Feldspars optical properties

Most natural alkali feldspars are unmixed in the sense that they contain both Na-rich and K-rich phases, but the highest-temperature forms of alkali feldspar do form a complete series of solid solutions. On the basis of their optical properties, four separate series of minerals are recognized, viz.,... 

The optics of the various feldspars have turned out to be quite complicated and are only slowly being unraveled. For any particular specimen, the various optical properties depend quite critically on both composition and thermal history or structural state. For soil scientists, it seems pertinent to remark that one should not attempt to characterize closely the feldspars in sand fractions by their optical properties without a considerable amount of experience and/or the advice of a highly skilled petrologist who recognizes the peculiarities of these minerals. As an example of their complexities, consider the twin laws for feldspars that... 

On the other hand, the curves of refractive index can be used to estimate the composition of a plagioclase, especially if it is possible to invert the specimen to the maximum high-temperature form first (J. R. Smith [1958]). For the alkali feldspars, however, the values of the refractive indices are quite sensitive to small amounts of the anorthite molecule in solid solution (or to traces of Ba, Sr, Rb, and Fe ), and furthermore this property is fairly dependent on the thermal state or history of the specimen. Refractive index is not a particularly suitable optical property for estimating chemical composition for the alkali feldspars, but is fairly useful for the plagioclases. 

As Smith and Gay [1958] have emphasized, the detailed study of a plagioclase specimen requires single crystal, powder diffraction, chemical analysis, refractive index, and optical orientation data at least. This is similarly true for the alkali feldsparsi especially since some optical properties are sensitive to compositional impurities as much as structural state. It would seem that such a careful study of feldspars in the fine sand fraction may repay handsome... 

Morey, G. W., and W. T. Chen, 1955. The action of hot water on some feldspars. Am. Mineral. 40 997. Muir, I. D., 1962. The paragenesis and optical properties of some ternary feldspars. Norsk Geol. Tids. 42 477. 

Besides mechanical properties, fillers change the optical and electrical properties, chemical and weathering resistance, flammability and density of polymers. In particular, most fillers destroy optical translucency, unless their refractive index (see Table 4.4) is the same as that of the polymer. Close agreement between the fibre and the polymer refractive index is required for translucency. Feldspar has a refractive index very similar to that of several polymers and so filled, translucent products can be obtained, but it is abrasive to processing equipment. 

A related experiment on a crystal of Na-feldspar (NaAlSisOg), a mineral that is also widely used in dosimetry, studied cyclotron resonance in order to determine the effective mass of the carriers. Optically Detected Cyclotron Resonance (OCDR) at 23 GHz was combined with HFEPR at 93 GHz. In order to carry out the OCDR experiment it was necessary to enhance the emission by doping the feldspar with a low concentration of copper, but this was not felt to affect the cyclotron resonance properties. The OCDR spectrum at 23.1 GHz consisted of a single broad line with a peak at 0.65 T and a FWHM of 0.2 T. The line was interpreted as cyclotron resonance with an isotropic effective mass of 0.79me. It... 

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