Jahnsite-(CaFeMg)

Chemical formula: CaFe<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-(CaFeMg) is a rare, hydrated phosphate forming aggregates of small, tabular crystals with a honey-yellow to brown color.

## Characteristics Jahnsite-(CaFeMg) is a hydrated phosphate belonging to the jahnsite group, forming a series with jahnsite-(CaMnMg) and other members of the group. It occurs as aggregates of small, tabular or bladed crystals, which rarely exceed 1 mm in length. Crystals often form radial aggregates, rosettes, or fan-shaped forms, growing on other minerals in rock cavities. ## Physical Properties This mineral is characterized by a Mohs hardness of 4 and a density of approximately 2.74 g/cm³. It has a vitreous luster and is transparent to translucent. It exhibits perfect cleavage in one direction, which is visible on some crystals. ## Colors and Varieties Jahnsite-(CaFeMg) most commonly ranges in color from honey-yellow, through yellowish-brown, to dark brown. Its color is directly related to its iron content. No named varieties of this mineral are distinguished. ## History and Name Jahnsite-(CaFeMg) was first described in 1979 by Paul B. Moore and Taka Takaharu. The name refers to its chemical composition – the dominance of calcium (Ca), iron (Fe), and magnesium (Mg) in specific structural positions – and honors Richard H. Jahns (1915-1983), a professor of geology at Stanford University, for his contributions to the study of granitic pegmatites. ## Uses Due to its rarity and small crystal size, jahnsite-(CaFeMg) has no industrial applications. It is solely a mineral of scientific and collector's interest, sought after by specialists and collectors of rare minerals.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
White
Density
2.74
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Jahnsite-(CaFeMg) is recognized by its characteristic small, tabular crystals forming radial or fan-shaped aggregates. Key features include its honey-yellow to brown color, vitreous luster, and occurrence in cavities of granitic pegmatites. ## Distinguishing from Similar Minerals It can be confused with other minerals from the jahnsite group or other microcrystalline phosphates. Final differentiation from jahnsite-(CaMnMg) or other members of the series requires advanced chemical analyses (e.g., EDS). Visually, it may resemble some zeolites or other secondary phosphates, but it differs from them in crystal form and environment of occurrence. ## Crystal Forms Crystals are typically thin, tabular or bladed, elongated in one direction. They most often occur as radial, spherical, or fan-shaped aggregates and rosettes.

Geological environment

## Genesis Jahnsite-(CaFeMg) is a secondary phosphate mineral. It forms as a result of hydrothermal alteration of primary phosphates (such as triphylite) in complex granitic pegmatites. It crystallizes in the late stage of pegmatite evolution, in rock cavities and fissures. ## Mineral Associations It often co-occurs with other phosphate minerals, such as rockbridgeite, strengite, phosphosiderite, hureaulite, vivianite, as well as with quartz, albite, and muscovite. ## Localities The most important and classic localities for this mineral are in the USA, including the Tip Top Mine in Custer County (South Dakota), where it was discovered. It is also known from other pegmatites in South Dakota (e.g., in the Keystone area) and from the Palermo No. 1 pegmatite in North Groton, New Hampshire.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, distinct radial aggregates or rosettes of intense, honey-yellow color are most valued by collectors. Contrast with the rock matrix and the absence of damage to delicate crystals are important. The size of the aggregates also increases the value of the specimen, although they rarely exceed a few millimeters in diameter. ## Popular Localities The most sought-after specimens come from the type locality – the Tip Top Mine in South Dakota, USA. Specimens from this location are considered classic and serve as a standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Distilled water can be used, but prolonged soaking should be avoided due to the mineral's hydrated nature. Ultrasonic cleaners should be avoided. ## What to Avoid Avoid contact with acids and other chemicals that can damage delicate crystals. The mineral is sensitive to high temperatures, which can lead to its dehydration (loss of water) and destruction of its structure. It should not be exposed to prolonged direct sunlight. ## Storage Jahnsite-(CaFeMg) specimens are brittle and susceptible to mechanical damage. It is best to store them in separate, padded collector's boxes, away from harder minerals. They should be protected from dust, humidity, and sudden temperature changes.

Sources

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