Jahnsite-(MnMnFe)
Chemical formula: Mn<sup>2+</sup>Mn<sup>2+</sup>Fe<sup>2+</sup><sub>2</sub>Fe<sup>3+</sup><sub>2</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>2</sub>·8H<sub>2</sub>O
Jahnsite-(MnMnFe) is a rare, hydrated phosphate forming small, platy crystals with a characteristic orange-brown color.
Properties
- Mohs hardness
- 4
- Luster
- Vitreous
- Streak
- Yellow-brown
- Density
- 2.75
- Cleavage
- Good on {001}
- Fracture
- Uneven
- Transparency
- Translucent to transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Key diagnostic features include its orange-brown color, vitreous luster, characteristic crystal habit (small, platy or prismatic), and occurrence in granitic pegmatites. Reaction to acids (solubility) can be helpful but requires sample destruction. ## Distinguishing from Similar Minerals Jahnsite-(MnMnFe) can be confused with other secondary phosphates of similar color, such as rockbridgeite, strengite, or members of the eosphorite group. Final differentiation from other members of the jahnsite group (e.g., jahnsite-(CaMnFe) or jahnsite-(MnMnMg)) is impossible without advanced chemical analysis (e.g., EDS). ## Crystal Forms It forms thin, platy crystals with a rectangular outline or short, prismatic crystals. These crystals often group into radial aggregates, rosettes, or fan-like clusters, growing on other minerals in rock cavities.
Geological environment
## Genesis Jahnsite-(MnMnFe) is a secondary mineral, formed by hydrothermal alteration of primary phosphate minerals (such as triphylite-lithiophilite) in the oxidation zones of granitic pegmatites rich in phosphorus, manganese, and iron. ## Mineral Associations It often co-occurs with other secondary phosphates, such as rockbridgeite, hureaulite, strengite, stewartite, vivianite, as well as with quartz, muscovite, and albite. ## Localities The most important and classic locality, from which the best specimens originate, is the Tip Top Mine in Custer County, South Dakota, USA. This mineral has also been identified in other pegmatites worldwide, including Brazil (Minas Gerais region), Portugal, and Germany, although specimens from these localities are generally less spectacular.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those from the Tip Top Mine in South Dakota. Samples with well-formed, sharp crystals forming aesthetic, radial or rosette-like aggregates are highly valued. An intense, orange-brown color and good contrast with the host mineral (e.g., white albite) significantly increase the specimen's value. Crystal size is also important, although they rarely exceed a few millimeters. ## Popular Localities By far the most desired and well-known locality is the Tip Top Mine in South Dakota, USA. Specimens from this location are considered the global standard for this mineral.
Care and storage
## Cleaning Utmost caution is recommended. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, use a soft brush and distilled water, then immediately and thoroughly dry the specimen. The mineral is brittle, so avoid any mechanical pressure. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral. Do not use ultrasonic cleaners. As a hydrated mineral, it is sensitive to high temperatures, which can lead to dehydration and destruction. Also, avoid prolonged exposure to moisture. ## Storage Specimens should be stored in stable conditions, away from heat sources and direct sunlight. It is best to keep them in sealed collector boxes to protect them from dust and mechanical damage. Due to its brittleness, it should not be stored in contact with harder minerals.