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A wealth of data from the Herschel and Planck satellites and now from ALMA,revealing cold dust thermal emission, is available for astronomicalenvironments ranging from interstellar clouds, cold clumps, circumstellarenvelops, and protoplanetary disks. The interpretation of these observationsrelies on the understanding and modeling of cold dust emission and on theknowledge of the dust optical properties. The aim of this work is to provideastronomers with a set of spectroscopic data of realistic interstellar dustanalogues that can be used to interpret the observations. Glassy silicates ofmean composition (1-x)MgO - xSiO2 with x = 0.35, 0.40 and 0.50 weresynthesized. The mass absorption coefficient (MAC) of the samples was measuredin the spectral domain 30 - 1000 $\mu$m for grain temperature in the range 300K - 10 K and at room temperature in the 5 - 40 $\mu$m domain. We find that theMAC of all samples varies with the grains temperature. In the FIR/submm, andabove 30K, the MAC value at a given wavelength increases with the temperatureas thermally activated absorption processes appear. The studied materialsexhibit different and complex behaviors at long wavelengths (lambda $\geq$ 200to 700 $\mu$m depending on the samples) and the MAC cannot be approximated by asingle power law in ${\lambda}^{-\beta}$. These behaviors are attributed to theamorphous nature of dust and to the amount and nature of the defects withinthis amorphous structure. Above 20 $\mu$m, the measured MAC are much higherthan the MAC calculated from interstellar silicate dust models indicating thatthe analogues measured in this study are more emissive than the silicates incosmic dust models. This has important astrophysical implications becausemasses are overestimated by the models. Moreover, constraints on elementalabundance of heavy elements in cosmic dust models are relaxed

Low temperature MIR to submillimeter mass absorption coefficient of interstellar dust analogues I: Mg-rich glassy silicates