Ojuelaite

Chemical formula: Zn<sup>2+</sup>Fe<sup>3+</sup><sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>·4H<sub>2</sub>O

Ojuelaite is a rare zinc iron arsenate, forming characteristic yellow-green, radial aggregates of acicular crystals.

## Characteristics Ojuelaite is a hydrated zinc iron arsenate belonging to the arthurite group. This mineral forms very characteristic, tiny, acicular or capillary crystals, which most often occur as spherical or radial aggregates. Individual crystals are usually too small to be observed with the naked eye, but their clusters form spectacular, silky or dull crusts and spherical forms on the host rock. ## Physical Properties Ojuelaite crystals are very brittle. The mineral has a Mohs hardness of 3. The luster is described as silky, especially noticeable in fibrous aggregates, or dull. It is a translucent mineral. Its density is approximately 3.4 g/cm³. ## Colors and Varieties Ojuelaite occurs in characteristic shades from canary yellow to yellow-green. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Ojuela Mine in Mapimí, Durango State, Mexico. It was first described in 1981 by George

Properties

Mohs hardness
3
Luster
Silky
Streak
Light yellow
Density
3.4
Cleavage
Good on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Ojuelaite is most easily recognized by its characteristic mode of occurrence – radial or spherical aggregates composed of very fine, acicular crystals. Its canary yellow to yellow-green color and silky luster are also key. It occurs in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals It can be confused with other secondary arsenate minerals of similar color and form, such as mimetite or adamite. Adamite rarely forms such delicate, acicular aggregates, more often occurring as fan-shaped clusters of better-formed crystals. Legrandite, also found in the Ojuela Mine, has a more intense, lemon-yellow color and forms larger, prismatic crystals. Final distinction often requires chemical analysis. ## Crystal Forms Crystals are elongated, acicular or capillary, usually not exceeding a few millimeters in length. They form radial, stellate, or spherical aggregates, as well as crusts with a fibrous structure.

Geological environment

## Genesis Ojuelaite is a secondary mineral, formed in the oxidation (weathering) zones of polymetallic hydrothermal deposits rich in arsenic, zinc, and iron. It forms in vugs and fissures in rocks, most often on a limonite substrate. ## Mineral Associations It most often co-occurs with other arsenates, such as adamite, legrandite, scorodite, paradamite, and mimetite. It is also accompanied by goethite, limonite, and occasionally smithsonite. ## Localities The most important and classic locality, from which the best specimens originate, is the Ojuela Mine in Mapimí, Durango State, Mexico. It has also been identified in several other localities worldwide, including the mines of the Lavrion area in Greece and Tsumeb in Namibia; however, specimens from these places are much rarer and less spectacular than the Mexican ones.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued ojuelaite specimens are characterized by an intense, vibrant, yellow-green color. The form of the aggregates is crucial – perfect, spherical or "fluffy" clusters of acicular crystals without damage fetch the highest prices. Contrast with the host rock on which they occur, and the presence of associated minerals, e.g., purple adamite, also create visually attractive compositions. ## Popular Localities By far the most desired and well-known on the collector's market are specimens from the type locality – the Ojuela Mine in Mexico. They set the standard against which all other finds are compared.

Care and storage

## Cleaning Ojuelaite specimens are extremely delicate and brittle. Only use compressed air to remove dust. Any contact with water or mechanical cleaning, even with a soft brush, risks irreversible damage to the acicular crystals. ## What to Avoid Avoid contact with water, chemicals (including acids and detergents), and ultrasonic cleaners. The crystals are sensitive to shocks and impacts. Do not expose specimens to direct sunlight, which can cause fading. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from dust, vibrations, and mechanical damage. Due to its brittleness, ojuelaite is not suitable for mounting in jewelry or for frequent handling.

Sources

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