
A basic carbonate of copper with the chemical formula Cu3(OH)2(CO3)2. Azurite is normally associated with copper ores and often occurs with malachite. Azurite is mono-clinic. It may be massive or may occur in tabular, prismatic, or equant crystals. Invariably blue, azurite was originally used extensively as a pigment. Hardness is 3½–4 (Mohs scale) and specific gravity is 3.8. Notable localities for azurite are at Tsumeb, Southwest Africa, and Bisbee, Arizona. See also Copper.
Environment
Secondary (weathered) zone of copper ore deposits.
Crystal descriptionCommonly crystallized, often in large, well-formed, equidimensional, deep blue crystals and in rosette aggregates. Also in slender blue needles or even hairs. Frequently altered completely or in part to malachite. Commonly forms botryoidal growths like (and interlayered with) malachite, in crusts, stalactites, or stalagmites; also massive and earthy.
Physical propertiesLight blue to almost black. Luster glassy; hardness 3Ɖ-4; specific gravity 3.8; streak blue; fracture conchoidal; cleavage 1 good and 2 poor. Brittle; transparent in very thin flakes.
CompositionCopper carbonate (69.2% CuO, 25.6% CO 2 , 5.2% H 2 O).
TestsCopper blue color is distinctive. Fuses on charcoal, and with careful treatment will give copper bead in reducing flame. Dissolves in hydrochloric acid with effervescence. Drop of solution on platinum wire gives fine blue copper flame; green acid solution turns blue with addition of ammonia.
Distinguishing characteristicsMost other blue, disseminated minerals are harder. Its effervescence in acid distinguishes it from other secondary copper compounds for which it might be mistaken (relatively rare linarite is the most likely).
OccurrenceOne of the very desirable mineral collectibles. Azurite forms under conditions that are identical with those of malachite, with which it is always associated. Although azurite is rarer than malachite, and apparently less stable, fine occurrences are common and much too numerous to list. Morocco has provided some good crystals, and several Arizona and Utah mines still have good crystals. Azurite and malachite stains on the rocks have served as valuable prospecting guides. Found in fine crystals at Chessy, France, from which it received its British name chessylite. South Australia and New South Wales gave comparable examples. Sharp brilliant crystals, the finest known and up to 6 in. (15 cm) long, were found at Tsumeb, Namibia. Clifton and Bisbee, Arizona, were noted for fine azurite crystals in their early days. Fine malachite pseudomorphs have come from Tsumeb and from Arizona.
A common habit at many mines is crystallized masses which seem to grow freely and uniformly, like the aragonite balls described above. Very lustrous and almost black groups from Guangdong, China, are notable. In Northern Territory, near Alice Springs, Australia, there are flat discs of very blue azurite, a similar habit apparently grown in a thin clayey layer. Azurite also grows in hollow concretionary masses with crystal-linings, a habit for which Chessy was perhaps the best example, with an occasional malachite-coated cuprite octahedron.
| Azurite | |
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Azurite from China with large crystals and light surface weathering. |
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| General | |
| Category | Carbonate mineral |
| Chemical formula | Cu3(CO3)2(OH)2 |
| Strunz classification | 05.BA.05 |
| Crystal symmetry | Monoclinic 2/m |
| Unit cell | a = 5.01 Å, b = 5.85 Å, c = 10.35 Å; β = 92.43°; Z=2 |
| Identification | |
| Molar mass | 344.67 g/mol |
| Color | Azure-blue, Berlin blue, very dark to pale blue; pale blue in transmitted light |
| Crystal habit | Massive, prismatic, stalactitic, tabular |
| Crystal system | Monoclinic Prismatic |
| Twinning | Rare, twin planes {101}, {102} or {001} |
| Cleavage | Perfect on {011}, fair on {100}, poor on {110} |
| Fracture | Conchoidal |
| Tenacity | brittle |
| Mohs scale hardness | 3.5 to 4 |
| Luster | Vitreous |
| Streak | Light Blue |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.773 (measured), 3.78 (calculated) |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.730 nβ = 1.758 nγ = 1.838 |
| Birefringence | δ = 0.108 |
| Pleochroism | Visible shades of blue |
| 2V angle | Measured: 68°, calculated: 64° |
| Dispersion | relatively weak |
| References | [1][2][3] |
Azurite is a soft, deep blue copper mineral produced by weathering of copper ore deposits. It is also known as Chessylite after the type locality at Chessy-les-Mines near Lyon, France.[2] The mineral, a carbonate, 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.[4] 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 (لاژورد), an area known for its deposits of another deep blue stone, lapis lazuli ("stone of azure").
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Azurite is one of the two basic copper(II) carbonate minerals, the other being bright green malachite. Simple copper carbonate (CuCO3) is not known to exist in nature. Azurite has the formula Cu3(CO3)2(OH)2, with the copper(II) cations linked to two different anions, carbonate and hydroxide. Small crystals of azurite can be produced by rapidly stirring a few drops of copper sulfate solution into a saturated solution of sodium carbonate and allowing the solution to stand overnight.
Azurite crystals are monoclinic, and when large enough to be seen they appear as dark blue prismatic crystals.[2][3][5] 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 effervesce upon treatment with hydrochloric acid.
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 descriptions are provided by ligand field theory.
Azurite is unstable in open air with respect to malachite, and often is pseudomorphically replaced by malachite. This weathering process involves the replacement of some 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:
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 saltwater aquariums.
Azurite was used as a blue pigment for centuries. 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 or Armenian stone, in addition 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, 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. True lapis lazuli was chiefly supplied from Afghanistan during the Middle Ages while 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.[6]
Heating can be used to distinguish azurite from purified natural ultramarine blue, a similar but much more expensive pigment, as described by Cennino D'Andrea Cennini. Ultramarine withstands heat, but azurite turns to black copper oxide. However, gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.
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.
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| Small specimen of Azurite from China. |
The intense color of azurite makes it a 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.
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 very similar malachite, producing a striking color combination of deep blue and bright green that is strongly indicative of the presence of copper ores.
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 a German term for "kobolds"—that is, they are called nickels.
| Wikimedia Commons has media related to: Azurite |
| Wikisource has the text of the 1911 Encyclopædia Britannica article Azurite. |
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