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 .