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• Novel method developed to evaluate the energy effectiveness of microwave-assisted fragmentation. • Two new parameters are introduced to assess the heating and weakening impacts of microwave irradiation on rocks/ores. • Single-mode and multi-mode microwave cavities are studied and compared. • Effects of sample size with respect to wavelength and its distance from microwave waveguide are investigated. • Microwave weakening impact is significantly enhanced by improving cavity configurations. Enhancing the energy-effectiveness of microwave-assisted fragmentation is one of the overlooked areas in the published literature. A novel calorimetric technique is proposed to quantify the effect of inductive heating within a microwave cavity on subsequent rock/ore fragmentation. Two new parameters are introduced to identify, differentiate, and quantifiably evaluate the effects of microwave-induced energy and thermally induced fracturing on mechanical strength degradation. Additionally, the influences of sample geometry and position and orientation with respect to the microwave waveguide are experimentally investigated to determine optimum conditions for microwave treatment. Comparisons with published literature show that establishing optimum electromagnetic heating conditions significantly improves microwave-induced weakening. Microwave-assisted degradation has significant potential application to in-situ mining, tunnelling, rock breakage, and comminution. [ABSTRACT FROM AUTHOR]