Malachite

Last updated
Malachite
Malachite, Zaire.jpg
General
Category Carbonate mineral
Formula
(repeating unit)
Cu2CO3(OH)2
Strunz classification 5.BA.10
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/a
Identification
Formula mass 221.1 g/mol
ColorBright green, dark green, blackish green, with crystals deeper shades of green, even very dark to nearly black commonly banded in masses; green to yellowish green in transmitted light
Crystal habit Massive, botryoidal, stalactitic, crystals are acicular to tabular prismatic
Twinning Common as contact or penetration twins on {100} and {201}. Polysynthetic twinning also present.
Cleavage Perfect on {201} fair on {010}
Fracture Subconchoidal to uneven
Mohs scale hardness3.5–4
Luster Adamantine to vitreous; silky if fibrous; dull to earthy if massive
Streak light green
Diaphaneity Translucent to opaque
Specific gravity 3.6–4
Optical propertiesBiaxial (–)
Refractive index nα = 1.655 nβ = 1.875 nγ = 1.909
Birefringence δ = 0.254
References [1] [2] [3] [4]

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. [4]

Contents

Etymology and history

The entrance to the Neolithic era malachite mine complex on the Great Orme Great Orme Copper Mine - geograph.org.uk - 819.jpg
The entrance to the Neolithic era malachite mine complex on the Great Orme

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". [5] The mineral was given this name due to its resemblance to the leaves of the mallow plant. [6]

Malachite 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. [7] [8]

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. [9] Since then, malachite has been used as both an ornamental stone and as a gemstone.

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 at present, [10] but G.N Vertushkova reports the possible discovery of new deposits of malachite in the Urals. [11] 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. [12]

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. [13] [14]

Use

The funerary mask of the Red Queen of Palenque is made from a mosaic of malachite. Ajuar funerario de la Reina Roja - 8.jpg
The funerary mask of the Red Queen of Palenque is made from a mosaic of malachite.

Malachite was used as a mineral pigment in green paints from antiquity until c. 1800. [16] 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, [17] which features a huge malachite vase, and the Malachite Room in Castillo de Chapultepec in Mexico City. [18] Another example is the Demidov Vase, part of the former Demidov family collection, and now in the Metropolitan Museum of Art. [19] "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. [20]

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. [21] Marbodus recommended malachite as a talisman for young people because of its protective qualities and its ability to help with sleep. [22] It has also historically been worn for protection from lightning and contagious diseases and for health, success, and constancy in the affections. [22] 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. [22]

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. [23]

Ore uses

Copper nugget example Natural copper nugget.jpg
Copper nugget example

Simple methods of copper ore extraction from malachite involved thermodynamic processes such as smelting. [24] 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 ore. [24] [25]

The basic word equation for this reaction is:

Copper carbonate + heat --> carbon dioxide + copper oxide (color changes from green to black). [24] [25]

Copper oxide + carbon --> carbon dioxide + copper (color change from black to copper colored). [24] [25]

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. [26] [27] Sulfuric acid is the most common leaching agent for copper oxide ores like malachite and eliminates the need for smelting processes. [28]

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

Cu2(OH)2CO3 + 2H2SO4 --> 2CuSO4 + CO2 +3H2O. [28]

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. [29] Open-pit and underground mining practices can cause environmental degradation through habitat and biodiversity loss. [30] [31] Acid mine drainage can contaminate water and food sources to negatively impact human health if improperly managed or if leaks from tailing ponds occur. [31] [32] The risk of health and environmental impacts of both traditional metallurgy and newer methods of hydrometallurgy are both significant, [31] however, water conservation and waste management practices for hydrometallurgy processes for ore extraction, such as for malachite, are stricter and relatively more sustainable. [33] 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. [34] The copper carbonate dust from malachite is poisonous, therefore lapidary work involving malachite requires the use of a mask. [35]

See also

Related Research Articles

Bioleaching is the extraction of metals from their ores through the use of living organisms. This is much cleaner than the traditional heap leaching using cyanide. Bioleaching is one of several applications within biohydrometallurgy and several methods are used to recover copper, zinc, lead, arsenic, antimony, nickel, molybdenum, gold, silver, and cobalt.

Chalcopyrite Copper iron sulfide mineral

Chalcopyrite ( KAL-ko-PY-ryt) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.

Basic copper carbonate Chemical compound

Basic copper carbonate is a chemical compound, more properly called copper(II) carbonate hydroxide. It is an ionic compound consisting of the ions copper(II) Cu2+
, carbonate CO2−
3
, and hydroxide OH
.

Galena natural mineral form of lead sulfide

Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver.

Azurite Copper carbonate mineral

Azurite is a soft, deep-blue copper mineral produced by weathering of copper ore deposits. During the early 19th century, it was also known as chessylite, after the type locality at Chessy-les-Mines near Lyon, France. The mineral, a basic carbonate with the chemical formula Cu3(CO3)2(OH)2, has been known since ancient times, and was mentioned in Pliny the Elder's Natural History under the Greek name kuanos (κυανός: "deep blue," root of English cyan) and the Latin name caeruleum. Since antiquity, azurite's exceptionally deep and clear blue has been associated with low-humidity desert and winter skies. The modern English name of the mineral reflects this association, since both azurite and azure are derived via Arabic from the Persian lazhward (لاژورد), an area known for its deposits of another deep-blue stone, lapis lazuli ("stone of azure").

Chalcanthite

Chalcanthite, whose name derives from the Greek, chalkos and anthos, meaning copper flower, is a richly colored blue/green water-soluble sulfate mineral CuSO4·5H2O. It is commonly found in the late-stage oxidation zones of copper deposits. Due to its ready solubility, chalcanthite is more common in arid regions.

Copper extraction

Copper extraction refers to the methods used to obtain copper from its ores. The conversion of copper consists of a series of physical and electrochemical processes. Methods have evolved and vary with country depending on the ore source, local environmental regulations, and other factors.

Aurichalcite

Aurichalcite is a carbonate mineral, usually found as a secondary mineral in copper and zinc deposits. Its chemical formula is (Zn,Cu)5(CO3)2(OH)6. The zinc to copper ratio is about 5:4.

Cuprite

Cuprite is an oxide mineral composed of copper(I) oxide Cu2O, and is a minor ore of copper.

Dioptase

Dioptase is an intense emerald-green to bluish-green copper cyclosilicate mineral. It is transparent to translucent. Its luster is vitreous to sub-adamantine. Its formula is Cu6Si6O18·6H2O (also reported as CuSiO2(OH)2). It has a hardness of 5, the same as tooth enamel. Its specific gravity is 3.28–3.35, and it has two perfect and one very good cleavage directions. Additionally, dioptase is very fragile, and specimens must be handled with great care. It is a trigonal mineral, forming 6-sided crystals that are terminated by rhombohedra.

Copper(II) oxide Chemical compound - an oxide of copper with formula CuO

Copper(II) oxide or cupric oxide is the inorganic compound with the formula CuO. A black solid, it is one of the two stable oxides of copper, the other being Cu2O or copper(I) oxide (cuprous oxide). As a mineral, it is known as tenorite. It is a product of copper mining and the precursor to many other copper-containing products and chemical compounds.

Linarite

Linarite is a somewhat rare, crystalline mineral that is known among mineral collectors for its unusually intense, pure blue color. It is formed by the oxidation of galena and chalcopyrite and other copper sulfides. It is a combined copper lead sulfate hydroxide with formula PbCuSO4(OH)2. Linarite occurs as monoclinic prismatic to tabular crystals and irregular masses. It is easily confused with azurite, but does not react with dilute hydrochloric acid as azurite does. It has a Mohs hardness of 2.5 and a specific gravity of 5.3 - 5.5.

In ore deposit geology, supergene processes or enrichment are those that occur relatively near the surface as opposed to deep hypogene processes. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary (hypogene) sulfide ore minerals and redistribute the metallic ore elements. Supergene enrichment occurs at the base of the oxidized portion of an ore deposit. Metals that have been leached from the oxidized ore are carried downward by percolating groundwater, and react with hypogene sulfides at the supergene-hypogene boundary. The reaction produces secondary sulfides with metal contents higher than those of the primary ore. This is particularly noted in copper ore deposits where the copper sulfide minerals chalcocite Cu2S, covellite CuS, digenite Cu18S10, and djurleite Cu31S16 are deposited by the descending surface waters.

In situ leach

In-situ leaching (ISL), also called in-situ recovery (ISR) or solution mining, is a mining process used to recover minerals such as copper and uranium through boreholes drilled into a deposit, in situ. In situ leach works by artificially dissolving minerals occurring naturally in a solid state. For recovery of material occurring naturally in solution, see: Brine mining.

Whim Creek Copper Mine

The Whim Creek Copper Mine is an operating copper oxide mine, located in the Pilbara region of Western Australia.

Duftite

Duftite is a relatively common arsenate mineral with the formula CuPb(AsO4)(OH), related to conichalcite. It is green and often forms botryoidal aggregates. It is a member of the adelite-descloizite Group, Conichalcite-Duftite Series. Duftite and conichalcite specimens from Tsumeb are commonly zoned in color and composition. Microprobe analyses and X-ray powder-diffraction studies indicate extensive substitution of Zn for Cu, and Ca for Pb in the duftite structure. This indicates a solid solution among conichalcite, CaCu(AsO4 )(OH), austinite, CaZn(AsO4)(OH) and duftite PbCu(AsO4)(OH), all of them belonging to the adelite group of arsenates. It was named after Mining Councilor G Duft, Director of the Otavi Mine and Railroad Company, Tsumeb, Namibia. The type locality is the Tsumeb Mine, Tsumeb, Otjikoto Region, Namibia.

Calumetite

Calumetite is a natural rarely occurring mineral. It was discovered in 1963 at the Centennial Mine near Calumet, Michigan, United States. Calumetite was first discovered along with anthonyite. It has a chemical formula of Cu(OH,Cl)
2
•2(H
2
O)
.

Pseudomalachite

Pseudomalachite is a phosphate of copper with hydroxyl, named from the Greek for “false” and “malachite”, because of its similarity in appearance to the carbonate mineral malachite, Cu2(CO3)(OH)2. Both are green coloured secondary minerals found in oxidised zones of copper deposits, often associated with each other. Pseudomalachite is polymorphous with reichenbachite and ludjibaite. It was discovered in 1813. Prior to 1950 it was thought that dihydrite, lunnite, ehlite, tagilite and prasin were separate mineral species, but Berry analysed specimens labelled with these names from several museums, and found that they were in fact pseudomalachite. The old names are no longer recognised by the IMA.

Tsumebite

Tsumebite is a rare phosphate mineral named in 1912 after the locality where it was first found, the Tsumeb mine in Namibia, well known to mineral collectors for the wide range of minerals found there. Tsumebite is a compound phosphate and sulfate of lead and copper, with hydroxyl, formula Pb2Cu(PO4)(SO4)(OH). There is a similar mineral called arsentsumebite, where the phosphate group PO4 is replaced by the arsenate group AsO4, giving the formula Pb2Cu(AsO4)(SO4)(OH). Both minerals are members of the brackebuschite group.

Chalconatronite

Chalconatronite is a carbonate mineral and rare secondary copper mineral that contains copper, sodium, carbon, oxygen, and hydrogen, its chemical formula is Na2Cu(CO3)2•3(H2O). Chalconatronite is partially soluble in water, and only decomposes, although chalconatronite is soluble while cold, in dilute acids. The name comes from the mineral's compounds, copper ("chalcos" in Greek) and natron, naturally forming sodium carbonate. The mineral is thought to be formed by water carrying alkali carbonates (possibly from soil) reacting with bronze. Similar minerals include malachite, azurite, and other copper carbonates. Chalconatronite has also been found and recorded in Australia, Germany, and Colorado.

References

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Further reading