Investigation of Maximum Reflectivity and Other Optical Constants, Determination of Anisotropism in Polarized Light, and Spectroscopic Research
For numerous years optical methods of approach have engaged the attention of research workers in various parts of the world. For the most part these have followed three main directions, namely, investigation of maximum reflectivity and other optical constants, determination of anisotropism in polarized light, and spectroscopic research. Of these the determination of optical constants of ore minerals seems to have attracted many workers. This type of investigation was probably started by Drude when he obtained for stibnite such optical constants as the indices of refraction and coefficients of absorption, using a spectrometer equipped with rotatable nicols in the collimator and telescope tubes.
Two years later, the same worker determined the optical constants of various metals and alloys, using method of Kunndt of diverting a beam of light through a metal prism. Methods of Drudes were also employed by Forsterling to determine the reflectivity and other optical constants of hematite. Substantial advance in this part of the field was not attained until the development of photometric methods for determining reflectivity, first by Orcel and Schneiderhohn, later by Frick and Moses. The appliance of these methods has made possible the precise determination of the reflecting power of various ore minerals and has placed another valuable tool in the hands of workers in the field of ore mineral study with the aide of a microscope.
Observations on the anisotropism of ore minerals which seem to have been started early in this century by Koenigsberger who used a Savart plate, a plate of known refractive index, a reflecting prism and a contrast biplate of smoky quartz. His work was followed by that of F. E. Wright, who then developed methods for determining the degree of anisotropism involving the use either of a special cleavage plate of calcite or of two biprisms of quartz, called the bi-quartz wedge plate. Other workers in this field include Glaser, Orcel, Sampson and Goranso. Spectrographic studies of the ore minerals that can be examined under a microscope seem to have been much less popular than the other two approaches from the physical side.
The names of but five workers in this part of the field have come to the attention of writters, namely Schneiderhohn, Claussen, Berthelot and Orcel and Harcourt. Of these Schneiderhohn conducted spectrographic analyses on the platinum ores of the Bushveld Complex, Claussen did similar work on galena, sphalerite, and pyrite from many localities, and Harcourt used this method to distinguish enargite and famatinite. At present these are the only five that have come to my notice. To complete the picture on the optical side, mention must be made of the extensive notes of Guild, Schneiderhöhn, McKinstry and Stephens on the effect of strong light sources on silver minerals, and also of the investigations of Myers on the advantages of oblique illumination.
Moreover, a research was conducted on the diagnostic value of color in identifying ore minerals. By using either a comparison microscope or two microscopes equipped with a comparison ocular, and with the color contrasts intensified by the use of color-screens, he was able to establish the fact that the absolute color value of any opaque mineral is constant. Still another worker, Gaubert, investigated such physical properties as form, transparency and streak, the last mentioned property being studied either on a glass slide or on an unpolished quartz plate.
Another attractive avenue of investigation followed by many during the last ninety years has led in the direction of quantitative macroscopic and microscopic analysis. While it is true that most of the efforts in this direction have been confined to determining the quantitative relations of the rock-forming minerals, some investigators at least have turned their attention towards a quantitative determination of the opaque minerals in ores.
