Jahnsite-(NaMnMg)

Chemical formula: (Na,Ca)(Mn<sup>2+</sup>,Fe<sup>3+</sup>)(Mg,Fe<sup>3+</sup>)<sub>2</sub>Fe<sup>3+</sup><sub>2</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>2</sub>(H<sub>2</sub>O)<sub>8</sub>

A rare phosphate from the jahnsite group, forming small, tabular crystals of amber color and vitreous luster.

## Characteristics Jahnsite-(NaMnMg) is a complex hydrated phosphate of sodium, manganese, and magnesium, belonging to the jahnsite subgroup. This mineral forms very small, usually not exceeding 1 mm, tabular or bladed crystals. It most often occurs as radial or spherulitic aggregates, as well as coatings and efflorescences on other minerals. Its appearance is characteristic due to its amber-yellow to orange-brown color and intense, vitreous luster. ## Physical Properties Jahnsite-(NaMnMg) crystals exhibit a hardness of approximately 4 on the Mohs scale. They have a vitreous luster and are transparent to translucent. The mineral's density is about 2.73 g/cm³. It is a brittle mineral with an uneven fracture. ## Colors and Varieties This mineral occurs in shades from amber-yellow and golden-yellow to orange-brown. No color or commercial varieties have been distinguished. ## History and Name Jahnsite-(NaMnMg) was first described in 2004 by Anthony R. Kampf, Paul B. Moore, Ian M. Steele, Joseph J. Pluth, and Joseph V. Smith. The name refers to its chemical composition – the dominance of sodium (Na), manganese (Mn), and magnesium (Mg) – and its belonging to the jahnsite group. The group name honors Richard H. Jahns (1915-1983), an American geologist and mineralogist from the California Institute of Technology, for his contributions to the study of granitic pegmatites. ## Uses Jahnsite-(NaMnMg) has no industrial application. It is solely an object of interest for collectors specializing in rare phosphate minerals and micromounts.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
Light yellow
Density
2.73
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Jahnsite-(NaMnMg) can be recognized by its characteristic occurrence in the form of small, tabular crystals forming radial or spherulitic aggregates. Key visual features include an amber-yellow to orange-brown color, vitreous luster, and occurrence in cavities within phosphate pegmatites. ## Distinguishing from Similar Minerals This mineral can be confused with other members of the jahnsite group or other fine-grained phosphates of similar color. Positive identification is almost exclusively possible using advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD), which allow for the determination of precise composition and structure. ## Crystal Forms Crystals are small, tabular or bladed, often flattened parallel to {010}. They typically form fan-shaped, radial, or spherical (spherulitic) aggregates, as well as druses and coatings in rock fissures.

Geological environment

## Genesis Jahnsite-(NaMnMg) is a secondary phosphate mineral. It forms as a result of hydrothermal processes in the late stage of crystallization of complex granitic pegmatites, rich in phosphates. It crystallizes from post-magmatic solutions in cavities and fissures. ## Mineral Associations This mineral co-occurs with other phosphates. In its type locality (Tip Top mine in South Dakota, USA), it was found in association with jahnsite-(CaMnMg), rockbridgeite, strengite, leucophosphite, taranakite, vivianite, and montgomeryite. ## Localities The most important and type locality for jahnsite-(NaMnMg) is the Tip Top mine, near Custer in Custer County, South Dakota, USA. This is the only confirmed and detailed described locality for this mineral worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of jahnsite-(NaMnMg) specimens depends on several factors. Highly valued are specimens with well-formed, sharp crystals forming aesthetic, radial aggregates. The intensity and saturation of the amber color are important. Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. Contrast with the matrix mineral also enhances visual value. ## Popular Localities The only source of collector specimens is the Tip Top mine in South Dakota, USA. Material from this locality is 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 solubility in acids, contact with water and any chemical agents should be avoided. ## What to Avoid Ultrasonic cleaners, strong streams of water, acids, and organic solvents should be strictly avoided. The mineral is brittle and sensitive to mechanical shocks. Prolonged exposure to intense sunlight may not be advisable, although detailed data on this is lacking. ## Storage It is recommended to store specimens in stable conditions, away from dust and moisture. The safest form of storage, due to the small size and brittleness of the crystals, are "micromount" boxes or sealed display cases that protect against mechanical damage.

Sources

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