Azurite
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| Azurite |
Azurite with large crystals and light surface weathering. Shilu Mine, Guangdong Province, China |
| General |
| Category |
Carbonate mineral |
Formula
(repeating unit) |
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 |
| Formula 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").
Mineralogy
Azurite is one of the two basic copper(II)
carbonate minerals, the other being bright green
malachite. Simple copper carbonate (CuCO
3) is not known to exist in nature. Azurite has the formula Cu
3(CO
3)
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.
Color
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.
Weathering
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 (CO
2) units with water (H
2O), changing the carbonate:hydroxide ratio of azurite from 1:1 to the 1:2 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 saltwater
aquariums.
Uses
Fresh, unweathered azurite crystals showing the deep blue of unaltered azurite. From
Špania Dolina,
Slovakia
Pigments
Ground azurite for use as a pigment
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.
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.
Collecting
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.
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 very
similar malachite, producing a striking color combination of deep blue
and bright green that is strongly indicative of the presence of copper
ores.
Gallery of azurite mineral specimens
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 a
German term for "
kobolds"—that is, they are called
nickels.
