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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

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 break in any other directions. The common micas, muscovite, and biotite, cleave in one direction with extreme facility. Mineral gypsum also has one perfect cleavage that is most clearly seen when the section is evaluated under petrographic polarizing microscope. In thin sections of minerals, these cleavages are seen as parallel straight lines when viewed more closely under geological polarizing microscopes. Minerals having more than one cleavage usually show more than one set of parallel cracks under polarizing microscopes, those of the one set intersecting the others. As the mineral section is being evaluated with the aid of polarizing microscopes for geologists, it must be understood that these cleavages have a definite crystallographic orientation. In the case of micas, the one perfect cleavage is parallel to the basal pinacoid and it can be seen more clearly visible under polarized microscopes. The three cleavage directions of mineral calcite are parallel to the faces of a rhombohedron and this is more fascinatingly interesting when viewed under polarizing microscopes. It must be understood also, that they are not necessarily parallel to faces actually present in the crystal. Calcite for example, rarely crystallizes with the form of the rhombohedron to the faces of which its cleavages are parallel. The forms presented by calcite crystals are numerous and bewildering to the young beginners, but any one of these crystals may be cleaved in the three directions, which are those of the faces of what is known as the fundamental rhombohedron. This constancy of cleavage direction for any mineral, which shows cleavage at all, supplies another useful means of identification most especially when the evaluation is aided by a petrographic polarizing microscope.

 

            We find, however, a certain amount of variation in the different sections of any mineral. Micas, for instance, have one perfect cleavage, but in thin sections we see many examples in which no cleavage is visible. The explanation is perfectly simple. Consider the analogous case of a book. The cleavage flakes of the mica correspond to the leaves of the book. Let the book rest flat on the table and imagine it cut in vertical section. The cut surface would show a series of parallel lines produced by the intersection of the plane of section with the leaves. The horizontal section would not, however, show any cleavage, because it never cuts across a leaf. It is necessary to point out, however, that oblique sections do not always show cleavage. One might expect that, that the horizontal section already considered is the only one not cutting across the cleavage, it would be the only one not showing the cracks in section. This is not the case. If we consider a series of sections varying from the perpendicular to the parallel, we find the cracks becoming less and less conspicuous, until, long before the parallel condition is reached, the cleavages become invisible.

 

            We have considered the case of micas in some details as they supply a very simple case, which serves as a peg on which to hang an important principle. If the a case of a mineral showing two cleavages is understood, the presence of cleavage and the angle of inclination of cleavage must be also considered. Hornblende, monoclinic in crystal system, is a most common rock-forming mineral with two cleavage directions that are parallel to the prism faces. This can be seen more clearly exhibited when the mineral is evaluated meticulously with the aid of the polarized microscopes for geologists.

 

            A transverse section of four such faces would give a rhomb with angles of 125-degree and 55-degree, and these therefore, are the cleavage angles typically exhibited by this mineral in thin section as evaluated under petrographic polarizing microscope. A vertical section, however, shows one cleavage-direction only. The reason should be quite clear after a little consideration. It should be understood that the angles formed by the intersection of the cleavages, as seen in oblique sections, would actually vary with the degree of obliquity if they were visible. It should be remembered, however, that if section and cleavage were not inclined at or near a right angle, the cleavage cracks would not be visible. Consequently, when the two cleavages of hornblende are seen in section, that section is approximately transverse, and the characteristic angles are exhibited. Augite is another monoclinic mineral with prismatic cleavages, but there they are almost at right angles. This difference in the cleavage angles is extremely important as a means of distinction between the two minerals.



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Wednesday, February 13th, 2008 at 3:30 am
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Minerals Characteristics in Ordinary Transmitted Light
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