  The motion of diamagnetic dust particles in interstellar magnetic fields is
studied numerically with several different sets of parameters. Two types of
behavior are observed, depending on the value of the critical number $R$, which
is a function of the grain inertia, the magnetic susceptibility of the material
and of the strength of rotation braking. If $R\leq10$, the grain ends up in a
static state and perfectly aligned with the magnetic field, after a few braking
times. If not, it goes on precessing and nutating about the field vector for a
much longer time. Usual parameters are such that the first situation can hardly
be observed. Fortunately, in the second and more likely situation, there
remains a persistent partial alignment which is far from negligible, although
it decreases as the field decreases and as $R$ increases. The solution of the
complete equations of motion of grains in a field helps understanding the
details of this behavior. One particular case of an ellipsoidal forsterite
silicate grain is studied in detail and shown to polarize light in agreement
with astronomical measurements of absolute polarization in the infrared.
Phonons are shown to contribute to the progressive flattening of extinction and
polarization towards long wavelengths. The measured dielectric properties of
forsterite qualitatively fit the Serkowski peak in the visible.
