  Context. AGB stars go through a process of strong mass-loss that involves
pulsations of the atmosphere, which extends to a region where the conditions
are adequate for dust grains to form. Radiation pressure acts on these grains
which, coupled to the gas, drive a massive outflow. The details of this process
are not clear, including which molecules are involved in the condensation of
dust grains.
  Aims. To study the role of the SiO molecule in the process of dust formation
and mass-loss in M-type AGB stars.
  Methods. Using the IRAM NOEMA interferometer we observed the $^{28}$SiO and
$^{29}$SiO $J=3-2$, $v=0$ emission from the inner circumstellar envelope of the
evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission
profiles to compare the observations to models using a molecular excitation and
ray-tracing code for SiO thermal emission.
  Results. We observed circular symmetry in the emission distribution. We also
found that the source diameter varies only marginally with radial velocity,
which is not the expected behavior for envelopes expanding at an almost
constant velocity. The adopted density, velocity, and abundance laws, together
with the mass-loss rate, which best fit the observations, give us information
on the chemical behavior of the SiO molecule and its role in the dust formation
process.
  Conclusions. The results indicate that there is a strong coupling between the
depletion of gas phase SiO and gas acceleration in the inner envelope. This
could be explained by the condensation of SiO into dust grains.
