Wallkilldellite-(Fe)

Chemical formula: Ca₂Fe²⁺₃(As⁵⁺O₄)₂(OH)₄·9H₂O

Wallkilldellit-(Fe) is a rare calcium and iron arsenate, forming hexagonal crystals of brown to brownish-yellow color.

## Characteristics Wallkilldellit-(Fe) is a hydrated calcium and iron arsenate, belonging to the wallkilldellit group. This mineral forms small, hexagonal, tabular crystals, often grouped into rosettes or spherical aggregates. Its color ranges from brown to brownish-yellow. It is a secondary mineral, forming in the oxidation zones of ore deposits. ## Physical Properties Wallkilldellit-(Fe) crystals are very soft, with a Mohs hardness of 2-3. They have a vitreous to resinous luster and are translucent. The mineral's density is approximately 3.0 g/cm³. It exhibits distinct cleavage in one plane. ## Colors and Varieties This mineral occurs in shades from brown to brownish-yellow. No distinct color varieties or commercial varieties have been identified. ## History and Name Wallkilldellit-(Fe) was first described in 1999 by G. Favreau, J. E. Dietrich, G. Ghilini, P. G. Orlandi, J. L. Meisser, and M. Ciriotti. Its name refers to its chemical composition – it is an analog of wallkilldellit with dominant iron (Fe) in place of manganese. The group name comes from the Wallkill River near Franklin, New Jersey, USA, where the first mineral of this series was discovered. ## Uses Wallkilldellit-(Fe) has no industrial applications. It is of purely scientific and collector's interest as a rare secondary mineral.

Properties

Mohs hardness
2-3
Luster
Vitreous, Resinous
Streak
Light brown
Density
3.0
Cleavage
{0001}
Transparency
Translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Wallkilldellit-(Fe) can be identified by its characteristic small, hexagonal crystals forming rosette-like aggregates. Key features include its brownish-yellow color, low hardness, vitreous or resinous luster, and pale brown streak. It occurs in paragenesis with other secondary arsenate minerals. ## Distinguishing from Similar Minerals It can be confused with other minerals from the wallkilldellit group, such as wallkilldellit-(Mn). Differentiation requires advanced chemical analysis (EDS/WDS) to determine the dominant element (iron or manganese). It may also resemble other secondary arsenates of similar color, but the crystal form and aggregates are often a distinguishing feature. ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline. These crystals often combine into characteristic, rosette-like or spherulitic aggregates.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation zones (gossans) of polymetallic deposits rich in arsenic. It results from the weathering of primary ore minerals, especially arsenopyrite and other iron sulfides and arsenides. ## Mineral Associations It co-occurs with other secondary minerals such as pharmacosiderite, scorodite, beudantite, jarosite, goethite, and also with quartz. ## Localities The most important and type locality is the Roua mines (Clue de Roua) in Daluis, Alpes-Maritimes department, France. It is a very rare mineral, and its occurrence has been confirmed in only a few places worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. Contrast with the rock matrix and absence of mechanical damage are also important. Due to the small size of the crystals, specimens visible to the naked eye are particularly sought after. ## Popular Localities The only source of high-quality collector's specimens is the type locality – the Roua mines in France. Specimens from this location are the standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its low hardness and potential reactivity, washing in water, and especially using ultrasonics, should be avoided. ## What to Avoid Contact with chemicals, acids, and detergents should be avoided. The mineral is susceptible to scratches and impacts. As a hydrated mineral, it may be sensitive to changes in temperature and humidity, which can lead to its dehydration and destruction. ## Storage Wallkilldellit-(Fe) specimens should be stored under stable conditions, in sealed containers or display cases, away from direct sunlight and heat sources. It is best to store it separately to avoid abrasion by harder minerals.

External references

Sources

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