A Co vermiculite intercalation compound (VIC) magnetically behaves as an Ising-like Heisenberg antiferromagnet on a triangular lattice. Magnetic properties of a Co VIC have been studied using superconducting quantum interference device (SQUID) dc magnetization and SQUID ac magnetic-susceptibility measurements. This compound undergoes magnetic phase transitions at T-N1 (=16.3 K) and T-N2 (=10.7 K), and shows magnetic anomalies at T-p1 (=7.7 K), T-p2 (=5.9 K), and T-p3 (approximate to3 K). The c-axis spin order and the in-plane spin order occur at T-N1 and T-N2, respectively. The magnetic phase diagram for Hparallel toc consists of four phases whose boundaries meet at a multicritical point (T(m)approximate to12 K,H(m)approximate to25 kOe). The freedom of rotation around an axis perpendicular to the spin plane where three sublattice-magnetization vectors lie could be frozen out below T-p1 or T-p2. A superparamagnetic behavior of Arrhenius type is observed around T-p3.