Textural and Mineralogical Controls on Microwave-Induced Cracking in Granites

Abstract

Microwave irradiation has been considered as a potential method for weakening rock in mining and civil engineering applications, and numerous studies have demonstrated the strength-reducing effects. SEM-based automated mineralogy provides new opportunities to examine the mineralogical controls on microwave-induced cracking. This study employed a combined approach of optical microscopy and automated mineralogical analysis of scanning electron microscopy to investigate the roles of mineralogy and texture in microwave-induced cracking of granitic rocks. Most rocks on Earth, such as granite, are composed of relatively weak microwave absorbing minerals, compared to those tested in prior investigations on ores. This study examined three types of natural granite specimens, selected for their varying proportions of weak microwave absorbers (albite, amphibole, biotite, orthoclase, and quartz), and their contrasting textures (perthitic, granophyric and oikocrystic) and grain sizes (fine and coarse grained). Microwave irradiation experiments at 3.2 kW and 2.45 GHz led to the generation of macroscopically and microscopically visible cracks and lower P-wave velocities after irradiation. The optical investigations revealed that coarse-grained (1–5 mm) granites developed extensive networks of narrow cracks; whereas, fine-grained (<1 mm) granites of similar composition developed few cracks which were comparatively wider. Quantitative assessment of the spatial relationships between these cracks and the host minerals showed that intragranular cracks developed along cleavage planes of albite and amphibole, potentially in response to thermal expansion of brittle grains. Intergranular cracking occurred adjacent to thermally conductive or highly expansive grains such as quartz and biotite. In these specimens, cracking appears to be driven by contrasts among the chemical, mineralogical, thermal and microwave properties of the constituent minerals, and strong absorbers are not essential. The limited dataset from this study suggests that granitoid rocks may be potential targets for industrial applications of microwave irradiation.