Amazonite

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 jewellery for well over three thousand years, as attested by archaeological finds in Middle and New Kingdom 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 that 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

General

Category Tectosilicate minerals, feldspar group, alkali series, microcline variety

Formula KAlSi3O8

Crystal system Triclinic

Identification

Color Green, blue-green

Crystal habit Prismatic

Cleavage Perfect

Fracture Uneven, splintery

Tenacity Brittle

Mohs scale hardness 6.0–6.5

Luster Vitreous

Streak White

Diaphaneity Translucent, opaque

Specific gravity 2.56–2.58

Refractive index 1.522–1.530

Birefringence −0.008

Pleochroism Absent

Dispersion None

Ultraviolet fluorescence Weak; olive-green

Other characteristics Radioactive 14.05% (K)

References :214–215

Etymology

Amazonite was already being used to make jewelry in ancient Egypt. Later, scientists derived the name from the Amazon River, where it has not yet been found.

In contrast, Alexander von Humboldt (1769–1859) reported on an Indian tribe living on the Rio Negro who wore amulets made of amazonite. These supposedly originated from a land where women lived without men (Amazons).

Characteristics

The Mohs hardness of amazonite corresponds to that of microcline (6 to 6.5). Both exhibit a glass-like luster on their surfaces. However, amazonite is generally less transparent, being more translucent to opaque. Its density, at 2.56 to 2.59 g/cm 3, is slightly higher than that of microcline (2.54 to 2.57 g/cm 3). Its refractive index is 1.522 to 1.530. 

Color

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 suggests 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 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 color may be the aggregate effect of several mutually inclusive and necessary factors.

Varieties

Since microcline, together with orthoclase, sanidine, and albite, forms a mixed series as the final member, the term amazonite has been adopted for all blue to green, lead-bearing varieties of these minerals. Amazonite must therefore be considered a collective term for the varieties microcline amazonite, orthoclase amazonite, albite amazonite, and the various plagioclase amazonites.

Occurrence

Amazonite is a mineral of limited occurrence. In Bronze Age Egypt, it was mined in the southern Eastern Desert at Gebel Migif. In early modern times, it was obtained almost exclusively from the area of Miass in the Ilmensky Mountains, 50 miles (80 km) southwest of Chelyabinsk, Russia, where it occurs in granitic rocks.

Amazonite is now known to occur in various places around the world. Those places are, among others, as follows:

Australia:

Eyre Peninsula, Koppio, Baila Hill Mine (Koppio Amazonite Mine)

China:

Baishitouquan granite intrusion, Hami Prefecture, Xinjiang: found in granite

Libya:

Jabal Eghei, Tibesti Mountains: found in granitic rocks

Mongolia:

Avdar Massif, Töv Province: found in alkali granite

Ethiopia:

Konso Zone

South Africa:

Mogalakwena, Limpopo Province; Khâi-Ma, Northern Cape; Kakamas, Northern Cape; Ceres Valley, Western Cape

Sweden:

Skuleboda mine, Västra Götaland County: found in pegmatite

United States:

Colorado: 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

Virginia: Morefield Mine, Amelia County: found in pegmatite; Rutherford Mine, Amelia County

Pennsylvania: Media, Delaware County; Middletown, Delaware County

Use as a gemstone

Amazonite is usually cut in a table or cabochon shape for jewelry, but is also processed in spherical form into necklaces and decorative objects (e.g., sculptures). However, due to its structurally excellent cleavage, it is very sensitive to pressure and difficult to set.

When polished, amazonite takes on a light green color. Due to its color similarity, amazonite can be confused with chrysoprase, green jade, greenish minerals and varieties of the serpentine group, and turquoise. 

Health

A 2021 study by the German Institut für Edelsteinprüfung (EPI) found that the amount of lead that leaked from an 11 g (0.39 oz) sample of amazonite into an acidic solution simulating saliva exceeded European Union standard DIN EN 71-3:2013's recommended amount by five times. This experiment was to simulate a child swallowing amazonite, and could also apply to new alternative medicine practices such as inserting the mineral into oils or drinking water for days.

In fiction

Amazonite is one of the minerals in the video game Motherload. It appears as shiny polygons and is the rarest mineral after diamonds, and therefore commands a high price.

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