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Friday, March 2, 2012

Azurite Hydrous Copper Carbonate a Blue Gem.

Botrydoil Azurite from the Apex Mine of Utah.
Photo by Rob Lavinsky

Azurite is one of the lesser known copper ores that is produced by the weathering of copper deposits. The mineral is also known as chessylite named after the Chessy-les-Mines near the city of Lyon in France. This mineral has been known to the ancients and is mentioned in Pliny the Elder’s, Natural History.  Because of its deep blue color has also been used as a pigment that is a deep blue color. Azurite is usually found in the desert where the low humidity causes its cousin malachite to lose water.  This is where it is often found in the company of its cousin malachite.  Both minerals are essentially the same copper carbonate except azurite contains more water in its crystals.  Over time this water will evaporate causing the azurite to change to malachite.

Typically azurite crystals are monoclinic that when they are large enough to be seen will appear is dark blue prismatic crystals. In nature azurite crystals are massive to nodular with some of them forming stalactitic masses. Any specimen of azurite over time will lighten in color as its surface retrogrades in the malachite. The mineral is soft with a Mohs hardness of 3.5 to 4. It displays a specific gravity ranging from 3.77 to 3.89. The mineral azurite is destroyed by the application of heat losing both carbon dioxide and water forming a type of black powdery copper oxide. A drop of acid placed on the specimen will cause it to effervesce like any other carbonate.

Azurite and malachite
Photo by Rob Lavinsky

Some of the earliest uses of azurite are a blue pigment in artist’s paints. In this use it was finely ground, and then mulled with linseed oil and turpentine. The mineral is capable of producing many different shades of blue depending upon the fineness to which it is ground. When it is mixed with oil, as in oil paints it turns slightly green. On the other hand if it is mixed with egg yolk it turns a gray green. In older paintings the azurite has turned into malachite displaying a greenish color. Many times what was azurite was mislabeled as lapis lazuli that in times past was a name that was applied to many other blue pigments. True lapis lazuli that was used as another blue pigment came primarily from Afghanistan during the Middle Ages. Azurite was a common mineral found in Europe in the same timeframe.

Lapis Lazuli, notice the difference in color from azurite.

There were sizable deposits of azurite that were mined near Lyons, France during the Middle Ages. Azurite was also mined during the 12th century in Saxony and the various silver mines that were located there.

If azurite is heated it turns black whereas the more expensive natural ultramarine remains blue when it is heated. If the mineral is gently heated it produces a deep blue pigment that is often used by the Japanese in some of their painting techniques.

Azurute surrounded by malachite.
Photo by Rob Lavinsky

Occasionally azurite finds uses beads and other types of jewelry; it is also used as a decorative stone. The thing that keeps it from being used more commonly as a gemstone is its softness and the fact that it weathers easily reverting back to malachite.

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