Malachite

Occurrence

Malachite often results from the supergene weathering and oxidation of primary sulfidic copper ores and is often found with azurite (Cu₃(CO₃)₂(OH)₂), goethite, and calcite. Except for its vibrant green color, the properties of malachite are similar to those of azurite and aggregates of the two minerals occur frequently. Malachite is more common than azurite and is typically associated with copper deposits around limestones, the source of the carbonate.

Large quantities of malachite have been mined in the Urals, Russia. Ural malachite is not being mined as of 2006, but G.N Vertushkova reports the possible discovery of new deposits of malachite in the Urals. It is found worldwide, including in the Democratic Republic of the Congo; Gabon; Zambia; Tsumeb, Namibia; Mexico; Broken Hill, New South Wales; Burra, South Australia; Lyon, France; Timna Valley, Israel; and the Southwestern United States, most notably in Arizona.

Anthropogenic malachite historically was believed to be the primary component of the patina that forms on copper and copper alloy structures exposed to open-air weathering, but atmospheric sources of sulfate and chloride (such as air pollution or sea winds) typically favour the formation of brochantite or atacamite. Malachite can also be produced synthetically, in which case it is referred to as basic copper carbonate or green verditer.

Structure

Malachite crystallizes in the monoclinic system. The structure consists of chains of alternating Cu²⁺ and OH⁻ ions, with a net positive charge, woven between isolated triangular CO₃²⁻ ions. Thus, each copper ion is conjugated to two hydroxyl ions and two carbonate ions; each hydroxyl ion is conjugated with two copper ions; and each carbonate ion is conjugated with six copper ions.

Use

Malachite was used as a mineral pigment or dye in green paints from antiquity until c. 1800. The pigment is moderately lightfast, sensitive to acids, and varying in color. This natural form of green pigment has been replaced by its synthetic form, verditer, among other synthetic greens.

Malachite is also used for decorative purposes, such as in the Malachite Room in the Hermitage Museum, which features a huge malachite vase, and the Malachite Room in Castillo de Chapultepec in Mexico City. Russian tsars obtained malachite for decorating their castles, paneling the walls, and for beautiful inlaid works. Another example is the Demidov Vase, part of the former Demidov family collection and now in the Metropolitan Museum of Art. "The Tazza", a large malachite vase, one of the largest pieces of malachite in North America and a gift from Tsar Nicholas II, stands as the focal point in the centre of the room of Linda Hall Library. In the time of Tsar Nicolas I decorative pieces with malachite were among the most popular diplomatic gifts. It was used in China as far back as the Eastern Zhou period. The base of FIFA World Cup Trophy has two layers of malachite. There is a malachite fireplace surround in Cliffe Castle, Keighley, using malachite mined in the Urals.

Symbolism and superstitions

A 17th-century Spanish superstition held that having children wear a lozenge of malachite would help them sleep and keep evil spirits at bay. Marbodus recommended malachite as a talisman for young people because of its protective qualities and its ability to help with sleep. It has also historically been worn for protection from lightning and contagious diseases and for health, success, and constancy in the affections. During the Middle Ages, wearing it engraved with a figure or symbol of the Sun was customary to maintain health and to avert depression to which Capricorns were considered vulnerable.

In ancient Egypt, the colour green (wadj) was associated with death and the power of resurrection, as well as new life and fertility. Ancient Egyptians believed that the afterlife contained an eternal paradise, referred to as the "Field of Malachite", which resembled their lives, but with no pain or suffering. During these times, the green pigment made from malachite was often used for eye preparations during the burial process. These preparations were an essential tool in the funerary equipment, even in modest burials.

Ore uses

Simple methods of copper ore extraction from malachite involved thermodynamic processes such as smelting. This reaction involves the addition of heat and a carbon, causing the carbonate to decompose, leaving copper oxide and an additional carbon source such as coal converts the copper oxide into copper metal.

The basic word equation for this reaction is:

Copper carbonate + heat → carbon dioxide + copper oxide (color changes from green to black).

Copper oxide + carbon → carbon dioxide + copper (color change from black to copper colored).

Malachite is a low-grade copper ore, but due to increased demand for metals, more economic processing such as hydrometallurgical methods (using aqueous solutions such as sulfuric acid) are being used, as malachite is readily soluble in dilute acids. Sulfuric acid is the most common leaching agent for copper oxide ores like malachite and eliminates the need for smelting processes.

The chemical equation for sulfuric acid leaching of copper ore from malachite is:

Health and environmental concerns

Mining for malachite for ornamental or copper ore purposes involves open-pit mining or underground mining depending on the grade of the ore deposits. These mining practices can cause environmental degradation through habitat and biodiversity loss. Acid mine drainage can contaminate water and food sources to impact human health negatively if improperly managed or if leaks from tailing ponds occur. The risk of health and environmental impacts of both traditional metallurgy and newer methods of hydrometallurgy are both significant, but water conservation and waste-management practices for hydrometallurgy processes for ore extraction, such as for malachite, are stricter and relatively more sustainable. New research is also being conducted on better alternatives to methods such as sulfuric acid leaching, which has high environmental impacts, even under hydrometallurgy regulation standards and innovation.