Amazonite with smoky quartz, sitting atop white Cleavelandite
Feldspar is the most common mineral on Earth, and comes in many guises, each forming in different conditions such as lava chemistry or intensity of metamorphism, making them useful indicators to crystallisation conditions. Some of them are also visually stunning minerals, such as these electric blue crystals of microcline. This potassium rich variant occurs in silica rich granitic rocks, that have been refined by the Earth's still from mantle magmas or melted crust. When they crystallise alongside quartz in pegmatites some amazing specimens can grow. the quartz is smoky due to colour centres activated by natural radiation from minerals in the granites.
The name comes from the Amazon river, though how it came about is uncertain. Sources include Russia, Brazil, Madagascar and Colorado in the pegmatite belts where many gems are found. The colour was initially thought to be caused by impurities of copper, since it produces similar hues in other minerals, but the latest research suggests lead and water provide its electric green-blue shades.
It has been popular since ancient Egyptian times, used in carvings and cabochon gems, and is rarely treated, though it is soft and should not be exposed to harsh chemicals. This specimen, measuring 8.5x6cm includes bright white as well as blue feldspar and came from Colorado.
Loz
Image credit: Exceptional Minerals
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