Physicochemical characterization and expansive-porous evolution of green obsidian from cerro de las navajas under controlled thermal treatments

This work presents the first systematic study on the behavior of green obsidian from Cerro de las Navajas (Mexico) under controlled thermal treatments, considering temperature, sample size, and holding time as the main variables. Two types of samples were analyzed: bulk samples and compacted powders. The glass transition (650 °C) and the onset of significant expansion in bulk samples (900 °C) occurred at lower temperatures than those reported for other obsidians, suggesting a higher content of structural OH groups. Under optimized conditions, the bulk samples reached porosities up to 85%, predominantly closed (99%), with densities of 0.36 g/cm3 and thermal conductivity of 0.28 W·m−1·K−1. At temperatures above 1150 °C, rapid pore coalescence was observed, a stage prior to structural collapse. In contrast, powder samples did not exhibit expansion. Expansion was found to be strongly influenced by sample size and the amount of initial vesicles, which act as nucleation centers. The amorphous character was preserved after thermal treatments. The results demonstrate that obsidian, when subjected to controlled thermal treatments, develops properties with potential for technological applications where high porosity and low density are critical.