Andyrobertsite
Chemical formula: KCd<sup>2+</sup>Cu<sup>2+</sup><sub>5</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>4</sub>[As<sup>5+</sup>(OH)<sub>2</sub>O<sub>2</sub>]·2H<sub>2</sub>O
Andyrobertsite is a rare, hydrated potassium, cadmium, and copper arsenate, forming characteristic, intensely blue prismatic crystals.
Description
## Characteristics Andyrobertsite is a complex, hydrated arsenate belonging to the group of rare minerals. It occurs as slender or stout prismatic crystals, which often form divergent, radial aggregates or chaotic clusters. Its most characteristic feature is its intense, deep color, ranging from blue to greenish-blue. Crystals are usually small but well-formed and embedded in voids within the host rock. ## Physical Properties This mineral is characterized by a vitreous luster and is transparent to translucent. Its Mohs hardness is 3, making it relatively soft and susceptible to scratching. The calculated density is approximately 4.01 g/cm³. ## Colors and Varieties Andyrobertsite is valued for its vivid, uniform color, which ranges from azure-blue to sky-blue, sometimes with a greenish tint. No color or commercial varieties are distinguished, which is typical for such rare minerals. ## History and Name The mineral was described in 1999 by Mark A. Cooper and Frank C. Hawthorne. Its name honors Andrew C. Roberts (born 1950), a Canadian mineralogist with the Geological Survey of Canada, specializing in the study and characterization of new and rare mineral species. The type locality, or the first place of discovery, is the famous Tsumeb mine in Namibia. ## Uses Due to its extreme rarity and chemical composition (arsenic and cadmium content), andyrobertsite has no industrial applications. It is solely an object of scientific interest and a prized acquisition in advanced collections of rare minerals.
Diagnostic features
## Identification Key identifying features of andyrobertsite are its intense blue color, prismatic crystal habit often forming radial aggregates, and its origin from the only known locality – the Tsumeb mine. Vitreous luster and relatively low hardness are also helpful in identification. ## Distinguishing from Similar Minerals Andyrobertsite can be confused with other blue secondary minerals from Tsumeb, such as azurite, which, however, rarely forms similar acicular crystals and more often occurs in granular or massive forms. It is distinguished from other rare, blue arsenates (e.g., keyite) mainly by its crystal habit. Definitive identification is almost exclusively possible using advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Crystal Forms It forms elongated, prismatic crystals, terminated by pyramidal faces. It is most commonly found as fan-shaped or radial aggregates, growing from voids in the rock.
Geological environment
## Genesis Andyrobertsite is a secondary mineral that formed in the oxidation zone of a polymetallic, hydrothermal ore deposit. It forms in vugs and small fissures within massive tennantite ore, as a result of the weathering of primary ore minerals. ## Mineral Associations This mineral coexists with other rare arsenates and secondary minerals characteristic of the Tsumeb mine. Its most common associations include prosperite, legrandite, köttigite, adamite, and tennantite. ## Localities The only confirmed locality for andyrobertsite in the world is the Tsumeb mine in the Oshikoto Region, Namibia. This is a classic example of a mineral known from only one locality, making it extremely rare.
Rarity
Extremely rare
Collector aspects
## Quality Criteria For collectors, the most desirable specimens are those with sharp, well-formed, and undamaged crystals of a deep, saturated blue color. Aesthetic, radial aggregates embedded in a contrasting rock matrix are highly valued. The size of the crystals and the entire specimen, as well as the presence of rare associated minerals, significantly increase its value. ## Popular Localities All known specimens of andyrobertsite come from the Tsumeb mine in Namibia. This mine has been closed for years, meaning that new specimens do not appear on the market, and those available in collector circulation are highly prized by systematic mineral specialists and collectors of Tsumeb minerals.
Care and storage
## Cleaning Andyrobertsite specimens should be cleaned with the utmost care. Due to its toxic composition (arsenic, cadmium), all handling should be done with gloves, avoiding dust inhalation. For dust removal, a soft brush is best. Contact with water should be kept to an absolute minimum; if necessary, use distilled water and immediately dry the specimen. ## What to Avoid The mineral is sensitive to chemicals, especially acids, which can destroy it. Ultrasonic cleaners, high temperatures (risk of dehydration and color loss), and prolonged exposure to direct sunlight, which theoretically can cause fading, should be avoided. It is soft, so it must be protected from scratching. ## Storage It is recommended to store specimens in closed, stable containers or display cases to protect them from dust, moisture, and mechanical damage. It is important to clearly label the mineral as toxic to prevent accidental, careless contact.