  A wealth of data from the Herschel and Planck satellites and now from ALMA,
revealing cold dust thermal emission, is available for astronomical
environments ranging from interstellar clouds, cold clumps, circumstellar
envelops, and protoplanetary disks. The interpretation of these observations
relies on the understanding and modeling of cold dust emission and on the
knowledge of the dust optical properties. The aim of this work is to provide
astronomers with a set of spectroscopic data of realistic interstellar dust
analogues that can be used to interpret the observations. Glassy silicates of
mean composition (1-x)MgO - xSiO2 with x = 0.35, 0.40 and 0.50 were
synthesized. The mass absorption coefficient (MAC) of the samples was measured
in the spectral domain 30 - 1000 $\mu$m for grain temperature in the range 300
K - 10 K and at room temperature in the 5 - 40 $\mu$m domain. We find that the
MAC of all samples varies with the grains temperature. In the FIR/submm, and
above 30K, the MAC value at a given wavelength increases with the temperature
as thermally activated absorption processes appear. The studied materials
exhibit different and complex behaviors at long wavelengths (lambda $\geq$ 200
to 700 $\mu$m depending on the samples) and the MAC cannot be approximated by a
single power law in ${\lambda}^{-\beta}$. These behaviors are attributed to the
amorphous nature of dust and to the amount and nature of the defects within
this amorphous structure. Above 20 $\mu$m, the measured MAC are much higher
than the MAC calculated from interstellar silicate dust models indicating that
the analogues measured in this study are more emissive than the silicates in
cosmic dust models. This has important astrophysical implications because
masses are overestimated by the models. Moreover, constraints on elemental
abundance of heavy elements in cosmic dust models are relaxed
