  We present evidence, via a large survey of 191 new spectra along with
previously-published spectra, of a divide in the 3-$\mu$m spectral properties
of the low-albedo asteroid population. One group ("Sharp-types" or ST, with
band centers $<$ 3 $\mu$m) has a spectral shape consistent with carbonaceous
chondrite meteorites, while the other group ("not-Sharp-types" or NST, with
bands centered $>$ 3 $\mu$m) is not represented in the meteorite literature but
is as abundant as the STs among large objects. Both groups are present in most
low-albedo asteroid taxonomic classes, and except in limited cases taxonomic
classifications based on 0.5-2.5-$\mu$m data alone cannot predict whether an
asteroid is ST or NST.
  Statistical tests show the STs and NSTs differ in average band depth,
semi-major axis, and perihelion at confidence levels $\ge$98\%, while not
showing significant differences in albedo. We also show that many NSTs have a
3-$\mu$m absorption band shape like Comet 67P, and likely represent an
important small-body composition throughout the solar system. A simple
explanation for the origin of these groups is formation on opposite sides of
the ammonia snow line, with the NST group accreting H2O and NH3 and the ST
group only accreting H2O, with subsequent thermal and chemical evolution
resulting in the minerals seen today. Such an explanation is consistent with
recent dynamical modeling of planetesimal formation and delivery, and suggests
that much more outer solar system material was delivered to the main asteroid
belt than would be thought based on the number of D-class asteroids found
today.
