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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

As it is often convenient to refer to these colors in terms of Newton’s scale, it is necessary to examine this more closely. Examining these colors, beginning at the thin end of the wedge, it can be found that the first few colors are dark gray, light gray, white, yellow, and red. These constitute what is known as the first order. The succeeding colors include at intervals, bands of red and pink, the pink becoming paler and paler until it is indistinguishable from pale gray or white. These repeated bands supply a useful means of splitting up the scale into two orders, each band terminating an order. It is difficult to state how many orders are exhibited by such a wedge, as, to some eyes, a color high up in the scale may appear to be pink or green, while to others it is simply a white or gray. However, seven of these orders are distinctly visible. Higher colors may be referred to as whites of a high order. No two orders exhibit quite the same colors, but only practice will enable the viewer to state definitely on examining mineral fragments between crossed nicols, to what order exactly the color belongs. The first order colors are fairly characteristic. Those of the second and third orders are the brightest in the scale. Those of the higher orders become more and more pale. It is well for the beginners to realize that it is not advisable to use the words high and bright as synonyms. The higher colors in the scale are not bright.

 

            An anisotropic section, colorless in ordinary light, shows when between crossed nicols a color that is due entirely to interference. But the case of an anisotropic mineral section, which absorbs some constituent of white light, is not quite so simple. Muscovite, which is colorless in ordinary light, polarizes in bright pinks and greens. Biotite, with approximately the same birefringence, usually shows only a grayish brown color. The reason for this is, of course, that biotite shows interference colors and absorption color simultaneously. Tourmaline and brown hornblende supply other examples of this masking of the polarization color. While brown sphene, whose polarization color is a high order gray shows between crossed nicols a color not noticeably different from the absorption color. One can only say of such a section of sphene, that it is anisotropic.

 

            Few minerals, placed between crossed nicols of petrographic polarizing microscope, show what are known as ultra-polarization colors. These colors are not to be found in Newton’s scale, and are shown only by a few minerals, all with a very low birefringence as found when they are evaluated under polarized light of the geological polarizing microscope. Chlorite, a monoclinic micaceous mineral, and zoisite, an orthorhombic mineral allied to epidote, occasionally shows these ultra-blues and ultra-browns. The most satisfactory explanation of these polarization colors is that the section is isotropic for some colors and anisotropic for others.



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Friday, February 15th, 2008 at 5:12 am
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Characters of Minerals Between Crossed Nicols
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