Hendricksite
Chemical formula: KZn<sup>2+</sup><sub>3</sub>(Si<sub>3</sub>Al)O<sub>10</sub>(OH)<sub>2</sub>
Hendricksite is a rare mineral from the mica group, a zinc variety of biotite, characterized by its dark color and platy habit.
Properties
- Mohs hardness
- 2.5-3
- Luster
- Vitreous to sub-metallic
- Streak
- Pale brown
- Density
- 3.3
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features of hendricksite include its micaceous, platy habit, perfect cleavage in one direction, dark, brownish-black color, and occurrence in a specific geological environment (metamorphic zinc deposits). The flexibility of thin lamellae is also a key indicator. ## Distinguishing from Similar Minerals Hendricksite is difficult to distinguish from other dark micas, such as biotite or phlogopite, without advanced chemical analysis (e.g., EDS) that can confirm high zinc content. In the context of the Franklin deposit, its association with franklinite, willemite, and zincite is a strong diagnostic clue. ## Crystal Forms This mineral forms pseudohexagonal, tabular crystals of varying thickness. It occurs as single crystals, but more often as scaly, platy, or packet aggregates of crystals. Crystals may be intergrown with calcite or other associated minerals.
Geological environment
## Genesis Hendricksite is a mineral of metamorphic origin. It forms as a result of contact or regional metamorphism of zinc and manganese ore deposits, precipitated in carbonate rocks (marbles). Its typical environment is zinc skarns. ## Mineral Associations In its type locality (Franklin, New Jersey, USA), hendricksite co-occurs with franklinite, willemite, zincite, calcite, andradite, beryl, hardystonite, and esperite. In other localities, it may be accompanied by sphalerite, galena, and other ore minerals. ## Localities The most important and classic locality for hendricksite is the Franklin-Sterling Hill deposit in New Jersey, USA, from which the best specimens originate. It is also known from Långban in Sweden, the Broken Hill mine in Australia, and several locations in Russia (Urals, Siberia).
Rarity
Rare
For collectors
## Quality Criteria The most prized hendricksite specimens are those with well-formed, sharp crystals of pseudohexagonal outline. Crystal size and luster are also important. Specimens where hendricksite is aesthetically set on a contrasting matrix (e.g., white calcite) or occurs in association with other rare minerals from the Franklin deposit (such as willemite or franklinite) are particularly sought after. ## Popular Localities By far the most desired specimens by collectors come from historical finds in the Franklin and Sterling Hill mines in New Jersey, USA. These localities are classic for this mineral and have provided the best known specimens in the world.
Care and storage
## Cleaning Hendricksite specimens should be cleaned very carefully, using a soft brush to remove dust. Compressed air can be used. Due to its platy structure, avoid washing under running water, which could penetrate between the lamellae and cause them to separate. If liquid is necessary, distilled water and very gentle drying are recommended. ## What to Avoid Avoid contact with acids and other chemicals. The mineral is soft and susceptible to mechanical damage, such as scratches or bending and breaking of lamellae. It should not be heated or exposed to sudden temperature changes. ## Storage Hendricksite specimens are best stored in separate, padded boxes to protect them from scratching and damage to the delicate edges of the lamellae. They should be protected from dust and moisture.