Azurite nodule, bisected...
The bright blue and green copper carbonate minerals azurite and malachite are known as secondary minerals, because in the geological sequence that leads to lovely specimens like the one in the photo they form from pre existing elements and minerals, replacing them with something more appropriate for the environmental conditions. In the case of these beauties the journey likely started with hot copper rich fluid being expelled from a crystallising granite somewhere in the depths of the Earth. Conditions were anoxic and the copper was carried in a loose bond with an element such as sulphur or chlorine, until the fluid encountered a change in chemical or pressure/temperature conditions that forced some elements out of solution.
They probably crystallised as sulphide minerals, in this case either forming or replacing a nodule in the sediments. Later, as the rocks containing the nodule were slowly uncovered by erosion and oxygen and carbonate rich water started to percolate down from the surface the sulphides were transformed into carbonates, while retaining the shape of the initial concretion.
Loz
Image credit: LGF Foundation.
