Felsőbányaite
Chemical formula: Al<sub>4</sub>(S<sup>6+</sup>O<sub>4</sub>)(OH)<sub>10</sub>·4H<sub>2</sub>O
Felsőbányaite is a rare, hydrated aluminum sulfate, forming delicate, pearly aggregates of white or slightly colored hue.
Properties
- Mohs hardness
- 1.5
- Luster
- Pearly
- Streak
- White
- Density
- 2.42
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features of felsőbányaite include its exceptional softness, pearly luster, occurrence in spherical or radial aggregates, and reaction to water. It is one of the most delicate minerals. ## Distinguishing from Similar Minerals It can be confused with other white, secondary sulfates or carbonates, such as gypsum, calcite, or hydrozincite. Felsőbányaite differs from them by its significantly lower hardness, characteristic aggregate form, and sensitivity to water, which, for example, calcite does not exhibit. Compared to gypsum, it is much softer and more brittle. ## Crystal Forms It most often forms spherical, hemispherical, and reniform aggregates with a radial or concentric structure. Fibrous, rosette-like masses or small, hexagonal tabular crystals are less common.
Geological environment
## Genesis Felsőbányaite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits, especially in environments rich in aluminum and sulfates. It forms as a result of the action of acidic mine waters on clay rocks, shales, or other aluminosilicate minerals. It is a typical mineral of mine fire zones and waste dumps. ## Mineral Associations It often occurs in association with other secondary minerals, such as gypsum, alunite, epsomite, melanterite, as well as with ore minerals (pyrite, sphalerite, galena) and quartz. ## Localities The most important and classic locality is Baia Sprie (formerly Felsőbánya) in Romania. Other known localities include the Baccu Locci mine in Sardinia (Italy); Kladno and Rakovník in the Czech Republic; Baia Mare in Romania; the Marcel mine in Radlin (Poland); and also some localities in Germany (dumps in the Zwickau area) and France.
Rarity
Rare
For collectors
## Quality Criteria Specimens most valued by collectors are those with well-formed, spherical aggregates with a distinct, radial internal structure and a clean, white color. Contrast with the rock matrix and lack of damage are important, which is difficult to achieve due to the mineral's fragility. Specimens from the historical locality of Baia Sprie in Romania have particular value. ## Popular Localities Classic specimens, considered exemplary, come from Baia Sprie in Romania. High-quality spherical aggregates have also been found in the Baccu Locci mine in Sardinia, Italy.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Contact with water is absolutely inadvisable, as the mineral is partially soluble in it and may suffer permanent damage or disintegration. ## What to Avoid Avoid all contact with water and other liquids, including cleaning agents. The mineral is very soft and brittle, so it must be protected from impacts, scratches, and vibrations. High temperatures, which can lead to dehydration and destruction of the structure, should also be avoided. ## Storage Felsőbányaite requires storage in dry conditions. It is best kept in a closed, padded collector's box (a so-called "membrane box"), which protects against dust, moisture, and mechanical damage.