Jahnsite-(MnMnMn)
Chemical formula: Mn<sup>2+</sup>Mn<sup>2+</sup>Mn<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-(MnMnMn) is a rare, hydrated phosphate forming aggregates of small, tabular crystals with a characteristic yellowish-brown to orange-brown color.
Properties
- Mohs hardness
- 4
- Luster
- Vitreous
- Streak
- Yellow-brown
- Density
- 2.7-2.8
- Cleavage
- Good on {001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Jahnsite-(MnMnMn) is identified by its characteristic color (shades of brown and orange), crystal habit (small, tabular or bladed), and specific occurrence environment – oxidation zones of granite pegmatites rich in phosphates. Vitreous luster and relatively low hardness are also helpful features. ## Distinguishing from Similar Minerals This mineral can be confused with other secondary manganese and iron phosphates, such as rockbridgeite, hureaulite, strengite, or members of the eosphorite group. Final differentiation from other very similar members of the jahnsite group (e.g., jahnsite-(CaMnMn) or jahnsite-(NaFeMg)) requires advanced chemical analyses (EDS/WDS). ## Crystal Forms Crystals are typically small, tabular or bladed, often elongated. They usually form radial, fan-shaped, or spherulitic aggregates. They rarely occur as single, well-formed crystals in vugs.
Geological environment
## Genesis Jahnsite-(MnMnMn) is a secondary mineral, formed by hydrothermal alteration or weathering of primary phosphate minerals (mainly triplite) in the oxidation zones of complex granite pegmatites. It crystallizes in cavities and rock fissures. ## Mineral Associations This mineral often co-occurs with other secondary phosphates, such as rockbridgeite, hureaulite, strengite, phosphosiderite, vivianite, eosphorite, as well as with quartz, microcline, and muscovite. ## Localities The most important and classic locality, from which the best specimens originate, is the Tip Top mine in Custer County, South Dakota, USA. Other known localities include the Foote mine in North Carolina (USA), Hagendorf-Süd in Bavaria (Germany), and several localities in Brazil (e.g., in Minas Gerais state).
Rarity
Rare
For collectors
## Quality Criteria Most valued by collectors are specimens with well-formed, sharp crystals forming aesthetic, radial or fan-shaped aggregates. Intense, orange-brown color is desirable. Contrast with the rock matrix (e.g., white quartz) and the presence of rare associated minerals are also important. Crystal size rarely exceeds a few millimeters, so specimens with larger crystals are exceptionally valuable. ## Popular Localities By far the most sought-after and highly prized specimens of jahnsite-(MnMnMn) come from the type locality – the Tip Top mine in South Dakota, USA. They are considered the global standard for this mineral.
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 fragile crystals. ## What to Avoid The mineral is sensitive to acids and strong chemicals. It should be protected from prolonged exposure to moisture, which can lead to its slow decomposition. High temperatures and sudden thermal changes should also be avoided. ## Storage Jahnsite-(MnMnMn) specimens are best stored in stable conditions, in closed boxes or display cases, away from direct sunlight and heat sources. Due to its brittleness, it should be avoided to place it in contact with harder minerals.