Aluminocoquimbite
Chemical formula: Al<sub>2</sub>Fe<sup>3+</sup><sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>6</sub>(H<sub>2</sub>O)<sub>12</sub>·9H<sub>2</sub>O
Aluminocoquimbite is a rare, hydrated aluminum and iron sulfate, forming small, purple or pink hexagonal crystals.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.06-2.11
- Cleavage
- Indistinct on {0001}
- Fracture
- Conchoidal to uneven
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Features facilitating identification include: characteristic pale pink or purple color, hexagonal crystal outline, low hardness, and vitreous luster. A key, though destructive, diagnostic feature is its solubility in water. Its occurrence in paragenesis with other secondary sulfates in dry environments is also an important clue. ## Distinguishing from Similar Minerals Aluminocoquimbite is most often confused with **coquimbite**. Coquimbite usually has a more intense, dark purple color, while aluminocoquimbite is lighter. However, a definitive distinction between these two minerals requires chemical analysis (e.g., EDS) to determine which element – aluminum or iron – dominates the structure. From fibrous sulfates, such as **halotrichite**, it is distinguished by its crystal habit (prismatic/platy, not acicular). ## Crystal Forms It forms short, prismatic or platy crystals with a hexagonal base. It often occurs as radial or granular aggregates, as well as coatings and efflorescences on other minerals.
Geological environment
## Genesis Aluminocoquimbite is a secondary mineral, formed in the oxidation zones of sulfide ore deposits (mainly pyrite) in aluminum-rich rocks. It forms in dry climates where evaporation exceeds precipitation. It is also found as a product of volcanic exhalations (fumaroles) and in burning coal mine dumps. ## Mineral Associations It co-occurs with a whole range of other secondary sulfates, such as coquimbite, rhomboclase, voltaite, copiapite, alunogen, halotrichite, and szomolnokite. Pyrite and melanterite are also often associated with it. ## Localities This is a very rare mineral. The most important localities include: Byellorusskoye deposit in Kazakhstan (type locality); fumaroles in the Ravat River valley in Tajikistan; mine dumps in Kladno and Libušín in the Czech Republic; Pecs in Hungary; and Vesuvius in Italy.
Rarity
Very rare
For collectors
## Quality Criteria The quality of aluminocoquimbite specimens is primarily assessed based on crystal development and size. Well-formed, sharply terminated crystals with a distinct, hexagonal shape are most valued, even if they are small (millimeter-sized). The intensity and purity of the color are also important – specimens with a distinct pink or purple hue are preferred. An attractive matrix and association with other rare sulfates increase its value. ## Popular Localities Specimens from the fumaroles in Ravat, Tajikistan, are considered the best for collectors, yielding sharp, well-formed microcrystals. Czech localities (e.g., Kladno) also provide good quality specimens, often in rich parageneses.
Care and storage
## Cleaning The mineral is soluble in water, so its cleaning must be done exclusively dry. A soft, dry brush or a gentle stream of compressed air can be used to remove dust. Any contact with liquids should be avoided. ## What to Avoid Contact with water and other liquids, which can damage or completely dissolve it, must be strictly avoided. The mineral is sensitive to high air humidity, which can lead to its gradual decomposition (deliquescence). It should also be protected from high temperatures, which cause the loss of crystallization water and destruction of its structure. ## Storage Aluminocoquimbite specimens require storage in stable, dry conditions. The best solution is to place them in a sealed, airtight container (e.g., a "membrane box" type or a box with a seal) along with a moisture-absorbing agent, such as silica gel.