Ardana was a Federation member world, widely known for its capital city of Stratos, an anti-gravity elevated metropolis. It was also known as the only known planet that mines zenite.
Native to the planet were a humanoid race, Ardanans, who were divided into two castes, a labor class of Troglytes and an intellectual ruling class of Stratos city-dwellers. The caste system was riven with conflict. Despite its membership in the Federation, Ardana employed torture and capital punishment.
In 2269 the USS Enterprise came to obtain zenite for use to prevent a bacterial botanical plague on Merak II. (TOS: "The Cloud Minders")
Background
According to a non-canon source, The Worlds of the Federation, the star that Ardana orbits is named Mu Leonis A, Ardana being its second planet.
It has been speculated that this planet's zenite has been an enormous source of wealth, enabling the aristocratic population such luxuries as a floating city. Although it seems likely, no revelation was ever made regarding whether or not Kirk and Spock made a full report on Ardana's social policies to the Federation, or whether the Federation could do anything about it. However, in giving the Troglytes the filter masks to protect against the mind impairing zenite gas, they made the planet's underclass a much more potent political force to oppose the oppression.
According to DS9: "Accession", Ardana's policies of caste system oppression, executing political dissidents, and public torture would be in direct contradiction to Federation guidelines established by the 2370s for practices accepted by its member worlds.