  We present new room-temperature 1100 - 1800 cm^{-1} spectra of melilite
silicates and 600 - 2000 cm^{-1} spectra of three randomly orientated
fine-grained carbonates to determine the possible carrier(s) of a 6.9~micron
absorption feature observed in a variety of dense astronomical environments
including young stellar objects and molecular clouds. We focus on the low-mass
post-AGB star Sakurai's Object which has been forming substantial quantities of
carbonaceous dust since an eruptive event in the 1990s. Large melilite grains
cannot be responsible for the 6.9-micron absorption feature because the
similarly-shaped feature in the laboratory spectrum was produced by very low
(0.1 per cent by mass) carbonate contamination which was not detected at other
wavelengths. Due to the high band-strength of the 6.9-micron feature in
carbonates, we conclude that carbonates carry the astronomical 6.9~micron
feature. Replacement of melilite with carbonates in models of Sakurai's object
improves fits to the 6 - 7-micron Spitzer spectra without significantly
altering other conclusions of Bowey's previous models except that there is no
link between the feature and the abundance of melilite in meteorites. With
magnesite (MgCO3), the abundance of 25-micron-sized SiC grains is increased by
10 - 50 per cent and better constrained. The mass of carbonate dust is similar
to the mass of PAH dust. Existing experiments suggest carbonates are stable
below 700~K, however it is difficult to ascertain the applicability of these
experiments to astronomical environments and more studies are required.
