  We present 48 Herschel/PACS spectra of evolved stars in the wavelength range
of 67-72 $\mu$m. This wavelength range covers the 69 $\mu$m band of crystalline
olivine ($\text{Mg}_{2-2x}\text{Fe}_{(2x)}\text{SiO}_{4}$). The width and
wavelength position of this band are sensitive to the temperature and
composition of the crystalline olivine. Our sample covers a wide range of
objects: from high mass-loss rate AGB stars (OH/IR stars, $\dot M \ge 10^{-5}$
M$_\odot$/yr), through post-AGB stars with and without circumbinary disks, to
planetary nebulae and even a few massive evolved stars. The goal of this study
is to exploit the spectral properties of the 69 $\mu$m band to determine the
composition and temperature of the crystalline olivine. Since the objects cover
a range of evolutionary phases, we study the physical and chemical properties
in this range of physical environments. We fit the 69 $\mu$m band and use its
width and position to probe the composition and temperature of the crystalline
olivine. For 27 sources in the sample, we detected the 69 $\mu$m band of
crystalline olivine ($\text{Mg}_{(2-2x)}\text{Fe}_{(2x)}\text{SiO}_{4}$). The
69 $\mu$m band shows that all the sources produce pure forsterite grains
containing no iron in their lattice structure. The temperature of the
crystalline olivine as indicated by the 69 $\mu$m band, shows that on average
the temperature of the crystalline olivine is highest in the group of OH/IR
stars and the post-AGB stars with confirmed Keplerian disks. The temperature is
lower for the other post-AGB stars and lowest for the planetary nebulae. A
couple of the detected 69 $\mu$m bands are broader than those of pure
magnesium-rich crystalline olivine, which we show can be due to a temperature
gradient in the circumstellar environment of these stars. continued...
