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, […]

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 […]

Up to the present wedge were used merely to demonstrate the effects of vibration in thickness of an anisotropic section of a mineral. Now, we have to consider some other uses to which wedge may be put.
 
            It has been suggested that, by practice, the beginner mineralogist may acquire the ability to recognize the particular […]

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 […]

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 […]

All these polarization colors in the case of anisotropic sections are due to the amount of relative retardation effected in the section – the difference between the velocities of the two rays traveling in the section, or, expressed differently, the difference between the two refractive indices, the retardation increasing, of course, with increase of thickness. […]

We are now to discuss the reasons why a section in one position gives a polarization color when viewed between crossed nicols of the geological polarizing microscopes, and in another becomes extinguished. It has already been seen that first, if the light leaves the section vibrating as it entered (as it left the polarizer) it […]

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 […]

Some mineral sections have no power to produce any illumination. They are singly refracting and consequently are quite dark between crossed nicols of petrographic polarizing microscopes. They remain in that condition during a complete rotation of the stage of the polarizing microscope, and are said to be isotropic. All sections of the minerals crystallizing in […]

When minerals are to be examined between crossed nicols, both analyzer and polarizer but be placed in the axis of the microscope. And moreover, the two nicols must be rotated to such positions that, with an empty stage, no light reaches the eye. In order to simplify this adjustment each nicol is provided with a […]