Minehillite
Chemical formula: (K,Na)₂Ca₂₈Zn²⁺₅Al₄Si₄₀O₁₁₂(OH)₁₆
Minehillite is a rare, colorless or white zinc, potassium, and calcium silicate of the reyerite group, known exclusively from the Franklin Mine in the USA.
Properties
- Mohs hardness
- 4
- Luster
- Pearly
- Streak
- White
- Density
- 2.93
- Cleavage
- {0001}
- Transparency
- Transparent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Minehillite is identified by its characteristic form – it creates small, hexagonal, tabular crystals gathered in radial aggregates or rosettes. Key features include pearly luster on cleavage surfaces, white color, low hardness, and occurrence in the specific geological environment of the Franklin Mine. ## Distinguishing from Similar Minerals It can be confused with other white, platy minerals from the reyerite group or zeolites. It is distinguished from reyerite by the presence of zinc in its composition. From visually similar zeolites (e.g., spherulitic thomsonite), it is distinguished by lower hardness and hexagonal crystal form. Certain identification requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline. These crystals most often occur in the form of radial or spherulitic (spherical) aggregates and rosettes.
Geological environment
## Genesis Minehillite is a secondary mineral, formed as a result of hydrothermal processes within metamorphosed, stratiform zinc and manganese ore deposits. It forms in rock fractures and voids during the late stage of mineralization. ## Mineral Associations This mineral occurs in association with other rare minerals typical of Franklin, such as franklinite, willemite, zincite, as well as calcite, barite, rhodochrosite, and various zeolites. ## Localities The only confirmed occurrence of minehillite in the world is its type locality: the Franklin Mine, in Franklin, Sussex County, New Jersey, USA.
Rarity
Extremely rare
For collectors
## Quality Criteria The most valuable minehillite specimens are those with well-formed, sharp, and undamaged crystalline aggregates in the form of rosettes or radial fans. Specimens where the white, pearly minehillite aggregates contrast with a colorful rock matrix, for example, with black franklinite or pink rhodochrosite, are also highly prized. The size of the aggregates and the richness of their occurrence on the specimen also significantly increase its value. ## Popular Localities The only source of minehillite specimens is the historic Franklin Mine in New Jersey, USA. All specimens available on the collector's market originate from this single locality.
Care and storage
## Cleaning Minehillite specimens are extremely delicate and fragile. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove dust. Water and any chemical agents should be avoided. ## What to Avoid Contact with water, ultrasonic cleaners, chemical agents, and sudden temperature changes must be strictly avoided. Its low hardness (4 on the Mohs scale) makes it susceptible to scratches, and its perfect cleavage makes it prone to mechanical damage, such as impacts or drops. ## Storage It is recommended to store specimens only in stable, sealed collector boxes (e.g., "perky boxes"), lined with soft material. They should be protected from dust, vibrations, and direct contact with harder minerals.