Hillesheimite
Chemical formula: (K,Ca,Ba,◻)₂(Mg,Fe²⁺,Ca,◻)₂[(Si,Al)₁₃O₂₃(OH)₆](OH)·8H₂O
Hillesheimite is a very rare silicate mineral, forming colorless, bladed crystals with a pearly luster, found in only one location in the world.
Properties
- Mohs hardness
- 4
- Color
- Yellow to brownish
- Luster
- Pearly
- Streak
- White
- Density
- 2.16
- Cleavage
- Perfect parallel to (010) and distinct on (100) and (001)
- Fracture
- Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identification of hillesheimite is practically only possible under laboratory conditions using advanced techniques such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). In collector's conditions, identification relies on knowledge of the provenance (Eifel, Germany), the characteristic form of thin, bladed crystals forming radial aggregates, and co-occurrence with other rare zeolite minerals. ## Distinguishing from Similar Minerals It can be confused with other zeolite minerals of similar appearance, such as thomsonite or natrolite, which also form acicular or radial aggregates. Definitive differentiation requires specialized analytical studies. ## Crystal Forms It forms very thin, bladed or lath-like crystals, elongated in one direction. These crystals combine into radial, stellate, or haphazardly tangled aggregates.
Geological environment
## Genesis Hillesheimite forms under hydrothermal conditions in vesicles and fissures within volcanic rocks, such as phonolitic and nephelinitic lavas. It crystallizes from aqueous solutions at relatively low temperatures during the late stages of volcanic activity. ## Mineral Associations This mineral co-occurs with other zeolite and related minerals, such as phillipsite, chabazite, gismondine, willhendersonite, apophyllite, natrolite, calcite, and thomsonite. ## Localities The only confirmed occurrence of hillesheimite in the world is its type locality: the Graulay (or Gräulay) quarry near the town of Hillesheim in the Eifel mountains, in the Rhineland-Palatinate state of Germany.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of hillesheimite specimens is assessed based on the richness and aesthetics of the crystal aggregates on the rock matrix. Specimens with well-formed, radial aggregates that stand out clearly from the background are most valued. Co-occurrence with other rare minerals is also important, as it increases the scientific and collector's value of the specimen. Due to the microscopic size of the crystals, all specimens fall into the "micromount" category.
Care and storage
## Cleaning Due to the extreme delicacy and small size of the crystals, mechanical cleaning is inadvisable and practically impossible without damaging the specimen. If absolutely necessary, compressed air can be carefully used from a safe distance to remove loose dust. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. The mineral is very soft and susceptible to mechanical damage. It should not be heated or soaked in water, as it is a hydrated mineral and may undergo changes. ## Storage Hillesheimite specimens should be stored exclusively in specialized "micromount" boxes, protecting them from dust, shocks, and humidity changes. Avoid touching specimens with fingers.