Kruijenite
Chemical formula: Ca₄Al₄(S⁶⁺O₄)F₂(OH)₁₆·2H₂O
Kruijenite is a very rare calcium and aluminum sulfate, forming microscopic, colorless, tabular crystals.
Properties
- Mohs hardness
- 3
- Color
- Pale greenish-yellow to colourless
- Luster
- Vitreous
- Streak
- White
- Density
- 2.573
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Transparent
- Crystal system
- Tetragonal
Diagnostic features
## Identification Identification of kruijenite is impossible without advanced laboratory techniques. It requires chemical analysis (EDS) to confirm its composition (presence of Ca, Al, S, F) and X-ray diffraction (XRD) to determine its crystal structure. Visually, under high magnification, characteristic, hexagonal, tabular crystals forming rosette-like clusters can be observed. ## Distinguishing from similar minerals Kruijenite co-occurs with other rare sulfates from the ettringite group, such as ettringite or sturmanite. Distinguishing it from these minerals, as well as from other microscopic sulfates and secondary minerals, is only possible based on chemical composition analysis and crystallographic data. ## Crystal forms Kruijenite forms thin, tabular crystals with a hexagonal outline. These crystals typically combine into rosette-like or spherical aggregates up to 40 micrometers in size.
Geological environment
## Genesis Kruijenite is a secondary mineral, formed under low-temperature hydrothermal processes. In its type locality (Caspar quarry), it crystallized in cavities within xenoliths of metamorphic rocks (gneisses) that were incorporated into basaltic lava. Its formation is associated with the interaction of sulfate- and fluorine-rich solutions with minerals of the surrounding rocks. ## Mineral associations This mineral co-occurs with other rare secondary minerals. In its type locality, it was found in association with ettringite, thaumasite, aragonite, calcite, tobermorite, apophyllite-(KF), and hydroxyellestadite. ## Localities The only confirmed occurrence of kruijenite in the world is its type locality: the Caspar quarry in the Bellerberg volcanic complex, near Ettringen, in the Eifel mountains, Rhineland-Palatinate, Germany. This is a classic locality for many rare secondary minerals.
Rarity
Extremely rare
For collectors
## Quality criteria As a microscopic mineral, kruijenite is not evaluated according to typical criteria such as size or purity. The collector's value of a specimen primarily depends on the richness and quality of the microscopic crystal aggregates on the rock matrix. The most desirable specimens are those with well-formed, sharp crystals forming photogenic, rosette-like clusters, clearly visible under a microscope. Association with other rare minerals is also important. ## Popular localities The only source of kruijenite specimens is the Caspar quarry in Germany. Material from this locality is extremely rare and sought after by specialized collectors of microscopic minerals (so-called micromounts).
Care and storage
## Cleaning Due to its extreme rarity and microscopic nature, kruijenite specimens should not be cleaned under amateur conditions. Any intervention risks irreversible damage to the material. Specimens are found on a delicate rock matrix and should only be analyzed under laboratory conditions. ## What to avoid Avoid any contact with chemicals, water, as well as changes in temperature and humidity. Vibrations and shocks can cause microscopic aggregates to detach from the substrate. The specimen should be protected from dust. ## Storage Kruijenite specimens should be stored exclusively in specialized, sealed "micromount" containers that protect against dust, humidity, and mechanical damage. Storage under stable room conditions is crucial for preserving the integrity of the specimen.