The Guide to Rocks and Minerals

In focus: Azurite

Chemical Composition: Cu3(CO3)2(OH)2
Group: Carbonate
Colour: Azure blue, Blue, Light blue, Dark blue
Streak: light blue
Hardness: 3.5-4
Density: 3.8-3.9
Luster: Vitreous
Cleavage: Perfect in one direction

Azurite (a Copper Carbonate Hydroxide) is a very popular semi-precious stone because of its beautiful colour which ranges from light to dark blue. Azurite is typically found as tabular or prismatic crystals with smooth vitreous faces. The name of this mineral comes from the Arabic word for blue. Azurite was used extensively as a dye for paints and fabrics during the Middle Ages and Renaissance, but there are records which suggest Azurite was used for this long before, during the forth dynasty in Egypt (around 2575 BC).

Azurite, like its close cousin Malachite, is associated with copper deposits and can be found across the world. The most notable occurrences are in: Lasal, Utah; Bisbee, Arizona and New Mexico, Mexico; Tsumeb, Nambia; Shaba, the Congo; Morocco; Australia and in many locations in Europe.

As mentioned above Azurite is in many ways very similar to Malachite. There are striking similarites in their chemical composion and location, and above all, Malachite can create an Azurite pseudomorph - an exact copy of an Azurite crystal (though instead of being blue, it would be green). So how can two minerals which are so similar present themselves in such different colours? The answer to that lies in their chemical difference, which is small but significant. Azurite has one extra CuCO3 molecule, which means that chemically it is more prone to oxidation processes. In the presence of water Azurite will slowly "mature" into the more stable Malachite by combining with an extra Cu(OH)₂ molecule. This will result in two molecules of Malachite for each Azurite molecule

Azurite - Cu(OH)₂-2(CuCO₃)

Malachite - Cu(OH)₂-CuCO₃

The oxidation process of Azurite, although very slow, is a continuous process, resulting in the creation of pseudomorphs and the striking changes of colour. For rock collectors this is a very beautiful and exciting process, but it has an ugly side in other areas, especially when azurite has been used in paints (see below).

Mineral uses

as a pigment
as an ornamental stone
for jewellery
as an insignificant source of copper ore

Azurite as pigment

The natural variety of pigment is obtained by simply grinding, washing and sieving Azurite. Coarsely ground Azurite produces a dark blue, while finely ground Azurite produces a lighter tone. The addition of oil will add a greenish tint to the colour, and when mixed with egg yolk the pigment turns to greenish-gray.

Azurite pigmet has been used extensively in paint and this is where the oxidative process is mostly visible. Old paintings which started with beautiful blue skies have over the centuries seen these skies turn an ugly green hue.

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