Aluminite
Chemical formula: Al<sub>2</sub>S<sup>6+</sup>O<sub>4</sub>(OH)<sub>4</sub>·7H<sub>2</sub>O
Aluminite is a hydrated aluminum sulfate, most often forming white, earthy aggregates and concretions in clay and coal deposits.
Properties
- Mohs hardness
- 1-2
- Luster
- Dull
- Streak
- White
- Density
- 1.66-1.82
- Cleavage
- Good on {010}
- Fracture
- Earthy
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Key diagnostic features of aluminite are its extreme softness (hardness 1-2), low density, white color, and earthy appearance. Its characteristic adherence to the tongue, resulting from its porous structure, is also distinctive. It dissolves and effervesces in hydrochloric acid, which is an important distinguishing feature. ## Distinguishing from Similar Minerals Aluminite can be confused with other white, earthy minerals, such as kaolinite, halloysite, or gibbsite. Kaolinite and halloysite are clay minerals that do not react with hydrochloric acid. Gibbsite, although also a weathering product rich in aluminum, is significantly harder than aluminite (2.5-3.5 on the Mohs scale). ## Crystal Forms Well-formed crystals are extremely rare and appear as tiny, acicular forms visible only under high magnification. The vast majority of specimens occur as compact, earthy aggregates, as well as spherical or reniform concretions and crusts.
Geological environment
## Genesis Aluminite is a secondary mineral. It forms as a result of sulfuric acid-rich waters acting on aluminum-rich rocks, mainly clays and clay shales. Sulfuric acid most often originates from the weathering of iron sulfides, such as pyrite and marcasite. It is a typical mineral for oxidation zones in lignite deposits and clay sediments. ## Mineral Associations This mineral often occurs in association with gypsum, native sulfur, pyrite, marcasite, gibbsite, epsomite, and various clay minerals, such as halloysite or kaolinite. ## Localities Classic and most important localities for aluminite include Halle in Saxony-Anhalt (Germany), which is its type locality. Significant finds also come from Newhaven in Sussex (England), from the vicinity of Paris (France), from Hungary (e.g., Eger), and from the United States (including Utah and Colorado).
Rarity
Not very common
For collectors
## Quality Criteria The collector's value of aluminite is moderate. Most prized are specimens of pure, white color, forming well-developed, spherical or reniform concretions with an aesthetic form. Specimens from classic, historical localities, such as Halle or Newhaven, are more sought after. The presence of associated minerals forming interesting assemblages also increases value. ## Popular Localities The most known and valued specimens by collectors come from historical localities in Germany (Halle) and England (Newhaven). Good quality concretions are also found in some localities in France and Hungary.
Care and storage
## Cleaning Aluminite is extremely soft and porous, requiring very careful cleaning. The safest method is to use a soft brush to remove loose dust. Wet cleaning should be avoided, as water can be absorbed and weaken the specimen's structure. ## What to Avoid This mineral is sensitive to water and can partially disintegrate upon contact with it. It should be protected from acids and other chemicals. As a hydrated mineral, it is also sensitive to high temperatures, which can cause dehydration and destruction of its structure. Due to its low hardness, it is susceptible to scratches and mechanical damage. ## Storage Aluminite specimens are best stored in a dry place, in a sealed box or display case, to protect them from dust and moisture. It is advisable to line the container with a soft material to prevent abrasions and shocks.