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We present new room-temperature 1100 - 1800 cm^{-1} spectra of melilitesilicates and 600 - 2000 cm^{-1} spectra of three randomly orientatedfine-grained carbonates to determine the possible carrier(s) of a 6.9~micronabsorption feature observed in a variety of dense astronomical environmentsincluding young stellar objects and molecular clouds. We focus on the low-masspost-AGB star Sakurai's Object which has been forming substantial quantities ofcarbonaceous dust since an eruptive event in the 1990s. Large melilite grainscannot be responsible for the 6.9-micron absorption feature because thesimilarly-shaped feature in the laboratory spectrum was produced by very low(0.1 per cent by mass) carbonate contamination which was not detected at otherwavelengths. Due to the high band-strength of the 6.9-micron feature incarbonates, we conclude that carbonates carry the astronomical 6.9~micronfeature. Replacement of melilite with carbonates in models of Sakurai's objectimproves fits to the 6 - 7-micron Spitzer spectra without significantlyaltering other conclusions of Bowey's previous models except that there is nolink between the feature and the abundance of melilite in meteorites. Withmagnesite (MgCO3), the abundance of 25-micron-sized SiC grains is increased by10 - 50 per cent and better constrained. The mass of carbonate dust is similarto the mass of PAH dust. Existing experiments suggest carbonates are stablebelow 700~K, however it is difficult to ascertain the applicability of theseexperiments to astronomical environments and more studies are required.

Sakurai's Object revisited: new laboratory data for carbonates and melilites suggest the carrier of 6.9 $\mu$m excess absorption is a carbonate