Azurite
Azurite or Azure spar is a soft, deep-blue copper mineral produced by weathering of copper ore deposits. During the early 19th century, it was also known as chessylite, after the type locality at Chessy near Lyon, France. The mineral, a basic carbonate with the chemical formula Cu322, has been known since ancient times, and was mentioned in Pliny the Elder's Natural History under the Greek name and the Latin name caeruleum. Copper gives it its deep blue color.
Mineralogy
Azurite has the formula Cu322, with the copper cations linked to two different anions, carbonate and hydroxide. It is one of two relatively common basic copper carbonate minerals, the other being bright green malachite. Aurichalcite is a rare basic carbonate of copper and zinc. Simple copper carbonate is not known to exist in nature, due to the high affinity of the ion for the hydroxide anion.Azurite crystallizes in the monoclinic system. Large crystals are dark blue, often prismatic. Azurite specimens can be massive to nodular or can occur as drusy crystals lining a cavity.
Azurite has a Mohs hardness of 3.5 to 4. The specific gravity of azurite is 3.7 to 3.9. Characteristic of a carbonate, specimens effervesce upon treatment with hydrochloric acid. The combination of deep blue color and effervescence when moistened with hydrochloric acid are identifying characteristics of the mineral.
Color
The optical properties of minerals such as azurite, and malachite are characteristic of copper. Many coordination complexes of copper exhibit similar colors. According to crystal field theory, the color results from low energy d-d transitions associated with the d9 metal center.Weathering
Azurite is unstable in open air compared to malachite, and often is pseudomorphically replaced by malachite. This weathering process involves the replacement of some of the water units with carbon dioxide, changing the carbonate:hydroxide ratio of azurite from 1:1 to the 1:2 ratio of malachite:From the above equation, the conversion of azurite into malachite is attributable to the low partial pressure of carbon dioxide in air.
Azurite is quite stable under ordinary storage conditions, so that specimens retain their deep blue color for long periods of time.
Occurrences
Azurite is found in the same geologic settings as its sister mineral, malachite, though it is usually less abundant. Both minerals occur widely as supergene copper minerals, formed in the oxidized zone of copper ore deposits. Here they are associated with cuprite, native copper, and various iron oxide minerals. It also often occurs along with quartz, cerussite, chrysocolla, olivenite and baryte.Fine specimens can be found at many locations. Among the best specimens are found at Bisbee, Arizona, and nearby locations, and have included clusters of crystals several inches long and spherical aggregates and rosettes up to in diameter. Similar rosettes are found at Chessy, Rhône, France. The best crystals, up to in length, are found at Tsumeb, Namibia. Other notable occurrences are in Utah; Mexico; the Ural and Altai Mountains; Sardinia; Laurion, Greece; Wallaroo, South Australia; Brazil and Broken Hill.
Uses
Pigments
Azurite is unstable in air, however it was used as a blue pigment in antiquity. Azurite is naturally occurring in Sinai and the Eastern Desert of Egypt. It was reported by F. C. J. Spurrell in the following examples; a shell used as a pallet in a Fourth Dynasty context in Meidum, a cloth over the face of a Fifth Dynasty mummy also at Meidum and a number of Eighteenth Dynasty wall paintings. 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, Armenian stone, and azurro della Magna. 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. Lapis lazuli was chiefly supplied from Afghanistan during the Middle Ages, whereas azurite was a common mineral in Europe at the time. Sizable deposits were found near Lyon, France. It was mined since the 12th century in Saxony, in the silver mines located there.Heating can be used to distinguish azurite from purified natural ultramarine blue, a more expensive but more stable blue pigment, as described by Cennino D'Andrea Cennini. Ultramarine withstands heat, whereas azurite converts to black copper oxide. However, gentle heating of azurite produces a deep blue pigment used in Japanese painting techniques.
Azurite pigment can be synthesized by precipitating copper(II) hydroxide from a solution of copper(II) chloride with lime and treating the precipitate with a concentrated solution of potassium carbonate and lime. This pigment is likely to contain traces of basic copper chlorides.