Natrodufrénite

Chemical formula: NaFe₂+Fe₃+₅(PO₄)₄(OH)₆·₂H₂O

A rare, secondary sodium iron phosphate, forming characteristic dark green, radial aggregates of fibrous crystals.

## Characteristics Natrodufrénite is a rare, hydrated sodium iron phosphate belonging to the dufrénite group. It typically occurs as spherical or radial aggregates, composed of very fine, fibrous or acicular crystals. Individual crystals are rarely visible to the naked eye. Specimens usually have a dark green, olive-green to almost black color and often cover the surfaces of host rocks in the form of thin coatings or crusts. ## Physical Properties Natrodufrénite crystals are very small and brittle. The mineral exhibits a silky or vitreous luster, and in the case of dense aggregates, it may appear dull. It is a relatively soft mineral, with a Mohs hardness of about 3.5. It is opaque or at most translucent on thin edges. ## Colors and Varieties Natrodufrénite occurs in shades of green, from olive-green, through dark green, greenish-black, to almost black. Its streak is characteristically olive-green. No distinct varieties of this mineral are recognized. ## History and Name The mineral was first described in 1981 by P.J. Dunn, D.R. Peacor, and R.C. Rouse. Its name refers to its chemical composition – the presence of sodium (Latin: *natrium*) – and its relationship to the structurally similar mineral, dufrénite. ## Uses Due to its rarity and small crystal size, natrodufrénite has no industrial applications. It is solely of interest to collectors specializing in rare phosphate minerals and microminerals.

Properties

Mohs hardness: 3.5
Luster: Silky
Streak: Olive-green
Density: 3.35
Cleavage: Perfect on {100}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of natrodufrénite is its mode of occurrence: radial or spherical aggregates of very fine, fibrous crystals with a dark green color. The characteristic olive-green streak is also important. Occurrence in the oxidation zones of iron deposits or phosphate pegmatites is an additional clue. ## Distinguishing from Similar Minerals Natrodufrénite is visually almost impossible to distinguish from other minerals of the dufrénite group (e.g., dufrénite, burangaite) without advanced chemical analysis (EDS) or X-ray diffraction (XRD). It can also be confused with other secondary iron phosphates, such as rockbridgeite or strengite, which form similar aggregates but often differ in hue or crystal form. Unlike many of them, natrodufrénite is characterized by the presence of sodium. ## Crystal Forms The mineral forms aggregates of fibrous, acicular, or bladed crystals, arranged in radial, spherulitic (spherical), or fan-shaped forms. It also forms crusts and coatings.

Geological environment

## Genesis Natrodufrénite is a secondary mineral. It forms in the oxidation (weathering) zones of deposits rich in iron and phosphorus. It is most commonly found in sedimentary iron deposits and in granitic pegmatites, where it forms as a result of the alteration of primary phosphate minerals, such as triphylite. ## Mineral Associations It often co-occurs with other secondary phosphate minerals, such as rockbridgeite, strengite, phosphosiderite, hureaulite, as well as iron minerals like goethite and hematite. In pegmatites, it is associated with quartz, feldspars, and muscovite. ## Localities Among the most important and well-documented natrodufrénite localities worldwide are the Bull Moose Mine in Custer (South Dakota, USA), where it was discovered, as well as pegmatites in the Hagendorf-Süd area (Bavaria, Germany). It also occurs in Spain (La Paloma mine, Murcia), Portugal, and the Iron Knob iron mine in South Australia.

Rarity

Rare

For collectors

## Quality Criteria Specimens most valued by collectors are those with well-formed, distinct spherical or radial aggregates of intense, dark green color. Contrast with a light host rock (matrix) on which the crystals are deposited is important. The size of the aggregates and the absence of mechanical damage to the delicate fibers significantly increase the value of the specimen. Specimens from classic, well-documented localities, such as the Bull Moose Mine or Hagendorf, are particularly sought after. ## Popular Localities Collectors most highly value specimens from the type locality at the Bull Moose Mine in South Dakota (USA) and from classic localities in the Hagendorf pegmatites in Germany. These localities have yielded the best-defined and most aesthetic specimens of the mineral.

Care and storage

## Cleaning Extreme caution is advised. It is best to use compressed air to remove dust. Any mechanical contact, even with a soft brush, can permanently damage the delicate, acicular crystals. If wet cleaning is necessary, use only distilled water and apply it as a gentle spray or with a pipette, avoiding a strong stream. ## What to Avoid Ultrasonic cleaners must be strictly avoided, as they will destroy the aggregates. Avoid contact with acids and other chemicals that may react with phosphates. Specimens are fragile and sensitive to impacts and shocks. ## Storage Natrodufrénite specimens should be stored in stable conditions, away from dust and vibrations. The best solution is sealed "micromount" boxes or display cases, which protect against mechanical damage and dust accumulation. Avoid placing heavier specimens nearby.

Sources

IMA Mineral List (CNMNC)