Rostite

Chemical formula: AlS<sup>6+</sup>O<sub>4</sub>(OH)·5H<sub>2</sub>O

Rostite is an extremely rare, hydrated aluminum sulfate, forming microscopic, colorless or white crystals and efflorescences.

## Characteristics Rostite is a very rare mineral from the sulfate group. It occurs as microscopic, tabular or bladed crystals, which rarely exceed 0.2 mm in length. Most often, it forms earthy, powdery or chalk-like aggregates, efflorescences, and crusts of white color. Due to the small size of its crystals, its macroscopic features are difficult to observe, and identification requires advanced research methods. ## Physical Properties Rostite crystals exhibit a vitreous to pearly luster on cleavage surfaces. The mineral is very soft, with a Mohs hardness of approximately 2. It is transparent to translucent. The calculated density is 1.91 g/cm³. ## Colors and Varieties Rostite is usually colorless or white. No colored or commercial varieties are distinguished. ## History and Name The mineral was first described in 1979 by Czech mineralogists F. Čech, J. Jarkovský, F. Novák, and J. Ševců. The name honors Rudolf Rost (1894-1976), a Czech mineralogist and professor at Charles University in Prague, for his contributions to mineralogical research, including studies of minerals from Libušín, where rostite was discovered. ## Uses Due to its extreme rarity and microscopic size, rostite has no industrial application and is solely an object of scientific interest and for specialized collectors of rare minerals.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
1.91
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Macroscopic identification of rostite is practically impossible. Identification requires specialized analytical techniques, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). Its occurrence as white, earthy efflorescences in mine fire environments is a certain indication, but not a diagnostic feature. ## Distinguishing from Similar Minerals Rostite can be confused with many other secondary sulfates, such as gypsum, chalcanthite (in dehydrated form), melanterite, and especially with other hydrated aluminum sulfates like alunogen or jurbanite. Differentiation from these minerals is possible only based on structural and chemical analysis. ## Crystal Forms It forms very small, flattened, tabular crystals with a hexagonal or rhombic outline. It occurs mainly as loose, earthy or powdery aggregates, and brittle crusts and efflorescences.

Geological environment

## Genesis Rostite is a secondary mineral, forming under specific conditions. Its typical formation environment includes burning mine dumps and coal deposits, where it forms as a result of reactions of volcanic gases (fumaroles) with clay minerals in barren rocks. It forms in low-temperature zones (below 100°C). ## Mineral Associations It co-occurs with other minerals typical of mine fire environments, such as native sulfur, sal ammoniac, alunogen, gypsum, tschermigite, mascagnite, and boussingaultite. ## Localities The most important localities worldwide, from which the best-studied specimens originate, are: - Libušín and Kladno in the Czech Republic (type locality). - Burning dumps near Pecs in Hungary. - Coal seam fires in Burning Groutite, near Durango, Colorado, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a rostite specimen is primarily assessed based on the confirmation of its identity using analytical methods. The collector's value is enhanced by the richness of the efflorescence on the rock matrix and the presence of well-formed, though still microscopic, crystals visible under high magnification. Association with other rare minerals from the same paragenesis is also important. ## Popular Localities The most prized specimens by specialized collectors are those from the type locality in the Czech Republic (Libušín, Kladno) and from the Hungarian Pecs basin, from which many well-documented samples originate.

Care and storage

## Cleaning Rostite specimens should not be wet cleaned. The mineral is water-soluble. Any contaminants should be removed mechanically, with the utmost care, using, for example, a soft brush or compressed air from a safe distance. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to moisture and can dehydrate or recrystallize in dry air. It should be protected from temperature and humidity changes. ## Storage Rostite specimens must be stored in sealed, dry containers, preferably with a desiccant (e.g., silica gel). They should be kept under stable room conditions, away from heat sources and direct sunlight.

Sources

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