Franklinfurnaceite
Chemical formula: Ca₂Mn²⁺₃Mn³⁺Fe³⁺Zn²⁺₂Si₂O₁₀(OH)₈
A rare mineral from the chlorite group, discovered in the famous Franklin deposit in New Jersey, USA, characterized by its dark brown color.
Properties
- Mohs hardness
- 3
- Streak
- Dark brown
- Density
- 3.66
- Cleavage
- {001}
- Fracture
- Micaceous
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Franklinfurnaceite is identified by its appearance – small, dark brown, hexagonal flakes forming rosette-like aggregates. Key features also include perfect cleavage, low hardness, and a dark brown streak. However, final identification requires advanced analytical methods, especially in the context of its typical locality. ## Distinguishing from Similar Minerals It can be confused with other dark lamellar minerals, such as biotite, stilpnomelane, or other chlorites. Distinguishing it "in the field" is practically impossible. Its unique chemical composition and co-occurrence with other minerals from the Franklin deposit, such as willemite, franklinite, or zincite, set it apart. ## Crystal Forms It forms small, pseudohexagonal, tabular, or lamellar crystals. It is most commonly found in the form of rosette-like or lamellar aggregates, growing on other minerals.
Geological environment
## Genesis Franklinfurnaceite formed as a result of metamorphic processes acting on zinc, iron, and manganese ore deposits. It is a product of hydrothermal alterations within the metamorphosed, Precambrian carbonate rocks (marbles) of the Franklin deposit. ## Mineral Associations This mineral co-occurs with a rich array of other minerals typical of this locality. It is most often found in association with willemite, franklinite, zincite, calcite, andradite, as well as rarer species such as esperite, hodgkinsonite, or gahnospinel. ## Localities The only confirmed locality for franklinfurnaceite in the world is its type locality – the Franklin Mine in Franklin, Sussex County, New Jersey, USA. It is an endemic mineral to this single deposit.
Rarity
Very rare
For collectors
## Quality Criteria As a "micromount" type mineral, its collector's value depends on the quality and size of the crystals, although these rarely reach significant dimensions. Specimens with well-formed, sharp flakes forming aesthetic rosettes are most prized. Association with other rare minerals from Franklin is also important, especially if they form a contrasting background. ## Popular Localities The only source of specimens is the historic Franklin Mine in New Jersey, USA. Collector material comes exclusively from old collections or from material obtained from mine dumps.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its softness and lamellar structure, avoid washing with water, and especially mechanical cleaning, which could damage the delicate crystals. ## What to Avoid Avoid contact with chemicals, including acids and strong detergents. The mineral is susceptible to mechanical damage, such as scratches, impacts, or abrasion. It should not be heated or subjected to ultrasound. ## Storage Franklinfurnaceite specimens, being microscopic and fragile, are best stored in sealed "micromount" boxes, which protect them from dust and mechanical damage. Avoid placing heavier minerals in direct contact.