malachite

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malachite

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[Middle English melochite, from Latin molochītes, from Greek molokhītis, from malakhē, molokhē, mallow, of Semitic origin.]

For more information on malachite, visit Britannica.com.

A bright-green, basic carbonate of copper [Cu₂CO₃(OH)₂]. Malachite is the most stable copper mineral in natural environments in contact with the atmosphere and hydrosphere. It occurs as an ore mineral in oxidized copper sulfide deposits; as a stain on fractures in rock outcrops; as a corrosion product of copper and its alloys (except in industrial-urban environments, where the basic copper sulfate dominates); as suspended particles in streams and in alluvial sediments; and as encrustations on bronze artifacts in seawater and on cocco-liths floating in the oceans. It can be distinguished from other green copper minerals by its effervescence in acid. The combination of hardness (3.5–4 on Mohs scale) ideal for carving, color variation in concentric layers, and adamantine-to-silky luster has made malachite a highly prized ornamental stone. Its rare blocky-tabular crystals up to 5 mm (0.2 in.), its pseudomorphs after azurite crystals to 2 cm (0.8 in.), and its more common felty tufts perched on bright blue azurite are eagerly sought by mineral collectors. Malachite is an important copper ore mineral in supergene copper oxide deposits formed by weathering of primary copper sulfide deposits. See also Azurite; Carbonate minerals; Copper.

A carbonate of copper; green in color; harder than marble; usually employed as a highly polished veneer.

A precious stone (a variety of topaz) of basic copper carbonate. Folklore held that it preserved the cradle of an infant from spells.

Environment

Secondary (weathered) zone of copper ore deposits.

Usually in fibrous and silky crusts and masses; individualized single crystals are rare. When found they are usually silky-appearing twins with a re-entrant angle showing at the top. Also massive and earthy, often as thin films staining rock.

Light to dark green. Luster usually silky, crystals vitreous; hardness 3Ɖ-4; specific gravity 3.9-4.0; streak green; fracture usually splintery; cleavage basal, visible in the rare single crystals and sometimes across crusts. Brittle; opaque to translucent.

Copper carbonate (71.9% CuO, 19.9% CO ₂ , 8.2% H ₂ O).

Dissolves readily in hydrochloric acid with the release of bubbles. Gives all copper tests: solution color, bead test, copper bead on charcoal, and flame color.

Its intense green color shows it to be a copper mineral, but, in case of any doubt, a test could be made to make sure that it is not a chromium or nickel green. Likely to be confused with numerous copper sulfates, arsenates, and phosphates, but the effervescence as it dissolves in the hydrochloric acid eliminates them and proves it to be a carbonate.

The most common and most stable of the secondary ores of copper, always forming as a consequence of the weathering of primary copper sulfides. In its environment, it is so abundant that it constitutes an important ore, frequently appearing in the capping over a copper deposit, together with azurite, cuprite, and native copper.

Malachite usually forms fibrous crusts and masses. In a very compact form it was extensively used in Russia for carvings and mosaics. Since it varies considerably in hardness and may be poorly consolidated, only the hardest masses are suitable for this use. Several African countries still produce this dense lapidary material. Some years ago specimens were abundant at the famous Copper Queen Mine at Bisbee, Arizona, though most Arizona occurrences are soft, almost velvety crusts of slender needles, stalactites, and stalagmites, layered masses frequently alternating with bands of azurite, a constant associate.

Crystals of malachite are rare and usually small; few are over 1/16 in. (2 mm) across. The best have come from Germany, Russia, Namibia, Zaire, and sw. U.S. Large pseudomorphs composed of radiating velvety malachite needles, which seem to start from several centers on each surface, result from the alteration of dark blue crystals of azurite. Most recently the largest malachite crystals have come from the Ongonja Mine, Namibia, but previously they came from other localities, particularly Arizona and Morocco. A few large pseudomorphs after giant azurites like those of Tsumeb have been found in Michoacán, Mexico.

For other uses, see Malachite (disambiguation).

Malachite
Malachite, Zaire
General
Category	Carbonate mineral
Chemical formula	CuCO3.Cu(OH)2
Identification
Molar mass	221.1 g/mol
Color	Bright green to blackish green
Crystal habit	Massive, botryoidal, stalactitic
Crystal system	Monoclinic - prismatic
Cleavage	Perfect on [201] fair on [010]
Fracture	Conchoidal to splintery
Mohs scale hardness	3.5 - 4.0
Luster	Dull/vitreous in large quantities, silky in crystal form
Streak	light green
Diaphaneity	Transparent to translucent
Specific gravity	3.6 - 4
Optical properties	Biaxial (-)
Refractive index	nα = 1.655 nβ = 1.875 nγ = 1.909
Birefringence	δ = 0.254
References	[1][2]

Malachite is a carbonate mineral normally known as "copper carbonate" with the formula CuCO₃.Cu(OH)₂. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms. Pseudomorphs after more tabular or blocky azurite crystals also occur.

Contents 1 Etymology and history 2 Occurrence and historical uses 3 Gallery 4 References 5 See also

Etymology and history

The stone's name derives (via Latin and French) from Greek molochitis, "mallow-green stone", from molochē, variant of malachē, "mallow". Malachite was used as a mineral pigment in green paints from antiquity until about 1800. The pigment is moderately lightfast, very sensitive to acids and varying in color. The natural form was being replaced by its synthetic form, verditer amongst other synthetic greens. It is also used for decorative purposes, such as in the Malachite Room in the Hermitage, which features a large malachite vase (unknown scale). "The Tazza", one of the largest pieces of malachite in North America and a gift from Tsar Nicholas II, stands as the focal point in the center of the room of Linda Hall Library.

Occurrence and historical uses

Malachite often results from weathering of copper ores and is often found together 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 together. 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. It is found in the Democratic Republic of Congo; Zambia; Tsumeb; Namibia; Russia; Mexico; Broken Hill, New South Wales; England; Lyon; and in the Southwestern United States especially in Arkansas and Arizona. In Israel, malachite is extensively mined at Timna valley, often called King Solomon's Mines, although research has revealed an interruption in mining activity at the site during the 10th century, the time of Solomon.^[3] Archeological evidence indicates that the mineral has been mined and smelted at the site for over 3,000 years. Most of Timna's current production is also smelted, but the finest pieces are worked into silver jewelry.

In Greek mythology, the throne of Demeter, goddess of grain and harvest, was fashioned from malachite and adorned with golden pigs and ears of barley.^[4]

Gallery

Malachite from the Democratic Republic of Congo	A polished slice of Malachite	Malachite	Malachite crystals atop blue azurite, with brown cuprite on the white kaolinite
Ball-and-stick model of malachite's unit cell	Polished Malachite

References

1. ^ http://webmineral.com/data/Malachite.shtml Webmineral data
2. ^ http://mindat.org/min-2550.html Mindat
3. ^ Parr, Peter J review of "Timma: Valley of the Biblical Copper Mines" by Beno Rothenberg Bulletin of the School of Oriental and African Studies, University of London, Vol. 37, No. 1, In Memory of W. H. Whiteley (1974), pp. 223-224
4. ^ http://alex.edfac.usyd.edu.au/BLP/websites/ROBMATULEWICZ/gods.html The Palace of Olympus (University of Sydney, Australia)

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
• Mineral Galleries
• Virtual tour of the Malachite Room

See also

• aventurine
• dioptase

Look up malachite in Wiktionary, the free dictionary.

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