GRAIN SIZE DISTRIBUTION OF FE0 GLOBULES IN LUNAR AGGLUTINITIC GLASS: FIRST RESULTS FROM APOLLO 17 SOIL 78421

The areal distribution of submicron sized Fe⁰, mostly nanophase globules, in lunar agglutinitic glass darkens the Moon's albedo and affects the uv-vis-ir reflectance spectra of lunar soils. No quantitative estimate of the distribution is available. We have imaged Fe⁰ globules in 436 agglutinate segments from polished grain mounts of the 90-150µm size fraction of 13 lunar soils of diverse maturity. Backscattered electron (BSE) images were obtained using a JEOL analytical FEG-SEM at magnifications generally ranging from x3,000 to x30,000 using a 15kv beam at 12 nanoamp sample current with a resolution of about 4nm. We report the first results of 2-D grain size distribution of 3505 Fe⁰ globules in Apollo 17 soil 78421 determined from 90 BSE images of glassy segments of 13 agglutinates. Each image was uploaded using common software and placed under a grid scaled to the magnification for measuring the diameter of the exposed surface of each globule. Numbers of globules within 8 size intervals were counted for plotting a cumulative curve. In soil 78421, their mean size is 12.9F (=129 nm) with a standard deviation of 1.9F, i.e., they are extremely well sorted. A semi-quantitative estimate puts the total area of exposed Fe⁰ at about 5% of the glassy surface of agglutinates.