Azurite

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This article is about the carbonate mineral. For the FDPSO vessel, see Azurite FDPSO. For the green fluorescent protein derivative, see Green fluorescent protein § GFP derivatives. For the mountain, see Azurite Peak.
Azurite
2V angle
Measured: 68°, calculated: 64°
Dispersionrelatively weak
References[2][3][4]

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.[3] 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.[5] Copper (Cu2+) gives it its blue color.[6]

Mineralogy

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)

carbonate minerals, the other being bright green malachite. Aurichalcite is a rare basic carbonate of copper and zinc.[7] Simple copper carbonate (CuCO3) is not known to exist in nature, due to the high affinity of the Cu2+
 ion for the hydroxide anion HO−
.[8]

Azurite crystallizes in the

monoclinic system.[9] Large crystals are dark blue, often prismatic.[3][4][7] Azurite specimens can be massive to nodular or can occur as drusy crystals lining a cavity.[10]

Azurite has a

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.[7][10]

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.[11][12]

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 carbon dioxide (CO2) units with water (H2O), changing the carbonate:hydroxide ratio of azurite from 1:1 to the 1:2 ratio of malachite:[7]

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.[13]

Occurrences

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

oxidized zone of copper ore deposits. Here they are associated with cuprite, native copper, and various iron oxide minerals.[7]

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 2 inches (51 mm) in diameter. Similar rosettes are found at Chessy, Rhône, France. The best crystals, up to 10 inches (250 mm) in length, are found at Tsumeb, Namibia. Other notable occurrences are in Utah; Mexico; the Ural and Altai Mountains; Sardinia; Laurion, Greece; Wallaroo, South Australia; Brazil and Broken Hill.[10]

Uses

Pigments

Azurite is unstable in air, however it was used as a

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 Lyons, France. It was mined since the 12th century in Saxony, in the silver mines located there.[16]

Heating can be used to distinguish azurite from purified natural

Cennino D'Andrea Cennini. Ultramarine withstands heat, whereas azurite converts to black copper oxide.[17] However, gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.[18]

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.[19]

  • Ground azurite for use as a pigment
    Ground azurite for use as a pigment
  • The background of Lady with a Squirrel by Hans Holbein the Younger was painted with azurite
    The background of Lady with a Squirrel by Hans Holbein the Younger was painted with azurite
  • The greenish tint of the Madonna's mantle in Raphael's Madonna and Child Enthroned with Saints is due to azurite weathering to malachite
    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.[20] However, its softness and tendency to lose its deep blue color as it weathers leaves it with fewer uses.[21]
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.[13]

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.[7]

History

Azurite was known in the pre-classical

ancient Roman wall paintings but Roman writers certainly knew about its use as a pigment.[22] The fusing of glass and azurite was developed in ancient Mesopotamia.[23]

Gallery

See also

References

  1. S2CID 235729616
    .
  2. ^ Handbook of Mineralogy
  3. ^ a b c Mindat.org
  4. ^ a b Webmineral.com Webmineral Data
  5. ^ The Ancient Library: Smith, Dictionary of Greek and Roman Antiquities, p.321, right col., under BLUE Archived December 20, 2005, at the Wayback Machine
  6. ^ "Minerals Colored by Metal Ions". minerals.gps.caltech.edu. Retrieved 2023-03-01.
  7. ^
    ISBN 047157452X
    .
  8. ISBN 9780750659246
    .
  9. S2CID 95738208
    .
  10. ^
    ISBN 0442276249
    .
  11. ^ Nassau, K. (1978). "The origins of color in minerals". American Mineralogist. 63 (3–4): 219–229.
  12. ^ Klein & Hurlbut 1993, pp. 260–263.
  13. ^
    doi:10.1080/00357529.1991.11761595
    .
  14. ^ 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.) Oxford University Press 1993, p. 23–24
  15. ISBN 978-0521452571
    .
  16. ISBN 0-8317-7739-7
  17. doi:10.1179/019713678806029166
    .
  18. doi:10.1080/03094227.1987.9638544
    .
  19. JSTOR 1505860
    .
  20. S2CID 219714562
    .
  21. ISBN 9781402768293
    . Retrieved 18 September 2021.
  22. OCLC 312267983
    .
  23. ISBN 0924171952
    .

External links

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