Pleochroic

In thin sections natural graphite is translucent, strongly pleochroic, and uniaxial. It has a negative sign of birefringence and two extinctions per revolution under crossed Nicol prisms. The atomic number of carbon accounts for its low absorption coefficient for x-rays and electrons. 

Mehrfarbetidruck, m. multicolor priutiug. mehrfarblg, a. multicolor(ed). polychromatic pleochroic. 

H4Co2Mo10O38]4-.79 Crystals of the ammonium salt are pleochroic and transmit red, blue-green and yellow light along the three molecular axes. 

From about 1930 onwards, developments in the field of naphthoquinone dyes concentrated on the use of naphthazarin and intermediates for the preparation of violet, blue, and green acid and disperse dyes [1]. More recently there has been interest in the synthesis and color and constitution properties of simple colored naphthoquinones, stimulated by the fact that such dyes have similar tinctorial properties to the anthraquinones but a smaller molecular size. The naphthoquinones provide a useful alternative to the anthraquinones for certain specialized applications, e g., as pleochroic dyes with improved solubility for liquid-crystal displays. As a result, research interest in these chromogens remains unabated, even though they have failed to make any major impact as textile dyes [2-8],... 

Fig. 8. Partly corroded molybdenite (MoS2) fragments, strongly pleochroic, floating in a metal-rich sulfidic melt. Heated in a plasma-jet melting apparatus, chilled from above 2000 °C x 2000, oil immersion...

Like its ferrous analogue, cobalt hydroxide readily absorbs oxygen, yielding a brown mass. When dissolved in acids stable cobaltous salts are obtained, but when dissolved in hot, concentrated aqueous potassium hydroxide and allowed to cool, cobaltous oxide gradually crystallines in microscopic prisms 5 which are deposited as a violet powder. The crystals are pleochroic, of density 3-597 at 15° C-, permanent in air and insoluble in ammonium hydroxide. They are soluble in acetic acid, in sodium hydroxide, and in hot solutions of ammonium chloride. 

The first nematic guest-host prototype nematic guest-host display device contained a nematic liquid crystal (4-butoxybenzoic acid) and a pleochroic dye (methyl red or indophenol blue) sandwiched between two (Nesa) electrodes dTn 12/im) rubbed uniaxially, but with no additional orientation layer, see Figure 3.14. One polariser was fixed to the front substrate surface with its direction of maximum absorption parallel to the rubbing direction and, therefore, the nematic director. 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

The tetrahydrate, 2FeCl3.4HaO, first obtained by Roozeboom, crystal-lisesTn The rhombic system. The crystals appear pleochroic in polarised light, the colours ranging from yellow to brown. They melt at 73 5° C. 

Molybdite or molybdenum ochre forms orthorhombic crystals, and occurs with molybdenite, from which it is probably derived. It consists essentially of the trioxide MoOg, but analysis has shown that its composition is probably expressed by the formula Fe203-3Mo03.7jH20 it being, in fact, a hydrated ferric molybdate. The sample examined was of a yellow colour, possessed a fibrous structure and a silky lustre, and was pleochroic. 

Figure 6 Cosmic jewels an aggregate of tiny blue hibonite crystals from the Murray CM chondrite. This entire object is no more than 100 xm in maximum size. The color of these sapphire-like gems is real and is due to the presence of substantial titanium in the crystal structure. The color variation is partly due to the crystals being compositionaUy zoned (their margins are more titanium-rich), and also to their being pleochroic under plane polarized light the color is visible only when the crystals are in specihc orientations relative to the polarizers on the...

If the microstructure is too fine, and the material is pleochroic, one can attempt to identify the azimuths for maximum and minimum absorption of linearly polarized light (171 in optically biaxial material, these need not coincide with the local extinction directions. The use of optical pleochroism as just described is one example of how studies limited to observations between crossed polars do not exhaust the information available from light microscopy. 

Polarized measurements using single crystal flakes reveal no in-plane and out-ofplane dichroic character of bands at 110, 91 and 83 cm for muscovite, phlogopite and biotite, respectively, and proved the assignment to vibrations of the oxygen atoms that constitute the cage in which K is located (Laperche and Prost 1991). Weak pleochroic... 

Much attention has been given to the analysis of OH stretching bands of micas occurring in an isolated region of the spectrum ranging from about 3750 to 3550 cm (Farmer 1974). Using polarized radiation, the pleochroic scheme of IR absorption bands, measured on oriented crystal sections, makes it possible to impose defined constraints on the orientation of the OH dipoles. In a fundamental polarised IR study, Tsuboi (1950) determined the position of the H atom in the structure of muscovite. The variation of absorption intensity of the OH stretching band with the direction of the electric vector of... 

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