Herbertsmithite

Chemical formula: Cu²⁺₃Zn²⁺(OH)₆Cl₂

Herbertsmithite is a rare copper and zinc hydroxychloride, distinguished by its intense green color and trigonal crystals.

## Characteristics Herbertsmithite is a mineral from the atacamite group, being a hydrated copper and zinc chloride. It forms small, well-developed crystals with a tabular or rhombohedral habit, often gathered in rosette-like or radial aggregates. Its most characteristic feature is its vivid, emerald-green or blue-green color, which makes it an attractive micromount mineral. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3-3.5. It has a vitreous luster and is transparent. Its density is quite high, approximately 3.85 g/cm³, which is typical for copper-bearing minerals. ## Colors and Varieties Herbertsmithite occurs in shades from light green, through blue-green, to intense emerald-green. No named varieties are distinguished, and color differences depend on slight variations in chemical composition and crystallization conditions. ## History and Name The mineral was first described in 1972. Its name honors the British mineralogist Herbert Smith (1872-1953), who was the curator of the mineral collection at the Natural History Museum in London and the inventor of the mineralogical refractometer. The discovery was made at the Los Tres Presidentes mine in Chile. ## Uses Due to its rarity and small crystal size, herbertsmithite has no industrial applications. However, it is a valued and sought-after collector's stone, especially among micromount mineral enthusiasts.

Properties

Mohs hardness: 3-3.5
Luster: Vitreous
Streak: light green
Density: 3.85
Cleavage: on {10_11}
Fracture: Uneven
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Herbertsmithite can be identified by its characteristic emerald-green color, trigonal crystal form, and co-occurrence with other secondary copper minerals. A light green streak is also a helpful diagnostic feature. ## Distinguishing from Similar Minerals It can be confused with other green secondary copper minerals, such as atacamite, paratacamite, or brochantite. It is distinguished from atacamite and paratacamite (which crystallize in other systems) by its trigonal crystal symmetry. Accurate identification often requires advanced research methods, such as X-ray diffraction (XRD). ## Crystal Forms Herbertsmithite crystals are usually small, with a tabular or rhombohedral habit. They often form aggregates in the shape of rosettes, radial clusters, or crusts on the host rock.

Geological environment

## Genesis Herbertsmithite is a secondary mineral, formed in the oxidation zones (gossans) of copper and zinc ore deposits. It forms in arid and saline climates as a result of the alteration of primary sulfide minerals by chloride-rich waters. ## Mineral Associations It often co-occurs with other secondary copper and zinc minerals, such as paratacamite, atacamite, anglesite, gypsum, hemimorphite, and iron oxides (goethite, hematite). ## Localities The most important and type locality is the Los Tres Presidentes mine in the Caracoles district, Antofagasta region, Chile. It is also known from several other localities worldwide, including Iran, Greece (Lavrion), and the United States (Arizona).

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp crystals of intense, emerald-green color. Rich clusters of crystals forming visually attractive rosettes or carpets on a small piece of host rock are also highly valued. Due to the small size of the crystals, aesthetics and specimen quality are crucial, especially for micromounts. ## Popular Localities By far the most sought-after and classic specimens come from the type locality in Chile (Los Tres Presidentes Mine). Specimens from other places, such as mines in the Iranian province of Yazd, are also valued, but Chilean ones are considered exemplary for this species.

Care and storage

## Cleaning Herbertsmithite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. After wet cleaning, the specimen must be thoroughly dried. ## What to Avoid The mineral is sensitive to acids and strong chemicals, which can damage it or alter its color. Ultrasonic cleaners and steam cleaning should be avoided. As a secondary mineral, it may be unstable in conditions of high humidity. ## Storage Collector specimens of herbertsmithite, especially those in micromount form, are best stored in closed, padded boxes to protect them from dust and mechanical damage. Exposure to direct sunlight, which could cause the intense color to fade over time, should be avoided.