azurite

 

(ăzh'ərīt) , blue mineral, the basic carbonate of copper, occurring in monoclinic crystals or masses that range from transparent to translucent and opaque. It is usually associated with malachite, which it resembles except in color; when the two minerals are very closely associated, the stone is called azurmalachite. Beautiful crystals of azurite are found in the United States in Arizona and New Mexico and in France at Chessy (for which the mineral is sometimes called chessylite); they are used for ornamental purposes. The mineral is an important ore of copper.

Azurite
Azurite from China with large crystals and light surface weathering.
General
Category	Mineral
Chemical formula	Cu3(CO3)2(OH)2
Identification
Molecular Weight	344.67 gm
Color	Light Blue-Azure Blue-Dark Blue
Crystal habit	massive, prismatic, stalactitc, tabular
Crystal system	Monoclinic
Twinning	Rare, across {101}, {102} or {001}
Cleavage	Perfect on the {011}, fair on the {100}
Fracture	Concoidal
Tenacity	brittle
Mohs Scale hardness	3.5 to 4
Luster	vitreous
Birefringence	δ = 0.108
Dispersion	relatively weak
Pleochroism	Visible
Streak	Light Blue
Density	3.77 - 3.89, Average = 3.83

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^[1] 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^[2] 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 ^{[3] [4] [5]} 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. azurite can easily be identified because it fizzes with hydrochloric acid.

Uses

Pigments

Azurite has been used as a blue mineral 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 into malachite tend to 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 as a 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.

Historical Trivia

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 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.

Chemistry

Composition

Azurite is one of two basic copper(II) carbonate minerals that occur naturally, the other being bright green malachite. Pure copper carbonate is unstable in water, and is not known to exist in nature. Azurite consists chemically of two parts copper(II) carbonate to one part copper(II) hydroxide:

2CuCO₃ • Cu(OH)₂

Weathering

Azurite is unstable in open air with respect to malachite, and often is pseudomorphically replaced by malachite. The weathering process consists of the replacement of one-fourth of the carbon dioxide (CO₂) units in azurite with units of water (H₂O). This small change transforms the 2-to-1 carbonate-to-hydroxide ratio of azurite into the 1-to-1 ratio of malachite:

2( 2CuCO₃ • Cu(OH)₂ ) + H₂O → 3( CuCO₃ • Cu(OH)₂ ) + CO₂

From the above equation it is reasonable to assume that one factor in why freshly mined azurite begins a slow conversion to malachite is the low vapor pressure of carbon dioxide in open air. Assuming comparable levels of moisture, this moves the equilibrium towards malachite. Storage in a carbon-dioxide rich atmosphere thus would presumably help slow the azurite weathering process by moving the equilibrium point back towards the mineral containing higher levels of CO₂.

Color

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The trivial difference in composition between azurite and malachite raises an interesting question: How can such a small change in ratios produce such a drastic color difference?

In transition metal minerals such as azurite and malachite, intense colors typically arise from the formation of coordination complexes. A coordination complex is a metal ion (cation) surrounded in a geometrically precise fashion by molecules or ions called ligands. A ligand can be either a polar molecule such as water, or a negatively charged ion (anion) such as carbonate. The formation of a coordination complex often changes significantly both the chemical behavior of a metal ion and its response to light, as described in detail by crystal field theory. The rules for naming of such complexes often result in long, archaic-sounding names.

For example, copper(II) ions in solution tend to gather four polar molecules around in the shape of a flat (coplanar) square^[6], with the more negative ends of the molecules pointed towards the copper ion:

 +X-     -X+

     Cu++

 +X-     -X+

While it is easy to see how the charge of the copper(II) could attract the more negative end of such molecules, the flat arrangement (versus the more intuitive tetrahedral arrangement) is a consequence of the strong shaping effects of electron orbitals within the incomplete inner (transition) electron shell of the copper(II) ion. Realizing that the inner shell electrons that control ligand positions are the same ones that capture light makes the powerful impact of the ligands on color easier to understand. Just as passengers can unexpectedly rock a large sightseeing boat rushing to view a spectacular sight on one side, the redistribution of inner electrons due to the presence of enticing ligands can create a precise distribution of charge that selectively captures particular colors or, more precisely the quanta or photons of those colors.^[7]

In the absence of ligands such as water, copper(II) ions are, rather surprisingly, colorless. When water molecules become available in sufficient numbers, they attach to the corners (X's) shown above to give the blue color typical of copper sulfate dissolved in water. If ammonia is added to the water, ammonia displaces the four water molecules to form the much deeper blue complex known as tetraamminecopper(II). The addition of ammonia thus provides a simple and vivid test for the presence of copper(II) in a solution. In both of these complexes the more negative oxygen or nitrogen atoms are closer to the copper(II) ion and coplanar with it—that is, the oxygens or nitrogens form the corners of the squares. The more distant hydrogens are relatively unaffected by the electron orbital geometry of the copper(II) ion, and so may lie above or below the surface of the square:

    H H   H H               H H   H H
     \|   |/                 \|   |/
      O   O                 H-N   N-H
        Cu++                    Cu++
      O   O                 H-N   N-H
     /|   |\                 /|   |\
    H H   H H               H H   H H

tetraaquacopper(II)    tetraamminecopper(II)

In contrast to solutions, the ways in which ligands can surround a metal ion in a crystal are constrained by way atoms and molecules are arranged within that crystal. Thus while azurite and malachite are constructed from the same set of copper(II) ions and potential ligands (specifically CO₃^2- carbonate ions and OH^- hydroxide ions), their different crystal structures result in unique geometries in the way those ligands cluster around the copper(II) ions. Specifically, azurite contains two unique copper(II) coordination complexes, and malachite contains an entirely different pair of copper(II) coordination complexes. Since none of these coordination geometries are shared between the two minerals, it can be deduced that one of the two copper(II) coordination species in azurite is responsible for the deep blue color. This species is lost when azurite converts to malachite, resulting in an abrupt switch to the bright green coloration of the malachite complexes.

Of all four of these coordination geometries, only one has the flat coplanar geometry characteristic of many other blue copper complexes. All of the other copper(II) species in both azurite and malachite are either pyramidal, with one additional ligand attached above the square, or octahedral, with two additional ligands attached above and below the square. The only square coplanar complex occurs in one-third of the azurite copper(II) ions and looks like this:

              OH-    CO3--

                  Cu++

              CO3--  OH-

transdicarbonatotransdihydroxocuprate(II) -4

The similarity of this species to the blue water and ammonia complexes makes it the best candidate for providing the intense blue of azurite.

For readers interested in viewing azurite^[8]and malachite^[9] crystal structures interactively, a JavaMage animation of these and other minerals is available at the Loyola University's Interactive Minerals website, created by Steve Pavkovic.

Synthesis

An idea of just how intense the blue of the azurite complex is can be obtained by slowly dripping a small quantity of copper sulfate solution into a saturated solution of sodium carbonate while stirring rapidly. The result is a solution whose blue is so intense that the equivalent amount of copper solution in ammonia literally pales in comparison. The most likely culprit in this case is a close relative of the azurite species, but with water (H₂O) replacing the hydroxide anions (OH^-):

              H2O    CO3--

                  Cu++

              CO3--  H2O

transdicarbonatotransdiaquacuprate(II) -2

The above complex is metastable even at very low concentrations, in the sense that a solution of it looks and behaves very much like a supersaturated solution of azurite. If left overnight in an open container, the complex decomposes through the formation of small crystals of azurite on the sides of the container, leaving the carbonate solution once again colorless. Including the sodium cations that balance the charge of the complex, the crystallization reaction is:

3( Na₂(Cu(CO₃)₂(H₂0)₂) ) → 2CuCO₃•Cu(OH)₂ + 2Na₂CO₃ + 2NaHCO₃ + 4H₂O

In other words, when the azurite forms the surplus carbonate anions and water molecules bound to the copper(II) cations are returned to the solution, with a slight net increase in acidity (buffered by the formation of bicarbonate anions) to balance the formation of copper hydroxide in the azurite crystals.

Toxicity

All copper minerals are toxic and should not be ingested. However, the toxicity risk is far less than that of heavy metal minerals such as cinnabar (mercury sulfide), so ordinary handling of azurite poses no significant threat. In common with soluble copper minerals and copper metal itself, azurite poses a significant toxicity threat to aquatic life and should not be used as a decorative stone in fresh or (especially) salt-water aquariums.

References

See also

• List of minerals

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