Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.527k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 527
Mathematics
Logic
Pyroxene, Reflectance, Spectral
Scientific paper
We are developing a new approach to quantitative spectral characterization of remotely sensed objects. The work is an iterative and integrated theoretical and laboratory investigation. We are relating the spectral signature of the two types of carbonaceous chondrites (CV and CM) with their chemistry, mineralogy, and grain size distribution. In the future, the laboratory and theoretical portions of the study will be related to remote observations of asteroid surfaces. Radiative theory is being used to model the laboratory reflectance spectra of members of each of the two classes of carbonaceous chondrites (the theoretical mixtures will be generated using the known chemistry, mineralogy, optical constants and grain size distributions which have been previously determined) and verify the goodness of fit of the theoretical models to the laboratory spectra of meteorites. In using theoretical modeling assumptions about the physical and chemical properties of materials must be made. For the wavelength range we intend to consider (0.2-5 micrometers), we have previously obtained reasonable results using optical constants derived from reflectance spectra. That is, using a powdered sample in a narrow grain size range we invert a Hapke-based theoretical reflectance calculation to obtain an absorption coefficient. The primary disadvantage to this method is that it requires an assumption regarding the behavior of the index of refraction. However, as the index typically does not vary strongly in this wavelength interval, this is a reasonable assumption. Assumptions about the grain size of the materials being modelled are also important considerations when using Hapke-based theoretical modeling techniques. Commonly a single grain size range is deemed appropriate for a specific calculation and from that range an average grain size is estimated for the material. However, as most materials, including planetary regoliths and meteorites, are not composed of single mineralogies or single particle grain sizes we have investigated the disadvantages of making assumptions about a single grain size to represent the entire grain size distribution. For this preliminary study we selected pyroxene separates that were ground into narrow size intervals and for which an average grain size for each interval had been determined by Scanning Electron Microscope. Using a Hapke-based theoretical model we calculated the absorption coefficient. Then using a cumulative power-law distribution we calculated an absorption coefficient of the mixture. The average grain size of the cumulative distribution was determined to be 51.8 micrometers. This compound absorption coefficient was then used to compute a reflectance spectra of the pyroxene with a grain size of 52 micrometers. This spectrum was compared with the theoretical spectrum of the pyroxene in which the size distribution was a single size interval with an average grain size of 52 micrometers. Differences in the strengths of the resulting 2 micrometers absorption feature were nearly 7%, although no change was observed in the 1 micrometer absorption feature. Thus, suggesting that using a single grain size value to determine the absorption coefficient of a poly-grain size material needs to be reevaluated. Deriving the absorption coefficient based on a cumulative power-law distribution rather than a single average grain size is necessary, especially in multi-component mixtures where each mineral component, whether it is in the matrix or appears as an inclusion or chondrule, will likely have a unique grain size distribution. When modelling surfaces or materials with several mineralogic materials present, the failure to consider a cumulative size distribution will effect the estimates of mineral abundances.
Calvin Wendy M.
King Trude V. V.
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