Guanacoite

Chemical formula: MgCu<sup>2+</sup><sub>2</sub>Mg<sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>4</sub>(H<sub>2</sub>O)<sub>4</sub>

Guanacoite is an extremely rare, hydrated copper magnesium arsenate, forming blue, acicular crystals.

## Characteristics Guanacoite is a very rare secondary mineral from the arsenate group. It occurs as slender, prismatic, or acicular crystals, often forming radial or divergent aggregates. Its most characteristic feature is its vivid blue to greenish-blue color. Crystals are usually small, but their intense color makes them easily noticeable against the host rock. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3. It has a vitreous luster and is translucent. Its density is significantly higher than average, at about 3.75 g/cm³, which is typical for minerals containing heavy elements like arsenic and copper. ## Colors and Varieties Guanacoite occurs in shades from pure blue to greenish-blue. Color variation depends on subtle differences in chemical composition. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 2003 by a team of mineralogists (J. Schlüter, K.-H. Klaska, G. Adiwidjaja, G. Gebhard). Its name comes from its discovery locality – the Guanaco mine in Antofagasta Province, Chile, which is its only known occurrence. ## Applications Due to its extreme rarity, guanacoite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for specialized mineral collectors.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
Light blue
Density
3.75
Cleavage
Good on {010}
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Key features for identifying guanacoite are its characteristic blue color, acicular or slender prismatic crystal habit forming radial aggregates, and its occurrence in paragenesis with other secondary copper arsenates. Hardness (3) and relatively high density are also helpful. ## Distinguishing from Similar Minerals Guanacoite can be confused with other blue secondary minerals. It differs from linarite by crystal habit and paragenesis (linarite is a sulfate). It differs from azurite by habit (azurite rarely forms needles) and lack of reaction with hydrochloric acid. Unlike chrysocolla, guanacoite forms distinct crystals and has greater hardness. Certain identification often requires advanced studies. ## Crystal Forms Crystals are slender, prismatic, with an acicular habit. They typically occur as small, divergent or radial aggregates, growing on the host rock.

Geological environment

## Genesis Guanacoite is a secondary mineral, forming in the oxidation (weathering) zone of polymetallic deposits rich in arsenic and copper. It crystallizes in rock cavities and fractures as a result of surface water action on primary ore minerals. ## Mineral Associations This mineral occurs in association with other rare secondary minerals, such as olivenite, conichalcite, arhbarite, and gypsum. ## Localities The only confirmed occurrence of guanacoite in the world is its type locality – the Guanaco mine, in the Santa Catalina district, Antofagasta Province, in the Atacama Desert, Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria The collector's value of guanacoite specimens is primarily determined by its extreme rarity. Most prized are specimens with well-formed, distinct crystals of intense blue color, forming rich, aesthetic coatings on a small rock matrix. The presence of associated minerals can enhance the specimen's attractiveness. ## Popular Localities All collector specimens come from a single location in the world – the Guanaco mine in Chile. This makes every guanacoite specimen a valuable "topotype" acquisition (from the type locality).

Care and storage

## Cleaning Guanacoite specimens should be cleaned with the utmost care, preferably using a soft brush dry to remove dust. If absolutely necessary, a damp cotton swab with distilled water can be used, avoiding oversaturation of the specimen. Ultrasonic cleaners are strictly prohibited. ## What to Avoid As a hydrated arsenate, guanacoite is chemically and physically sensitive. Contact with acids and other chemicals should be avoided. The mineral is sensitive to high temperatures, which can cause dehydration and damage. It is also brittle and susceptible to mechanical damage. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from dust, light, and damage. Stable humidity and temperature conditions should be maintained.

Sources

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