The Guide to Rocks and Minerals

In focus: Amazonite

Amazonite is a Potassium Aluminum Silicate (KAlSi₃O₈). It is a green variety of mocrocline feldspar. The stone is named after the Amazon River, although it is doubtful that Amazonite itself has ever been found in the Amazon area. Apparently the Spanish explorers who named the mineral confused Amazonite with some other mineral from the Amazon basin. Which mineral is now unknown, but in fact Amazonite is frequently mistaken for other green minerals, for example jade.

Amazonite is a feldspar. Feldspar is a group of minerals which are very abundant in the earth's crust. Most common feldspars are divided into two groups: alkali feldspars (KAlSi₃O₈ through NaAlSi₃O₈ series), of which Amazonite is a member, and the plagioclase feldspars (NaAlSi₃O₈ through CaAl₂Si₂O₈ series). Actually, Amazonite is a green variety of microcline. Microcline is a potassium-rich alkali feldspar, which is an important igneous rock forming tectosilicate mineral. As well as green (Amazonite), microcline can be clear, white, pale-yellow or brick-red.

It was originally thought that the green colour of Amazonite was the result of copper impurities, because copper compounds often have blue and green colours. However, studies by Hoffmeister and Rossman (1985) have suggested that the green colour is the result of small amount of lead and water in the feldspar.

Amazonite was once obtained almost exclusively from the Ilmen mountains in Russia. However, since 1876, the Pikes Peak district of Colorado, U.S.A., has became the most important source. Indeed the best green crystals yes found have come from scattered pegmatitic pockets in schist near Crystal Peak and Pike Peak in Colorado. These green crystals found there are often associated with smoky quartz. Amazonite was also found in other parts of the United states, and in Madagascar in pegmatite (a coarse-grained granite, sometimes rich in rare elements such as uranium, tungsten, and tantalum).

Mineral uses

gemstones for jewellery
suitable for carving and polishing, often used as an ornamental stone

Identification

Amazonite is the only green microcline feldspar. The colour is lost by heating to about 300^oC and can be restored by irradiation if heating did not exceed 500^oC, which would result in the loss of structural water. During natural processes both natural radiation and structural water are essential to produce chromophoric mono- or tri-valent lead, and therefore the natural bright green colour of Amazonite.

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