Amazonite, also known as Amazonstone, is a green tectosilicate mineral, a variety of the potassium feldspar called microcline. Its chemical formula is (KAlSi3O8), which is polymorphic to orthoclase.
Its name is taken from that of the Amazon River, from which green stones were formerly obtained, though it is unknown whether those stones were amazonite. Although it has been used for over two thousand years, as attested by archaeological finds in Egypt and Mesopotamia, no ancient or medieval authority mentions it. It was first described as a distinct mineral only in the 18th century.
Green and greenish-blue varieties of potassium feldspars which are predominantly triclinic are designated as amazonite. It has been described as a “beautiful crystallized variety of a bright verdigris-green” and as possessing a “lively green colour.” It is occasionally cut and used as a gemstone.
Amazonite is a mineral of limited occurrence. Formerly it was obtained almost exclusively from the area of Miass in the Ilmensky Mountains, 50 miles southwest of Chelyabinsk, Russia, where it occurs in granitic rocks.
Amazonite is now known to occur in various places around the globe. Those places are, among others;
- Baishitouquan granite intrusion, Hami Prefecture, Xinjiang: found in granite.
- Jabal Eghei, Tibesti Mountains: found in granitic rocks.
- Avdar Massif, Töv Province: found in alkali granite.
- Mogalakwena, Limpopo Province
- Khâi-Ma, Northern Cape
- Kakamas, Northern Cape
- Ceres Valley, Western Cape
- Deer Trail, Arapahoe County.
- Custer County.
- Devils Head, Douglas County.
- Pine Creek, Douglas County.
- Crystal Park, El Paso County.
- Pikes Peak, El Paso County: found in coarse granites or pegmatite.
- St. Peter’s Dome, El Paso County.
- Tarryall Mountains, Park County.
- Crystal Peak, Teller County.
- Morefield Mine, Amelia County: found in pegmatite.
- Rutherford Mine, Amelia County.
- Media, Delaware County.
- Middletown, Delaware County.
For many years, the source of amazonite’s color was a mystery. Some people assumed the color was due to copper because copper compounds often have blue and green colors. A 1985 study suggest that the blue-green color results from quantities of lead and water in the feldspar. Subsequent 1998 theoretical studies by A. Julg expand on the potential role of aliovalent lead in the color of microcline.
Other studies suggest the colors are associated with the increasing content of lead, rubidium, and thallium ranging in amounts between 0.00X and 0.0X in the feldspars, with even extremely high contents of PbO, lead monoxide, (1% or more) known from the literature. A recent 2010 study also implicated the role of divalent iron in the green coloration. These studies and associated hypotheses indicate the complex nature of the color in amazonite, in other words the aggregate effect of several mutually inclusive and necessary factors.