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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

We already know that a section of quartz that is cut parallel to the principal crystallographic axis c polarized in yellow of the first order, the highest polarization color possible in sections of quartz of the standard thickness. All minerals crystallizing in the hexagonal or tetragonal system or dimetric are similar in this respect, that, although the highest possible polarization color for any mineral is a function of the birefringence, they all exhibit their highest polarization color when cut in such a direction that the crystallographic axis c lies in the plane of section. This means that, for such a section, the difference in velocity of the two rays penetrating it, is at its maximum. And as such sections show straight extinction, the two vibration directions are respectively parallel to and perpendicular to the length of the crystal. These particular directions, being the vibration directions of the rays with the two extremes of velocity for the crystal, are known as ether axes. That of the fastest ray is referred to as the X ether axis and that of the slowest ray as the Z ether axis. There are two cases: First, where c coincides with Z, the double refraction is positive. Second, where c coincides with X, the double refraction is negative. The wedge may be used to determine which of these two cases a particular vertical section of a dimetric mineral represents.

 

            Before this can e done, however, it is necessary to learn something more about the wedge. It has been known that its two vibration directions are respectively parallel and perpendicular to the length, but it is not yet determined if which of these is a fast ray and which one is a slow ray. In order to make this determination, we require a vertical section of a dimetric mineral whose sign is known. A small crystal of quartz meets these requirements admirably. It is well known that quartz is positive and therefore in a vertical section the length is the direction of slow vibration.

 

            The section is placed on the stage between crossed nicols and brought to a position of maximum illumination, and the wedge inserted into the eyepiece slot of the polarizing light microscope. Opposition between crystal and wedge might be obtained with the crystal either N.E. is to S.W., or N.W is to S.E. Suppose that opposition is obtained when the c axis is N.E. is to S.W., the length of the wedge being N.E. is to S.E. It has been known that c is slow, and that a is fast, and as opposition is taking place, the ray that came quickly through the quartz crystal comes slowly through the wedge, and vice versa. Therefore, the light vibrating parallel to the length of the wedge is penetrating slowly and the light vibrating perpendicular to the length is penetrating more rapidly.

 

            The character of the wedge has been determined. And it has been found as a slow wedge in terms of the kind of ray vibrating longitudinally. This fact having been determined, the knowledge thus obtained may be applied in order to determine the sign of any vertical section of a dimetric mineral.

 

            Mineral apatite, for example, is elongated along the c axis. A section parallel to c, tested with the wedge between crossed nicols of the petrographic polarizing light microscope, is seen to give compensation with its length N.W. is to S.E.

 

            A wedge and crystal must be in opposition, and it has been known that the length of the wedge is slow, then the length of the apatite must be fast, c = X and the sign is negative. The test may be applied to any vertical section of a dimetric mineral if one knows the position of the crystallographic axis c in the section.



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Friday, February 15th, 2008 at 5:17 am
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Characters of Minerals Between Crossed Nicols
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