Experimental and computational study of light scattering by irregular particles with extreme refractive indices: hematite and rutile

We present measurements of the complete scattering matrix as a function of the scattering angle of randomly oriented irregular hematite and rutile particles. The measurements were made at a wavelength of 632.8 nm in the scattering angle range from 5–174 degrees. Apart from their astronomical interest, these two samples are extremely interesting from a theoretical point of view, because they both have high real parts of the refractive index (about 3.0 for the hematite and 2.73 for the rutile). In addition, the hematite sample has a high imaginary part of the refractive index k, with values between 10-1 and 10-2, whereas rutile is a non-absorbing material ($k\approx 0$) at the studied wavelength. The scattering patterns of these mineral particles are quite similar to each other but show remarkable differences when compared to the results obtained for irregular mineral particles with moderate real parts of the refractive index. The measured results for both samples were compared with results of Mie calculations for projected surface equivalent spheres and T-matrix calculations for various spheroidal and cylindrical shapes. Both the experimental and theoretical results presented in this work seem to indicate that the scattering behavior of irregular mineral particles that have a high real part of the refractive index is not very dependent on the shape of the particles. In this case, Mie theory may give reasonable results despite the irregular shapes of the particles.