Jahnsite-(CaMnFe)
Chemical formula: CaMn²⁺Fe²⁺₂Fe³⁺₂(PO₄)₄(OH)₂·8H₂O
Jahnsite-(CaMnFe) is a rare, hydrated phosphate forming small, tabular crystals of a characteristic golden-brown color.
Properties
- Mohs hardness
- 4
- Luster
- Vitreous
- Streak
- White
- Density
- 2.86
- Cleavage
- Good on {001}
- Fracture
- Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Jahnsite-(CaMnFe) can be identified by its characteristic habit of thin, tabular crystals, often forming fan-shaped or radial aggregates. Key features also include its golden-brown color, vitreous luster, and occurrence in pegmatite cavities. A white streak is an additional diagnostic clue. ## Distinguishing from Similar Minerals It can be confused with other phosphates from the jahnsite group or similar secondary minerals found in pegmatites, such as members of the rockbridgeite group or strengite. Final differentiation from very similar members of its own subgroup (e.g., jahnsite-(CaMnMn) or jahnsite-(CaMnMg)) requires advanced chemical analyses (EDS/WDS). ## Crystal Forms Crystals are typically thin, tabular or bladed, elongated in one direction. Most often, they form radial, fan-shaped, or spherulitic aggregates. Less commonly, they are found as single, well-formed crystals.
Geological environment
## Genesis Jahnsite-(CaMnFe) is a secondary phosphate mineral. It forms as a result of hydrothermal alteration of primary phosphates, such as triplite, in complex granitic pegmatites. It crystallizes in rock cavities and fissures. ## Mineral Associations It often co-occurs with other phosphate minerals, such as hureaulite, strengite, rockbridgeite, vivianite, as well as with quartz, albite, and microcline. In its type locality, it was found in association with triplite, hureaulite, and rockbridgeite. ## Localities The most important and classic locality for this mineral is its type locality – the Fletcher Mine in Groton, Grafton County, New Hampshire, USA. It is also known from several other pegmatites worldwide, including the Tip Top Mine in Custer County, South Dakota, USA, as well as from Brazil (Minas Gerais) and Germany (Hagendorf, Bavaria).
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp crystals forming aesthetic, radial or fan-shaped aggregates. An intense, golden-brown color and good luster are important. Contrasting placement on a light rock matrix, for example on quartz, significantly enhances the specimen's appeal. Due to the small size of the crystals, quality is often assessed under magnification. ## Popular Localities Specimens from the type locality, the Fletcher Mine in New Hampshire, have particular historical value. High-quality, aesthetic aggregates also come from the Tip Top Mine in South Dakota, which is a known source of many rare phosphate minerals.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to Avoid The mineral is sensitive to acids and other strong chemicals. Prolonged exposure to humidity should be avoided, as it is a hydrated mineral and changes in humidity can affect its structure. Due to its relatively low hardness, it is susceptible to scratching. ## Storage Jahnsite-(CaMnFe) specimens are best stored in separate, padded display boxes to prevent contact with harder minerals. Protect from dust and sudden temperature changes. Due to the small size of the crystals, it is often stored in "micromount" boxes.