Influence of Chemical Composition and Microvesiculation on the Chromatic Features of the Obsidian of Sierra de las Navajas (Hidalgo, Mexico)

The obsidian of Sierra de las Navajas is well known for its green color and gold hue. In order to relate these features with compositional and microtextural characteristics, we have carried out a microanalytical study by Wave Dispersion System associated to Electron Probe Micro-Analyzer, Scanning Electron Microscope observation, and X-rays micro-tomographic analyses of samples showing different colors (dark to light green, sometimes with bands of different color intensity) and hues (changing, uniform, no hue). In accordance with previous studies, the green color of the obsidian seems to be related to a high iron content, probably in its reduced state. However, no significant difference in composition occurs between dark and light green samples. The SEM observation and microtomographic study revealed the absence of microcrystals and the occurrence of vesicles of different size, shape, and orientation. Lighter green colors are shown by highly vesiculated surfaces, whereas non-vesiculated samples are darker. On the surfaces with a high concentration of coarse vesicles, a uniform golden hue is observed. Decreasing vesiculation gives a hue changing with the angle of incident light. However, when the vesicularity approaches zero, no hue is visible. The iso-orientation of vesicles along preferential directions and their distribution in bands determine the variation in color intensity and hue on differently oriented surfaces. Microvesiculation also influences other characteristics that were important features for the use of obsidian in the past, such as fracture, transparency, and roughness.