Gordaite
Chemical formula: NaZn<sup>2+</sup><sub>4</sub>(S<sup>6+</sup>O<sub>4</sub>)(OH)<sub>6</sub>Cl·6H<sub>2</sub>O
Gordaite is a rare, secondary sulfate mineral, forming characteristic, sky-blue aggregates of fine, tabular crystals.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.61
- Cleavage
- Perfect on {0001}
- Fracture
- Micaceous
- Transparency
- Transparent to Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Characteristic features of gordaite include its light blue color, the formation of rosette-like or spherulitic aggregates composed of fine, tabular crystals, and its occurrence in the oxidation zones of polymetallic deposits. It reacts with acids. ## Distinguishing from Similar Minerals Gordaite is sometimes confused with other blue secondary copper and zinc minerals, such as aurichalcite or rosasite. It is distinguished from them by its characteristic crystal habit (hexagonal tablets), the formation of spherical aggregates, and its mineral paragenesis, especially its co-occurrence with anglesite, hemimorphite, and zinc sulfates. ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline. These crystals almost always occur as radial, spherical, or rosette-like aggregates, less commonly as single, scattered flakes on the surface of the host rock.
Geological environment
## Genesis Gordaite is a secondary mineral, forming in the oxidation zones (gossans) of polymetallic deposits rich in zinc. It forms under desert conditions, as a result of reactions between sulfate and chloride-rich solutions and zinc ore minerals, such as sphalerite. ## Mineral Associations It most commonly co-occurs with other secondary minerals, such as anglesite, hemimorphite, linarite, chalcanthite, brochantite, gypsum, halite, as well as quartz and iron oxides (goethite, hematite). ## Localities The most important and classic locality for gordaite is the San Francisco Mine in Sierra Gorda (Antofagasta region, Chile). This mineral has also been identified in other dry, desert localities, including the Red Gill and Roughton Gill mines in Cumbria (England, United Kingdom) and several places in Greece (Lavrion mines).
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, spherical or rosette-like aggregates of intense blue color, contrastingly set on a small rock matrix. The absence of damage to the delicate crystals and the aesthetic arrangement of gordaite clusters are also important. The size of the aggregates rarely exceeds a few millimeters. ## Popular Localities Specimens from the type locality, the San Francisco Mine in Chile, are by far the most sought after and highly valued. Material from other localities, such as England, is also interesting for collectors specializing in minerals from a given locality, but specimens from Chile are considered exemplary.
Care and storage
## Cleaning Gordaite specimens are extremely delicate and sensitive to water. Wet cleaning should be avoided. Only very careful dust removal using a soft brush or a stream of compressed air from a distance is permissible. ## What to Avoid Contact with water, which can damage or dissolve the crystals, must be strictly avoided. The use of any chemical agents, ultrasonic cleaners, and exposure to high temperatures and direct sunlight, which can cause fading, are also inadvisable. ## Storage Specimens should be stored in a dry place, preferably in a closed, airtight display box, away from sources of moisture. Due to its fragility, the mineral should be protected from shocks and contact with harder minerals.