Azurite
A deep-blue copper mineral used as pigment since antiquity.
Azurite, also known as Azure spar or chessylite, is a soft, deep-blue copper mineral that forms through the weathering of copper ore deposits. Its deep blue color is due to the presence of copper(II) cations. The mineral is a basic carbonate with the chemical formula Cu3(CO3)2(OH)2, and it crystallizes in the monoclinic system, often forming large, dark blue prismatic crystals, though it can also appear as massive, nodular, or drusy formations. It has a Mohs hardness of 3.5 to 4 and a specific gravity between 3.7 and 3.9. A key identifying feature is that it effervesces when treated with hydrochloric acid, a characteristic of carbonates. Azurite has been known since ancient times and was described by Pliny the Elder in his *Natural History* under the Greek name *kuanos* and the Latin *caeruleum*. During the early 19th century, it was also called chessylite after its type locality at Chessy near Lyon, France. The mineral is unstable in open air and often weathers into the more stable green mineral malachite, a process that involves the replacement of water with carbon dioxide. Despite this, azurite remains stable under normal storage conditions. Notable occurrences include Bisbee, Arizona; Chessy, France; and Tsumeb, Namibia. Historically, it was ground into a pigment known as mountain blue or Armenian stone, used from ancient Egypt through the Middle Ages, though it was often confused with lapis lazuli. Its softness and tendency to weather limit its use in jewelry, but it remains a popular specimen for collectors.
- formula
- Cu3(CO3)2(OH)2
- hardness
- 3.5 to 4 (Mohs)
- specific_gravity
- 3.7 to 3.9
- crystal_system
- Monoclinic
- color
- Deep blue
- type_locality
- Chessy near Lyon, France
Lore & Background
Azurite crystallizes in the monoclinic system, with large crystals often prismatic and dark blue. Specimens can be massive, nodular, or occur as drusy crystals lining a cavity. The mineral effervesces upon treatment with hydrochloric acid, a characteristic of carbonates. Its deep blue color results from copper(II) ions and low energy d-d transitions. Azurite is unstable in open air compared to malachite and often pseudomorphically replaced by malachite, a process involving replacement of water with carbon dioxide. Under ordinary storage conditions, azurite is quite stable and retains its color for long periods.
Reader's Guide
Azurite has been used as a blue pigment since ancient times, with examples found in Egyptian Fourth Dynasty pallets, Fifth Dynasty mummy cloth, and Eighteenth Dynasty wall paintings. It was known as mountain blue, Armenian stone, and azurro della Magna. When mixed with oil it turns slightly green; with egg yolk it turns green-grey. Heating distinguishes azurite from ultramarine: azurite converts to black copper oxide, while ultramarine withstands heat. Azurite is used occasionally as beads and jewelry, though its softness and tendency to weather limit its use. It is a popular collector's stone, and the notion that specimens must be carefully protected from bright light, heat, and open air may be an urban legend. Azurite is a good surface indicator of weathered copper sulfide ores, usually found with malachite.
Did You Know?
- Azurite was mentioned in Pliny the Elder's Natural History under the Greek name kuanos, root of the English word cyan.
- Azurite is one of two relatively common basic copper(II) carbonate minerals, the other being malachite.
- The best azurite crystals, up to 10 inches in length, are found at Tsumeb, Namibia.
- Gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.
Frequently Asked Questions
What is Azurite?
Azurite is a soft, deep-blue copper mineral that forms as copper ore deposits undergo weathering. It goes by the alternate names Azure spar and chessylite, and has been recognized by humans since ancient times.
What is Azurite's chemical formula and crystal system?
Its formula is Cu3(CO3)2(OH)2, classifying it as a basic copper carbonate. It crystallizes in the monoclinic system and has a specific gravity ranging from 3.7 to 3.9.
How hard is Azurite on the Mohs scale?
Azurite rates between 3.5 and 4 on the Mohs hardness scale, making it soft enough to be scratched by a steel nail. This relative softness makes it easy to polish into cabochons but also means collectors must handle specimens with care.
Why is Azurite historically significant as a pigment?
It has been ground into a blue pigment since antiquity, and Pliny the Elder referenced it in his Natural History under the Greek name kuanos and the Latin caeruleum. Its intense, deep blue made it one of the most valued natural pigments before synthetic dyes became available.
Where is Azurite's type locality?
The type locality is the town of Chessy, near Lyon, France, where the mineral was first formally described. This origin is the source of its alternate name, chessylite.
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