Alum-(Na)

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

Alum-(Na) is a water-soluble sodium and aluminum sulfate, forming colorless or white octahedral crystals.

## Characteristics Alum-(Na), also known as sodium alum, is a hydrated sodium and aluminum sulfate belonging to the alum group. It forms characteristic, well-developed crystals in the shape of regular octahedra, which can reach significant sizes. It also occurs as granular, fibrous aggregates, as well as crusts and efflorescences. It is a brittle mineral, and its specimens are usually colorless or white. ## Physical Properties This mineral is characterized by low hardness, 2 on the Mohs scale, meaning it can be scratched with a fingernail. It has a vitreous luster and is transparent to translucent. Its density is low, approximately 1.67 g/cm³. A key property of sodium alum is its complete solubility in water. It has a sweetish-astringent taste, however, tasting minerals is strongly discouraged. ## Colors and Varieties Alum-(Na) is usually colorless or white. Impurities can impart yellowish or grayish hues, but it does not form colored varieties of commercial significance. ## History and Name The name "alum" comes from the Latin word *alumen*, used in antiquity to describe substances with an astringent taste, employed in medicine and dyeing. The prefix "-(Na)" was added as part of the International Mineralogical Association (IMA) systematics to emphasize the dominance of sodium in its chemical composition and to distinguish it from other minerals in the alum group, e.g., potassium alum (Alum-(K)). ## Uses Due to its solubility and brittleness, Alum-(Na) has limited significance as an ornamental stone. However, it is valued by mineral collectors, especially specimens with well-formed crystals. Synthetic alums, including sodium alum, are used in industry, among others, for water purification, in tanning, as a mordant in dyeing, and in paper production.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
1.675
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Cubic

Diagnostic features

## Identification The most important diagnostic features are: crystal form (octahedra), low hardness (2 on the Mohs scale), vitreous luster, and complete solubility in water. The reaction to moisture is immediate and destructive to the specimen. ## Distinguishing from Similar Minerals Alum-(Na) can be confused with other minerals from the alum group, such as Alum-(K). Definitive distinction requires chemical analysis. From visually similar fluorite, it is distinguished by significantly lower hardness and solubility in water. Halite, although also crystallizing in the isometric system and being soft, has perfect cubic cleavage, which sodium alum does not possess. ## Crystal Forms It crystallizes in the isometric system, most often forming perfectly developed, single crystals in the form of octahedra {111}. Less commonly, forms that are a combination of octahedron and cube are found. It also occurs as fibrous, granular aggregates, and as efflorescences and crusts on rocks.

Geological environment

## Genesis Alum-(Na) is a secondary mineral. It forms as a result of weathering of clay rocks or shales rich in aluminum and sulfide minerals (mainly pyrite). It forms in dry and desert conditions as a product of evaporation of sulfate solutions. It is also a typical product of volcanic activity, crystallizing from hot gases (sublimates) in fumaroles. ## Mineral Associations It co-occurs with other soluble sulfates, such as alunogen, melanterite, epsomite, as well as with gypsum, native sulfur (in volcanic environments), pyrite, and marcasite. ## Localities Significant occurrences of this mineral are found worldwide, mainly in places with suitable conditions. The most important include: the vicinity of Vesuvius and the Phlegraean Fields in Italy (as a product of volcanic exhalations), Vulcano island (Aeolian Islands), Alum mine in Nevada (USA), as well as numerous localities in the Atacama Desert in Chile.

Rarity

Not very common

For collectors

## Quality Criteria The most sought after by collectors are large, fully transparent, single crystals with sharp edges and smooth faces, exhibiting an ideal octahedral form. Specimens on a rock matrix are rarer and more highly valued. Co-occurrence with other well-formed minerals, e.g., sulfur, also enhances attractiveness. ## Popular Localities Localities associated with volcanic activity in Italy, such as Vesuvius, the vicinity of Naples, and Vulcano island, are considered classic, providing the best collector specimens. Historical specimens of excellent crystalline form originate from these places.

Care and storage

## Cleaning **ABSOLUTELY DO NOT USE WATER.** The mineral is completely soluble in water. Cleaning should only be done dry, using a very soft brush to remove dust or compressed air from a safe distance. ## What to Avoid Avoid contact with water, water vapor, and any moisture. High humidity can cause the crystal surface to become dull or begin to dissolve. Protect it from high temperatures, which can lead to dehydration and destruction of the crystal structure. ## Storage Alum-(Na) specimens must be stored in a dry place. It is recommended to keep them in tightly sealed containers or display cases with a desiccant (e.g., silica gel). It is not suitable for display in open showcases in rooms with fluctuating humidity.

External references

Sources

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