ALMOST all of the elements heavier than helium in the interstellar medium reside in small solid dust particles(1). Dust dominates the energy balance through its continuous absorption and emission of radiation. Its opacity provides the only direct means to assess the total mass in many interstellar regions : for example, in molecular clouds(2), in circumstellar disks undergoing planet formation(3-5) and even in some galaxies(6). But there are almost no laboratory measurements of small-particle opacities at low temperatures for any of the principal constituents of interstellar dust at the far-infrared and millimetre wavelengths at which most observations are made. Here we report a laboratory study of absorption by forsterite (Mg2SiO4), the pure magnesian form of the mineral olivine, which is a main constituent of interstellar dust We have measured millimetre-wave absorption spectra of synthesized amorphous and crystalline forsterite powders between 3.5 and 15 cm(-1) at temperatures below 30 K, the region most germane to estimates of interstellar dust mass. Our measurements indicate that the absorption per unit mass for this silicate is several times larger than that normally assumed in astronomical research(7-10). Estimates of the opacity of interstellar dust may have to be increased accordingly, causing the mass of interstellar particles derived from observations of the opacity to be lowered by a corresponding amount.