Azurite and Malachite (#35)

Lustrous bright blue compact intergrown rosettes of azurite to 1 ¾" centered in a mass of dull opaque green crystalline malachite.
North America La Sal, San Juan County, Utah, USA 10" x 7" x 5"

 

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Malachite and azurite are two copper carbonates that are minor ore minerals. Azurite, as its name might suggest, is the blue one, which obviously leaves malachite as the green one. You might wonder whether malachite was named alter the Old Testament prophet, but it was not. Its name derives from the Latin word for the green leaves of the mallows plant - changed from molochitus (in Latin) to malachite by the mid-17th century.

The chemical formulas are Cu2(CO3)(OH)2 (malachite) and Cu3(CO3)2(OH)2 (azurite), and the two minerals occur together frequently in the oxidized zones of copper ore deposits. If "oxidized zone" doesn't mean anything to you, read the Learn More for specimen #30. These are secondary minerals, meaning that they formed by alteration of the original copper ore minerals.

Now, why are the colors different? In Learn More for specimen #23, we talked about crystal field effects as a cause of color. Copper (Cu) atoms, in both malachite and azurite, are surrounded by 4 oxygen atoms essentially in a rough square. But in azurite, that square is more distorted that it is in malachite, and so the oxygen atoms approach the d orbitals of Cu differently. This leads to d orbital energies that are different in the two minerals, and so different parts of the spectrum are "removed" in moving the d electrons between orbitals.

In aquamarine (specimen #23), the Fe is a trace element, so the color is pale. In these two copper minerals, Cu is an essential and abundant element, and so the colors are bold.