Various Co²⁺-substituted tetrasilicic micas having high contents of Co²⁺, such as KCoMgLi(Si₄O₁₀)F₂, KCo₂Li(Si₄O₁₀)F₂ and KCo_2.5(Si₄O₁₀)F₂, were synthesized through solid-state reaction techniques at 1000, 800 and 800°C, respectively. Lattice constants a, b increased with increasing Co²⁺-substitutions, reflecting the increase in the ionic radius of Co²⁺ against Mg²⁺, while c decreased due to the flattening of octahedral sheets. Both the micas KCoMgLi(Si₄O₁₀)F₂ and KCo₂Li(Si₄O₁₀)F₂ exhibited pink color while the mica KCo_2.5(Si₄O₁₀)F₂ turned blue due to the trace amount of tetrahedral Co²⁺-substitution. Site preference of Co²⁺ ions depended on formation temperature and composition of micas. UV-visible spectra of Co²⁺-micas were similar to that of [Co(H₂O)₆]²⁺. This can be explained in terms of both the nephelauxetic and spectrochemical series of F^-<H₂O<O_a (apical oxygen). IR spectra showed that the tetrahedral distortion changes with increasing Co²⁺-substitutions, i.e., the interatomic distance of Si-O_a lengthens. The Co²⁺-mica KCoMgLi(Si₄O₁₀)F₂ showed paramagnetism while the larger Co²⁺-substituted mica KCo₂Li(Si₄O₁₀)F₂ showed antiferromagnetism. On the other hand, the mica KCo_2.5(Si₄O₁₀)F₂ having the highest Co²⁺-content exhibited only a weaker magnetic interaction between Co²⁺ ions, probably due to the local variations of octahedral vacancy and Co²⁺-occupancy into two octahedral sites M1, M2 and tetrahedral sites.