Grain-scale pore geometry primarily controls the fluid distribution in rocks, affecting material transport and geophysical response. The dihedral angle (Θ) in the olivine–fluid system is a key parameter determining the pore fluid geometry in mantle wedges. Both curved and faceted olivine–fluid interfaces define Θ in the system, generating the faceted–faceted (FF), faceted–curved (FC), and curved–curved (CC) angles. However, the effect of faceting on Θ under various pressure and temperature (P–T) conditions and fluid compositions have not been constrained, and its mineralogical understanding is unresolved. This study evaluates the facet-bearing Θ and their proportions in olivine–multicomponent aqueous fluid systems. Our results show that 1/3 of olivine–fluid Θ are facet-bearing angles irrelative to the P–T conditions and fluid compositions. Faceting produces larger dihedral angles than the CC angles. The grain boundary plane (GBP) distribution reveals that the GBPs of faceted interfaces at triple junctions were subjected to low Miller Index faces ((100), (010), and (101)). Moreover, calculating the FF angles from two adjacent low Miller index planes highly reproduces measured angle values based on the olivine crystal habit. Therefore, our study suggests that the FF angle is strongly affected by olivine crystallography. The presence of faceting increases Θ and critical fluid fraction (Φc) for percolation, thus decreases the permeability. In the mantle wedge, where olivine crystallographic preferred orientation (CPO) is expected, increasing the FF angle proportion with associated changes in fluid pore morphology will lead to the permeability anisotropy and consequent geophysical anomalies.