Azurite

Azurite is a soft, deep blue copper mineral produced by weathering of copper ore deposits. It is also known as Chessylite after the Chessy-les-Mines near Lyon, France, where striking specimens have been found. The mineral has been known since ancient times, and was mentioned in Pliny the Elder's Natural History under the Greek name kuanos ("deep blue," root of English cyan) and the Latin name caeruleum The blue of azurite is exceptionally deep and clear, and for that reason the mineral has tended to be associated since antiquity with the deep blue color of 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, an area known for its deposits of another deep blue stone, lapis lazuli ("stone of azure").

Mineralogy
Azurite  crystals are monoclinic, and when large enough to be seen they appear as dark blue prismatic crystals. Azurite specimens are typically massive to nodular, and are often stalactitic in form. Specimens tend to lighten in color over time due to weathering of the specimen surface into malachite. Azurite is soft, with a Mohs hardness of only 3.5 to 4. The specific gravity of azurite is 3.77 to 3.89. Azurite is destroyed by heat, losing carbon dioxide and water to form black, powdery copper(II) oxide. Characteristic of a carbonate, specimens effervece upon treatment with hydrochloric acid.

Pigments
Azurite was used as a blue pigment for centuries. It was formerly known as Azurro Della Magna (from 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. Older examples of azurite pigment may show a more greenish tint due to weathering into malachite.

Azurite was distinguished from (the much more expensive) purified natural ultramarine blue by heating (as described by Cennino D'Andrea Cennini). Ultramarine withstands heat, whereas azurite turns black (copper oxide). Gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.

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 limit such uses. Heating destroys azurite easily, so all mounting of azurite specimens must be done at room temperature. When tumbled, azurite takes a fine polish, showing a dazzling display of shades of blue and violet.

Collecting
The intense color of azurite makes it popular collector's stone. However, bright light, heat, and open air all tend to reduce the intensity of its color over time. To help preserve the deep blue color of a pristine azurite specimen, collectors should use a cool, dark, sealed storage environment similar to that of its original natural setting.

Prospecting
While not a major ore of copper itself, azurite is a good surface indicator of the presence of weathered copper sulfide ores. It is usually found in association with the chemically very similar malachite, producing a striking color combination of deep blue and bright green that is strongly indicative of the presence of copper ores.

History
The use of azurite and malachite as copper ore indicators led indirectly to the name of the element nickel in the English language. Nickeline, a principal ore of nickel that is also known as niccolite, weathers at the surface into a green mineral (annabergite) that resembles malachite. This resemblance resulted in occasional attempts to smelt nickeline in the belief that it was copper ore, but such attempts always ended in failure due to high smelting temperatures needed to reduce nickel. In Germany this deceptive mineral came to be known as kupfernickel, literally "copper demon". The Swedish alchemist Baron Axel Fredrik Cronstedt (who had been trained by Georg Brandt, the discoverer of the nickel-like metal cobalt) realized that there was probably a new metal hiding within the kupfernickel ore, and in 1751 he succeeded in smelting kupfernickel to produce a previously unknown (except in certain meteorites) silvery white, iron-like metal. Logically, Cronstedt named his new metal after the nickel part of kupfernickel. An unintended later consequence of his choice is that both Canadian and American coins worth one-twentieth of a dollar are now named after the German word for "demons"—that is, they are called nickels.

Composition


Azurite is one of two basic copper(II) carbonate minerals, the other being bright green malachite. Simple copper carbonate (CuCO3) is not known to exist in nature. In azurite, copper(II) is linked to two different anions, carbonate and hydroxide, the compound has the formula Cu3(CO3)2(OH)2. The optical properties (color, intensity) of minerals such as azurite and malachite are explained in the context of conventional electronic spectroscopy of coordination complexes. Relatively detailed description are provided by Ligand Field Theory. Blue-colored species akin to the azurite can be obtained by combining solutions of copper sulfate with a saturated solution of sodium carbonate.

Weathering


Azurite is unstable in open air with respect to malachite, and often is pseudomorphically replaced by malachite. The weathering process effect of the replacement of some the carbon dioxide (CO2) units with water (H2O). This change in the carbonate/hydroxide ratio of azurite into the 1-to-1 ratio of malachite:


 * 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 also incompatible with aquatic media, such as salt-water aquariums.

Toxicity
Minerals in general should not be ingested.