Apjohnite
Chemical formula: Mn<sup>2+</sup>Al<sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>4</sub>·22H<sub>2</sub>O
Hydrous manganese aluminum sulfate, forming delicate, silky white efflorescences and coatings.
Properties
- Mohs hardness
- 1.5-2
- Luster
- Silky
- Streak
- White
- Density
- 1.8
- Cleavage
- Perfect on {010}
- Fracture
- Fibrous
- Transparency
- Transparent to Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification The key diagnostic features of apjohnite are its mode of occurrence – delicate, silky, fibrous aggregates – and its extreme softness. Its origin (efflorescences on sulfide rocks) is also characteristic. Water solubility is a confirming feature, but this test destroys the sample. ## Distinguishing from Similar Minerals Apjohnite is visually indistinguishable from other minerals in the halotrichite group, such as halotrichite (with magnesium instead of manganese) or pickeringite. Certain differentiation requires advanced chemical analyses (e.g., EDS). It is distinguished from the fibrous variety of gypsum (satin spar) by its much greater fragility and water solubility. ## Crystal Forms Apjohnite crystals are monoclinic, with an acicular or hair-like habit. They almost always occur as fibrous, radial, tangled (felted) aggregates, and as crusts and efflorescences.
Geological environment
## Genesis Apjohnite is a typical secondary mineral. It forms in oxidation (weathering) zones of ore deposits, especially pyrite, in clay rocks, shales, and coal seams. It forms as an evaporation product of sulfate-rich waters on the walls of old mine workings, in caves, and on rock surfaces. ## Mineral Associations It most often co-occurs with other sulfates, such as melanterite, halotrichite, pickeringite, epsomite, as well as gypsum and alunogen. ## Localities Known occurrences of apjohnite include Algoa Bay in South Africa (type locality), Iquique in Chile, the Harz Mountains in Germany, as well as numerous localities in the USA (e.g., in Utah) and Hungary. In Poland, its presence has been confirmed in the "Szczęście Luizy" mine in Wałbrzych.
Rarity
Not very common
For collectors
## Quality Criteria The most valued by collectors are specimens with a large surface area, covered with a dense layer of pure, white, and silky fibers. Contrast with the dark host rock enhances attractiveness. Due to the mineral's delicacy, undamaged specimens, well-protected during transport, fetch higher prices. ## Popular Localities Localities in Chile (e.g., Alcaparrosa mine) and historical locations in Germany are considered classic and provide the best specimens.
Care and storage
## Cleaning Apjohnite is water-soluble, so it must not be cleaned wet under any circumstances. To remove dust, only a very soft brush or a gentle stream of compressed air from a distance should be used. ## What to Avoid Contact with water and other liquids must be strictly avoided. The mineral is sensitive to changes in humidity – in a humid environment, it can absorb water and dissolve, and in too dry an environment, it can lose its crystallization water and crumble. It should be protected from direct sunlight and high temperatures. ## Storage Apjohnite specimens require storage under stable conditions. The best solution is a tightly sealed container (e.g., a "membrane box" type or a display case with a lid) with a desiccant (e.g., silica gel) placed inside. Due to its extreme fragility, touching and moving the specimen should be avoided.