  We present a diagnostic tool to determine the forsterite abundance of the
dust ejected by AGB stars. Our method is based on a comparison between the
observed strength of spectral bands of forsterite and model calculations. We
show that the 11.3 {\mu}m forsterite band is a robust indicator of the
forsterite abundance of the current mass-loss period for AGB stars with an
optically thick dust shell. The 33.6 {\mu}m band of forsterite is sensitive to
changes in the density and the geometry of the emitting dust shell, and so a
less robust indicator. We apply this method to six high mass-loss rate AGB
stars, showing that AGB stars can have forsterite abundances of 12% by mass and
higher, which is more than the previously found maximum abundance of 5%.
