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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The observant mineral evaluator can easily notice that while some mineral sections polarize and extinguish as a whole, differences of color being easily explainable as the result of variations in thickness, some particular mineral fragment is divided into two areas, the dividing line being straight, and usually sharply defined. Of these two areas, one is in a position of extinction when the other is illuminated, and vice versa. Moreover, the polarization color of the two areas is unlike. Orthoclase feldspar will often show this difference of extinction position and also of polarization color, one area showing yellow and the other gray. These peculiarities are indicative of twinning, and often afford a useful means of distinction between different minerals. The reason for the different positions of extinction of the two areas is to be found in the fact that, as they have different crystallographic orientations, their rectangular vibration directions are not parallel, each to each. The reason for the difference of polarization color is to be found also in the fact that the two areas have different crystallographic orientations. They behave just as if they were two separate sections of the same mineral. This particular kind of twinning, resulting in the production of two such areas only, is known as simple twinning.

 

            The sections of plagioclase feldspars such as Labradorite show another type of twinning. In this case, there are not merely two such areas, but any number from three upwards in one crystal. When one of these fragments is examined between crossed nicols of the polarizing microscope, it shows alternating black and illuminated ribbons when one set is in extinction. It is worth noting that alternate ribbons extinguish simultaneously. This particular type of twinning, of which Labradorite has been taken as an example, is known as lamellar twinning.

 

            The fact that twinning is exhibited commonly by some minerals and never in others renders this particular character very useful in mineral identification. Simple twinning serves to distinguish orthoclase feldspar from quartz, and from plagioclase feldspar – which shows lamellar twinning.

 

            It is obvious that this test of twinning, the different positions of extinction, can only be observed in the case of anisotropic sections with the aid of the petrographic polarizing microscopes. This particular kind of evidence is not available in the case of minerals crystallizing in the cubic system, and it is therefore a matter of some difficulty to prove the presence of twinning in sections of minerals having such a high degree of symmetry. Some indication, however, is sometimes given by a re-entrant angle, though this is not a satisfactory proof of twinning.



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Friday, February 15th, 2008 at 5:08 am
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Characters of Minerals Between Crossed Nicols
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