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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 should be clearly understood that the anisotropic sections, for most positions of the stage of the polarizing microscope, transmit light of a certain color, but that, from time to time during rotation they become dark – they are said to be extinguish. And if the angle of rotation is taken, it will be found that extinction takes place at intervals of exactly 90 degrees when viewed under petrographic polarizing light microscopes. For instance, most sections of the mineral Apatite, a mineral that crystallizes in the hexagonal system, are elongated, colorless, with high refractive index, and a basal cleavage, when rotated between crossed nicols, show a color that is gray when evaluated closely under geological polarizing microscopes. This is extinguished when the edges are N.S. or E.W. The mineral Muscovite supplies another good example. This mineral usually shows no definite outline, but seen in ordinary light it is colorless, and shows a well-defined cleavage. The color between crossed nicols of the polarized microscope is bright pink or green, and this extinguishes when the cleavage is N.S. or E.W. Both these cases show straight extinction, that is, extinction takes place when some definite direction, edge or cleavage, is placed N.S. or E.W.

 

            Other minerals, however, although they extinguish at intervals of 90 degrees, do not extinguish in these positions, but at some other position during rotation. Consider Augite, the pale brown mineral with high refractive index and usually found showing one or two good cleavages. Most sections of this mineral augite, for instance extinguish when the edges of the fragment are not parallel to either of the rectangular cross-wires in the eyepiece of the petrographic polarizing microscope – placed N.S. and E.W. Such sections show oblique extinction, and it is sometimes useful to know the position at which extinction does take place. This angle of extinction is measured either from the N.S. or the E.W. cross-wire. In order to determine its value, the stage of the polarized microscope is first rotated until the mineral fragment is extinguished, when the angular position of the stage is read. The stage of the polarizing microscope is then rotated until the crystal edge is parallel to one of the cross-wires, when the stage reading is again taken. The difference between the two readings gives the angle of extinction. It will be seen, however, that this is only one of two possible values. The other value would have been obtained if rotation had brought the edge parallel to the other cross-wire. These two values are, of course, complementary, and it is usual to take the smaller value as the angle of extinction.



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Friday, February 15th, 2008 at 5:16 am
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Characters of Minerals Between Crossed Nicols
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