  Several astrophysically relevant solid oxides and silicates have extremely
small opacities in the visual and near-infrared in their pure forms. Datasets
for the opacities and for the imaginary part k of their complex indices of
refraction are hardly available in these wavelength ranges. We aimed at
determining k for spinel, rutile, anatase, and olivine, especially in the
near-infrared region. Our measurements were made with impurity-containing,
natural, and synthetic stardust analogs. Two experimental methods were used:
preparing small sections of natural minerals and synthesizing melt droplets
under the electric arc furnace. In both cases, the aborption properties of the
samples were measured by transmission spectroscopy. For spinel (MgAl2O4),
anatase, rutile (both TiO2), and olivine ((Mg,Fe)2SiO4), the optical constants
have been extended to the visual and near-infrared. We highlight that the
individual values of k and the absorption cross section depend strongly on the
content in transition metals like iron. Based on our measurements, we infer
that k values below 10^(-5) are very rare in natural minerals including
stardust grains, if they occur at all. Data for k and the absorption cross
section are important for various physical properties of stardust grains such
as temperature and radiation pressure. With increasing absorption cross section
due to impurities, the equilibrium temperature of small grains in circumstellar
shells increases as well. We discuss why and to what extent this is the case.
