  Hydrous silicates occur in various cosmic environments, and are among the
minerals with the most pronounced bands in the far infrared (FIR) spectral
region. Given that Herschel and ALMA will open up new possibilities for
astronomical FIR and sub-mm spectroscopy, data characterizing the dielectric
properties of these materials at long wavelengths are desirable. We aimed at
examining the FIR spectra of talc, picrolite, montmorillonite, and chamosite,
which belong to four different groups of phyllosilicates. We tabulated
positions and band widths of the FIR bands of these minerals depending on the
dust temperature. By means of powder transmission spectroscopy, spectra of the
examined materials were measured in the wavelength range 25-500 mum at
temperatures of 300, 200, 100, and 10 K. Room-temperature measurements yield
the following results. For talc, a previously unknown band, centered at 98.5
mum, was found, in addition to bands at 56.5 and 59.5 mum. For montmorillonite,
several bands at wavelengths <110 mum were detected, including a band at 105
mum with an FWHM of about 10 mum. Picrolite shows a sharp 77 mum FIR band.
Chamosite is characterized by bands in the 72-92 mum range, and a prominent
band at 277 mum. At decreasing temperature, most of the bands shift to shorter
wavelengths. Examining a potential counterpart of the 105mum band in the
spectra of HD 142527 and HD 100546, we find that the broad band in the spectra
of these young stars - extending from 85 to 125 mum - cannot be due to
montmorillonite or any of the hydrous silicates we studied, since these
materials have sharper bands in the FIR wavelength range than previously
assumed, especially at low temperatures.
