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").
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 effervesce upon treatment with
hydrochloric acid.
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. 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.
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.
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.
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.
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.
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.
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. Small crystals of azurite can be obtained 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 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.
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