CuZnCl(OH)_3_

Chemical formula: CuZnCl(OH)<sub>₃</sub>

Gillardite is a very rare secondary copper and zinc mineral, forming characteristic rosette-like aggregates of green, tabular crystals.

## Characteristics Gillardite is a very rare halide from the atacamite group, chemically a basic copper and zinc chloride. It occurs as small, hexagonal or pseudohexagonal crystals with a tabular habit. These crystals often form fan-shaped or rosette-like aggregates, less frequently appearing as single, scattered flakes on the surface of the host rock. Its structure is closely related to herbertsmithite. ## Physical Properties Gillardite crystals exhibit a hardness of approximately 3 on the Mohs scale. They have a vitreous luster, and a pearly luster on cleavage surfaces. They are transparent to translucent. The mineral's density is approximately 3.96 g/cm³. ## Colors and Varieties Gillardite has a characteristic emerald-green color. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was named after Dermot Gillard, an Australian mineral collector who first found its specimens. It was recognized by the International Mineralogical Association (IMA) in 2004. The type locality, from which the first described specimens originated, is the 132 North mine in the Widgiemooltha region of Western Australia. ## Uses Due to its extreme rarity and small crystal size, gillardite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and a subject of scientific research.

Properties

Mohs hardness: 3
Luster: Vitreous, Pearly
Streak: Pale green
Density: 3.96
Cleavage: Perfect on {0001}
Fracture: Micaceous
Transparency: Transparent to Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Characteristic features of gillardite include its emerald-green color, hexagonal crystal outline, and tendency to form rosette-like and fan-shaped aggregates. Excellent cleavage and a pearly luster on its surfaces are also helpful in identification. ## Distinguishing from Similar Minerals Gillardite can be confused with other secondary copper minerals of similar color, such as brochantite, malachite, or other minerals from the atacamite group (e.g., herbertsmithite). It is distinguished from malachite by its lack of reaction with hydrochloric acid (it does not effervesce). It is distinguished from brochantite and other copper sulfates by its crystal form (hexagonal plates) and chemical tests for the presence of chlorine. Final distinction from herbertsmithite requires advanced analyses (e.g., XRD). ## Crystal Forms Gillardite forms thin, tabular crystals with a hexagonal or trigonal outline. They most often occur as radial, rosette-like, or fan-shaped aggregates. Single, isolated crystals are less common.

Geological environment

## Genesis Gillardite is a secondary mineral, forming in the oxidation zones (gossans) of copper and zinc ore deposits. It forms under dry and saline conditions, as a result of the alteration of primary sulfide minerals by chloride-rich waters. ## Mineral Associations This mineral co-occurs with other secondary copper and zinc minerals. It is most commonly found in association with paratacamite, herbertsmithite, gypsum, quartz, goethite, and hematite. ## Localities The most important and type locality for gillardite is the 132 North mine in Widgiemooltha, Western Australia. It is also known from several other localities worldwide, including the Esperanza mine in the Atacama region of Chile and Laurion, Greece.

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. An intense, emerald-green color and good contrast with the surrounding host rock are highly valued. Due to the mineral's rarity, even microscopic but well-defined crystals are considered valuable. ## Popular Localities By far the most prized and classic gillardite specimens come from the type locality - the 132 North mine in Western Australia. Material from this mine is considered exemplary for this species.

Care and storage

## Cleaning Gillardite specimens should be cleaned with the utmost care, using only compressed air to remove dust. Due to its softness and delicacy, contact with water, brushes, and other mechanical tools should be avoided. ## What to Avoid The mineral is sensitive to chemicals, especially acids, which can damage or completely dissolve it. Ultrasonic cleaners, sudden temperature changes, and prolonged exposure to moisture should be avoided. ## Storage The safest way to store gillardite is to place it in a sealed, padded collector's box (e.g., a "membrane box" or perky box), which protects it from dust, mechanical damage, and moisture. It should be stored under stable room conditions.