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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

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. This sandwich rests on a glass slip and is covered by another and much thinner slip of glass, but these may be ignored. We are concerned here only with the substance in contact with the minerals making up the rock.


            It is well known that colorless transparent substances are visible only when they differ in optical density or refractive index from the fluid in which they are immersed. And the greater the difference between the two substances, the more conspicuous will be their limiting surfaces. An instructive experiment is to powder common glass and drop it in water. Better still, powder a fragment of the mineral cryolite and drop the powder in water. It is with the greatest difficulty that the mineral can be seen when immersed in the liquid, and one is inclined to believe that it has passed into solution. It is invisible, however, merely because the two substances, cryolite and water, have almost exactly the same refractive index.


            We are now in a position to make a general statement. A transparent solid is more or less visible according to whether it is more or less different in refractive index from the fluid in which it is immersed. The minerals in thin section may be considered to be immersed in a liquid – the Canada balsam. On examining the margins of the transparent minerals in thin section, we find, by working round the edges of the section where the minerals are in contact with Canada balsam, that some of the edges stand out conspicuously, while where other minerals come in contact with Canada balsam, the junction is comparatively inconspicuous. The minerals are in contact with the mounting medium also in their upper and lower surfaces, and these supply us with additional information. In consequence of the method of preparation of thin sections, these surfaces are rough. They are grooved and pitted by the powders employed in the grinding. Those minerals-differing in refractive index from that of the mounting medium show these rough surfaces when carefully focused. On the other hand, those constituents approximating in refractive index to the Canada balsam always appear to be smooth in consequence of this approximation. Clearly, then, we have here a means of determining by mere inspection whether or not a particular mineral has a refractive index which is near to or distant from that of Canada balsam, 1.54.


            But it is necessary to determine whether the refractive index of the mineral in question is higher or lower than that of Canada balsam. And a means of making this determination is supplied by BECKE’S TEST. Using the high power objective magnification of petrographic polarizing microscope, focus carefully an edge of the mineral where it is in contact with the mounting medium, choosing an edge as clean as possible. If a small fragment of the mineral has become detached from the rest of the rock and thus lies surrounded by Canada balsam, it will supply the ideal conditions. Rack the tube up and down a little on each side of the position of focus, being careful not to push the objective of the polarizing microscope through the slide, an accident that occasionally happens to beginners. It will be seen that, when the fragment is slightly out of focus when viewed, it surrounded by, or encloses, a ring of light, which expands or contracts as the tube of the polarized microscope is alternately raised and lowered.


            There are two cases. The refractive index of the mineral is (1.) higher, and (2.) lower, than that of Canada balsam, 1.54. It is convenient to consider only what happens when the objective is being raised. In the first case, when the mineral is denser than the mounting medium, the ring of light will contract. It will pass from the Canada balsam to the mineral. When the mineral is the less dense substance, the light passes from the mineral to the balsam.


            In both cases, as the objective is raised, the light passes from the substance with the lower, to that with the higher refractive index. It is thus possible, by this test, to eliminate all those minerals, which differ from one under consideration in being higher or lower than Canada balsam in refractive index. And the method of elimination will sometimes be found extremely useful in making identifications. A concave mirror should always be used in making this test, and a diaphragm of the petrographic polarizing microscope will be found useful in difficult cases. The existence of this more intense illumination depends on the property of total internal reflection, as the light impinges on a surface limiting a dense substance. In some cases it is possible by this method to compare the refractive indices of two minerals in contact, but the contact is not always sufficiently good to give results.


            As examples of case 1, topaz, apatite, or olivine gives good results. Fluor, nosean, and Leucite supply good examples of case 2. When reference is made to the refractive index of a doubly refracting mineral, it must be understood that the means refractive index is referred to, also, that that value has been obtained when using the sodium light.

Wednesday, February 13th, 2008 at 3:34 am
Minerals Characteristics in Ordinary Transmitted Light
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Click Here For Best Selection Of High Quality Polarizing Microscope