Jahnsite-(CaMnMg)

Chemical formula: CaMn<sup>2+</sup>Mg<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-(CaMnMg) is a rare, hydrated phosphate forming prismatic crystals of yellow to brown color, found in granitic pegmatites.

## Characteristics Jahnsite-(CaMnMg) is a complex, hydrated phosphate mineral belonging to the jahnsite group. It typically forms small, prismatic or tabular crystals, often gathered in radial aggregates or rosettes. Specimens have a characteristic, intense color, ranging from pale yellow, through honey-brown, to dark brown, which is associated with the iron and manganese content. ## Physical Properties This mineral is characterized by a vitreous luster and is usually translucent to opaque. Its Mohs hardness is approximately 4, making it relatively soft. The density is slightly higher than average for minerals, at about 2.73 g/cm³. ## Colors and Varieties The color of jahnsite-(CaMnMg) is its significant feature and ranges from pale yellow, through golden-yellow, orange-brown, reddish-brown, to almost black. This variability depends on the proportion of iron and manganese ions in its structure. No distinct commercial or color varieties are distinguished. ## History and Name The name "jahnsite" honors the American geologist and mineralogist Richard H. Jahns (1915–1983), a professor of geology at Stanford University, for his contributions to the study of pegmatites. The suffix -(CaMnMg) specifies the chemical composition of this particular member of the jahnsite group, indicating calcium, manganese, and magnesium as dominant cations in specific structural positions. The mineral was first described by Paul B. Moore and Jun Ito in 1978 based on material from the Tip Top mine in South Dakota, USA. ## Uses Jahnsite-(CaMnMg) has no industrial applications. It is a mineral of purely scientific and collectible importance, valued by advanced collectors of rare phosphate minerals.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
Light brown
Density
2.73
Cleavage
Good on {001}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Jahnsite-(CaMnMg) can be identified by its characteristic prismatic crystals, often forming radial or fan-shaped aggregates. Key features include an intense, yellow-brown color, vitreous luster, and occurrence in a paragenesis typical of granitic pegmatites, especially in association with other phosphates. ## Distinguishing from Similar Minerals This mineral can be confused with other members of the jahnsite group, and accurate identification requires chemical analysis (EDS). It can also be mistaken for other yellow or brown phosphates, such as rockbridgeite, hureaulite, or strengite. Differentiation is based on crystal habit, hardness, and specific paragenesis. Jahnsite often forms more defined, prismatic crystals than massive rockbridgeite. ## Crystal Forms Crystals are typically elongated, prismatic along the [001] axis, and tabularly flattened. They often occur as radial aggregates, rosettes, and also as coatings and crusts on other minerals.

Geological environment

## Genesis Jahnsite-(CaMnMg) is a secondary mineral, formed in the hydrothermal zones of complex granitic pegmatites. It forms as a result of the alteration and oxidation of earlier primary phosphates, such as triphylite or lithiophilite, in the presence of solutions rich in calcium, magnesium, and iron. ## Mineral Associations This mineral often co-occurs with other phosphates, such as rockbridgeite, hureaulite, strengite, phosphosiderite, as well as with quartz, microcline, muscovite, albite, and goethite. ## Localities The most important and classic type locality (localitas typica) is the Tip Top mine, Custer County, South Dakota (USA). This is the source of the best known specimens of this mineral. Other known localities include the Foote mine in North Carolina (USA), as well as occurrences in Brazil (in the state of Minas Gerais) and in Germany (Hagendorf, Bavaria).

Rarity

Rare

For collectors

## Quality Criteria The most highly valued specimens of jahnsite-(CaMnMg) are characterized by well-formed, sharp crystals with an intense, golden-yellow or orange-brown color. High luster and translucency of the crystals enhance their attractiveness. Specimens where radial jahnsite aggregates create an aesthetic contrast with the matrix mineral, e.g., with white albite or quartz, are particularly sought after. Crystal size is also important, although they rarely exceed a few millimeters. ## Popular Localities Undoubtedly, the most prized and sought-after specimens by collectors 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. Due to possible water sensitivity, prolonged soaking should be avoided. If liquid is necessary, distilled water or isopropyl alcohol is recommended, followed by immediate, gentle drying. ## What to Avoid Avoid contact with acids and other aggressive chemicals that can damage the mineral. Ultrasonic cleaners or steam cleaners should not be used. The mineral is relatively soft, so it must be protected from scratching by harder minerals. Sudden temperature changes should also be avoided. ## Storage Jahnsite-(CaMnMg) specimens should be stored in separate, padded boxes or display cases to prevent mechanical damage and contact with other minerals. Due to its fragility, it should be handled with care.

Sources

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