  CoSe$_2$O$_5$ has a crystal structure consisting of zig-zag chains of edge
shared CoO$_6$ octahedra running along the c axis, with the chains separated by
Se$_2$O$_5^{2-}$ units. Magnetic susceptibility measurements indicate a
transition at 8.5 K to an ordered state. We investigate here the nature of this
magnetic ordering using magnetization and specific heat measurements in
addition to powder neuttron diffraction. A transition to long-range
antiferromagnetic order is found below $T_N$ = 8.5 K as identified by magnetic
susceptibility measurements and magnetic Bragg reflections, with a propagation
vector $\mathbf{k}$ = 0. The magnetic structure shows that the moments align
perpendicular to the c-axis, but with the spins canting with respect to the a
axis by, alternately, $+8^\circ$ and $-8^\circ$. Interestingly, the low-field
magnetic susceptibility does not show the anticipated cusp-like behavior
expected for a well-ordered antiferromagnet. When the susceptibility is
acquired under field-cooling conditions under a 10 kOe field, the the usual
downturn expected for antiferromagnetic ordering is obtained. Heat capacity
measurements at low temperatures indicate the presence of gapped behavior with
a gap of 6.5 K.
