  We report the clear detection of the 2175A dust absorption feature in the
optical afterglow spectrum of the gamma-ray burst (GRB) GRB070802 at a redshift
of z=2.45. This is the highest redshift for a detected 2175A dust bump to date,
and it is the first clear detection of the 2175A bump in a GRB host galaxy,
while several tens of optical afterglow spectra without the bump have been
recorded in the past decade. The derived extinction curve gives A_V=0.8-1.5
depending on the assumed intrinsic slope. Of the three local extinction laws,
an LMC type extinction gives the best fit to the extinction curve of the host
of GRB070802. Besides the 2175A bump we find that the spectrum of GRB070802 is
characterized by unusually strong low-ionization metal lines and possibly a
high metallicity for a GRB sightline ([Si/H]=-0.46+/-0.38,
[Zn/H]=-0.50+/-0.68). In particular, the spectrum of GRB070802 is unique for a
GRB spectrum in that it shows clear CI absorption features, leading us to
propose a correlation between the presence of the bump and CI. The gas to dust
ratio for the host galaxy is found to be significantly lower than that of other
GRB hosts with N(HI)/A_V=(2.4+/-1.0)x10^21 cm^-2 mag^-1, which lies between
typical MW and LMC values. Our results are in agreement with the tentative
conclusion reached by Gordon et al. 2003 that the shape of the extinction
curve, in particular the presence of the bump, is affected by the UV flux
density in the environment of the dust.
