Dufrénite

Chemical formula: Ca₀.₅Fe₂+Fe₃+₅(PO₄)₄(OH)₆·₂H₂O

Dufrénite is a mineral from the phosphate group, forming characteristic, radial aggregates of dark green to almost black color.

## Characteristics Dufrénite is a hydrated iron phosphate, belonging to the group of the same name. It most often occurs as spherical or radial aggregates, as well as in the form of fibrous or spherulitic masses and crusts. Well-formed, single crystals are very rare and small, usually forming microscopic, bladed forms. Aggregates typically have a silky or dull luster on fracture surfaces. ## Physical Properties This mineral is relatively soft, with a Mohs hardness ranging from 3.5 to 4.5. It is quite brittle and has an uneven fracture. Its density ranges from 3.1 to 3.4 g/cm³. Crystals are usually translucent at the edges, while massive aggregates are opaque. The luster is most often vitreous; on fibrous aggregates, it can be silky, and in massive forms, it is dull or earthy. ## Colors and Varieties Dufrénite is characterized by a limited but typical color palette. Olive green, dark green, and greenish-black are dominant, with brownish-green shades being less common. The dark color results from the presence of both divalent and trivalent iron in its structure. There are no named color varieties or commercial types. ## History and Name The mineral's name, given in 1833 by Alexandre Brongniart, honors the French geologist and mineralogist Pierre-Armand Dufrénoy (1792-1857). It was described based on specimens from the locality of Chéniers, France. The mineral's chemical formula has undergone many revisions over the years, and the current, complex formula reflects the intricate structure of the dufrénite group. ## Uses Dufrénite has no industrial significance. However, it is a mineral of interest and sought after by collectors, especially specimens with well-formed, radial aggregates from classic localities.

Properties

Mohs hardness: 3.5-4.5
Luster: Vitreous, Silky, Dull
Streak: Yellow-green
Density: 3.1-3.4
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification A key diagnostic feature of dufrénite is its mode of occurrence – typically as spherical or radially fibrous aggregates. Its characteristic dark green to greenish-black color and relatively low hardness are also distinctive. The streak, ranging from yellowish-green to olive-green, is also helpful in identification. ## Distinguishing from Similar Minerals Dufrénite is sometimes confused with other secondary phosphates of similar appearance, such as cacoxenite, rockbridgeite, or strengite. It differs from cacoxenite by its usually darker, greener color (cacoxenite is typically yellow to brown). Rockbridgeite has a very similar appearance and composition, and distinguishing them often requires advanced chemical analyses (XRD). Strengite forms similar spherulites but usually has a purple or pink color. ## Crystal Forms Although it crystallizes in the monoclinic system, well-formed, single crystals are extremely rare and appear as small, bladed or acicular forms. The dominant and most recognizable forms are spherical (spherulitic) aggregates with a radial internal structure, as well as reniform, botryoidal masses, and crusts.

Geological environment

## Genesis Dufrénite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits, especially those containing iron sulfides (such as pyrite or marcasite) in the presence of phosphate minerals, e.g., apatite. It also forms in granitic pegmatites as a product of the alteration of primary iron and manganese phosphates, such as triphylite-lithiophilite. It can also occur in some sedimentary iron deposits. ## Mineral Associations It most often co-occurs with other secondary phosphate minerals, such as rockbridgeite, strengite, phosphosiderite, vivianite, as well as iron oxyhydroxides (goethite, lepidocrocite) and quartz. In pegmatites, it is often accompanied by triphylite, lithiophilite, and other phosphates. ## Localities Significant dufrénite localities worldwide include Wheal Phoenix in Cornwall (United Kingdom); mines near Siegen in North Rhine-Westphalia (Germany); Chanteloube and Chéniers in France (type locality); the Palermo Mine in New Hampshire and various localities in Cherokee County, Alabama (USA). It is also known from pegmatites in Sapucaia do Norte, Minas Gerais (Brazil).

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, distinct spherical or radial aggregates of intense, dark green color. High luster, absence of mechanical damage, and aesthetic placement on the rock matrix, often in contrast with quartz or other minerals, are important. Specimens from classic, historic localities, such as Cornwall or the German Siegerland, are particularly sought after. The size of the aggregates also affects value – larger, well-defined spherulites are rarer and more desirable. ## Popular Localities Localities that yield the most prized collector specimens include historic mines in Cornwall, United Kingdom; the Siegerland region in Germany; and pegmatite localities in New Hampshire and Alabama in the USA. Specimens from these places are considered classics for this mineral.

Care and storage

## Cleaning Dufrénite specimens should be cleaned very carefully, preferably using a soft brush to remove dust. Fibrous and acicular aggregates are extremely delicate and can be damaged even by light touch. The use of water is not recommended, as the mineral may be sensitive to moisture, and water can penetrate porous aggregates, weakening them. ## What to Avoid Ultrasonic cleaners, chemical agents, acids, and detergents should be absolutely avoided, as they can destroy or chemically alter the mineral. Dufrénite is sensitive to high temperatures and prolonged exposure to direct sunlight, which can lead to fading or brittleness. It should be protected from moisture and sudden temperature changes. ## Storage Dufrénite specimens are best stored in closed, stable containers or display cases to protect them from dust, moisture, and mechanical damage. Due to its brittleness, it should not be stored in contact with harder minerals. It is advisable to place it in a separate box with soft padding.

Sources

IMA Mineral List (CNMNC)