The effect of initial oxidation state on crystallization of basaltic glass

Iron-rich silicates mined as basalt may be processed as glass fibers and, woven to textile mats, it may be used as heat insulation. High-temperature stability is however a limiting factor because at least three process of alteration occur upon heat treatment that affect mechanical and chemical properties, micron to nano-crystallization, oxidation and cation-enrichment at the glass surface with a few microns depth. We here evaluate the crystallization behavior of synthetic Fe3+-rich basalt (SB) heat treated in air (negligible oxygen potential) in comparison with previously studied Fe2+-rich natural basalt (NB) heat treated in air and in Ar (i.e., high and low oxygen potential, respectively). Initial crystals growing in Fe2+-rich basalt are micron-sized dendrites, and with temperature of heat treatment they become increasingly granular and are identified as pyroxene. Pyroxene in SB form smaller dendrites; crystallization textures and nucleation and growth rates, derived from crystal size distribution, are independent of the temperature of heat treatment in the range between 850 and 1030 °C. Here, the activation energy of pyroxene vanishes, though crystallization rates are smaller than those of NB. Whereas in NB pyroxene growth occurs diffusion limited, in SB it is likely to be limited by the attachment of ions to new crystal surface.