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We present the results of X-ray and magnetic neutron diffraction studies on a single crystal of a first stage CoCl2intercalated graphite which is a 2D XY system in which the Kosterlitz-Thouless theory can be tested. The correlation length of the in-plane ferromagnetism increases as the temperature is lowered and diverges at 4.5 K. The c-axis antiferromagnetic order appears first at 6.5 K, but the mechanism of the 2D ordering is not modified. For this reason the 2D analysis is justified. We have then analysed the magnetic intensities and the correlation lengths as functions of temperature and shown the presence of defects (vortices) which lower the measured magnetization in the ferromagnetic planes as predicted in the 2D XY systems.