Nanophase Iron Globules in Lunar Soil

Micrometeoritic impacts on lunar soils produce melt and vapor. A patina of condensed vapor is deposited on lunar grains, the melt forms agglutinitic glass. In lunar soils, agglutinitic glass and rinds of grains host submicron-sized globules of pure Fe0 (Fe-rich globules larger than 1 micron usually contain other elements such as Ni, P, and S). Observation and measurement of such small size requires either back scattered electron (BSE) imaging with a high-resolution SEM or transmitted electron imaging with a TEM. The two techniques impose different limitations on the size-range of measurements. Resolution of BSE imaging of polished thin sections or grain mounts of lunar soils is at best around 4-5nm (JEOL 6340F field-emission (FE)-SEM at JSC). Therefore, Fe0 globules below 10nm in cross-sectional diameter are not truly measured. The upper limit of a millimeter or so is not a hindrance. In fact, it is an advantage because whole grains can be observed and mapped at varying magnifications. Angstrom-scale resolution of TEM images is more than sufficient to observe and measure the smallest of Fe0 globules that are about 1nm in cross-section. Microtoming edges of lunar grains; however, puts an upper size limitation of 50nm, at best, on the wafer, which more or less limits measuring Fe0 globules up to 30nm or so. Clearly, SEM and TEM techniques complement each other in obtaining the complete range of size distribution of Fe0 globules in lunar soils. Below we describe, in brief, our method of determining the size distribution of Fe0 globules in agglutinitic glass using BSE-SEM imaging and size measurement. Although our work is incomplete, we also include a table of results obtained so far, which understandably would be refined as we collect more data.