Microscopic-photometric measurement of polymodal clay orientation using circular-polarized light and interference colours

Abstract

Orientation measurements are important in many branches of geosciences in both field and laboratory studies. For fine-grained especially clayey samples, particulate and other physical methods are unsuitable. This is because numerous individual measurements are needed for a desirable level of accuracy, and sophisticated sample preparations and equipment are required. Existing photometric techniques were evaluated. They fail to deliver circular vectorial data (both magnitude and direction) for preferred orientation, and cannot detect bimodal or polymodal patterns. A modified method is developed to overcome these pitfalls. The pertinent principles related to clay orientation in thin sections, relative retardation and interferece colours are reviewed. Spectral distributions of interference colours are calculated and plotted to facilitate the selection of a suitable wavelength to winnow the light. A sensitive-tint plate of gypsum is used to alter the relative retardation and hence the interference colours of oriented clay in thin sections. The interference colours of unlike vibration direction are separated from those of alike vibration and isotopic components (including voids) by a monochromatic filter at a chosen wavelength, and detected by a photoelectric sensor (selenium photocell). Circular-polarized light was employed to eliminate the extinction phenomenon which was regarded as unwanted noise. Details on calibration, equipment setup and procedures are given. The application of circular statistics to the orientation data is described and illustrated with results of selected specimens. © 1988.