluminosilicate, forming colorless to white, tabular crystals with a pearly luster.

## Characteristics Amstallite is a mineral from the silicate group, chemically classified as a hydrated calcium aluminosilicate. It occurs as very small, tabular or bladed crystals, rarely exceeding 1 mm in length. It typically forms radial or randomly oriented aggregates. It is colorless to white, with a characteristic pearly luster on cleavage surfaces. ## Physical Properties This mineral is relatively soft, with a hardness of approximately 2 on the Mohs scale. Its crystals are transparent to translucent. It exhibits perfect cleavage in one direction, which is the cause of its pearly luster. The calculated density is 2.33 g/cm³. ## Colors and Varieties Amstallite is typically colorless or white. No colored or commercial varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality near Amstall in the municipality of Mühldorf in Lower Austria. It was first described in 1987 by G. Niedermayr, W. Postl, A. Beran, and K. Schmets after being found in 1984. It is the only confirmed occurrence of this mineral in the world. ## Applications Due to its extreme rarity and microscopic crystal size, amstallite has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized collectors of rare minerals.

Properties

Mohs hardness
2
Luster
Pearly
Streak
White
Density
2.33
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Amateur identification of amstallite is practically impossible. It requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). Visually, it can be suspected based on its occurrence in a characteristic geological environment at its only known locality. A characteristic feature is small, tabular crystals with a pearly luster. ## Distinguishing from Similar Minerals It can be confused with other white or colorless secondary minerals with a pearly luster, such as gypsum, talc, or some micas. However, definitive differentiation requires specialized laboratory equipment. ## Crystal Forms Amstallite forms very small, thin, tabular crystals with a hexagonal outline, elongated in one direction. These crystals often combine into radial or irregular aggregates.

Geological environment

## Genesis Amstallite is a secondary mineral, formed as a result of hydrothermal processes occurring in fracture zones and tectonic faults. It occurs within biotite-plagioclase gneisses that have undergone significant deformation and brecciation. It formed under low-temperature conditions, likely as a result of the alteration of primary minerals. ## Mineral Associations This mineral co-occurs with minerals such as calcite, laumontite, stilbite, scolecite, heulandite, quartz, adularia, albite, titanite, apatite, epidote, clinozoisite, chlorite, and hematite. ## Localities The only confirmed occurrence of amstallite in the world is its type locality - a quarry near the village of Amstall in the Dunkelsteinerwald Forest, Mühldorf municipality, Lower Austria.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of an amstallite specimen is primarily assessed based on the abundance and development of microscopic crystals on the rock matrix. The most desirable specimens are those with clearly visible, well-formed crystalline aggregates contrasting with the substrate. Due to the microscopic nature of the mineral, its "purity" and "color" are not as critical criteria as they are for macroscopic minerals. ## Popular Localities The only source of specimens is the type locality in Lower Austria. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Amstallite specimens are extremely delicate and typically microscopic in size. Mechanical or chemical cleaning is not recommended. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and any cleaning tools. The crystals are very soft (hardness 2) and brittle, susceptible to scratches and mechanical damage. They should be protected from changes in temperature and humidity. ## Storage Specimens should be stored exclusively in specialized, sealed "micromount" boxes, protecting them from dust, shocks, and damage. Display is not recommended due to the mineral's sensitivity and small size.

Sources

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