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Click Here For Best Selection Of High Quality Polarizing Microscope

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In optical mineralogy, a twinned crystal is formed of two or more individual crystals of the same species and joined together according to a definite law. There are many distinguished twins known in the field of optical mineralogy. If viewed closely with the aid of polarized microscope and crystals are found simply united by a common plane, then it is said to be joined together as contact twins. However, when crystal appear to cross each other in a complex but symmetrical way as seen clearly exhibited under polarizing microscope, then it is joined by penetration twin. Simple twins is said to consist just two individuals, which can be seen more clearly exhibited when viewed with the aid of petrographic polarizing light microscope. A repeated or polysynthetic twin on the other hand consists of several individuals, often in lamellar form.

            The description of the geometric relationship between the individuals of twinned crystal in the field of optical mineralogy field of study is in terms of twin axes or twin planes. In optical mineralogy, a twin axis is defined as a line of rotation about which one twin can be brought into the orientation of the other and usually the rotation as observed under petrographic polarizing microscope is about 180-degree. A twin axis is said to be the most convenient way of describing most twins. A twin plane on the other hand is a plane of reflection across which the twins are mirror images. It is but very obvious that a twin plane cannot be parallel to a plane of symmetry of the crystal since the mirror images across such a plane are identical. It is also important to note that the twin plane must not be confused with the composition plane. In the field of optical mineralogy, a composition plane is defined as the plane that actually unites the two individuals. But it is also interesting to note that these two planes sometimes in some cases they coincide.

            As observed in the field of optical mineralogy, normal twins have a twin axis normal to the composition plane. Parallel twins have a twin axis that lies in the composition plane parallel to a crystal edge. Complex twin on the other hand can be visualized as a combination of normal and parallel-twinning, the twin axis lying in the composition plane normal to a crystal edge.

            It has been observed in optical mineralogy that there are ways several ways in which twin crystals may form. It is known that growth or primary twins form during the growth of the crystal and it is most often observed at the beginning of growths. Secondary twins on the other hand form subsequent to crystal growth. For instance, deformation twins result from the rotation of part of the crystal into a twin orientation during deformation. With several experimentations made in optical mineralogy, calcite is found to develop deformation twins easily, if squeezed in a vice. Transformation twins on the other hand result from the change of symmetry of certain crystals on changes of temperature and pressure. For instance, monoclinic alkali-feldspar may invert to a triclinic form on cooling, and in so doing will nucleate numerous twins in a cross-hatched arrangement. Leucite twins in a similar way on changing from cubic to a lower symmetry on cooling.



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Thursday, April 16th, 2009 at 2:32 am
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Optical Mineralogy
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