Strätlingite

Chemical formula: Ca<sub>2</sub>Al(Si,Al)<sub>2</sub>O<sub>2</sub>(OH)<sub>10</sub>·2.25H<sub>2</sub>O

Strätlingite is a rare calcium aluminum silicate, forming colorless or white, hexagonal crystals, found in altered limestone rocks.

## Characteristics Strätlingite is a hydrated calcium aluminum silicate, belonging to the group of rare minerals. It typically occurs as very small, thin, tabular crystals with a hexagonal outline. It also forms rosette-like or spherulitic aggregates, and sometimes coatings and efflorescences. Individual crystals are most often colorless to white, with a pearly or vitreous luster on basal surfaces. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It is light, with a density close to 2.14 g/cm³. Crystals exhibit perfect cleavage in one direction, which is characteristic of its layered structure. It is transparent to translucent. ## Colors and Varieties Strätlingite is usually colorless or white. No colored varieties or commercial names are distinguished for it, which is typical for minerals of primarily scientific and collector interest. ## History and Name The mineral's name comes from its type locality in the Strätling quarry, near Marburg in Hesse (Germany). It was described and recognized as a new mineral species in 1975. ## Uses Strätlingite has no industrial applications. Its significance is limited to scientific research in mineralogy and petrology, and to advanced collections specializing in rare minerals or minerals from specific localities.

Properties

Mohs hardness
2
Luster
Pearly, Vitreous
Streak
White
Density
2.14
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Strätlingite is difficult to identify without specialized equipment. It is distinguished by its occurrence as very small, hexagonal, tabular crystals, often forming rosette-like aggregates. Its low hardness and pearly luster are also characteristic. It typically occurs in thermally altered carbonate rocks. ## Distinguishing from Similar Minerals It can be confused with other secondary minerals of similar appearance, such as ettringite, thaumasite, or some zeolites. Differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS), due to the small crystal sizes and similarity of forms. ## Crystal Forms Crystals are thin, tabular, with a hexagonal outline. They often form fan-shaped, rosette-like, or spherulitic (spherical) aggregates. They can also occur as fine-grained masses or coatings on other minerals.

Geological environment

## Genesis Strätlingite is a secondary mineral, formed under low-temperature alteration (contact metamorphism) conditions. It forms in xenoliths of limestone or marl rocks that have been incorporated into magmatic intrusions, such as basalts. It results from the reaction of rock minerals with hot solutions. ## Mineral Associations It often co-occurs with other minerals formed under similar conditions, such as ettringite, afwillite, portlandite, calcite, hydrocalumite, mayenite, and minerals from the tobermorite group. ## Localities The most important strätlingite localities worldwide include the type locality in Strätling (Hesse, Germany), as well as other places in Germany (e.g., Eifel). It is also known from Boișoara in Romania, Klöch in Austria, Fort-Portal in Uganda, Hatrurim in Israel, and several locations in Japan and Russia.

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of strätlingite primarily depends on the quality and development of the crystals. Specimens with well-formed, sharp crystals creating aesthetic, rosette-like aggregates are most valued. Association with other rare minerals is also important, as it enhances the scientific and visual value of the specimen. Due to their small size, specimens are typically "micromount" type. ## Popular Localities Specimens from the type locality in Germany (Strätling) have particular historical value. Well-crystallized samples from the Hatrurim Complex in Israel and from Boișoara in Romania are also highly prized.

Care and storage

## Cleaning Strätlingite specimens are extremely delicate and fragile. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove loose dust. Avoid water and any chemical agents. ## What to Avoid Absolutely avoid contact with water, acids, detergents, and other chemicals. The mineral is very soft, so it must be protected from scratching and impacts. It should not be subjected to ultrasonic cleaning. It is sensitive to changes in temperature and humidity. ## Storage Strätlingite specimens should be stored in stable conditions, in a closed, padded collector's box, to protect them from mechanical damage, dust, and humidity changes. Due to the small size of the crystals, "micromount" boxes are ideal.

Sources

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