Effects of particle optical properties on grain size measurements of aeolian dust deposits

Particle size data are holding crucial information on the sedimentary environment at the time the aeolian dust deposits were accumulated. However, only the correct granulometric data and its proper interpretation can be used in this kind of investigations.
Various aspects of aeolian sedimentation (wind strength, distance to source(s), possible secondary source regions and modes of sedimentation and transport) can be reconstructed from proper grain size distribution data. Laser diffraction methods provide much more accurate and reliable information on the major granulometric properties of wind-blown sediments compared to the sieve and pipette methods. 
The Fraunhofer and the Mie optical theories have been used for laser diffraction grain size measurements. The two different approaches need different “background” information on the measured medium. During a measurement with Fraunhofer theory, it is assumed that the particles are relatively large (over 25-30 ┬Ám – about 40 times larger than the wavelength of the ligth) and opaque. The Mie theory could offer more accurate data on the smaller fraction (clay and fine silt), but the knowledge of the refraction and absorption indices is needed (as it is a solution for Maxwell's electromagnetic field equations), which is a difficult question in the case of polymineralic samples.

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