  Differentiated asteroids are rare in the main asteroid belt despite evidence
for ~100 distinct differentiated bodies in the meteorite record. We have sought
to understand why so few main-belt asteroids differentiated and where those
differentiated bodies or fragments reside. Using the Sloan Digital Sky Survey
(SDSS) to search for a needle in a haystack we identify spectral A-type
asteroid candidates, olivine-dominated asteroids that may represent mantle
material of differentiated bodies. We have performed a near-infrared spectral
survey with SpeX on the NASA IRTF and FIRE on the Magellan Telescope.
  We report results from having doubled the number of known A-type asteroids.
We deduce a new estimate for the overall abundance and distribution of this
class of olivine-dominated asteroids. We find A-type asteroids account for less
than 0.16% of all main-belt objects larger than 2 km and estimate there are a
total of ~600 A-type asteroids above that size. They are found rather evenly
distributed throughout the main belt, are even detected at the distance of the
Cybele region, and have no statistically significant concentration in any
asteroid family. We conclude the most likely implication is the few fragments
of olivine-dominated material in the main belt did not form locally, but
instead were implanted as collisional fragments of bodies that formed
elsewhere.
