| Mengyin of Shandong province is one of the four big main diamond producing areas in our country, which is rich in the colored diamonds. Colored diamonds include many brown ones. Through making researches on the relation between the color and the lattice imperfection of brown diamonds from Mengyin, it is helpful to find new color enhancement methods, and to promote the market value of brown diamonds accordingly, which has high economic significance.A series of researches on the original stone and the sliced sample have been used in this article,such as micro-observation,optical (orthogonal polarized light),spectroscopy (low temperature photoluminescence, FTIR, Raman, ultraviolet-visible absorption), luminescence characteristics(cathodoluminescence, thermoluminescence, ultraviolet-fluorescence), microscopic morphology(transmission electron microscope), and so on. It reveals its internal lattice imperfection information, and to seek the relation between unusual extinction and microscopic characteristics of the plastic deformation in diamond, consequently to discuss the possible forming mechanism of each kind of lattice imperfection and the relationship with the color. Main recognitions are following:1. There are eleven kinds of crystal surface erosion shapes on the brown diamond selected in this research. The whole original stones' color is quite even. After slicing, the internal color presents obvious uneven distribution. The color distribution has three kinds: even color, stripped color, ringed or massive color.2. The infrared spectrum has demonstrated bond C-H, N-H, O-H, slippage plane {111}, olivine, pyrope, and so on, which cause mass of absorption peaks. They have demonstrated different defect information individually.3. Diamond crystal has formed dislocation destruction on the slippage plane, which may induce the valence bonds' break in certain circumstances, and form the center of structure defects. These defect absorption light may cause brown on the-slippage plane, that is, diamond's brown relates to the defects produced with accompany of the slippage plane. They result from the development of plastic deformation after crystallization. Plastic deformations degree and brown present positive correlation. The samples, whose plastic deformations are intense, represent distinct brown bands or brown regions after slicing.4. In the low temperature photoluminescence experiment, the samples participating in the tests have the characteristic peaks (575.92nm,612.59nm,617nm,626.89nm,and so on) , which are casued by plastic deformations, and attribute the luminescent center related with dislocations. To a certain extent, it indicates that the brown diamond from Mengyin, Shandong has experienced various degree plastic deformation in its forming process, which causes the brown.5. The samples' intrinsic peaks of Laman spectrum have a universe drift to the low wave value direction. It generally indicates that the structural feature of the diamond has had certain changes. Meantime, cooperating with the result of micro-enlargement,orthogonal polarized light and cathodoluminescence observations, it may be thought that these changes of the spectrum peaks are related to the internal stress of brown diamond.6. The colors of the samples participating in ultraviolet-visible absorption spectrum test are primarily brown and tan. The samples, which obviously present the 416nm characteristic peak in the absorption spectrum atlas, are caused by N3. They are all I aAB types and mostly tan.7. The TEM shows that there are high density dislocations in the samples. In a sense, it indicates that this kind of diamond samples have a certain extent plastic deformation. At the same time, the spectrometer of the transmission electron microscope demonstrates that the diamond samples have the inclusions of sylvite, salt, olivine, pyrope, diopsideand so on .The inclusion types have a very high correspondence with the ones tested by infrared spectrum.
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