Rock fracturing under microwave irradiation: development of analytical methods and investigation of the contribution of bound water

Abstract

Microwave irradiation has been considered as a potential method for weakening rock in mining and civil engineering applications. Most rocks in Earth’s crust are predominantly composed of silicate minerals, which are weak microwave absorbers, and the behavior of these rocks under microwave irradiation is not completely understood. This contribution examines the mineralogical and textural controls governing microwave-induced cracking in granites and the influence of the presence of hydrated minerals. A review of existing methods to characterize cracks indicated that there is no single method that provides comprehensive data on crack location, morphology, mineralogy, textural controls, and reduction of rock strength. Therefore this study developed an integrated suite of methods to assess cracks. Mechanical properties are quantified using relative P-wave velocity measurements and Uniaxial Compressive Strength tests. The morphology of microwave-induced cracks is assessed using new micro-computed tomography and 3D imaging tools. The mineralogical association of cracks is quantitatively described using a combination of optical microscopy, Scanning Electron Microscopy, automated mineralogy, and point counting. Results show that contrary to hypotheses presented in the literature, the color, water, or iron content of constituent minerals cannot be used to predict the microwave behavior of a rock. The texture of the rock is an important governing factor. Coarse grained rocks develop complex networks of narrow cracks whereas fine grained rocks will develop few wider cracks. Fine pre-existing weaknesses in granite do not appear to be affected by microwave irradiation and are easily discernable from microwave-induced damage. Adjacent mineral grains with contrasting properties (microwaving, thermal, physical or chemical) appear to be critical to the development of cracks. If microwave technology is applied to rocks made up of minerals that are weak microwave absorbers, the ideal candidates constitute coarse grained (1-5 mm) rocks containing randomly disseminated minerals with contrasting properties. This includes granites, granodiorites, diorites, and gabbros, conglomerates, breccias, coarse sandstones, and metamorphic rocks without small-scale fabrics. Granite hosted ore deposits such as porphyry copper, certain skarn (tin, tungsten, and molybdenum), epithermal deposits, intrusion-related gold, and rare earth element-bearing deposits and IOCG deposits hosted in breccias could also be viable candidates.