Wendwilsonite

Chemical formula: Ca₂Mg(As₅+O₄)₂·₂H₂O

Wendwilsonite is a rare calcium magnesium arsenate, forming striking pink to reddish-purple crystals, prized by collectors.

## Characteristics Wendwilsonite is a hydrated calcium magnesium arsenate belonging to the roselite group. It forms characteristic, well-developed crystals with a prismatic or tabular habit, often aggregated into radial or fan-shaped clusters. Specimens typically exhibit an intense, attractive color, ranging from pink to reddish-purple, making them sought-after collector's minerals. Crystals rarely exceed a few millimeters in length and most often grow on host rocks, creating striking contrasts. ## Physical Properties Wendwilsonite crystals display a vitreous luster and are transparent to translucent. The mineral's hardness on the Mohs scale is 3-4, classifying it as relatively soft. Its density ranges from 3.51 to 3.58 g/cm³. ## Colors and Varieties The color of wendwilsonite is its most distinctive feature. It typically occurs in shades from pink, through pinkish-red, to reddish-purple. The intensity of the color depends on the cobalt content, which substitutes for magnesium in the crystal structure. Wendwilsonite forms a continuous solid solution series with roselite (where cobalt predominates) and talmessite (where magnesium is fully replaced). Specimens of intermediate composition, with noticeable cobalt content, may exhibit deeper, more saturated colors. ## History and Name The mineral was named in honor of Wendell E. Wilson (born 1946), an American mineralogist, founder and editor-in-chief of "The Mineralogical Record" magazine, in recognition of his contributions to mineralogy. It was first described in 1987 by Pete J. Dunn, Bozidar D. Sturman, and Nels A. Nelen after examining material from the Sterling Hill mine in New Jersey (USA) and Bou Azzer in Morocco. ## Uses Wendwilsonite has no industrial applications. It is solely a mineral of scientific and collector's interest.

Properties

Mohs hardness: 3-4
Luster: Vitreous
Streak: Pale reddish white
Density: 3.51-3.58
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Wendwilsonite can be identified by its characteristic pinkish-red to purple color, vitreous luster, and typical prismatic crystals forming fan-shaped or radial aggregates. It often occurs in association with other arsenates, such as erythrite or talmessite, on dolomitic or calcitic rocks. ## Distinguishing from Similar Minerals Wendwilsonite is visually almost impossible to distinguish from roselite without advanced chemical analyses (EDS, WDS). By definition, wendwilsonite is the magnesium-dominant variety, while roselite is cobalt-dominant. Roselite tends to exhibit more intense, red colors, but this is not a rule. From erythrite, which also forms pink coatings and acicular crystals, wendwilsonite differs in crystal form – wendwilsonite crystals are typically thicker, more prismatic, and less often form fibrous aggregates. ## Crystal Forms Crystals are monoclinic, usually short-prismatic in habit, elongated along the crystallographic b-axis. They are often flattened tabular. Most commonly, they form fan-shaped, radial, or rosette aggregates of small crystals; less frequently, they occur as single, isolated crystals.

Geological environment

## Genesis Wendwilsonite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal ore deposits rich in arsenic, cobalt, and nickel. It crystallizes in fissures and cavities of carbonate rocks, such as dolomites and calcites, which constitute the gangue of the deposit. ## Mineral Associations This mineral often co-occurs with other arsenates, especially roselite, talmessite, and erythrite. Depending on the location, it may also be associated with: calcite, dolomite, quartz, pharmacolite, picropharmakolite, goethite, and various cobalt and nickel minerals. ## Localities The most important and well-known wendwilsonite localities worldwide are the Bou Azzer region in Morocco, from which the best collector's specimens originate. This mineral has also been identified in the Sterling Hill mine in Ogdensburg, New Jersey (USA) – this is one of its type localities. Other confirmed occurrences include Schneeberg in Saxony (Germany) and the Veta Negra mine in the Atacama region (Chile).

Rarity

Rare

For collectors

## Quality Criteria The most highly prized wendwilsonite specimens are characterized by an intense, saturated reddish-purple or deep pink color. Key factors are good crystal development, transparency, and size. Specimens with distinct, isolated crystals or aesthetic, fan-shaped aggregates that create an attractive contrast with the light host rock (matrix) are particularly sought after. Undamaged specimens with a clear luster command the highest prices. ## Popular Localities By far the best and most desired wendwilsonite specimens by collectors come from the Bou Azzer region in Morocco. This locality is renowned for providing specimens with exceptional color and excellent crystal form. Specimens from other localities, such as Sterling Hill or Schneeberg, are much rarer on the collector's market and primarily hold historical and scientific significance.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If washing is necessary, use only distilled water and avoid prolonged soaking. After cleaning, the specimen should be allowed to air dry completely at room temperature. ## What to Avoid The mineral is sensitive to acids and strong chemicals, which can damage it. Ultrasonic cleaners and steam cleaning should be avoided. It should not be heated or exposed to sudden temperature changes. As an arsenate, mineral dust is toxic – avoid inhaling it and wash hands after contact with raw specimens. ## Storage Wendwilsonite should be stored in stable conditions, away from direct sunlight, which can cause color fading over time. It is best kept in a closed box or display cabinet to protect it from dust and mechanical damage. Due to its softness, it should not come into contact with harder minerals.

Sources

IMA Mineral List (CNMNC)