Gemini Station was a Federation facility active in the 23rd century. In appearance, the station resembled two K-class space stations attached at their bases. The modules could be separated at the base and operate independently.
In the fourth year (2269-70) of its five-year mission, the USS Enterprise was conducting research into the origin of the universe at Gemini Station. An experiment successfully formed a cloud of quark gluon plasma, but it grew exponentially and overwhelmed the station. Spock, trapped aboard the space station, relied on Chekov's ingenuity as acting science officer to help rescue him. They then needed to collapse the cloud before it became a hazard. (TOS - Year Four comic: "Issue 5")