Malachite

Malachite is a copper carbonate hydroxide mineral, with the formula Cu2CO3(OH)2. This opaque, green-banded mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses, in fractures and deep, underground spaces, where the water table and hydrothermal fluids provide the means for chemical precipitation. Individual crystals are rare, but occur as slender to acicular prisms. Pseudomorphs after more tabular or blocky azurite crystals also occur.

Malachite

General

Category Mineral

Physical properties

Color Green

Stripe Black

Gloss Metallic in crystals, matte in earthy masses,

Transparency translucent

Crystal system Monoclinic

Crystalline habit Massive, botryoidal, stalactitic, granular, fibrous

Exfoliation Perfect

Fracture Conchoidal

Hardness 5-6

Specific weight 3.75-3.95

Density 4.22 g/cm³

Refractive index nα = 1.655

nβ = 1.875

nγ = 1.909

Birefringence δ = 0.254

Optical properties Biaxial (–)

Etymology and history

The stone's name derives (via Latin: molochītis, Middle French: melochite, and Middle English melochites) from Greek Μολοχίτης λίθος molochites lithos, "mallow-green stone", from μολόχη molochē, variant of μαλάχη malāchē, "mallow". The mineral was given this name due to its resemblance to the leaves of the mallow plant. Copper (Cu2+) gives malachite its green color.

Malachite was mined from deposits near the Isthmus of Suez and the Sinai as early as 4000 BCE.

It was extensively mined at the Great Orme Mines in Britain 3,800 years ago, using stone and bone tools. Archaeological evidence indicates that mining activity ended c. 600 BCE, with up to 1,760 tonnes of copper being produced from the mined malachite.

Archaeological evidence indicates that the mineral has been mined and smelted to obtain copper at Timna Valley in Israel for more than 3,000 years. Since then, malachite has been used as both an ornamental stone and as a gemstone.

The use of azurite and malachite as copper ore indicators led indirectly to the name of the element nickel in the English language. Nickeline, a principal ore of nickel that is also known as niccolite, weathers at the surface into a green mineral (annabergite) that resembles malachite. This resemblance resulted in occasional attempts to smelt nickeline in the belief that it was copper ore, but such attempts always ended in failure due to high smelting temperatures needed to reduce nickel. In Germany this deceptive mineral came to be known as kupfernickel, literally "copper demon." The Swedish alchemist Baron Axel Fredrik Cronstedt (who had been trained by Georg Brandt, the discoverer of the nickel-like metal cobalt) realized that there was probably a new metal hiding within the kupfernickel ore, and in 1751 he succeeded in smelting kupfernickel to produce a previously unknown (except in certain meteorites) silvery white, iron-like metal. Logically, Cronstedt named his new metal after the nickel part of kupfernickel.

Classification

In the outdated 8th edition of the mineral classification according to Strunz, malachite still belonged to the common mineral class of “carbonates, nitrates and borates ” and there to the section “anhydrous carbonates without foreign anions”, where it forms the “azurite-hydrozincite group” with the system number Vb/B.01 together with aurichalcite, azurite, hydrozincite, loseyite and rosasite.

In the Lapis systematics by Stefan Weiß, last revised in 2018 and formally based on the old systematics by Karl Hugo Strunz in the 8th edition, the mineral was given the system and mineral number V/C.01-020. This also corresponds to the section "Anhydrous carbonates, with foreign anions", where malachite, together with aurichalcite, azurite, brianyoungite, chukanovite, georgeite, glaucospherite, hydrozincite, kolwezite, loseyite, mcguinnessite, nullaginite, paradsasvarite, pokrovskite, rosasite, sclarite, and zinc rosasite, forms the "Azurite-Rosasite Series" with the system number V/C.01. 

Since the complete revision of Strunz's mineral classification in the 9th edition (2001), the mineral class of carbonates (and related minerals) has been reclassified, and borates form a separate class. Malachite is therefore now found in the mineral class of "carbonates and nitrates" and within it in the section "carbonates with additional anions; without H2O". This is further subdivided according to the cations involved in the compound, and the mineral is sorted according to its composition in the subsection "With Cu, Co, Ni, Zn, Mg, Mn". As the eponymous mineral, it forms the "malachite group" with the system number 5.BA.10 and the other members chukanovite, georgeite, glaucospherite, kolwezite, mcguinnessite, nullaginite, pokrovskite, rosasite, and zinc rosasite.

In the Dana classification of minerals, which is predominantly used in English-speaking countries, MineralName has the system and mineral number 16a.03.02.01. As in the outdated Strunz and subsequent Lapis classifications, this corresponds to the common class of "Carbonates, Nitrates, and Borates" and therein to the division "Carbonates - Hydroxyl or Halogen," where the mineral, along with chukanovite, nullaginite, and pokrovskite, is found in the "Malachite Group" with the system number 16a.03.02 within the subdivision "Carbonates - Hydroxyl or Halogen with (AB) 2 (XO) 3 Z q ".

Chemistry

In the ideal, pure composition of malachite (Cu 2 (CO 3)(OH) 2), the mineral consists of two parts copper (Cu), one part carbon (C), five parts oxygen (O), and two parts hydrogen (H). This corresponds to a mass fraction (weight percent) of 57.48 wt.% Cu, 5.43 wt.% C, 36.18 wt.% O, and 0.91 wt.% H or, in the oxide form, 71.95 wt.% copper(II) oxide (CuO), 19.90 wt.% carbon dioxide (CO 2), and 8.15 wt.% H 2 O. 

Natural mineral formations closest to this theoretical ideal composition were found in the area around Rio Marina on the Italian island of Elba. The analyzed samples consisted of 72.03 wt% CuO, 20.04 wt% CO 2, and 8.09 wt% H 2 O. 

Crystal structure

Malachite crystallizes monoclinic in the space group P 2 1 / a (space group no. 14, position 3) with the lattice parameters a = 9.50 Å; b = 11.97 Å; c = 3.24 Å and β = 98.7° as well as 4 formula units per unit cell. 

Characteristics

Malachite is characterized by its exclusively green color, which occurs in banded layers between light green and black-green. Malachite generally exhibits very strong pleochroism (multicolor), which, depending on the direction of the light, manifests itself in a color change from almost colorless to yellow-green to deep green. 

The mineral's Mohs hardness is approximately 3.5 to 4, and its density is 3.6 to 4.05 g/cm³, depending on the compound's purity. Its copper content is approximately 57 percent.

Due to its low hardness and relatively low density, malachite is prone to breakage. Sunlight causes it to pale, while water can diminish its luster and, in some cases, even cause a discoloration.

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 was historically believed to be the primary component of the patina which forms on copper and copper alloy structures exposed to open-air weathering; however, 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+ ions 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 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 wands and 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, panelling 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.

Use as a gemstone

Today, malachite is primarily used as a decorative stone in arts and crafts. In the Kremlin in Moscow, entire columns are made of malachite, which comes from the Urals. The banding, which occurs in various shades of green and is visible similar to wood grain, is particularly beautiful and highly valued. However, due to the toxic dust and grinding water it produces, its processing is complex and accordingly expensive.

It's relatively easy, albeit time-consuming, to coat the calcium carbonate-containing shells of mussels and snails, as well as marble ornamental objects, with a malachite coating. After several weeks of storage in a concentrated, aqueous copper sulfate solution, a more or less thick layer of malachite forms on the surface, releasing carbon dioxide (CO2). The shell limestone is gradually dissolved in the process.

Modern interior design

Malachite is still a source of inspiration for American artists such as Tony Duquette (1914–1999) in his own Dawnridge residence in Beverly Hills, USA, or for designers Kelly Wearstler or Jonathan Adler in their creations for their clients. The particular decorative motifs described by malachite can be taken up on new supports, thanks to modern printing techniques, whatever the support, such as on clothing or furnishing fabrics (malachite print): curtains, wall fabrics, as did the American fashion designer Halston.  

As a pigment

Finely ground malachite has been used since ancient times as a pigment known as “cold green” for wall paintings.

In a similar way to the malachite coating on shellfish, the mineral pigment malachite green can be produced from marble powder by mixing it with copper sulfate. The fineness of the powder determines the quality of the resulting pigment, which is used primarily in oil painting. A famous example of the use of malachite in painting is the green curtain in the background of Raphael 's Sistine Madonna. In the Colour Index, malachite is listed under the number CI Pigment Green 39. 

An organic dye of the same color called malachite green is not derived from the mineral malachite.

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, however, due to increase 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 as follows:

Cu2(OH)2CO3 + sulfuric acid 2H2SO4 → copper sulfate 2CuSO4 + carbon dioxide CO2 + water 3H2O (Reaction 1)

Other uses

In cosmetology, it is claimed that it has a protective action at several levels of the antioxidant defense process of cells, and a detoxifying effect.

In watchmaking, malachite can be used to make watch dials.

Symbolism and superstitions

A 17th-century Spanish superstition held that having a child 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 it was customary to wear it engraved with a figure or symbol of the Sun 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. 

Manipulations and imitations

Since malachite is a sought-after gemstone, but also quite soft (hardness 3.5 to 4 compared to glass 5 to 5.5), porous and sensitive due to its complete cleavage, rough stones are stabilized by incorporating a malachite dust-synthetic resin mixture. The same treatment method is also used to reconstruct larger, usable malachites from small fragments. These methods of manipulation are widespread with tumbled stones and " donuts " and are often observed during the azurite -malachite transformation phase. Stabilized and reconstructed stones can be recognized by their grainy, spotty appearance due to the filling of the gaps with filler. Malachite is often imitated by colored jasper, agate or marble. The "red malachite" (trade name) is actually red jasper. Malachite occurs naturally only in green shades. 

Esotericism

In esotericism, malachite is associated with Venus as a "planetary stone," with Capricorn as a " zodiac stone " alongside the main stones onyx and cat 's eye quartz, and with Friday as a weekday stone alongside emerald. Esotericists also use malachite as a healing stone for various ailments, including headaches, eye inflammation, arthritis, and irritation of the sciatic nerve. However, there is no scientific evidence for the listed effects.

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. Open-pit and underground mining practices can cause environmental degradation through habitat and biodiversity loss. Acid mine drainage can contaminate water and food sources to negatively impact human health 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, however, 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.

As a copper mineral, malachite also has toxic properties. Water in which malachite has been lying should therefore never be drunk. Sawing or crushing malachite without appropriate protective measures (dust mask, vacuum cleaner) is equally dangerous, as the fine dust releases considerable amounts of soluble copper and is accordingly toxic.

To prevent dents, cracks, and color changes, the mineral should be handled with care. While storing it in the dark is excessive, prolonged exposure to direct sunlight can cause visible fading. Even brief contact with water is not harmful. However, prolonged immersion in water usually takes away the stone's shine.

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