Jahnsite-(NaMnMn)

Chemical formula: NaMn₂+(Mn₂+Fe₃+)Fe₃+₂(PO₄)₄(OH)₂·₈H₂O

Jahnsite-(NaMnMn) is a rare, hydrated sodium, manganese, and iron phosphate, forming aggregates of small, tabular, brown crystals.

## Characteristics Jahnsite-(NaMnMn) is a complex, hydrated phosphate mineral belonging to the jahnsite group. It occurs as aggregates of very small, tabular or bladed crystals, rarely exceeding 1 mm in length. Crystals are often grouped into radial or acicular aggregates, as well as rosette-like or fan-shaped forms. Its appearance is usually inconspicuous due to the small size of the crystals. ## Physical Properties The mineral is characterized by a hardness of approximately 4 on the Mohs scale and a vitreous luster. It is brittle, and its density is about 2.73 g/cm³. It is transparent to translucent. ## Colors and Varieties Jahnsite-(NaMnMn) occurs in shades from yellowish-brown to dark brown and reddish-brown. No distinct color varieties or commercial varieties are recognized. ## History and Name The mineral's name refers to its chemical composition – it indicates the dominance of sodium (Na) in one structural site and manganese (Mn) in two other distinct positions. It is one of many minerals in the jahnsite group, and its full characterization was approved by the International Mineralogical Association (IMA). It was first described based on material from the Tip Top mine in Custer County, South Dakota, USA. ## Uses Jahnsite-(NaMnMn) has no industrial applications. It is of purely scientific and collector's interest, being an object of interest for specialists and advanced collectors of phosphate minerals.

Properties

Mohs hardness: 4
Luster: Vitreous
Streak: Pale brown
Density: 2.73
Cleavage: Good on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of jahnsite-(NaMnMn) under amateur conditions is practically impossible and requires advanced analytical methods, such as chemical analysis (EDS) and X-ray diffraction (XRD). Preliminary identification is based on the characteristic habit of small, tabular crystals, their brown color, and co-occurrence with other rare phosphates in granitic pegmatites. ## Distinguishing from Similar Minerals This mineral is extremely difficult to distinguish from other members of the jahnsite group, such as jahnsite-(CaMnMn) or jahnsite-(CaMnFe), without specialized chemical analysis. Visually, it can be confused with many other microcrystalline, brown phosphates or sulfates. ## Crystal Forms It forms very small, thin, tabular crystals, often with a hexagonal outline, elongated in one direction. These crystals usually occur as radial aggregates, fans, rosettes, or chaotic clusters on the surface of other minerals.

Geological environment

## Genesis Jahnsite-(NaMnMn) is a secondary mineral, formed in complex granitic pegmatites. It forms as a result of hydrothermal processes or weathering, which lead to the alteration of primary phosphate minerals, such as triphylite or lithiophilite. ## Mineral Associations This mineral co-occurs with other pegmatitic phosphates. In its type locality (Tip Top mine), it was found in association with quartz, muscovite, beryl, columbite, rockbridgeite, hureaulite, strengite, and vivianite. ## Localities The most important and best-documented occurrence of jahnsite-(NaMnMn) is its type locality – the Tip Top mine in Custer County, South Dakota, USA. This is a classic locality for many rare phosphate minerals. In addition, its presence has been reported in several other pegmatites worldwide, including in Brazil.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a jahnsite-(NaMnMn) specimen primarily depends on the abundance and development of its crystals. Specimens with clearly visible, sharp crystals forming aesthetic aggregates, such as radial rosettes, are most highly valued. The contrast with the host rock (matrix) and the presence of other rare associated minerals are also important. Due to the small size of the crystals, specimens are usually intended for "micromount" collections. ## Popular Localities The only source for obtaining valuable collector's specimens is the Tip Top mine in South Dakota, USA. Specimens from this locality are the standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only a soft brush to remove dust. Due to its brittleness and potential reactivity, washing in water, especially with chemical agents, should be avoided. If it is necessary to remove loose contaminants, compressed air can be used from a safe distance. ## What to Avoid Contact with acids and other chemicals must be strictly avoided. The mineral is brittle and susceptible to mechanical damage, so it should be protected from impacts and vibrations. It should not be heated or exposed to sudden temperature changes. ## Storage Collector's specimens of jahnsite-(NaMnMn) should be stored in stable conditions, preferably in a sealed "micromount" box, which protects them from dust and mechanical damage. Exposure to moisture and direct sunlight should be avoided.

Sources

IMA Mineral List (CNMNC)