  We have performed a resonant x-ray scattering study at the Co pre-K edge on a
single crystal of Ca3Co2O6. The measurements reveal an abrupt transition to a
magnetically ordered state immediately below T_N = 25 K, with a magnetic
correlation length in excess of 5500 {\AA} along the c-axis chains. There is no
evidence for modifications to the Co$^{3+}$ spin state. A temperature dependent
modulation in the magnetic order along the c-axis and an unusual decrease in
the magnetic correlation lengths on cooling are observed. The results are
compatible with the onset of a partially disordered antiferromagnetic structure
in Ca3Co2O6.
