In this subsection, we probe some interpretable characteristics of the GPEI model through feature importances (Section 3.2.1) and 2D contours for two of the compositional variables (Section 3.2.2). Particle size distribution visualizations are given in Section S3.1) and LOO-CV results are provided in Section S6. Plots with additional information for particle size distribution visualizations, feature importances, and 2D contours are given in Sections S1, S4 and S7, respectively.