Optical Properties of Graptolite Epiderm A Review

Abstract

Studies of optical properties of the skeletal materials of graptolites show progressive changes of reflec­tance, bireflectance, refractive index and absorptive index with maximum temperatures caused by depths of burial, metamorphism or tectonic disturbance. Samples of Lower Ordovician to Upper Silurian grapto­lites were selected from across Canada, at locations where the Conodont Alteration Indices of adjacent conodont collections provide temperature control. Maximum reflectance in oil and bireflectance appear to be the characters most diagnostic of maximum temperature and show progressively higher rates of in­crease at higher temperatures. These factors show potential for sensitive discrimination of temperatures at high levels of Conodont Alteration Indices. Besides depth of burial, variables that have some effect on the optical properties include the composi­tion of rock matrix, thermal metamorphism, mineralization and oxidation. The effects of morphological orientation and the taxonomical attributes of graptolites as yet are uncertain.