Jahnsite-(CaMnMn)

Chemical formula: CaMn<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-(CaMnMn) is a rare, hydrated phosphate forming aggregates of small, tabular crystals with a color ranging from yellow to brown.

## Characteristics Jahnsite-(CaMnMn) is a hydrated phosphate belonging to the jahnsite group. This mineral typically forms small, tabular or bladed crystals, often gathered into radial, rosette-like, or spherulitic aggregates. Individual crystals rarely exceed a few millimeters in size. It is a secondary mineral, forming in complex granitic pegmatites. ## Physical Properties Jahnsite-(CaMnMn) crystals exhibit a hardness of approximately 4 on the Mohs scale and are relatively brittle. They have a vitreous luster, and their density is about 2.75 g/cm³. They are transparent to translucent. ## Colors and Varieties Jahnsite-(CaMnMn) occurs in shades from yellow, through golden-yellow, orange-yellow, to brown and dark brown. Its color is characteristic and related to the presence of manganese and iron in its structure. No named varieties are distinguished. ## History and Name As a member of the jahnsite subgroup, it was named in honor of Richard H. Jahns (1915-1983), an American geologist and mineralogist from the California Institute of Technology, for his contributions to pegmatite research. The suffix "(CaMnMn)" specifies the chemical composition of the particular end-member in the group, indicating the dominance of calcium at the X site, manganese at the M1 site, and manganese at the M2 site in the crystal structure. The mineral was first described by P.B. Moore and J. Ito in 1978. ## Uses Jahnsite-(CaMnMn) has no industrial applications. It is solely a mineral of scientific and collecting interest, sought after by advanced collectors of rare phosphate minerals.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
Pale yellow
Density
2.75
Cleavage
Good on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Jahnsite-(CaMnMn) can be identified by its characteristic, small, tabular crystals gathered in radial or rosette-like aggregates. Its typical color – from yellow to brown – and vitreous luster are also distinctive. Its occurrence in a geologically specific environment, such as granitic pegmatites, is a key indicator. ## Distinguishing from Similar Minerals It can be confused with other secondary phosphates of similar color, such as rockbridgeite, strengite, or members of the eosphorite group. Differentiation often requires analysis of crystal habit (jahnsite forms characteristic tabular crystals) and knowledge of associated minerals. Definitive identification is primarily possible through analytical methods (XRD, EDS). ## Crystal Forms Crystals are most often thin-tabular, elongated, and flattened. They form fan-shaped, radial, spherulitic, or rosette-like aggregates. They also occur as thin coatings and granular aggregates.

Geological environment

## Genesis Jahnsite-(CaMnMn) is a secondary mineral that forms in the late stages of crystallization of complex granitic pegmatites. It results from hydrothermal alteration of earlier, primary phosphate minerals, such as triplite or lithiophilite, in the presence of calcium-rich solutions. ## Mineral Associations It often co-occurs with other phosphates, such as hureaulite, strengite, rockbridgeite, stewartite, mitridatite, cyrilovite, as well as with quartz, microcline, and muscovite. ## Localities The most important and classic localities for this mineral are in the USA, including the Tip Top Mine, Custer (South Dakota) – which is the type locality. It is also known from the Palermo No. 1 Mine in North Groton (New Hampshire) and other pegmatites in the New England region. Occurrences have also been reported in Brazil (Minas Gerais) and Namibia.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic, radial or rosette-like aggregates are most valued by collectors. An intense, golden-yellow or orange color is highly desirable. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to the small size of the crystals, specimens suitable for microscopic viewing (so-called "micromounts") are standard in collections. ## Popular Localities The best quality and most classic specimens of jahnsite-(CaMnMn) come from the Tip Top Mine in South Dakota, USA. Specimens from this locality are considered exemplary 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 brittle crystals. ## What to Avoid The mineral is sensitive to acids and strong chemical agents. Prolonged exposure to moisture should be avoided, as it is a hydrated mineral and changes in humidity can affect its stability. It should not be heated. ## Storage Collector specimens of jahnsite-(CaMnMn) are best stored in closed display boxes or cabinets to protect them from dust and mechanical damage. Due to its brittleness, contact with harder minerals should be avoided.

Sources

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