Tourmaline (#16)

A lustrous green gemmy single crystal, ex Andy Seibel.
South America Urobu mine, Aracuai, Minas Gerais, Brazil 1" x 1" x 1"

 

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Rocks, by definition, are made of minerals, so minerals occur in some form virtually everywhere in the world. Most mineral specimens, while of potential interest to geologists and mineralogists, are not visually appealing - that is, they look quite ordinary. Some localities, however, have produced such an abundance of striking specimens that they are essentially synonymous with "beautiful minerals." One such area is Minas Gerais. Minas Gerais ("General Mines" in Portuguese) is a state in Brazil, and it has yielded some of the world's most exquisite samples of tourmaline (both pink and green), topaz, emerald, aquamarine, and alexandrite. The state is also a source of gold, diamonds, hematite, quartz, and other non-gem minerals.

As with specimen #5 from Afghanistan, these minerals are associated with pegmatites in Minas Gerais. Pegmatites are coarse grained (that is, the crystals are unusually large) and associated with granitic rocks. Most of these pegmatites in Minas Gerais developed in mountain belts that formed from an orogeny (that is, a compressive mountain-building event due to plate tectonic activity) from 700 to 450 million years ago. Following that, from about 125 to 90 million years ago, the area experience uplift associated with the opening of the South Atlantic Ocean. These geologic events combined to generate the pegmatites in the mountains and then expose them by erosion. (In case you haven't looked at specimen #5, pegmatites are formed in the last stages of magmatic crystallization, when the concentration of volatiles (mostly water), and elements that don't easily enter the crystal structures of common minerals, is high.)