Zinc orthogermanate (Zn₂GeO₄) is a wide band-gap semiconductor that belongs to the rhombohedral  space group, being isostructural with the phenakite (Be₂SiO₄) and willemite (Zn₂SiO₄) orthosilicates. In addition to potential applications as luminescent hosts, catalysts and semiconductor devices, this material has been recently proposed for microwave (MW) dielectric applications, mainly as substrates or antennas. In this work, we discuss on the influence of the ceramic microstructure on the intrinsic and extrinsic MW dielectric responses of Zn₂GeO₄ synthesized by microwave-assisted solid-state reaction method and sintered at 1000 ^oC for 4h. This procedure led to dense ceramics with unusually large grains (up to 0.35 mm), which allowed us to use polarized micro-infrared spectroscopy (-FTIR) to determine the whole characteristics of the polar vibrational modes. In fact, we were able to resolve both types of orthogonal phonon modes from the polarized reflectivity spectra and fully describe the infrared phonon characteristics of the material. The obtained intrinsic dielectric parameters (from -FTIR) could be compared to the extrinsic MW ones (direct measurements). Also, an infrared spectroscopic mapping throughout the sample surface revealed the existence of a complex domain pattern inside the grains. More generally, we demonstrate the use of polarized micro-infrared spectroscopy to investigate large-grain ceramic materials. (Work partially supported by the Brazilian agencies CNPq and FAPEMIG.)