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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

It is often found that a crystal is not uniformly colored, but exhibits an arrangement of color bands, which are concentric with the exterior and is often splendidly displayed by the mineral under polarized microscopes. Nosean, for example, is commonly distinguished by a well-marked brown periphery that is majestically exhibited under geological polarizing microscope, although the center is colorless. Tourmaline, brown garnets, and pyroxenes frequently show a succession of concentric rings of colors that are fascinatingly interesting when viewed with the aid of polarizing microscopes. This variety of zoning is known as color zoning. A similar structure, already mentioned under the heading of inclusions, is exhibited by some minerals in consequence of their manner of arranging inclusions. Leucite is the most striking example, but a less conspicuous zoning by inclusions is sometimes seen in feldspars and pyroxenes.

 

            The minerals, which are grouped together as isomorphous, most commonly exhibit zoning and no better example can be found than the Albite-Anorthite series of plagioclase feldspars. It is often found that the crystal is not homogenous, but is built up of successive shells, each of which has a composition, which is different from that of its neighbors. The crystal has commenced its growth as one member of the series, but has received additions of ever varying composition. In consequence of this structure, the mineral being liable to decomposition and the several shells being unstable in different degrees, the weathering action has picked out the less stable layers and has emphasized the zonal nature. When the kaolinised layers have received a stain of iron oxide, as sometimes happens, the zoning becomes still more striking. But even when the feldspar is quite fresh, the zones are sometimes visible in consequence of the difference in refractive index between them. The color zoning of tourmaline, pyroxenes, and garnets, is also to be attributed to an isomorphous relationship. It happens to be the case that the different members of the particular series are differently colored and is usually produce wonderful microscope images under petrographic polarizing microscopes.



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Wednesday, February 13th, 2008 at 3:35 am
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Minerals Characteristics in Ordinary Transmitted Light
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