Effects of Very Fine Particle Size on Reflectance Spectraof Smectite and Palagonitic Soil

A montmorillonite clay and a palagonitic soil sample were sieved to produce a series of size fractions in order to investigate the role that extremely fine particle sizes play on absorption strengths in martian analog materials. Significant decreases in band strength for both samples were observed as particle size decreased. The less than 5-μm fractions had 1.4-, 1.9-, and 2.2-μm bands that were approximately half the strength of the absorptions from the 45to 75-μm size fractions. X-ray diffraction and loss on ignition measurements indicate that there are no significant changes in crystallinity, composition, or water content with particle size. Thus the reduction in spectral contrast is dominated by changes in the ratio of scattering to absorption with particle size. Furthermore, larger particles of montmorillonite clay are not individual crystals but are aggregates of much smaller particles. Nevertheless, the aggregate particles exhibit spectral properties consistent with the size of the aggregate, not the individual particles in the aggregate. This has important implications for the spectroscopic determination of the composition of the extremely fine martian dust. The abundance of very fine grained mineral phases such as phyllosilicates will be underestimated if comparisons to coarser or clumped laboratory samples are made. c © 1999 Academic Press