Jahnsite-(CaFeFe)

Chemical formula: {Ca}{Fe²⁺}{Fe²⁺₂}{Fe³⁺₂}(PO₄)₄(OH)₂·₈H₂O

Jahnsite-(CaFeFe) is a rare, brown phosphate mineral from the jahnsite group, forming small, platy crystals with a vitreous luster.

## Characteristics Jahnsite-(CaFeFe) is a hydrated calcium, ferrous, and ferric phosphate, belonging to the jahnsite subgroup. It occurs as small, platy or bladed crystals, often forming radial aggregates or rosettes. Its appearance is distinctive, although due to the small size of the crystals, magnification is often required for observation. ## Physical Properties This mineral is characterized by a hardness of 4 on the Mohs scale. Its luster is described as vitreous to slightly adamantine. It is a relatively brittle mineral. ## Colors and Varieties Jahnsite-(CaFeFe) occurs in various shades of brown, from light brown to dark 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 calcium (Ca) and iron (Fe) in two different structural positions, according to the nomenclature of the jahnsite group. It was formally described and approved as a new mineral species in 1995.

Properties

Mohs hardness: 4
Luster: vitreous-adamantine
Density: 0
Crystal system: Monoclinic

Diagnostic features

## Identification Jahnsite-(CaFeFe) is identified based on its characteristic crystal habit – thin, platy or bladed, often in the form of radial aggregates. The brown color, vitreous luster, and occurrence environment (granitic pegmatites) are key diagnostic features. ## Distinguishing from Similar Minerals It can be confused with other minerals from the jahnsite group or other brown secondary phosphates. Final distinction from very similar group members, such as jahnsite-(CaMnFe) or jahnsite-(CaMnMg), requires advanced chemical analyses (e.g., EDS). ## Crystal Forms It forms thin, platy crystals with a bladed habit, elongated along the crystallographic axis. These crystals often arrange into fan-shaped or radial aggregates and spherulitic clusters.

Geological environment

## Genesis It is a secondary mineral, formed as a result of hydrothermal processes in the late stages of granitic pegmatite crystallization. It forms in vugs and fissures, often as an alteration product of primary phosphates, such as triphylite. ## Mineral Associations It co-occurs with other phosphate minerals typical of pegmatites, such as rockbridgeite, strengite, hureaulite, vivianite, as well as with quartz, microcline, and muscovite. ## Localities The most important and best-documented occurrences of this mineral are found in the United States, particularly in pegmatite mines in New Hampshire (e.g., Palermo No. 1 mine in North Groton) and South Dakota (Keystone area).

Rarity

Very rare

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp crystals creating aesthetic, radial aggregates. Contrast with the rock matrix and absence of mechanical damage are also important. Due to the small size of the crystals, micromount specimens are the most common form in collections. ## Popular Localities The localities in New Hampshire, USA, especially historical finds from the Palermo No. 1 mine, are considered classic and provide the best specimens.

Care and storage

## Cleaning Specimens should be cleaned mechanically only, using a soft brush to remove dust. Due to its brittleness and potential reactivity, washing in water, and especially in chemical agents, should be avoided. ## What to Avoid Contact with acids and other chemicals that can damage or destroy delicate crystals should be avoided. The mineral is sensitive to rapid temperature changes and should not be heated. It should be protected from moisture. ## Storage It is recommended to store specimens in stable conditions, in closed display boxes or mineral cabinets, away from dust, moisture, and direct sunlight. Due to its brittleness, it should be avoided to place it in contact with harder minerals.

External references

https://www.mindat.org/min-10809.html