Azurite or Azure spar[ Krivovichev V. G. Mineralogical glossary. Scientific editor A. G. Bulakh. — St.Petersburg: St.Petersburg Univ. Publ. House. 2009. — 556 p. — ISBN 978-5-288-04863-0. (in Russian)] 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 (κυανός: "deep blue," root of English cyan) and the Latin name Cerulean.]
Mineralogy
Azurite has the formula Cu3(CO3)2(OH)2, with the copper(II) linked to two different anions, carbonate and hydroxide. It is one of two relatively common basic copper(II) carbonate minerals, the other being bright green malachite. Aurichalcite is a rare basic carbonate of copper and zinc. (1993). 047157452X, Wiley. 047157452X
Azurite crystallizes in the monoclinic system.
Azurite has a Mohs hardness of 3.5 to 4. The specific gravity of azurite is 3.7 to 3.9. Characteristic of a carbonate, specimens effervesce upon treatment with hydrochloric acid. The combination of deep blue color and effervescence when moistened with hydrochloric acid are identifying characteristics of the mineral.[
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Color
The optical properties (color, intensity) of minerals such as azurite and malachite are characteristic of copper(II). Many coordination complexes of copper(II) exhibit similar colors. According to crystal field theory, the color results from low energy d-d transitions associated with the d9 metal center.
Weathering
Azurite is unstable in open air compared to malachite, and often is replaced by malachite. This weathering process involves the replacement of some of the carbon dioxide (CO2) units with water (H2O), changing the carbonate:hydroxide ratio of azurite from 1:1 to the 1:2 ratio of malachite:[
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- 2 Cu3(CO3)2(OH)2 + H2O → 3 Cu2(CO3)(OH)2 + CO2
From the above equation, the conversion of azurite into malachite is attributable to the low partial pressure of carbon dioxide in air.
Azurite is quite stable under ordinary storage conditions, so that specimens retain their deep blue color for long periods of time.
Occurrences
Azurite is found in the same geologic settings as its sister mineral, malachite, though it is usually less abundant. Both minerals occur widely as supergene copper minerals, formed in the oxidized zone of copper ore deposits. Here they are associated with cuprite, native copper, and various iron oxide minerals.[ It also often occurs along with quartz, cerussite, chrysocolla, olivenite and baryte.]
Fine specimens can be found at many locations. Among the best specimens are found at Bisbee, Arizona, and nearby locations, and have included clusters of crystals several inches long and spherical aggregates and rosettes up to in diameter. Similar rosettes are found at Chessy, Rhône, France. The best crystals, up to in length, are found at Tsumeb, Namibia. Other notable occurrences are in Utah; Mexico; the Ural Mountains and Altai Mountains; Sardinia; Laurion, Greece; Wallaroo, South Australia; Brazil and Broken Hill.[
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Uses
Pigments
Azurite is unstable in air, however it was used as a blue pigment in antiquity.[Gettens, R.J. and Fitzhugh, E.W., "Azurite and Blue Verditer", in Artists' Pigments: A Handbook of Their History and Characteristics, Vol. 2: A. Roy (Ed.) (1993). Oxford University Press. pp. 23–24.] Azurite is naturally occurring in Sinai and the Eastern Desert of Egypt. It was reported by F. C. J. Spurrell (1895) in the following examples; a shell used as a pallet in a Fourth Dynasty (2613 to 2494 BCE) context in Meidum, a cloth over the face of a Fifth Dynasty (2494 to 2345 BCE) mummy also at Meidum and a number of Eighteenth Dynasty (1543–1292 BCE) wall paintings.[Andersen, Frank J. Riches of the Earth. W.H. Smith Publishers, New York, 1981, ]
Heating can be used to distinguish azurite from purified natural ultramarine blue, a more expensive but more stable blue pigment, as described by Cennino D'Andrea Cennini. Ultramarine withstands heat, whereas azurite converts to black copper oxide.
Azurite pigment can be synthesized by precipitating copper(II) hydroxide from a solution of copper(II) chloride with lime (calcium hydroxide) and treating the precipitate with a concentrated solution of potassium carbonate and lime. This pigment is likely to contain traces of basic copper(II) chlorides.
File:Azuritepigment.jpg|Ground azurite for use as a pigment
File:Lady with a Squirrel.jpg|The background of Lady with a Squirrel by Hans Holbein the Younger was painted with azurite
File:Madonna and Child Enthroned with Saints.jpg|The greenish tint of the Madonna's mantle in Raphael's Madonna and Child Enthroned with Saints is due to azurite weathering to malachite
Jewelry
Azurite is used occasionally as beads and as jewelry, and also as an ornamental stone.
Collecting
The intense color of azurite makes it a popular collector's stone. The notion that specimens must be carefully protected from bright light, heat, and open air to retain their intensity of color over time may be an urban legend. Paul E. Desautels, former curator of gems and minerals at the Smithsonian Institution, has written that azurite is stable under ordinary storage conditions.[
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Prospecting
While not a major ore of copper itself, the presence of azurite is a good surface indicator of the presence of weathered copper sulfide ores. It is usually found in association with the chemically similar malachite, producing a striking color combination of deep blue and bright green that is strongly indicative of the presence of copper ores.[
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History
Azurite was known in the pre-classical ancient world. It was used in ancient Egypt as a pigment, obtained from mines in Sinai Peninsula. Ancient Mesopotamian writers report the use of a special mortar and pestle for grinding it. It was also used in ancient Greece, for example on the Acropolis in Athens. It does not appear to have been used in ancient Roman wall paintings but Roman writers certainly knew about its use as a pigment.[Robert James Forbes, Studies in Ancient Technology, vol. 1, p. 216, Leiden: E. J. Brill, 1955 .] The glass fusing of glass and azurite was developed in ancient Mesopotamia.[Emmerich Paszthory, "Electricity generation or magic? The analysis of an unusual group of finds from Mesopotamia", p. 34 in, Stuart J. Fleming, Helen R. Schenck, History of Technology: The Role of Metals, University of Pennsylvania Museum of Archaeology, 1989 .]
Gallery
Azurite from China.jpg|Azurite crystals from China
File:Azurite from Arizona, collected by Dr John Hunter in the 18th century, Hunterian Museum, Glasgow.jpg|Azurite from Arizona, collected by Dr John Hunter in the 18th century, Hunterian Museum, Glasgow
File:Mineraly.sk - azurit.jpg|Fresh, unweathered azurite crystals showing the deep blue of unaltered azurite. From Špania Dolina, Slovakia
File:Azurite with Malachite - National Museum of Natural History - Washington, D.C..jpg|Azurite with Malachite, Copper Queen mine, Bisbee, Arizona
File:Azurite-23164.jpg|Azurite from Touissit, Morocco
File:Azurite-284714.jpg|Azurite, Morenci, Arizona
File:Azurite in siltstone, Malbunka mine NT.jpg|Azurite in siltstone, Malbunka mine, Northern Territory, Australia
File:Azurite-den07-03b.jpg|Azurite from Tsumeb, Namibia
File:Azurite Bisbee ROM.jpg|Azurite, cross-section through merged , Bisbee, Arizona
File:Batu Azurite.jpg|Azurite from Bandung, Indonesia
File:Sparkly Azurite Crystal 01.jpg|Azurite crystal, from the minerals collection at the Natural History Museum, London.
File:Azurite spheroids.jpg|Spheroidal azurite specimens from Utah
See also
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Basic copper carbonate
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Blue pigments
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List of inorganic pigments
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List of minerals
External links
