Effects of very fine particle size on reflectance spectra of 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 materia ls. Significant decreases in band strength for both samples were observed a s particle size decreased. The less than 5-mu m fractions had 1.4-, 1.9-, a nd 2.2-mu m bands that were approximately half the strength of the absorpti ons from the 45- to 75-mu m size fractions. X-ray diffraction and loss on i gnition measurements indicate that there are no significant changes in crys tallinity, composition, or water content with particle size. Thus the reduc tion in spectral contrast is dominated by changes in the ratio of scatterin g to absorption with particle size. Furthermore, larger particles of montmo rillonite clay are not individual crystals but are aggregates of much small er particles. Nevertheless, the aggregate particles exhibit spectral proper ties consistent with the size of the aggregate, not the individual particle s in the aggregate. This has important implications for the spectroscopic d etermination of the composition of the extremely fine martian dust. The abu ndance 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.