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Minerals A-E
Amazonite
Name
Amazonite
Chemistry
K(AlSi3O8)
Uses
Mineral specimen and in the ornamental trade
Color
Green or blue-green
Hardness
6-6.5
Specific gravity
2.54-2.69
Streak
White
Crystals
Prismatic, tabular, rhombohedral and often multifaced
Accompanied by:
Quartz , mica, tourmaline, topaz and garnet
Similar to:
Feldspars and dolomite
Amazonite belongs to the feldspar family.
Amazonite has the typical feldspar crystals with the difference that it is this interesting blue color. The blue is often unevenly dispersed through the crystals.
Amazonite belongs to the microline group. From the same group comes the other three well known feldspars. Labradorite, sunstone and moonstone. It is opaque. We have a batch of Southern African Amazonites in stock with well formed crystal shapes.
The feldspars are the most abundant and widespread minerals of the crust, or outermost part, of the Earth. Because of their abundance, feldspars are used in the classification of igneous rocks. They are also abundant in metamorphic rocks and in some sediments and sedimentary rocks, especially those formed in arid and semiarid regions.
Feldspars are also major constituents of moon rocks, which are similar to rocks of the Earth's crust. Feldspars are used in the manufacture of ceramics and ceramic glazes and as mild abrasives. A few varieties, including labradorite and orthoclase, are occasionally used as gems because they show an iridescent play of colors or a milky translucence. Milky varieties are termed moonstone.
Mineralogy
Feldspars are aluminosilicates, containing silicon and aluminum ions bound together by oxygen ions to form a three-dimensional framework of atoms (see silicate minerals). Other ions, principally potassium, sodium, and calcium, occupy sites within the framework and give rise to the three pure, or end-member species: orthoclase , albite, and anorthite. Extensive chemical variation, or solid solution, occurs between orthoclase and albite, with potassium and sodium substituting for each other. Feldspars in this range are called alkali feldspars. Complete solid solution between albite and anorthite is also possible, with calcium substituting freely for sodium. Because these two ions have different charges, the proportions of aluminum to silicon in the aluminosilicate framework must also vary to maintain electrical-charge neutrality. Feldspars between albite and anorthite in composition are termed plagioclase. There is little solid solution between orthoclase and anorthite.
Feldspars are generally light-colored minerals, white or buff to gray in color. One species, microcline, may also be light brick-red or even the green to blue-green variety called amazonite. Feldspars are slightly translucent and have a glassy, or vitreous, luster rather like that of glazed porcelain. Because their atomic framework has planes of weakness, feldspars exhibit good cleavage, breaking readily into blocky pieces with smooth sides. On the Moh's mineral hardness scale, feldspars are 6 slightly harder than a steel knife blade and about as hard as porcelain.
The high-temperature potassium feldspars, sanidine and orthoclase, are monoclinic in their crystal symmetry; the others are triclinic, although they retain the general atomic pattern of the monoclinic species. Because triclinic feldspars have nearly monoclinic symmetry, they commonly occur in complex intergrowths of crystals called twins, which mimic the higher monoclinic symmetry. Plagioclase in particular exhibits a prominent twinning called albite twinning. Thin, platelike crystals, oriented so as to be mirror images of each other, are intergrown. On a cleavage surface, this intergrowth causes a finely striped pattern characteristic of plagioclase. About a dozen different patterns of twinning have been recognized in the feldspars.
