Azurite - WikiHQ

Azurite or Azure spar 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 near Lyon, France. Copper (Cu2+) gives it its deep blue color.

Mineralogy

left|thumb|220px|Chemical structure of azurite. Color code: red = O, green = Cu, gray = C, white = H)

Azurite has the formula Cu3(CO3)2(OH)2, with the copper(II) cations 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. Simple copper carbonate (CuCO3) is not known to exist in nature, due to the high affinity of the ion for the hydroxide anion .

Azurite crystallizes in the monoclinic system. Large crystals are dark blue, often prismatic.

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.

Weathering

Azurite is unstable in open air compared to malachite, and often is pseudomorphically replaced by malachite. This weathering process involves the replacement of some of the water (H2O) units with carbon dioxide (CO2), changing the carbonate:hydroxide ratio of azurite from 1:1 to the 1:2 ratio of malachite:

Occurrences

thumb|Azurite from Burra Mine, South Australia

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. 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. Depending on the degree of fineness to which it was ground, and its basic content of copper carbonate, it gave a wide range of blues. It has been known as mountain blue, Armenian stone, and azurro della Magna (blue from Germany in Italian). When mixed with oil it turns slightly green. When mixed with egg yolk it turns green-grey. It is also known by the names blue bice and blue verditer, though verditer usually refers to a pigment made by chemical process. Older examples of azurite pigment may show a more greenish tint due to weathering into malachite. Much azurite was mislabeled lapis lazuli, a term applied to many blue pigments. As chemical analysis of paintings from the Middle Ages improves, azurite is being recognized as a major source of the blues used by medieval painters. Lapis lazuli (the pigment ultramarine) was chiefly supplied from Afghanistan during the Middle Ages, whereas azurite was a common mineral in Europe at the time. Sizable deposits were found near Lyon, France. It was mined since the 12th century in Saxony, in the silver mines located there.

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. However, gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.

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

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Jewelry

Azurite is used occasionally as beads and as jewelry, and also as an ornamental stone. However, its softness and tendency to lose its deep blue color as it weathers leaves it with fewer uses. Heating destroys azurite easily, so all mounting of azurite specimens must be done at room temperature.

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. The fusing of glass and azurite was developed in ancient Mesopotamia.

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 stalactites, 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