  Ground-based observations of `Barbarian' L-type asteroids at 1 to 2.5-$\mu$m
indicate that their near-infrared spectra are dominated by the mineral spinel,
which has been attributed to a high abundance of calcium-aluminum inclusions
(CAIs) -- the first solids to condense out of the protoplanetary disk during
the formation of the Solar System. However, the spectral properties of these
asteroids from 2.5 to 5-$\mu$m, a wavelength region that covers signatures of
hydrated minerals, water, and organics, have not yet been explored. Here, we
present 2 to 5-$\mu$m reflectance spectra of five spinel-rich asteroids
obtained with the NIRSpec instrument on the James Webb Space Telescope. All
five targets exhibit a $\sim$ 2.85-$\mu$m absorption feature with a band depth
of 3-6$\%$ that appears correlated in strength with that of the 2-$\mu$m spinel
absorption feature. The shape and position of the 2.85-$\mu$m feature are not a
good match to the 2.7-$\mu$m feature commonly seen in carbonaceous CM
meteorites or C-type asteroids. The closest spectral matches are to the Moon
and Vesta, suggesting commonalities in aqueous alteration across silicate
bodies, infall of hydrated material, and/or space weathering by solar wind H
implantation. Lab spectra of CO/CV chondrites, CAIs, as well as the minerals
cronstedtite and spinel, also show a similar feature, providing clues into the
origin of the 2.85-$\mu$m feature.
