Alum-(K)

Chemical formula: KAl(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·12H<sub>2</sub>O

Alum-(K) is a naturally occurring, hydrated potassium and aluminum sulfate, forming regular, octahedral crystals that are soluble in water.

## Characteristics Alum-(K), also known as potassium alum, is a mineral from the alum group. It typically forms very well-developed, isometric crystals in the shape of an octahedron, less commonly a cube. These crystals can be transparent and vitreous, with smooth, lustrous faces. It also occurs as fibrous or granular efflorescences and crusts (so-called 'blooms') on rocks or mine walls. It is a very brittle and light mineral. ## Physical Properties It is characterized by very low hardness, ranging from 2-2.5 on the Mohs scale, meaning it can be scratched with a fingernail. It has a vitreous luster and is transparent to translucent. Its most characteristic feature is its high solubility in water – contact with water causes its rapid destruction. It has an astringent, puckery taste, however, tasting minerals is highly inadvisable. ## Colors and Varieties Most often, it is colorless or white. Impurities can give it yellowish, grayish, or rarely reddish hues, depending on the presence of inclusions of other minerals or ions. There are no separate color or commercial varieties. ## History and Name The name "alum" comes from the Latin word *alumen*, used in antiquity to describe substances with astringent properties. For centuries, this term encompassed various sulfates. The modern name Alum-(K) was systematized by the International Mineralogical Association (IMA), where the suffix -(K) indicates potassium as the dominant cation in this particular variety of alum. These compounds have been used since ancient Egypt and Rome. ## Applications Natural potassium alum today is mainly of collector's and scientific interest. Its synthetic counterpart, however, is widely used in industry as a mordant in dyeing, for water purification, in tanning, in paper production, and also in cosmetics (e.g., as a deodorant or hemostatic agent).

Properties

Mohs hardness
2-2.5
Luster
Vitreous
Streak
White
Density
1.75-1.76
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Isometric

Diagnostic features

## Identification Key diagnostic features of potassium alum are its perfectly formed, octahedral crystals, very low hardness (2-2.5 on the Mohs scale), and high solubility in water. It has a characteristic astringent, sweet-sour taste, however, tasting it is dangerous and discouraged. It often occurs in the form of white efflorescences. ## Distinguishing from Similar Minerals It can be confused with other minerals from the alum group (e.g., soda alum), from which it can only be distinguished by chemical analysis. Halotrichite, also a white, fibrous sulfate, forms acicular aggregates, not regular octahedra. Other soluble sulfates, like epsomite, usually have a different crystal habit. It is distinguished from calcite and quartz by its extremely low hardness and solubility. ## Crystal Forms It crystallizes in the isometric system, forming classic, textbook octahedra. Crystals can be single or twinned. It also occurs in granular, massive aggregates, and as fibrous or powdery coatings and crusts on rocks.

Geological environment

## Genesis Alum-(K) is a secondary mineral. It forms mainly as a result of the weathering (oxidation) of sulfides (primarily pyrite and marcasite) in the presence of aluminum-rich rocks, such as clay shales or marls. It is also a typical product of volcanic activity, crystallizing from hot gases (fumaroles) near craters. It often forms as an efflorescence on the walls of old mines and on mine dumps. ## Mineral Associations It often co-occurs with other sulfates, such as alunogen, halotrichite, melanterite, epsomite, gypsum, as well as native sulfur (in volcanic environments) and clay minerals. ## Localities Significant localities with well-formed crystals are mainly volcanic areas. Historically known from Vesuvius and the Phlegraean Fields in Italy. It also occurs in Greece (Milos island), Germany (Bavaria), Hungary, and the USA (e.g., Alum Cave in Tennessee, where it was formerly mined, and near geysers in Yellowstone National Park).

Rarity

Not very common

For collectors

## Quality Criteria The most desired by collectors are large, fully transparent, single crystals in the form of a perfect octahedron, free from damage. Specimens on matrix are also highly valued, which is rare due to the mineral's formation process. Purity (lack of inclusions and dullness) and vitreous luster significantly increase the value of a specimen. ## Popular Localities Specimens from classic, volcanic localities in Italy (Vesuvius) are historically important. Good quality crystals have also been found in China (Hunan province) and in some mines in Germany. Due to its brittleness and solubility, well-preserved specimens from any locality are prized.

Care and storage

## Cleaning The mineral is extremely sensitive to water and moisture. **It must absolutely not be cleaned wet**, as it will dissolve. Dust can only be removed dry, using a soft brush or a very gentle stream of compressed air. ## What to Avoid Avoid all contact with water, water vapor, and high air humidity. Storage in a damp room (e.g., a basement) will lead to the destruction of the specimen. It should also be protected from sudden temperature changes. ## Storage Specimens of potassium alum must be stored in a dry environment. The best solution is a tightly sealed container (e.g., a "membrane box" type) or a display case with a moisture absorber (e.g., silica gel) placed inside. It should be isolated from other minerals that may require different conditions.

External references

Sources

Read more