Effect of the particle size and apparent density on the time required for the initial temperature increase during the microwave heating of Fe3O4 was investigated to clarify the mechanism of interaction between the magnetite powder and microwave at temperatures lower than the Curie point of Fe3O4 (585 degrees C). Samples in the form of powders and briquettes (density, 3.3 g/cm(3)) with particle sizes of 75-150 mu m, 45-75 mu m, and < 45 mu m were heated in multi-mode and maximum E- and H-field modes using a microwave generator at a frequency of 2.45 GHz. Results reveal that the microwave absorption capability of a sample with higher apparent density is lower because of its higher electrical conductivity, which causes a shallower penetration depth. Further, the effect of the particle size on the microwave heating of Fe3O4 at temperatures higher than the Curie point is different in the presence and absence of a strong electric field. This is attributed to a change in the heating mechanism from dielectric loss in the presence of the E-field to Joule loss in its absence. At temperatures lower than the Curie point, some of the small particles would be transparent owing to a greater penetration depth of the microwaves, which causes an early onset of temperature increase in the magnetite sample with a larger particle size. Moreover, the mechanism for the formation of spark and plasma during the microwave-induced heating is discussed by applying a novel approach to analyse the interactions between the powdery materials and microwaves. (C) 2018 Elsevier B.V. All rights reserved.