The most useful rocks for microscopic investigation are those igneous rocks known as plutonic. These may be looked upon as aggregations of minerals that have crystallized from the fused state that possess a certain interesting majestic images when evaluated under petrographic polarizing light microscopes. And as the individual mineral have not all formed simultaneously, it […]

It is often found that a crystal is not uniformly colored, but exhibits an arrangement of color bands, which are concentric with the exterior and is often splendidly displayed by the mineral under polarized microscopes. Nosean, for example, is commonly distinguished by a well-marked brown periphery that is majestically exhibited under geological polarizing microscope, although […]

For a full account of this property of solids and liquids, the students studying mineralogy is referred to textbooks on Light. We shall here consider only those aspects, which directly concern us.
 
            A rock section is an extremely thin slab of rock, which is sandwiched in between layers of a substance known as Canada balsam. […]

In some few cases, minerals may be seen to contain inclusions of other substances, which are useful in making identification. The best examples are seen in some specimens of leucite, which may show small fragments of other minerals, notably the green augite, which are usually arranged in rings. But it is also important to note […]

The light transmitted by the different minerals when evaluated in transmitted light of petrographic polarizing microscope varies both in amount and kind. Some minerals are almost opaque while others are perfectly transparent. In some case, the whole constituents of white light are allowed to pass through, these mineral are colorless. Other minerals, however, absorb some […]

Even a slight acquaintance with minerals as seen in hand specimens will be sufficient enough to enable the individual to recall examples of minerals that shows this tendency to break in definite planes or cleavage. Calcite for instance is known to split in three directions. In fact, calcite is so hard to or impossible to […]

Orthoclase feldspars supply us with a good example of mineral, which is liable to decomposition on exposure to atmospheric conditions. If the rock from which the section has been cut is ‘fresh,’ this mineral is quite colorless and transparent. But when decomposed, it has lost its transparency, and has become a mere aggregate of countless […]

The first examination of the mineral section should be made with the one inch objective where both polarizer and analyzer are being swung out of the axis of the petrographic polarizing microscope.
 
            As expected, it will be seen that the thin section being evaluated under the polarizing microscope is made up of several minerals, some […]