  The inner regions of protoplanetary discs (from $\sim$ 0.1 to 10 au) are the
expected birthplace of planets, especially telluric. In those high temperature
regions, solids can experience cyclical annealing, vaporisation and
recondensation. Hot and warm dusty grains emits mostly in the infrared domain,
notably in N-band (8 to 13~$\mu$m). Studying their fine chemistry through
mid-infrared spectro-interferometry with the new VLTI instrument MATISSE, which
can spatially resolve these regions, requires detailed dust chemistry models.
Using radiative transfer, we derived infrared spectra of a fiducial static
protoplanetary disc model with different inner disc ($< 1$ au) dust
compositions. The latter were derived from condensation sequences computed at
LTE for three initial $C/O$ ratios: subsolar ($C/O=0.4$), solar ($C/O=0.54$),
and supersolar ($C/O=1$). The three scenarios return very different N-band
spectra, especially when considering the presence of sub-micron-sized dust
grains. MATISSE should be able to detect these differences and trace the
associated sub-au-scale radial changes. We propose a first interpretation of
N-band `inner-disc' spectra obtained with the former VLTI instrument MIDI on
three Herbig stars (HD142527, HD144432, HD163296) and one T Tauri star (AS209).
Notably, we could associate a supersolar (`carbon-rich') composition for
HD142527 and a subsolar (`oxygen-rich') one for HD1444432. We show that the
inner disc mineralogy can be very specific and not related to the dust
composition derived from spatially unresolved mid-infrared spectroscopy. We
highlight the need for including more complex chemistry when interpreting
solid-state spectroscopic observations of the inner regions of discs, and for
considering dynamical aspects for future studies.
