Optical Constants of Monoclinic Anisotropic Crystals: Orthoclase

摘要

The optical constants, the real and imaginary parts of the complex refractive index of any substance, are useful in modeling the optical properties of that material. For biaxial crystals there are three sets of optical constants which may be observed at various directions in the crystal. For the monoclinic and triclinic crystal systems the optical axes of the crystal wander with frequency so that experiments to determine the optical constants have hitherto had to be made at different orientations for each frequency of interest. In the i.r. spectral region where the optical constants are subject to large spectral variations, a modification of classical dispersion theory has been developed to simultaneously determine the principal indices from a set of reflectance measurements of the crystal. The feldspars, with the exception of orthoclase and saninine, are rarely obtainable in the form of the large (approx. 1 cm on a side) single crystals required for the the dispersion theory analysis. Therefore, modeling experiments previously carried out with respect to feldspars in the i.r. region of the spectrum have had to rely on pseudo optical constants. The optical constants of the monoclinic crystal orthoclase(ideal composition 4(KA1Si3O8)) have been derived in the i.r. region of the spectrum by the use of dispersion theory. Values for the E parallel b orientation and for the a-c plane have been used, together with a theory of the emittance of the particulate materials, to simulate the spectrum of orthoclase powder.