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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 zoning of such crystals as those belonging to the Albite-Anorthite series are very diagnostic, and it has been pointed out that this particular character may be observed in ordinary light in consequence of the fact that the different shells have unequal refractive index indices and possibly different degrees of alteration. But this zoning is often quite inconspicuous until the examination is made between crossed nicol of the petrographic polarizing light microscopes. As its zone has its own position of extinction, the structure is still more conspicuous in this condition of the polarizing microscope, notwithstanding the fact that the same crystal may show lamellar twinning. Pyroxenes also may exhibit zoning between crossed nicols of the geological polarizing microscope, even in cases where such a character was invisible in ordinary light.

It has been also known that all sections of minerals crystallizing in the cubic system ought, when examined between crossed nicols of the polarizing microscopes, to be isotropic. But many sections of the mineral Leucite show gray polarization colors in intersecting ribbons even though it is usually considered to crystallize in the cubic system. The anomaly is that a cubic mineral is anisotropic, and that anomaly is bound to exist as long as we regard the mineral as belonging to this system. The mineral is not really a cubic mineral, but its crystals have a symmetry that is so nearly that of the trapezohedron, that it is a matter of some difficulty to place it elsewhere. Some sections of garnet exhibit similar anomalies.

The phenomena of aggregate polarization on the other hand is seen in those cases where a mineral has been decomposed with the production of an aggregate of smaller, anisotropic fragments of some other mineral or minerals. The alteration product is made up of small fragments with all possible orientations, and consequently there is never complete extinction of the pseudomorph – a fact that occasionally troubles students. If such a pseudomorph were looked upon as a crowd of small individuals, the difficulty disappears.

Friday, February 15th, 2008 at 5:13 am
Characters of Minerals Between Crossed Nicols
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