Goudeyite
Chemical formula: Cu²⁺₆Al(As⁵⁺O₄)₃(OH)₆·3H₂O
Goudeyite is a rare copper and aluminum arsenate, forming fine, acicular crystals of an intense blue-green color.
Properties
- Mohs hardness
- 3-4
- Color
- blue-green, yellow-green
- Luster
- Silky
- Streak
- greenish white
- Density
- 3.50
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Identification of goudeyite is based on its characteristic radial aggregate form, composed of fine, acicular crystals of an intense blue-green color. It occurs in the oxidation zones of copper and arsenic deposits. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Distinguishing from Similar Minerals Goudeyite is visually almost impossible to distinguish from other minerals of the mixite group, such as mixite itself, agardite, or petersite, which form identical shapes and have similar colors. It can also be confused with other secondary copper minerals, such as olivenite or conichalcite, but these typically form different crystal habits. Precise differentiation requires chemical analysis to determine the dominant cations. ## Crystal Forms Crystals are hexagonal, very elongated in the form of needles and hairs. They typically form radial, stellate, or spherical aggregates, as well as fibrous crusts and coatings.
Geological environment
## Genesis Goudeyite is a secondary mineral, formed in the oxidation (weathering) zones of hydrothermal copper and arsenic ore deposits. It forms in voids and fissures of rocks, often in rhyolites or rhyolitic tuffs, as a result of arsenic-rich waters acting on earlier copper minerals. ## Mineral Associations This mineral co-occurs with other secondary arsenate and sulfate minerals. It is most commonly associated with: olivenite, cornwallite, arthurite, metazeunerite, chalcanthite, brochantite, chrysocolla, quartz, and iron oxides. ## Localities The most important and type locality for goudeyite is the Majuba Hill mine in Pershing County, Nevada, USA, from which the best-known specimens originate. It has also been identified in several other locations worldwide, including the Clara mine in the Black Forest (Germany) and the Lavrion area in Greece, although these occurrences are of much lesser significance.
Rarity
Very rare
For collectors
## Quality Criteria The most prized goudeyite specimens are characterized by rich crusts with well-formed, radial aggregates of an intense blue-green color. Contrast with the light host rock and the absence of damage to the delicate needles are important. The size of the aggregates and the coverage of the rock surface are key factors influencing value. Specimens with well-defined, spherical aggregates are more desirable than irregular, fibrous coatings. ## Popular Localities By far the most sought-after and classic specimens come from the type locality - the Majuba Hill mine in Nevada, USA. Specimens from this location are considered the standard for this species.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Contact with water is not recommended due to the delicate and soluble nature of the mineral. Ultrasonic cleaners and steam cleaning should be avoided. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. The mineral is sensitive to high temperatures and humidity. Prolonged exposure to intense sunlight may potentially cause fading. ## Storage Goudeyite specimens should be stored under stable conditions, in a dry place, away from direct sunlight and heat sources. It is best to store them in closed collector's boxes to protect the delicate crystals from mechanical damage and dust.