## 2 Sample Selection and Characterization  
Fallout samples exhibiting textures that could potentially be attributed to liquid immiscibility, as seen in the works of [PERSON] et al. [3; 11], were investigated. Sample selection and preparation are reported in detail in [PERSON] et al. [3; 11]. Briefly, millimeter-scale samples were collected from a historic near-surface nuclear test. Several iron-rich samples with aerodynamic morphologies (smooth surface and spherical or tear-drop shaped) were chosen for analysis [11]. Samples were mounted in epoxy and polished to expose their approximate mid-plane cross section. After polishing to a 1 \(\upmu\)m surface roughness and carbon coated, backscattered electron images were obtained by Scanning Electron Microscopy (SEM) (Figure 1) with compositional analysis achieved by X-ray Energy Dispersive Spectroscopy (EDS) through SEM and Transmission Electron Microscopy (TEM) methods [3]. For this work, additional sample analysis was performed on selected samples using an FEI Inspect F Scanning Electron Microscope with an annular solid-state silicon backscatter electron (BSE) detector for imaging and an XFlash 6-60 Silicon Drift Detector for EDS analysis. For BSE imaging analysis, an electron beam of 20 keV was used, and EDS analysis was performed using a 10 keV electron beam for better spatial resolution. Additional measurements using a FEI Titan 80-300 S/TEM enabled two-phase EDS measurements for the L\({}_{\text{Fe}}\)-dispersed amoebaoids and offered better compositional estimates for L\({}_{\text{Si}}\)-dispersed amoeboids. TEM measurements were performed under 300 kV accelerating voltage. TEM samples were prepared using an FEI Nova 600i dual beam focused ion beam (FIB) to obtain 100 nm thick foils. Compositional analysis was obtained for bulk and liquid-immiscibility features of the samples using TEM-EDS mapping under STEM (Scanning Transmission Electron Microscopy) mode. Regions defining the entire amoeboid and each of the two-phase regions were used to estimate the bulk and two-phase compositions [3]. Any compositional analysis performed using SEM-EDS and S/TEM-EDS analysis will here on out be referred to as SEM and TEM, respectively. Two predominant microstructures were analyzed from the samples and denoted as follows: (1) L\({}_{\text{Si}}\)-dispersed where the silicon rich features (L\({}_{\text{Si}}\)) are dispersed in an iron-rich (L\({}_{\text{Fe}}\)) matrix (Figure 1A) and (2) L\({}_{\text{Fe}}\)-dispersed, where the iron-rich features are dispersed in a silicate-rich matrix (Figure 1B). Semi-quantitative estimates of the entire exposed amoeboid cross section compositions for both L\({}_{\text{Fe}}\)-dispersed and L\({}_{\text{Si}}\)-dispersed amoeboids were obtained by EDS. The electron beam was rastered over the amoeboid exposed cross sections during SEM and TEM measurements. The amoeboid compositions were assumed to be combinations of standard stoichiometric oxide compositions: SiO\({}_{2}\), FeO, Al\({}_{2}\)O\({}_{3}\), CaO, MgO. Compositional data for each of the two liquid phases (L\({}_{\text{Fe}}\) and L\({}_{\text{Si}}\)) of the L\({}_{\text{Si}}\)-dispersed microstructures were obtained using both SEM and TEM methods. However, only TEM data were obtained for the liquid compositions of L\({}_{\text{Fe}}\)-dispersed microstructures because the small diameters (sub-micron) of the dispersed features were beyond spatial resolution of EDS. Bulk compositions for L\({}_{\text{Fe}}\)-dispersed and L\({}_{\text{Si}}\)-dispersed microstructures were obtained using both SEM and TEM. The amoeboid bulk compositions are listed in Table 1 in oxide weight percent for the two types of amoeboids. Average bulk compositional data were obtained from the average SEM measurement of 23 L\({}_{\text{Si}}\)-dispersed amoeboids, with error reported as the standard error of the mean. For the L\({}_{\text{Fe}}\)-dispersed bulk compositions, SEM measurements on 11 amoeboids were averaged, with uncertainties calculated using the standard error of the mean.