Goldhillite
Chemical formula: Cu²⁺₅Zn²⁺(As⁵⁺O₄)₂(OH)₆·H₂O
Goldhillite is a rare copper and zinc arsenate, distinguished by its intense emerald-green color and transparent, platy crystals.
Properties
- Mohs hardness
- 3.5-0
- Luster
- Vitreous
- Streak
- Light green
- Density
- 4.199
- Cleavage
- {100}
- Fracture
- Irregular/Uneven
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Goldhillite can be recognized by its characteristic, vivid emerald-green color, transparency, and specific form of thin, platy crystals, often gathered in rosette-like aggregates. Its occurrence in association with other secondary copper and zinc minerals in the oxidation zones of deposits is also an important clue. ## Distinguishing from Similar Minerals Goldhillite is sometimes confused with other green, secondary arsenate and phosphate minerals of copper. It differs from malachite by its lack of reaction with hydrochloric acid and higher transparency. It differs from conichalcite by its more platy crystal habit (conichalcite most often forms spherical and radial aggregates). It differs from agardite by its crystal habit – agardite forms acicular or fibrous aggregates. Precise identification requires advanced analytical methods, such as X-ray diffraction (XRD) or EDS spectroscopy. ## Crystal Forms Crystals are thin, platy, with a hexagonal outline, arranged on the {100} plane. They often form rosette-like, fan-shaped, or spherical aggregates, as well as thin crusts and coatings on the host rock.
Geological environment
## Genesis Goldhillite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic ore deposits rich in copper, zinc, and arsenic. It forms as a result of the action of surface waters on primary ore minerals such as chalcopyrite, sphalerite, and arsenopyrite. ## Mineral Associations This mineral co-occurs with other secondary minerals of the oxidation zone. At its type locality (Gold Hill, Utah), it was found in association with austinite, azurite, conichalcite, malachite, olivenite, as well as quartz and goethite. ## Localities The most important and best-documented occurrence of goldhillite is its type locality: the central shaft of the Gold Hill Mine in Tooele County, Utah, USA. This is the only confirmed locality from which well-formed, identifiable specimens originate.
Rarity
Very rare
For collectors
## Quality Criteria Most valued by collectors are specimens with well-formed, sharp, transparent crystals of intense emerald-green color. Rich groups of crystals forming aesthetic rosettes or fans on a contrasting host rock (matrix) are highly prized. Crystal size is also important, although they rarely exceed 1 mm. Specimens from the type locality are particularly sought after. ## Popular Localities The only source of collector specimens is the Gold Hill Mine in Utah, USA. All valuable specimens on the market come from this single location.
Care and storage
## Cleaning Goldhillite specimens should be cleaned with the utmost care, using compressed air to remove dust. Contact with water, especially with acids or detergents, is not recommended. If wet cleaning is necessary, use only distilled water and a very soft brush, then immediately dry the specimen. ## What to Avoid Avoid all chemicals, including acids and strong bases, which can chemically react with arsenates. The mineral is soft and brittle, so it must be protected from impacts and scratches. It should not be heated or exposed to prolonged strong sunlight. ## Storage The best way to store goldhillite is to place it in a sealed, transparent "micromount" box, which protects delicate crystals from mechanical damage and dust. The specimen should be stored under stable room conditions, away from heat sources and humidity.