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www.mineralgallery.co.za Minerals K-O |
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Orthoclase |
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Name
Orthoclase
Chemistry
K[AlSi3O8]
Uses
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.
Color
Colorless, white, yellow to brown, meat red to reddish brown.
Hardness
6
Specific gravity
2.53-2.56
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. Milky varieties are termed moonstone.
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 Mohs 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.
The three different but closely related species of alkali feldspar are
sanidine,
orthoclase,
microcline.
The differences among these feldspars result from the different ways in which aluminum is distributed, or ordered, in the aluminosilicate atomic structures of each. Sanidine is stable at the highest temperatures. It can accommodate sodium in all proportions, and a complete compositional series (with a slight structural change) runs from pure potassium sanidine to pure albite. Members of this series that are more than two-thirds sodium are called anorthoclase and have triclinic symmetry.
Many igneous rocks contain orthoclase, which is more stable than sanidine at lower temperatures. The compositional range of orthoclase extends only partway to albite, and orthoclase may occur with albite in rocks. In some igneous rocks and in most metamorphic rocks, microcline is the common potassium feldspar. Microcline can accommodate only a little sodium and, like orthoclase, may occur with albite.
All the potassium feldspars can contain more sodium at high temperatures than at low. As a high-temperature feldspar cools, albite separates and appears as small grains, bleb, or streaks within it, in mixtures termed perthites. Perthitic texture may be coarse and easily visible as slight variations in color in the crystal, or it may be microscopic and practically invisible. The coarseness of the texture depends mostly on the rate at which the feldspar was cooled. The slower the cooling, the coarser the texture.
Alkali feldspars occur in many rocks. They are abundant in granites, a family of intrusive igneous rocks composed chiefly of alkali feldspars and quartz.
- Granites constitute the cores of mountain ranges; they are formed by the melting of the Earth's crust as mountains are built.
- Sanidine is found chiefly in some lavas, where it is preserved by rapid chilling. When cooled slowly, sanidine changes to orthoclase.
Crude crystals of microcline several feet wide (among the largest of any mineral) are commonly found in pegmatites, coarse-grained, granitelike rocks found in mountains. Pockets yield beautiful, sharp crystals, sometimes of the green variety amazonite.
Alkali feldspars are also common in many metamorphic rocks that have crystallized at high temperatures. Micas and other minerals common in lower temperature rocks tend to break down into feldspar as the temperature increases during metamorphism. Alkali feldspars weather to clays, forming deposits of china clay or kaolin.
