  Mid-IR (8 - 13 micron) interferometric data of four oxygen-rich AGB stars (R
Aql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) were
obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally
dispersed visibility data are analyzed by fitting a circular fully
limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength
is similar for all oxygen-rich stars. The apparent size is almost constant
between 8 and 10 micron and gradually increases at wavelengths longer than 10
micron. The apparent FDD diameter in the carbon-rich star V Hya essentially
decreases from 8 to 12 micron. The FDD diameters are about 2.2 times larger
than the photospheric diameters estimated from K-band observations found in the
literature. The silicate dust shells of R Aql, R Hya and W Hya are located
fairly far away from the star, while the silicate dust shell of R Aqr and the
amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to
the star at around 8 photospheric radii. Phase-to-phase variations of the
diameters of the oxygen-rich stars could be measured and are on the order of
15% but with large uncertainties. From a comparison of the diameter trend with
the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars
the overall larger diameter originates from a warm molecular layer of H2O, and
the gradual increase longward of 10 micron can be most likely attributed to the
contribution of a close Al2O3 dust shell. The chromatic trend of the Gaussian
FWHM in V Hya can be explained with the presence of AMC and SiC dust. The
observations suggest that the formation of amorphous Al2O3 in oxygen- rich
stars occurs mainly around or after visual minimum. However, no firm
conclusions can be drawn concerning the mass-loss mechanism.
