Study of the micro-vesiculation of the obsidian source of Sierra de las Navajas (State of Hidalgo, Mexico)

Sierra de las Navajas obsidian was highly exploited by the pre-colonial Mesoamerican people and was highly desirable for its very good quality and its characteristic green colour. In the archaeological obsidian mines, fragments with very variable macroscopic aspect occur, with colour ranging from very light to very dark green, and with variable hue. Physical properties, such as roughness and fracture, are variable as well. The surface of this obsidian is sometimes characterized by the occurrence of a diffuse micro-vesiculation. In order to investigate the 2-D and 3-D distribution of vesicles and to relate it to the macroscopic aspect and to the physical properties of obsidian, we have selected three samples showing different colour, roughness, hue and fracture. We have observed their surface by electron microscopy and we have investigated small volumes of the same specimen by X-rays micro-tomography. The results have shown that the hue sometimes shown by obsidian is strictly related with the occurrence of abundant, elongated and iso-oriented vesicles. The occurrence of coarse (hundreds of microns) vesicles also gives to the surface a high roughness. Finally, the shape of the vesicles controls the type of fracture (conchoidal or flat). Colour, hue, fracture and other physical characteristics, in their turn, influenced the use of obsidians by ancient and modern populations. Poorly or not vesiculated obsidian with perfect conchoidal fracture was heavily used for the production of blades, while vesiculated obsidian, showing a beautiful golden hue, seems to have been preferred for the manufacture of ornaments or magical and ritual objects.