  We explore experimentally possible explanations of the polarization curves of
the sunlight reflected by the Barbarian asteroids. Their peculiar polarization
curves are characterized by a large inversion angle, around 30 degrees, which
could be related to the presence of FeO-bearing spinel embedded in
Calcium-Aluminum Inclusions. In order to test this hypothesis, we have measured
the phase function and degree of linear polarization of six samples of Mg-rich
olivine and spinel. For each material, we have analyzed the light scattering
properties of a millimeter-sized grain and of two powdered samples with size
distributions in the micrometer size range. The three spinel samples show a
well-defined negative polarization branch with an inversion phase angle located
around 24-30 degrees. In contrast, in the case of the olivine samples, the
inversion angle is highly dependent on particle size and tends to decrease for
larger sizes. We identify the macroscopic geometries as a possible explanation
for the evident differences in the polarization curves between olivine and
spinel millimeter samples. Although the polarization behaviour in near
backscattering of the Barbara asteroid is similar to that of our spinel
mm-sized sample in random orientation, this similarity could result in part
from crystal retro-reflection rather than composition. This is part of an
ongoing experimental project devoted to test separately several components of
CV3-like meteorites, representative of the Barbarians composition, to
disentangle their contributions to the polarization behavior of these objects.
