Pleochroism

PLATO Plattnerite Platyhelminths Playfield Turf Plazjet Plectonema Plegine Pleochroism Pleurisy... 

The disadvantage of this procedure is that the minerals maybe physically or chemically altered during burning. Eor example, the refractive index of clay minerals is changed the color, birefringence, and pleochroism of micas is altered carbonates are destroyed and the iron sulfides are oxidized to iron oxides. 

The analyzer is removed and the color of the sample is observed in plane-polarized light. If the sample is colored, the stage is rotated. Colored, anisotropic materials may show pleochroism—a change in color or hue when the orientation with respect to the vibration direction of the polarizer is changed. Any pleochroism should be noted and recorded. 

Polychroismtis, m. polychroism, pleochroism. polychromatisch, a. polychromatic, polychrome. PolychromsMure,/. polychromic acid, polycyclisch, a. polycyclic. 

Fibers of tourmalines up to 8 inches long have been described from the Alps (Dietrich et al., 1966). Novad and Zak (1970) described shorter and thinner (diameter < 1 micrometer) tourmaline fibers from the Ural mountains. With high birefringence and pleochroism, tourmaline is readily identified by optical examination. It is distinguished from other fibrous silicates... 

Pleochroism-exhibits two different colours upon rotation of the stage of the polarising microscope... 

The observation of the colour and the degree of absorption associated with each index is of obvious value for identification purposes the larger the number of characteristics observed, the more certain the identification. Observations of pleochroism may also be useful as indications of certain features of molecular structure. (See Chapter VIII.)... 

Samarskite.—TYna tantaloniobate mineral has a widely varying composition, and was first discovered in the Urals. The presence of uranium in this mineral renders it radioactive. The presence of lead and helium has been detected. The ratio of lanthanides to yttrium is approximataly 1 6. In its crystal structure samarskite resembles yttrotantalite. Native crystals are brown, or velvet black, and become yellow orange when crushed showing pleochroism. 

For tetrachloro-l(2)-naphthalenone, Marckwald (1) found that only one of the two crystalline modifications was photochromic and that the color change was accompanied by an increase in the pleochroism of the crystal. Weigert (63,64) interpreted this effect in terms of a distortion of the crystal lattice. 

Measurements of electronic absorption spectra in the visible region not only lead to the evaluation of CFSE s, but they also provide useful information about the crystal chemistry of transition metal ions in the crystal structures and causes of colour and pleochroism of minerals. In this chapter, techniques for measuring absorption spectra of minerals are briefly described and some general applications of the optical spectra to basic crystal chemical properties, such as colour and pleochroism, are discussed. These examples also amplify many of the features of crystal field spectra outlined in chapter 3. 

Pleochroism is the property of a non-cubic crystal whereby certain wavelengths of polarized light are absorbed in different amounts in different crystallographic directions. Such differential absorption may produce different... 

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