Carbonates birefringence

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. 

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 effect of interatomic distance is strikingly illustrated by the fact that the birefringence of nitrates in which the nitrate ions are all parallel is much greater than that of carbonates of similar structure, though the interatomic distances in the nitrate ion are only slightly less than those in the carbonate ion ... 

Significant variations in the magnitude of the physical properties, such as the liquid crystal transition temperatures, viscosity, birefringence and the elastic and dielectric constants, of alkenyl-substituted compounds with a carbon-... 

Based on the evaluation of the results of physical measurements on polar alkenyl compounds described above, the position and configuration of the carbon-carbon double bond found in the apolar compounds is similar to that in the polar compounds, essentially for the same reasons. However, the combination of a low melting point and a high nematic clearing point of the bicyclohexane compounds (142,143 and 145) is remarkable. The birefringence and viscosity of these compounds is remarkably low. ... 

The high calcium content in the majolica found in Mexico City— 21.4% calculated as pure calcium carbonate compared with 5.9% in sherds of Teotihuacan—suggests that a calcium compound such as calcium or calcium magnesium carbonate may have been added to the majolica either as a temper or through deposition during burial. Petrographic examination of cross sections of representative Mexico City majolica sherds show heavy deposits of birefringent material with structures... 

Polymer characterization. The thermotropic behaviors of the polyester-carbonates prepared were examined under a polarized hot stage microscope (Sybron Co.). Normally the sample became softer at 120-150°C and melted with birefringence afterwards, depending on the molecular weight and composition of the polymer. The samples stayed bire-fringent until 350°c or higher. 

The inclusion of MHQ into the polyester-carbonate compositions gave polymers with strong birefringence... 

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