Kidwellite

Chemical formula: NaFe<sup>3+</sup><sub>9+x</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>11</sub>·3H<sub>2</sub>O (x ≈ 1/3)

Kidwellite is a rare, hydrated sodium iron phosphate, forming characteristic, radial aggregates with a silky luster and colors ranging from yellow to green.

## Characteristics Kidwellite is a hydrated sodium iron phosphate that most often occurs as spherical or hemispherical aggregates with a radial-fibrous structure. Individual crystals are very fine, acicular or hair-like, forming felt-like or velvety crusts on the host rock. Less commonly, it forms compact, earthy, or chalky masses. Its appearance is very characteristic, and the radial structures are often clearly visible to the naked eye. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 4. It has a silky luster, particularly noticeable on the surfaces of radial aggregates, which transitions to dull in the case of earthy forms. It is an opaque mineral or at most translucent on thin edges. Its density is approximately 3.4 g/cm³. ## Colors and Varieties Kidwellite exhibits colors ranging from light yellow, through yellowish-green, olive-green, to greenish-brown. Typically, individual specimens are not uniform in color, showing subtle tonal transitions within a single aggregate. No named varieties are distinguished. ## History and Name The mineral was first described in 1979 by D.R. Peacor, P.J. Dunn, and W.B. Simmons. Its name honors Albert Lewis Kidwell (1910-2008), an American geologist and mineral collector who specialized in phosphates from the Arkansas region and was the first to find this mineral in the Fodderstack and Coon Creek mines. The type locality is precisely these two mines in Polk County, Arkansas, USA. ## Uses Kidwellite has no industrial applications. It is solely a mineral of scientific and collector's interest, valued for its characteristic, aesthetic forms of occurrence.

Properties

Mohs hardness
4
Luster
Silky
Streak
Light yellow
Density
3.4
Cleavage
None
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Kidwellite is most easily recognized by its characteristic form of occurrence – spherical or hemispherical aggregates with a radial-fibrous internal structure. Key features also include a silky luster, yellowish-green color, and relatively low hardness. It often forms crusts on a limonite or quartz matrix. ## Distinguishing from Similar Minerals Kidwellite is sometimes confused with other secondary iron phosphates of similar form and color, such as strengite, rockbridgeite, or dufrenite. It differs from them by subtle color shades (kidwellite is usually lighter, more yellow) and the form of aggregates. Strengite often forms better-developed, thicker crystals, while rockbridgeite usually has a darker, almost black color. However, definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms This mineral forms very fine, acicular or hair-like crystals that almost always occur as radial, spherulitic aggregates. Single, isolated crystals are extremely rare and microscopic in size. Aggregates can form compact crusts or spherical forms up to several millimeters in diameter.

Geological environment

## Genesis Kidwellite is a secondary phosphate mineral, forming in oxidation zones (weathering) of deposits containing primary phosphate minerals (e.g., apatite) and iron minerals (e.g., pyrite, siderite). It forms at low temperatures in an iron- and phosphorus-rich environment, often in voids and fractures of sedimentary rocks, such as novaculites, or in pegmatites. ## Mineral Associations This mineral often co-occurs with other secondary iron phosphates. The most common associated minerals include strengite, rockbridgeite, dufrenite, beraunite, cacoxenite, as well as quartz, goethite, and limonite, which often form the matrix for its aggregates. ## Localities The most important and classic localities, from which the best specimens originate, are in Arkansas, USA, particularly in the Coon Creek and Fodderstack mines in Polk County. This mineral has also been found in Germany (Clara mine in the Black Forest, Rotläufchen mine near Waldgirmes), Portugal, Spain, the Czech Republic, and in small quantities in other locations worldwide.

Rarity

Rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, spherical or hemispherical aggregates of intense, vibrant color (from canary yellow to olive green). High silky luster and contrast with the rock matrix are important. Specimens where radial aggregates are numerous, undamaged, and aesthetically arranged on a small matrix achieve the highest value. The size of individual spherulitic clusters also affects attractiveness. ## Popular Localities By far the most sought-after and classic kidwellite specimens come from the type locality in Arkansas, USA (Coon Creek and Fodderstack mines). They are considered the global standard for this mineral. Good quality specimens also come from the German Clara mine, although they often have a slightly different morphology and color.

Care and storage

## Cleaning Kidwellite specimens should be cleaned very carefully, preferably dry, using a soft brush or compressed air to remove dust. Due to its fibrous and delicate nature, contact with water is not recommended, as it can penetrate the aggregate structure and be difficult to remove, and also potentially lead to chemical degradation of the mineral. Avoid ultrasonic cleaners. ## What to Avoid Avoid contact with all chemicals, including acids and strong bases. The mineral is sensitive to high temperatures and prolonged exposure to direct sunlight, which can cause fading. It should be protected from moisture and sudden temperature changes. ## Storage Kidwellite specimens are best stored in closed, dry containers or display cases to protect them from dust and mechanical damage. Due to its fragility, it should be kept away from other, harder minerals to prevent scratching and crushing of delicate fibers.

External references

Sources

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